Dirks Umn 0130E 21159.Pdf

Using binaural beat sensitivity to describe mechanisms that may enhance binaural interactions in single-sided-deafness cochlear-implant patients

A Dissertation SUBMITTED TO THE FACULTY OF UNIVERSITY OF MINNESOTA BY

Coral Erica Dirks

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

Peggy B. Nelson, Ph.D.

April 2020

Acknowledgements To:

Andrew and Peggy for believing in me,

Ethan and Pickles for loving me,

Mom, Wally, Sue, and Tom for strengthening me,

Dad for guiding me,

Lydia + Jordan, Isaiah, Jenna + Tom + Isla, and Ethan for uplifting me,

Alex for sticking with me,

Keith, Diane, Mark, Nisha, and Aaron for healing me,

Andy and Danny for supporting me, and

Jetty, Bijoy, Jared, Drew, Perry, and Chris for inspiring me.

Thank you.

Special thanks to:

• Center for Applied and Translational Sensory Science (CATSS), NIH grant R01

DC012262 (A.J.O.), and NIH grant F32DC016815-01 (C.E.D)

• Committee members Bert Schlauch and Matt Winn

• Research subjects

• Marge + Bink Hanson, Jan Blades, and Dick Hollander

• Michele and Travis Dirks

• Dave Scott

• Greg, Holly, Erick and Cailin Hollander

• Kris, Tor, Stella, and Haakon Hanson

• Brad, Deb, Olaf and Gunnar Hanson; Thomas Archeigo; Jessica, Kenny, and

Lauren Berglund

• Randy and Dianna Romsdahl

• Hunstad and Romsdahl extended families

• Marge Brady

• APC Lab: Magdalena Wojtczak, Heather Kreft, Anoo Mehta, Sara Madsen, Emily

Allen, Jordan Beim, Juraj Mesik, Chhayakanta Patro, Kelly Whiteford, Daniel

Guest, Hao Lu, Erin O’Neill, Zi Gao, Marion David, Jackson Graves, Lei Feng

• SPLICE Lab: Evelyn Davies-Venn, Courtney Glavin, Matt Waggenspack, Kristi

Oeding

• Winn Lab: Kate Teece, Steven Gianakas, Maria Paula Rodriguez, Hannah

Matthys, Emily Hugo, Lindsay Williams, Siuho Gong, Michael Smith

• Josh Bernstein, Olga Stakhovskaya, Kenneth Jensen, ORAU, and the Speech

and Language Pathology Center at Walter Reed National Military Medical Center

• Naomi Croghan and Cochlear Americas

• Jon Hartmann, Paul Manzanec, Brent Lucas, and Envoy Medical

• Adam Svec, Dorea Ruggles, Tess Koerner, and Trevor Perry

• SLHS Ph.D. cohort, especially Katie Bangert and Shriya Basu

• SNAP Fitness NE

• University Lutheran Church of Hope

“If I have seen further than others, it is by standing upon the shoulders of giants.”

--Isaac Newton

Table of Contents Acknowledgements i List of figures v List of tables vii Chapter 1: Introduction 1 A brief history of the cochlear implant 1 How CIs work 2 CI candidacy and outcome measures 5 SSD: spatial-hearing mechanisms and impairments 8 Treatment options and outcomes 14 Binaural integration: technical approaches to a new fitting philosophy 21 Description of Thesis Chapters 34 Chapter 2: Mechanisms of Localization and Speech Perception with Collocated and Spatially Separated Noise and Speech Maskers Under Single-Sided Deafness with a Cochlear Implant 36 Introduction 36 Experiment I: Localization 41 Materials and Methods 41 Procedure 46 Data Analysis 50 Results 51 Discussion 58 Experiment 2: Speech recognition with spatially colocated and separated noise and speech maskers 59 Materials and Methods 59 Results 63 Discussion 68 General Discussion 71 Conclusions 72 Chapter 3: Sensitivity to binaural temporal-envelope beats with single- sided deafness and a cochlear implant as a measure of tonotopic match 74 Introduction 74 Methods 76 Participants 76 Broadband stimuli and procedure 78 Narrowband stimuli and procedure 80 Results 82 Discussion 85 Conclusions 87

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Chapter 4: Spatial hearing and speech perception in cochlear-implant users with unilateral hearing loss using maps designed to optimize binaural interactions 88 Introduction 88 General methods 92 Participants 92 Mapping process 94 Testing protocol 97 Sound booth 98 Data analysis 99 Experiment I: Localization 100 Rationale 100 Stimuli 100 Procedure 101 Results 102 Discussion 105 Experiment II: Speech recognition with spatially collocated and separated noise and speech maskers 107 Rationale 107 Stimuli and listening configurations 107 Procedure 109 Data analysis 110 Results 111 Discussion 117 General discussion 118 Conclusion 124 Chapter 5: Conclusions 125 General Discussion 125 Future Directions 131 Bibliography 136

List of figures

FIGURE 2.1 AUDIOGRAMS OF ACOUSTIC EAR IN LISTENERS WITH SINGLE-SIDED DEAFNESS WITH A

COCHLEAR IMPLANT (SSD+CI). 42

FIGURE 2.2 LOCALIZATION ERROR [ROOT-MEAN-SQUARE (RMS) ERROR IN DEGREES] FOR LISTENERS WITH

SINGLE-SIDED DEAFNESS WITH A COCHLEAR IMPLANT (SSD+CI) IN THE SOUNDFIELD, ORGANIZED BY

STIMULUS GROUP. 53

FIGURE 2.3 LOCALIZATION ERROR (ROOT-MEAN-SQUARE IN DEGREES) ORGANIZED BY STIMULUS GROUP

FOR NORMAL-HEARING (NH) LISTENERS IN THE SOUND FIELD (FIRST PANEL), SINGLE-SIDED

DEAFNESS (SSD) LISTENERS IN THE SOUND FIELD (SECOND PANEL), AND NH LISTENERS IN THE

VOCODER SIMULATIONS (THIRD PANEL). 54

FIGURE 2.4 LOCALIZATION BIAS IN DEGREES FOR LISTENERS WITH SINGLE-SIDED DEAFNESS WITH A

COCHLEAR IMPLANT (SSD+CI) IN THE SOUND FIELD, ORGANIZED BY STIMULUS GROUP. 56

FIGURE 2.5 LOCALIZATION BIAS IN DEGREES ORGANIZED BY STIMULUS GROUP FOR NORMAL-HEARING

(NH) LISTENERS IN THE SOUND FIELD (FIRST PANEL), SINGLE-SIDED DEAFNESS (SSD) LISTENERS IN

THE SOUND FIELD (SECOND PANEL) AND NH LISTENERS UNDER VOCODER SIMULATION (THIRD

PANEL). 56

FIGURE 2.6 CONFUSION MATRICES FOR EACH STIMULUS TYPE IN LISTENERS WITH SINGLE-SIDED

DEAFNESS WITH A COCHLEAR IMPLANT (SSD+CI) IN THE BINAURAL LISTENING CONDITION. 57

FIGURE 2.7 SPEECH RECOGNITION THRESHOLDS FOR LISTENERS WITH SINGLE-SIDED DEAFNESS WITH A

COCHLEAR IMPLANT (SSD+CI). 63

FIGURE 2.8 SPEECH RECOGNITION THRESHOLDS FOR NORMAL-HEARING (NH) LISTENERS IN THE SOUND

FIELD (FIRST PANEL), NH LISTENERS IN THE VOCODER SIMULATION (SECOND PANEL), AND

LISTENERS SINGLE-SIDED DEAFNESS WITH COCHLEAR IMPLANT (SSD+CI) IN THE SOUND FIELD

(THIRD PANEL) GROUPS, RESPECTIVELY. 66

FIGURE 3.1 AUDIOGRAMS OF THE ACOUSTIC-HEARING EAR IN PARTICIPANTS WITH UNILATERAL HEARING

LOSS AND A CI IN THE CONTRALATERAL EAR. LINE COLOR AND TYPE DIFFERENTIATE INDIVIDUAL

SUBJECTS. 77

FIGURE 3.2 SENSITIVITY TO BINAURAL TEMPORAL-ENVELOPE BEATS FOR BASAL, MEDIAL, AND APICAL

ELECTRODES (ROWS 1, 2, AND 3, RESPECTIVELY) IN NINE LISTENERS (COLUMNS 1-9) WITH A

UNILATERAL CI AND ACOUSTIC HEARING IN THE CONTRALATERAL EAR. 83

FIGURE 4.1 AUDIOGRAMS OF THE ACOUSTIC-HEARING EAR IN PARTICIPANTS WITH UNILATERAL HEARING

LOSS AND A CI IN THE CONTRALATERAL EAR. 93

FIGURE 4.2 COMPARISON OF CHANNEL CENTER FREQUENCIES IN CLINICAL AND EXPERIMENTAL MAP

FREQUENCY ALLOCATION TABLES. 97

FIGURE 4.3 RMS ERROR AS A FUNCTION OF TIME. DATA ARE SEPARATED INTO PANELS BASED ON

STIMULUS TYPE: LOWPASS WORDS, HIGHPASS WORDS, AND UNFILTERED WORDS. 103

FIGURE 4.4 CONFUSION MATRICES FOR LOCALIZATION TASK DATA POOLED ACROSS SUBJECT AND TEST

SESSION. 103

FIGURE 4.5 SPEECH RECOGNITION THRESHOLD AS A FUNCTION OF TEST SESSION SEPARATED INTO

COLUMNS BASED ON MASKER TYPE AND ROWS BASED ON SPATIAL CONFIGURATION. 112

FIGURE 4.6 SPEECH RECOGNITION THRESHOLD MEASURED IN BABBLE ALONE AS A FUNCTION OF TEST

SESSION SEPARATED INTO PANELS BASED ON TEST CONDITION. 116

List of tables

TABLE 2.1 SSD+CI LISTENER DEMOGRAPHICS. 43

TABLE 2.2 LOCALIZATION STIMULUS DETAILS. 45

TABLE 2.3 PURE TONE COMPONENTS USED FOR MODULATED AND UNMODULATED INHARMONIC

COMPLEX TONE. 45

TABLE 2.4 CENTER FREQUENCIES FOR 16 SUB-BANDS OF ADVANCED BIONICS COCHLEAR IMPLANT. 49

TABLE 3.1 LISTENER DEMOGRAPHICS. 78

TABLE 4.1 SSD+CI LISTENER DEMOGRAPHICS. 94

TABLE 4.2 MEANS, T-STATISTICS, AND P-VALUES FOR TWO-TAILED PAIRED T-TESTS BETWEEN SRT MEANS

IN THE FIRST AND SECOND TEST SESSIONS. 114

TABLE 4.3 TEST STATISTICS FOR ONE-WAY REPEATED-MEASURES ANOVAS WITH A WITHIN-SUBJECTS

VARIABLE OF TEST SESSIONS 2-5 USING THE EXPERIMENTAL MAP ONLY. 115

TABLE 4.4 MEANS, T-STATISTICS, AND P-VALUES FOR TWO-TAILED PAIRED T-TESTS BETWEEN SRT MEANS

IN THE FIFTH AND SEVENTH TEST SESSIONS. 115

TABLE 5.1 SELF-PERCEIVED HEARING ABILITY ON THE SPEECH, SPATIAL, AND QUALITIES OF HEARING

QUESTIONNAIRE AS A FUNCTION OF TEST SESSION. 133

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Chapter 1: Introduction

A brief history of the cochlear implant Cochlear implants (CIs) are the most successful sensory prosthetic in the world.

Like many scientific breakthroughs, the idea of electrical stimulation of the auditory nerve was born out of curiosity. Alessandro Volta is credited with discovering electrical stimulation of the auditory system; he observed that passing current through his own head yielded auditory sensations. The mechanisms of electrical hearing, however, were investigated much later.

In the 1930s, German physicians placed a stimulating electrode in the middle ear space of people with and without middle ear structures and measured electrically evoked pitch perception (Stevens, 1937; Stevens & Jones, 1939).

Through a series of carefully controlled experiments, the researchers ruled out the middle ear as a source of electrical hearing. Around this time, an American scientist showed that electrical stimulation of a healthy inner ear could create auditory perceptions, which he called “electrophonic hearing” (Jones, Stevens, &

Lurie, 1940). Electrical stimulation of the healthy inner ear caused cochlear hair cells to release chemicals that signaled the auditory nerve to fire just as they would in response to acoustic stimulation. A group of Russian scientists also proved that the auditory nerve was a site of electrical stimulation; they demonstrated that direct stimulation of the auditory nerve produced no changes in pitch perception across a wide range of stimulus frequencies (Gersuni &

Volokhov, 1936).

The idea of using electrical stimulation as a treatment for deafness, however, only emerged in the 1950s. Up to this point, auditory sensations in response to electrical stimulation had only been documented in people with functioning inner ears. André Djourno, a basic scientist and electrophysiologist, and Charles Eyriès, an otolaryngologist, were the first to show that auditory sensations could still be produced in non-functioning inner ears. Djourno and

Eyriès implanted an induction coil into a patient with facial paralysis and bilateral hearing loss. During and after implantation, the patient could discriminate low- frequency sounds from high-frequency sounds in response to electrical stimulation (Djourno & Eyries, 1957; Djourno, Eyries, & Vallancien, 1957b,

1957a).

William House continued this work in the 1960s and, with the help of several collaborators, devised the first CI for human patients: a simple gold wire.

These discoveries have since stimulated decades of research across the globe into the biocompatibility, physiology, and psychophysics of cochlear implantation.

What emerged was a fully implanted, multi-channel electrode array with an external sound processor that successfully restores audibility of sound and facilitates speech intelligibility in people with hearing loss. Today, several companies manufacture CIs that have restored hearing to a combined total of more than 600,000 children and adults worldwide.

How CIs work Current CIs are transcutaneous; they have external and internal components that communicate across the skin of the temporal bone. The external sound

processor contains a microphone, sound processor, and transmitter. The microphone gathers sound from the environment and delivers it to the sound processor. The sound processor breaks the incoming sound into smaller packages of sound that can be delivered across the skin to the internal components. The external transmitter then sends power and the processed sound packages across the skin via radiofrequency to the internally implanted receiver/stimulator. The receiver/stimulator decodes the sound packages and uses those instructions to generate stimuli. A cable connects this component to the electrode array, which delivers the stimuli to the auditory nerve in the form of electrical pulses (Zeng, Rebscher, Harrison, Sun, & Feng, 2008).

The auditory sensations that CI users experience largely depend on the way sound processors manipulate sound. Signal processing schemes are similar across manufacturers. Over a given time window, the sound processor splits frequencies between ~150-8000 Hz (Richard J.M. Van Hoesel, 2012) into different “channels” of information that are often equal in number to the number of electrode contacts on the array; frequencies outside this range are discarded.

Within each channel, the temporal envelope of the sound in that time window is estimated and the temporal fine structure is removed. Advanced signal processing schemes are often introduced at this stage in an attempt to reduce eliminate noise and improve sound quality.

At this point, sound information is sent across the skin to the receiver/stimulator and the electrode array, which houses multiple electrode contacts are equally distributed along the length of the cochlea (although not to

the full extent.) Information within each spectral channel is delivered to a unique electrode contact on the array depending on the frequency content. To mimic spectral processing in the normal inner ear, high-frequency information is delivered to more basal electrode contacts whereas low-frequency information is delivered to more apical contacts. Temporal changes within a given channel/electrode are conveyed through a series of electrical pulses. The pulses are often equally spaced across time and mimic the temporal envelope of the original sound in terms of electrical current amplitude.

Despite technological advances in sound coding and implant design, some aspects of hearing cannot be successfully restored in CI users (Oxenham

& Kreft, 2014). The most significant design limitation of CIs is that stimulating electrodes sit in the scala tympani of the Organ of Corti, a fluid-filled space.

These types of solutions conduct electricity which cause electrical current to spread outward from an electrode contact. Rather than targeting one neuron, electrical spread stimulates several spiral ganglion neurons in the surrounding area leading to a lack of spectral resolution. Spectral resolution is critical for music and speech in noise perception. Furthermore, most signal-processing strategies remove the temporal fine structure of a sound, which is critical for music perception (Mehta & Oxenham, 2017; Smith, Delgutte, & Oxenham, 2002).

In summary, some spectral and temporal information is transmitted to the

CI user, but some important pieces are removed during processing. The pieces that are preserved and removed have important implications for how listeners perceive every day environmental sounds.

CI candidacy and outcome measures Hearing aids work best for people with mild to moderate hearing loss; they increase the intensity of low-level sounds above threshold and maintain the intensity of mid- and high-level sounds using compression. Hearing aids are less effective, however, for severe to profound types of hearing loss (H. Dillon, 2012).

At high output levels, hearing aids introduce spectral and temporal distortion into signals that degrade speech intelligibility (Arehart, Kates, Anderson, & Harvey,

2007; Tan & Moore, 2008). More severe types of hearing loss can be associated with poorer sensory, neurological, and metabolic cochlear health which further limits sound integrity (Yamasoba et al., 2013). Thus, people with severe and profound hearing loss primarily use hearing aids to enhance environmental sound awareness and aid in lip-reading.

First generation CI users achieved similar outcomes. When first marketed to the public in the 1980’s, single-channel CIs were intended for people with profound sensorineural hearing loss (>100 dB HL pure tone thresholds) whose amplified thresholds failed to reach 60 dB and could not understand speech with powerful, best-fit hearing aids (Niparko, 2009). With the introduction of the multichannel electrode array and advances in signal processing schemes, far more people perceived speech through the implant even in the absence of visual information (Sladen et al., 2018; Sladen, Carlson, et al., 2017). In a landmark study, Friesen et al. (2001) showed that some CI users could achieve ~80% open-set speech intelligibility scores in audio-only conditions in quiet and speech- shaped noise at high signal-to-noise ratios with as few as 4 channels. Optimal

performance was achieved with 8-10 channels for most CI users beyond which performance reached a plateau. Open-set speech intelligibility quickly superseded sound detection as the primary outcome measure.

Given that verbal communication was partially restored in most CI users, scientists and clinicians began to explore the potential benefit of implantation in other groups of people who obtain limited benefit from amplification. One population that struggles to benefit from hearing aids, despite good residual hearing, is people with normal to moderate thresholds in low-frequency regions and severe to profound hearing thresholds in the high frequencies. In these cases, periodicity and voicing cues are salient but not consonant-level information (i.e., speech “clarity”) hearing aid limitations (inability to provide sufficient gain, harmonic distortion, spread of excitation at high output levels).

High-frequency information can be restored through frequency compression or transposition, but this type of signal processing is unnatural and can interfere with low-frequency information (Simpson, Hersbach, & McDermott, 2005).

Cochlear implantation offered an alternative treatment option.

In the mid 1990s, the University of Iowa collaborated with Cochlear

Americas to create the first hybrid (or acoustic-electric) implant, whereby a hearing aid is coupled to a CI with a short electrode array (“Cochlear Implant:

Why Choose Us? | University of Iowa Hospitals & Clinics,” n.d.). Hybrid implants deliver acoustic stimulation to low-frequency regions and electrical stimulation to high-frequency regions in the same ear. Combined with "soft” surgery, an approach designed to reduce cochlear trauma during insertion (Lehnhardt,

1993), hybrid implantation had the potential to preserve residual acoustic hearing and improve high-frequency hearing sensitivity. Research consistently shows that acoustic+electric stimulation provides superior speech intelligibility outcomes in quiet and noise compared to preoperative performance. Postoperative performance under acoustic+electric stimulation is also better than listening with either mode alone in the implanted ear. Unfortunately, it is common for low- frequency hearing thresholds to decline after surgery (Boggess, Baker, &

Balkany, 1989). Hearing preservation remains an active line of research among surgeons and manufacturers today.

Around this time, research emerged showing that listening with two ears is often better than one, even in cases of combined acoustic and electric hearing.

Localization acuity and speech intelligibility in spatialized noise improved postoperatively in bilateral hearing aid users who received a CI in one ear and continued using a hearing aid in the contralateral ear (“bimodal” hearing) (Gifford,

Dorman, Shallop, & Sydlowski, 2010; Hughes, Neff, Simmons, & Moeller, 2014).

Spatial hearing also improved in hybrid CI users; postoperative outcomes under acoustic+electric stimulation were superior to preoperative best-aided performance as well as postoperative performance using either mode alone in the implanted ear (Ching, van Wanrooy, & Dillon, 2007). Clinically, spatial hearing outcomes are rarely measured. However, the benefit of listening with two ears spurred a trend in bilateral cochlear implantation. Work from the Binaural

Hearing and Speech Laboratory and others consistently demonstrate that people with bilateral severe to profound hearing loss typically benefit more from two CIs

than one (Kan & Litovsky, 2015). In some cases, binaural cochlear implantees perform better than bimodal implantees, likely because the type of the sound input is matched across ears (Adel, Nagel, Weissgerber, Baumann, & Macherey,

2019; Carlyon et al., 2010; Yawn et al., 2018).

In November 2000, the Federal Drug Administration (FDA) revised CI guidelines to include people with bilateral moderate to profound sensorineural hearing loss and limited benefit from amplification. Limited benefit was defined as preoperative test scores ≤ 50% sentence intelligibility in the ear to be implanted and ≤ 60% in the opposite ear or binaurally (Sladen et al., 2018). Given the high level of speech intelligibility that CIs consistently provided, "off-label" cochlear implantation became more wide-spread. Reports emerged showing that people who do not meet traditional CI indications benefit from a CI not only in terms of speech understanding but also tinnitus reduction and spatial hearing. This spurred the first change in CI candidacy criteria since 2001. On July 22, 2019,

MED-EL announced that its CI system received FDA approval for people with asymmetric hearing loss, including those with the most extreme case: single- sided deafness (SSD).

SSD: spatial-hearing mechanisms and impairments SSD refers to a condition in which people have normal hearing in one ear and severe-to-profound hearing loss in the other. People with asymmetric hearing loss, on the other hand, have some amount of hearing loss ranging from mild to severe in the better-hearing ear. It is estimated that approximately 200 new cases of SSD per million people are diagnosed each year (Baguley, Bird,

Humphriss, & Prevost, 2006), implying about 50,000 new cases per year in the

US adult population alone. It’s easy to assume that having two ears is like having two kidneys; the second provides additional support and, with some readjustment, a person can function normally without it. While it is possible to function with one ear, listening with two-ears is often more advantageous particularly in terms of spatial hearing.

One basic function of hearing is to locate objects in an observer’s surroundings. Three parameters define the orientation of an object in space relative to the observer: horizontal and vertical direction and distance between the source and observer. Sound localization in the vertical plane is primarily mediated via the spectral shape of sound above ~7000 Hz; as an object moves up and down, the ear, head, and torso reflect, diffract, and absorb sound, creating unique patterns of peaks and notches in the frequency spectrum

(Risoud et al., 2018). Since this is purely a physical effect, the vertical position of an object is learned throughout a listener’s lifetime and can be determined with one ear alone. Auditory distance is primarily determined by the intensity of a sound; as intensity increases, perceived distance between the object and observer decreases. In free-field conditions, the intensity of a sound decreases 6 dB for every doubling of source distance. Other factors that contribute to auditory distance perception are direct-to-reverberant energy ratio, spectrum, binaural cues, and dynamic cues (Zahorik, Brungart, & Bronkhorst, 2005). Similar to vertical localization, distance perception can be accomplished with one ear.

Three different mechanisms can be used to judge the horizontal position of an object: interaural level differences (ILDs), interaural timing differences

(ITDs) in the fine structure, and ITDs in the stimulus envelope (Blauert, 1997;

Middlebrooks & Green, 1991). ILDs refer to a difference in intensity across ears.

ITDs in the stimulus fine structure refer to minute temporal fluctuations in a signal. ITDs in the stimulus envelope refer to the broader temporal fluctuations in a signal. In humans with normal-hearing (NH), it is believed that ITDs in the fine structure are the dominant cue below about 1200 Hz (even when ITDs in the stimulus envelope are present) and that sensitivity is most acute between 500 and 800 Hz. Beyond about 1-meter, low frequencies diffract around the human head and reach the contralateral ear at about the same intensity as the ipsilateral ear. Above ~1000 Hz (the limit is somewhat unclear), the auditory nerve can no longer encode ITDs in the fine structure with maximum efficiency; the ILD mechanism does not appear to be fully activated either (Mills, 1958). Above

~1500 Hz, ILDs become the dominant localization cue. High frequencies cannot diffract around the head; therefore, the head acts as an acoustic barrier, attenuating high-frequency sound components as they travel to the contralateral ear. People with normal hearing can detect and discriminate differences in horizontal sound location between 1 and 10°, depending on the stimulus and the direction that sounds are coming from.

Sound localization is possible with one ear but requires substantial learning. In general, listeners tend to lateralize sound to the acoustic-hearing ear side. The lack of binaural input precludes people with unilateral hearing loss from

using timing cues to localize sound in the horizontal plane or benefit from binaural squelch and redundancy. Due to the head-shadow effect, this population can use the relative intensity of a sound above ~1.5 kHz to judge whether the source occurs on the better- or poorer-hearing. Monaural head-shadow cues are ambiguous for sounds with unknown intensities; therefore, a person must learn the average intensity of sound sources (e.g., how loud a common communication partner typically talks) in order to localize sound on intensity alone (Slattery &

Middlebrooks, 1994; Van Wanrooij & Van Opstal, 2004). Similarly, the pinna, head, and torso filter high-frequency components of a sound differently based on its incident angle. Over time, people learn to associate the resulting peaks and notches in the frequency spectrum above 4 kHz with different elevations and, to a lesser extent, horizontal position (Batteau, 1967; Middlebrooks & Green, 1991).

With experience (Firszt, Reeder, & Holden, 2017) and training (Yu et al., 2018), people can learn to use these cues to localize with moderate accuracy.

Unfortunately, monaural cues are fragile. The salience of spectral cues depends on the bandwidth of a sound (Goossens & Van Opstal, 1999;

Middlebrooks, 1992) and is easily disrupted by interfering sound (Pedley &

Kitterick, 2017) or physical changes to the pinna (P. M. Hofman & Van Opstal,

2003; Paul M. Hofman, Van Riswick, & Van Opstal, 1998). Moreover, the ability to extract monaural cues varies considerably across listeners. High-frequency hearing loss can reduce or eliminate spectral pinna cues (Agterberg, Hol, Van

Wanrooij, Van Opstal, & Snik, 2014). Sensitivity to differences in spectral shape also varies across listeners and is one factor that is known to affect vertical

sound source localization even more so than individual pinna cues (Andéol,

Macpherson, & Sabin, 2013; Majdak, Baumgartner, & Laback, 2014).

A higher-level function of hearing is speech communication. In quiet single-talker situations, speech communication is easy with one ear provided the signal is audible. In more complex listening situations, however, listening with two ears is often better than one. Similar to sound localization, it is possible to separate speech from noise or interfering talkers using monaural ILD-based mechanisms. Sound intensity is equal across ears when speech and maskers are collocated directly in front of the listener. When the masker moves toward one ear, however, the head and torso attenuate high-frequency components of the masker as it travels to the contralateral ear. The signal-to-noise ratio increases at the contralateral ear, allowing the listener to ignore information from the ear facing the masker. This is called the head-shadow effect (Dieudonné &

Francart, 2019a). When the masker faces a deaf ear, unilateral hearing loss can actually be advantageous. The signal falling on the deaf ear does not interfere or compete with the better-hearing ear which has the better signal-to-noise ratio.

Unilateral hearing loss is less desirable, however, when the masker is facing the better-hearing ear and the listener cannot reorient themselves in the listening environment. Listening with two ears ensures that the listener will always have access to a “better” ear (Firszt et al., 2017).

Two other mechanisms could be available to a person with SSD if hearing to one ear is restored. The first is binaural summation or redundancy. When a second ear with identical information is added, such as when speech and noise

are collocated in front of the listener, the brain uses the highly correlated information to better estimate the speech and masker components of the incoming signal. In addition to the head shadow effect, physically separating speech and noise introduces ITDs, a truly binaural phenomenon. Combined, these effects comprise the second mechanism: binaural squelch. In the literature, squelch is defined as the benefit of adding an ear with a poorer signal-to-noise ratio when speech and masker are spatially separated. The unique contribution of ITDs, recently called binaural contrast, can be estimated by taking the difference between binaural listening conditions where speech and masker are separated and then collocated. Broadly, the field has called the improvement in speech intelligibility due to spatial separation between target and masker as spatial release from masking, whether a person is listening with one or two ears.

In their new framework, however, Dieudonné and Francart (2019a) propose that spatial release of masking is the sum of monaural (head shadow) and binaural

(binaural contrast) contributions.

The incident angle of the target and masker sources influences the size of each effect (excluding redundancy). Physically, the head-shadow effect is largest when a sound source occurs at 60° on either side of the head relative to the face

(Culling, Jelfs, Talbert, Grange, & Backhouse, 2012). This suggests that spatial release from masking will be maximized for a person listening to speech at 60° on one side of the head and masker at 60° on the other side of the head. In the real world, however, it is more common and appropriate to face communication partners. Therefore, in the studies that follow, we measure the binaural benefit in

situations where speech is fixed at 0° azimuth and maskers are fixed at 60° on one side of the head or the other.

Spatial release from masking and its component parts also depend on the type of masker. Energetic maskers physically cover up (or mask) frequency components of the target signal in the auditory periphery. Purely energetic maskers, like speech-shaped noise, have a fixed frequency spectrum and statistically stationary temporal envelope (Oxenham & Kreft, 2014). Informational maskers, on the other hand, resemble the target signal in ways that make it challenging for the listener to tease apart the two. “Informational masking,” therefore, is defined as the amount of masking that cannot be explained in terms of peripheral frequency sensitivity (Oxenham, Fligor, Mason, & Kidd, 2003). The most effective informational masker for a target speech stimulus is a small number of same-gender talkers (Freyman, Balakrishnan, & Helfer, 2001;

Freyman, Helfer, McCall, & Clifton, 1999).

In sum, listening with two ears provides two types of cues – monaural and binaural – that can be used to localize sound and understand speech in noisy listening situations. People with unilateral hearing loss can learn to use monaural cues to localize sound with varied success. Those cues can be used to the listener’s advantage when listening to speech in noise but the benefit largely depends on the listener’s ability to manipulate her environment.

Treatment options and outcomes People with asymmetric hearing loss and SSD seek treatment for two primary reasons: tinnitus reduction and restoration of (spatial) hearing. The current

standard of care for this population is to fit them with devices that gather sound on the poorer hearing side and transmit it to the better-hearing ear. These can take the form of contralateral routing of signal (CROS) hearing aids, which comprise a transmitter and receiver coupled to the poorer and better ear, respectively, or an osseo-integrated hearing aid implanted on the mastoid behind the poorer ear. The former transmits sound acoustically via a wireless link and latter via bone conduction. The primary benefit of CROS and bone-anchored hearing aids is that they overcome the head-shadow effect. Sounds that the head would naturally attenuate are made audible in the better-hearing ear. This is particularly useful when listeners cannot easily situate themselves in the environment, such as when sitting at a table at a meeting and trying to hear a conversation on the side of the poorer ear.

CROS and bone-anchored hearing aids are less beneficial when noise is directed toward the poorer-ear side and target speech is coming from the front or better ear side. In this case, the head-shadow effect increases the SNR at the acoustic-hearing ear, making it favorable to have a deaf ear. CROS and bone- anchored hearing aids increase the level of the noise reaching the better ear, requiring target speech to increase proportionally in level to reach desired signal- to-noise ratios. Although both devices reduce performance in these situations,

Lin et al. (2006) and Niparko et al. (2003) suggest that listening with CROS hearing aids is more costly than with bone-anchored hearing aids. This is thought to be due to the fact that CROS hearing aids deliver frequencies above 5 kHz better than bone-conducted sound, which the skin and skull tissue attenuate

during transmission. Agterberg et al. (2019) corroborate this idea showing that, when fit with a bone-anchored hearing aid, people with near-normal hearing sensitivity in the better ear perform better on localization tasks than those with sensorineural hearing loss in the better ear. However, a recent meta-analysis

(Kitterick, Smith, & Lucas, 2016) found no statistically significant difference between aid types in spatially separated speech in noise tasks or on speech perception in quiet. Interestingly, the meta-analysis showed found listening with a bone-anchored hearing aid was more favorable than listening with CROS hearing aids when speech and noise were collocated from front.

Rerouting devices do not improve sound localization (Agterberg et al.,

2019) and, in the case of CROS hearing aids, actually interfere with monaural cues. Pedley and Kitterick (2017) showed that turning a CROS hearing aid on decreases sensitivity to spectral and level cues. They recorded short broadband noise bursts from three different locations at the level of the eardrum on a mannequin. Three different conditions were tested: unaided, CROS hearing aids attached but turned off, and CROS hearing aids attached and turned on. The recorded signals were delivered to NH listeners through insert earphones and manipulated to disrupt spectral or level cues. Spectral cues were disrupted by splitting the signals into different frequency bands and roving the levels of individual bands from trial to trial while preserving overall signal level. Level cues were disrupted by roving the intensity of the overall signal from trial to trial.

Localization accuracy was essentially the same in the unaided and CROS hearing aid attached and turned off conditions; accuracy was higher for

preserved level cues than spectral cues but, in both cases, performance exceeded chance (33%). When the CROS hearing aid was attached and turned on, performance decreased but only for certain sound locations. When spectral cues were preserved, errors were higher on the simulated poorer ear side but unaffected from the front or better ear side. When level cues were preserved, errors were higher on the simulated poorer ear side and for sounds originating from front.

The fitting of CROS or bone-anchored hearing aids, therefore, is a double- edged sword; they increase audibility of sounds occurring on the poorer ear side but at the cost of disrupting natural cues that help the listener accomplish

“spatial” hearing. Given these limitations, it come to no surprise that roughly half of people fit with rerouting devices reject them. In a sample of 178 bone- anchored hearing aid users, the most common reason for rejection was lack of perceived benefit (Wendrich, Kroese, Peters, Cattani, & Grolman, 2017).

Interestingly, one study found that the presence of tinnitus was a prognostic factor for bone-anchored hearing aid success, even though the aid had no effect on tinnitus (Faber et al., 2012). The same study found that hearing loss in the acoustic-hearing ear at 4 kHz was associated with success as well.

Van de Heyning et al. (2008) provided the first report of cochlear implantation as a treatment option for SSD, or the incapacitating tinnitus that can coexist with severe-to-profound sensorineural hearing loss. One common and beneficial side effect of cochlear implantation in people with bilateral severe-to- profound hearing loss is tinnitus suppression. Van de Heyning et al. extended

these findings to a group of 21 people with SSD. Tinnitus intensity and distress were measured using the Visual Analog Scale and Tinnitus Handicap

Questionnaire, respectively, over the course of 12 months. Statistically significant reductions in perceived intensity and handicap were observed immediately. VAS scores decreased from a score of 8.5 out of 10 preoperatively, where higher scores represent louder tinnitus, to 3.5 at 1-month and 2.2 at 3 months postoperatively when the CI was turned on. For most, tinnitus returned within minutes of turning the CI off, but at a lesser intensity; in three patients, tinnitus was completely eliminated postoperatively. Subsequent reports confirm this observation (Arndt et al., 2011; Gartrell et al., 2014; Mertens, De Bodt, & Van de

Heyning, 2016).

Subsequent reports also established that the provision of a CI could partially restore hearing to the poorer ear. Firszt et al. (2018), for instance, showed that speech intelligibility improves significantly after surgery in the implanted ear alone. CNC scores (n=39) increased from 0.04 proportion of key words correctly identified preoperatively to 0.49 at 6 months and 0.51 after 12 months. TIMIT sentence scores (n=38) in quiet increased from 0.01 proportion correct preoperatively to 0.37 and 0.38 postoperatively at 6 and 12 months, respectively.

The largest perceptual benefit of the implant, however, is observed in more complex listening situations. In terms of localization, the addition of a CI improves acuity when monaural level cues are disrupted. Firszt et al. (2018) measured RMS error in 22 adults with asymmetric hearing loss who were fitted

with a CI. 100 monosyllabic words were from an array of 15 speakers (5 inactive) equally distributed across a 140° arc in the horizontal plane. Mean pre-implant

RMS error was 48°. RMS error dropped to 32° after 6 and 12 months of listening experience. More recently, Litovsky et al. (2019) seven recent publications and showed that, on average, postoperative performance in SSD+CI patients is between 20-30° for non-moving broadband sounds, on par with bilateral CI users. In nine of their own SSD+CI listeners, they observed a 27° improvement in

RMS error from acoustic alone to acoustic+electric listening conditions; RMS error was 29° with the addition of the implant. They extended testing in four

SSD+CI listeners to measure detection and discrimination of moving sounds.

Three subjects could correctly identify a moving sound, two of which performed in the same range as NH listeners. None of these listeners, however, could detect direction (right- or left-moving) above chance. The fourth subject could not detect motion and was therefore excluded from the motion direction perception task.

Since CIs preserve relative intensity and periodicity in the stimulus envelope but remove fine structure, it was thought that, with the addition of a CI,

SSD listeners localize sound based on ILDs and possibly ITDs in the stimulus envelope but not in the fine structure. Dorman et al. (Dorman et al., 2015) tested this hypothesis in four SSD+CI listeners using lowpass, highpass, and wideband noises. Sensitivity to fine structure cues breaks down around 1.5 kHz whereas the head shadow effect begins to emerge above 1.5 kHz. Therefore, lowpass- filtered noises (filtered between 125 and 500 Hz) could convey ITDs in the fine

structure and envelope with essentially no contribution of ILDs. Highpass-filtered noises were filtered between 1.5-6 kHz, permitting access to ITDs in the stimulus envelope and ILDs but not ITDs in the fine structure. Finally, wideband words were bandpass filtered from 125 to 6000 Hz and introduced all potential binaural cues. Results confirmed that the addition of a CI partially restores ILD sensitivity;

RMS error was near chance in the lowpass noise condition but close to wideband performance levels in the highpass noise condition. These results are also consistent with the idea that fine structure ITDs are not reliably transmitted through the CI. Since ITDs were present in the stimulus envelope of both low- and high-pass noises, it remained unclear whether and to what extent envelope

ITD sensitivity was restored. Moreover, some CIs attempt to encode fine structure in the most apical channels by delivering pulses at zero crossings of the input signal (Magnusson, 2011). The first chapter of this dissertation helps resolve whether ITDs in stimulus envelope or fine structure can be restored with the provision of a CI and certain signal processing strategies.

The pattern of performance in spatialized speech in noise tasks corroborates the idea that the provision of a CI partially restores ILD perception.

Studies consistently measure a benefit of the CI compared to acoustic-hearing only when noise is directed toward the acoustic-hearing ear. In this case, the head-shadow effect attenuates noise at the poorer ear, improving its signal-to- noise ratio relative to the acoustic-hearing ear. Adding a CI gives listeners access to “better ear.” Few studies, however, find evidence of binaural integration (Williges et al., 2019). Summation and squelch are two binaural

hearing mechanisms in which adding an ear with the same or poorer signal-to- noise ratio improves speech intelligibility. Binaural benefit in these cases is due to redundant (or highly correlated) information and spatial-hearing cues when speech and noise are collocated and spatially separated, respectively. The unique contribution of binaural processing, called binaural contrast, is the difference between these two mechanisms when listening with two ears. In a review of the literature presented at the Conference of Implantable Auditory

Prostheses (including data from Chapter 1), Dieudonne and Francart (Dieudonné

& Francart, 2019b) found meager evidence of true binaural processing in speech intelligibility in noise tasks from two SSD+CI studies. Negative contrast (in which case binaural benefit is primarily due to redundant information, not binaural interactions) was observed in similar populations of bimodal and bilateral CI users.

Binaural integration: technical approaches to a new fitting philosophy At this point, three important questions arise. First, are SSD+CI listeners capable of experiencing true binaural interactions? CIs are currently fit as independent processing units whose primary function is to restore speech perception through the CI alone (Francart & McDermott, 2013). Other than overall loudness balancing, interaural comparisons are not taken into account during the fitting process. Second, what factors limit binaural integration in CI users and which can be modified? For the purposes of this dissertation, I will only focus on place of stimulation and temporal mismatch. Finally, what effect do changes in modifiable

factors have real-world outcomes? The second and third chapter of this dissertation are designed to address these questions in the SSD+CI population.

Data from similar CI populations and simulations of SSD+CI listening provide helpful insights.

The study of binaural interactions is rooted in the idea of binaural fusion: if interaural acoustic and/or electric stimulation can be fused into a single auditory percept then it is possible to study the effect of interaural disparities, such as

ITDs and ILDs. Early work in bilateral CI users showed that streams of electrical pulses could be fused and lateralized to one side by changing interaural pulse amplitude (R. J. M. van Hoesel & Clark, 1997; Richard J. M. van Hoesel & Tyler,

2003). In terms of place of stimulation, Kan et al. (Kan, Stoelb, Litovsky, &

Goupell, 2013) showed that perceptual fusion and ILD sensitivity are quite robust to interaural differences. Bilateral CI users in this study reported hearing one auditory image nearly 75% of the time even when the within-ear mismatch between a test electrode and the electrode perceptually matched to that in the contralateral ear were up to 6 electrode spacings apart. ILD sensitivity was nearly independent of interaural mismatch except at the most extreme mismatch of 8 electrode spacings.

ITD sensitivity, in contrast, is extremely sensitive to interaural frequency mismatch (Batra & Yin, 2004; Joris, Smith, & Yin, 1998; Kan & Litovsky, 2015). In the same study, Kan et al. (Kan et al., 2013) showed significant deficits in ITD sensitivity beyond 2 electrode spacings. Increasing interaural electrode mismatch led to increasingly larger ITD JNDs. In the most extreme cases of mismatch,

subjects lateralized fused images entirely to the left or right side regardless of

ITD; more than half the time, the fused image did not cross the midline when ITD was varied by more than 1 second. A number of studies since then have shown that, under careful control, ITD sensitivity is possible in people with residual acoustic hearing in the contralateral ear (Francart, Brokx, & Wouters, 2009;

Francart, Lenssen, & Wouters, 2011; Francart & McDermott, 2013; Lenssen,

Francart, Brokx, & Wouters, 2011). Francart et al. (Francart, Wiebe, & Wesarg,

2018) gathered data from 12 published studied and plotted the range of ITD

JNDs measured in bilateral and bimodal CI users. Bilateral CI users exhibited a wide range of sensitivity from 50 to <1600 microseconds. Bimodal users a smaller range of performance (~100-300 microseconds) that was lower (better) than the best ITD JNDs in the SSD+CI listeners.

In SSD+CI patients specifically, Francart et al. (2018) first screened for perceptual fusion at electrodes 12, 16, and 22 using band-limited acoustic click trains. To find the best acoustic match for each electrode, the frequency ranges in the acoustic-hearing ear that produced perceptual fusion were narrowed further using an ILD screening measure. A range of temporal delays were also tested to reduce any baseline ITD between the acoustic and electric ears and

“center” the fused image in the head. Only then were ITDs measured. Using this approach, they measured an ITD JNDs of ~400 microseconds at electrode 12 across subjects. Best ITD JNDs were highly variable at electrodes 16 and 22, ranging from as low as 200 microseconds to more than 1600 microseconds.

Binaural interactions have also been demonstrated in higher-level listening tasks, both in simulations of and actual SSD+CI listening. Binaural squelch occurs when the addition of a second ear with more degraded input or a poorer

SNR improves speech intelligibility relative to the monaural condition alone. Ma et al. (2016) vocoded sound to one ear and delivered unprocessed sound to the other in NH listeners to explore whether binaural interactions could be measured in SSD+CI listening simulations. In the vocoded and unprocessed ears, they measured the SNR required to achieve 50% correct speech intelligibility (SNR50) for CRM sentences in a speech-shaped noise masker. They also measured the

SNR required to achieve 71% correct speech intelligibility (SNR71) in the CI ear alone. When they presented target speech to both ears at SNR50, performance increased to ~63%. Interestingly, performance increased roughly another 10 percentage points to ~73% when the noise level in the CI ear was decreased to

SNR71 levels. The benefit of the second NH ear was most apparent when the noise in each ear was fixed at SNR50. Binaural benefit was observed whether the analysis and output filters in the vocoded ear were identical (perfect frequency-to-place alignment), shifted (basal shift of output versus analysis filters alone, some low frequency information removed), or realistic (basal shift of all output channels as well as frequency “compression” in the low-frequency channels, such that all analysis information was fit into the output). However, binaural benefit decreased significantly in the shifted and realistic conditions compared to the ideal condition. This study showed that binaural integration is

possible in SSD+CI listening but it remained unclear to what extent the benefit was due to squelch or better ear listening.

Shortly thereafter, Bernstein et al. (2016) isolated effects of binaural integration in SSD+CI listeners even without explicit attempts to frequency-match the electrode-to-neural interface. They measured contralateral unmasking for

CRM sentences in the presence of energetic and informational maskers at a variety of SNRs. In the monaural condition, a mixture of speech and masker to the acoustic hearing alone. In the binaural condition, a copy of the masker was added to the contralateral (CI) ear. In this way, Bernstein et al. eliminated the potential for head-shadow or better-ear effects in the binaural condition because the target was only presented to one ear. Any binaural benefit would have to be due to perceptual fusion of the masker across ears. As expected, the addition of a second ear improved speech intelligibility in SSD+CI listeners but only in the presence of strong informational masking (single same-gender talker) and at

SNRs between –4 and +4 dB. Release from masking was not observed when the masker was one or two different-gender talkers, where pitch differences could facilitate segregation, or in static noise where perceptual separation is not expected to improve speech intelligibility (Freyman et al., 1999).

In a follow-up study, Wess et al. (2017) directly manipulated spectral and temporal alignment in SSD+CI vocoder simulations to estimate the effect of mismatch in one or both dimensions on contralateral unmasking. They also measured the effect of spectral resolution independently. Due to spectral spread, the authors hypothesized that contralateral unmasking could be robust to some

frequency misalignment as long as output filters (or stimulated regions across ears) overlapped, producing some amount of interaural temporal envelope correlation for the masker. Temporal misalignment could also be tolerated in theory. Syllabic and phonemic temporal envelope modulations occur between 2-

5 Hz and 15-30 Hz, respectively. In terms of modulation period, this corresponds to 200 and 500ms for syllables and 30-60ms for phonemes. Binaural fusion is possible for latency differences of up to 12ms, beyond which normal-hearing listeners begin to perceive an “echo” or two separate streams of sound (Blauert,

1997; Brown, Stecker, & Tollin, 2015). Therefore, the authors hypothesized that contralateral unmasking could be with up to 12 ms of interaural latency differences, beyond which fusion and perceived spatial separation between target and masker would begin to break down. Note, interaural latency difference depend on frequency as well as CI manufacturer. The normal-hearing ear has a frequency-dependent delay; low frequencies take longer to arrive at the auditory nerve than high frequencies. MED-EL implements frequency dependent delays in their signal processor; the NH ear leads the CI ear by ~1 ms below 1.5 kHz above which the CI leads the NH ear (S. Zirn, Arndt, Aschendorff, & Wesarg,

2015). Cochlear and Advanced Bionics users experience 10.5-12.5ms and 9-

11ms delays between ears, respectively, whereby the CI ear lags.

In terms of spectral alignment, results showed reduced contralateral unmasking relative to no frequency mismatch within 2 ERB of misalignment

(Wess et al., 2017). Contralateral unmasking was eliminated beyond 4 ERB.

Unfortunately, this degree of mismatch is commonly observed in CI users. As

expected, contralateral masking was preserved but reduced with interaural delays up to 12 ms. Between 50-100 ms, contralateral unmasking was abolished; binaural performance was equal to monaural performance. Interestingly, a mismatch in more than one dimension was not additive; mismatch in one dimension had a small effect when a mismatch already existed in the other. The authors concluded that spectral and temporal mismatch impair contralateral unmasking but, of the two dimensions, spectral mismatch was the most detrimental. It has been shown that CI users can adapt to frequency-to-place mismatch (L. A. J. Reiss, Turner, Erenberg, & Gantz, 2007; M. A. Svirsky,

Silveira, Neuburger, Teoh, & Suárez, 2004; Mario A. Svirsky, Talavage, Sinha,

Neuburger, & Azadpour, 2015) but it is unclear the extent to which frequency-to- place mismatch affects binaural integration in actual SSD+CI users.

Fortunately, frequency alignment is one variable that can be adjusted easily in clinical CI software. The bigger question, however, is how to probe the frequency alignment of the electrode-to-neural interface on an individual basis.

The most efficient way is to image the cochlea after surgery (Noble, Labadie,

Gifford, & Dawant, 2013) and then combine images of the electrode array position with tonotopic maps of the standard cochleae (D. D. Greenwood, 1990;

Stakhovskaya, Sridhar, Bonham, & Leake, 2007). This method, however, exposes the patient to radiation, albeit a small dose, and cannot probe the underlying neural structure. It is possible that some regions along the basilar membrane do not have viable spiral ganglion connections, a factor that could affect speech intelligibility in CI users (Başkent & Shannon, 2006). CT and x-ray

images cannot detect these “dead regions” nor subsequent synaptic reorganization that could occur (Kandler, Clause, & Noh, 2009).

Another option is to compare the perceptual correlate of frequency, pitch, across the acoustic and electric hearing ears (Schatzer et al., 2014; Vermeire et al., 2008). These methods require the listener to compare the pitch in that an electrode elicits to pitch in the acoustic-hearing ear. Depending on the method, the listener will judge whether the pitch in one ear is higher or lower than the pitch in the other ear, quantify the difference in pitch between the acoustic and electric ear, or manually adjust the pitch in one ear to match the other. Pitch perception, however, is susceptible to sensory adaptation and non-sensory biases.

CI electrode arrays do not traverse the full length of the basilar membrane; the average cochlea is about 35 mm in length (2.5 cochlear turns, or 900°) whereas most electrode arrays, when fully inserted, reach a depth of 20 mm.

This often leads to a mismatch between the frequency information delivered to an electrode and its tonotopic place; the cochlea processes sound between 20 and 20000 Hz whereas CIs typically process frequencies between 200-8000 Hz

(242Hz and 7421Hz for Cochlear, 322Hz and 6346Hz for Advanced Bionics, and

149Hz and 7412 Hz for MED-EL FSP/FS4 strategies). The degree of mismatch, however, varies substantially across patients. Landsberger et al. (Landsberger et al., 2015) examined x-ray images from hundreds of implantees across the literature and found that the average insertion depth of the most apical electrode contact varied from 360° for the Cochlear Nucleus Contour Advance to 540° (the

deepest average insertion observed) for the MED-EL standard electrode array.

Wess et al. (2017) further analyzed these data and showed that the average frequency-to-place mismatch was 3.6-5.4 mm or 4-6 ERB, on average.

In a seminal study, Reiss et al. (2007) showed that pitch perception varies considerably in hybrid CI users, both within a test session up to 1 octave and slowly over time up to 2 octaves. Interestingly, they showed that speech perception scores in the acoustic, electric and combined conditions at the latest testing period were not correlated with early or late pitch sensations with one exception: higher pitch perception early in the study was positively correlated with speech perception through the electric modality alone. This led the authors to conclude that high pitch perception at hook-up may be related to the health of the auditory nerve, with higher pitch perception early on being linked to better neural survival in the basal region of the cochlea. In later studies, Reiss et al.

(2011) showed that early pitch perception at an electrode was more linked to its cochlear position and, in many cases, shifted to match its frequency allocation with experience. It was not uncommon, however, for pitch to shift in the opposite direction away from the assigned frequency allocation. Together, these results suggest that pitch perception should be tested immediately after hook-up to more accurately measure the underlying frequency-to-place interface.

Unfortunately, however, this is not the only issue that clinicians and researchers must contend with. Carlyon et al. (2010) measured pitch perception in new and experienced SSD+CI listeners and showed that the resulting pitch match for a given electrode depended not only on the starting pitch but also the

range of comparison frequencies that are presented. This was observed in all three methods they tested, each of which gave approximately the same pitch match for an electrode as the others with the same stimulus parameters.

Moreover, stimulus characteristics such as pulse rate affected the final pitch match but to a lesser extent. Adel et al. (2019) extended this observation to show that final pitch matches depend on the difference in stimulus quality between the acoustic and electric hearing ears; wideband pulse-like stimuli in the acoustic- hearing ear produced higher pitch matches at middle electrodes compared to sine-wave stimuli. Carlyon et al. (2010) proposed arbitrary guidelines for selecting reliable pitch matches but the number of matches that met those criteria in their study varied considerably within subject, across electrodes, and over time. This phenomenon is not unique to CI users. Goupell et al. (2019) replicated stimulus range effects on pitch perception in normal-hearing listeners as well as bilateral CI users.

Unlike pitch, ITD perception is thought to be less plastic in response to mismatch. ITDs are coded via a small number of “predetermined” neural circuits that, when disrupted during development, results in poor or absent ITD sensitivity in children that cannot be restored (Kan & Litovsky, 2015). This idea is corroborated by the fact that, in bilateral CI users, the best pitch-match for an electrode is often not the place at which best ITD sensitivity occurs (Long,

Eddington, Colburn, & Rabinowitz, 2003). If the goal is to restore binaural hearing, then it makes more sense to exploit binaural hearing mechanisms – or

ITDs – to frequency match the electrode to neural interface (J. G. W. Bernstein,

Stakhovskaya, Schuchman, Jensen, & Goupell, 2018).

To date, there are three studies in people with acoustic hearing in one ear and electric hearing in the other that have used ITD-based approaches to find the best frequency match for CI electrode contacts. The first, described at the beginning of this section, involves a series of bandpass fusion and ILD lateralization tasks to restrict the frequency range of potential matches in the acoustic-hearing ear (Francart et al., 2018). Static ITD sensitivity is then measured in narrower intervals in the restricted frequency range to find the place of best (lowest) ITD sensitivity. Note, fusion occurs over a wide frequency range in CI users which precludes it alone from being used to frequency align the electrode-to-neural interface (Oh et al., 2019, 2017; L. A. J. Reiss, Fowler,

Hartling, & Oh, 2018; L. A. J. J. Reiss, Eggleston, Walker, & Oh, 2016; L. Reiss,

Simmons, Anderson, & Molis, 2018). A second approach exposes listeners to bursts of pulse trains that systematically vary from left- to right-leading on a single trial (J. G. W. Bernstein et al., 2018). In this way, SSD+CI listeners have the opportunity to sense movement as opposed to small shifts of sound from midline. A final physiologically based approach is to measure brainstem activity in response to different pairings of acoustic-frequency and electric-contact stimulation. In bilateral CI users, He et al. (2010) and Hu and Dietz (2015) showed enhanced brainstem activity in response to binaural stimulation of presumably frequency-matched cochlear regions compared to the sum of monaural responses to the same stimulation. The electrode pairings that

produced this effect, called the binaural interaction component, were also those that displayed the best static ITD sensitivity. Similar objective measures that correlate with ITD sensitivity include the acoustic change complex (Haywood,

Undurraga, Marquardt, & McAlpine, 2015) and interaural phase modulation following response (Undurraga, Haywood, Marquardt, & McAlpine, 2016).

Each of these ITD-based studies described above, however, suffer from one major limitation. ITD sensitivity is optimal when the reference stimulus appears to originate from midline in the head (Koehnke, Culotta, Hawley, &

Colburn, 1995). This is not an issue for people with identical modes of stimulation across ears – whether acoustic or electric – because the baseline ITD will be nearly the same. In SSD, electric stimulation can arrive at the auditory nerve up to 12ms later than the acoustic stimulation, typically in a frequency-independent manner. Cochlear mechanics impose another frequency-dependent delay in the normal-hearing ear – also known as the traveling wave delay – whereby high- frequency innervation happens ~8ms sooner than low-frequency innervation.

Each of the approaches described above, therefore, must compensate for the baseline ITD before testing can begin.

One potential limitation of frequency matching is the removal of low- frequency information in the CI ear. Due to the lack of head shadow at low frequencies, however, SSD+CI listeners will still have access to fundamental frequency and periodicity cues in the acoustic-hearing ear that occur on the CI side. Sheffield et al. (2019) selectively deactivated apical electrodes in SSD+CI listeners and found that a substantial amount of low-frequency information could

be removed, and the listeners still benefit from perceived spatial separation in a speech in noise task. In other words, the acoustic ear will never be at a disadvantage in terms of signal-to-noise ratio at low frequencies. Low-frequency fine structure cues are also not readily available in most CIs.

In contrast, temporal alignment is easier to measure but cannot be changed in standard clinical software. Zirn et al. (2015) measured and compared

Wave V latencies for tone bursts and pulse trains in acoustic- and electric- hearing ears, respectively. The electric-hearing ears had been implanted with

MED-EL MAESTRO CI system. Results showed frequency-specific group delays in the acoustic- and electric-hearing ears. However, the slope of the delay as a function of frequency was steeper for the CI than the NH ear. Below 1 kHz, the acoustic-hearing ear arrived up to 1.5 ms sooner than the CI ear. Wave V latencies were nearly identical at 1 kHz (observed latencies of ~8 ms) above which the CI led the NH ear by ~1-1.5ms. This group later corrected for these delays and observed improved localization acuity in SSD+CI listeners.

Seebacher et al. (2019) implemented artificial CI processing delays in a group of

MED-EL SSD+CI listeners. In line with Zirn et al.’s physiological measurements, a group delay of 1 ms above 1 kHz produced the best localization accuracy in 12 subjects (median: ~25° for 1 ms delay, ~33° for no added delay). They also measured speech intelligibility for collocated target speech and speech-shaped noise masker. Statistically significant improvements were observed for group delays up to 1 ms beyond which performance dropped relative to the no delay condition.

Each study described above establishes that true binaural hearing is possible in SSD+CI listeners even though it is not readily observed in real-world simulations (e.g., localization of low-frequency sounds, squelch). These studies also describe factors, such as frequency and temporal misalignment, that limit binaural integration and could be modified in clinical practice. Theoretically, frequency and temporal alignment should improve binaural hearing outcomes in

SSD+CI patients, but to date evidence does not support this proposed solution.

Description of Thesis Chapters Chapter 2: It is known that localization and speech perception improve with the addition of a CI in cases of SSD. It is less clear, however, what cues the CI provides SSD listeners that affords better spatial hearing outcomes. The purpose of this chapter is to investigate the following questions: What mechanisms do

SSD+CI listeners use to localize sound? Does improved localization ability improve speech recognition in noisy spatial listening situations (C. Dirks, Nelson,

Sladen, & Oxenham, 2019)?

Chapter 3: Previous research shows that SSD+CI listeners use ILDs to localize sound but not ITDs in the stimulus envelope or fine structure. However, onset

ITD sensitivity can be measured in this population. The first purpose of this chapter is to determine whether listeners are sensitive to dynamic temporal envelope ITDs in the form of binaural temporal envelope beats. The second is to see whether SSD+CI listeners demonstrate frequency-selectivity for binaural temporal envelope beats. If so, this measure could be used to estimate the

frequency alignment of the electrode-to-neural interface (Dirks et al., under review).

Chapter 4: Traditionally, CIs are fit to optimize speech perception through the CI ear alone. In the case of SSD+CI, however, listeners may benefit more from a fitting strategy in which the CI complements information in the NH ear. One factor that is known to limit binaural hearing sensitivity in CI users is the frequency alignment of the electrode-neural interface. The purpose of this chapter is to see whether frequency-to-place matched maps improve binaural interactions compared to standard clinical maps. A localization experiment will be used to test for changes in sensitivity to ITDs in the stimulus envelope and fine structure as well as ILDs. A speech in noise experimental will be used to test for improved speech intelligibility under different maskers and spatial configurations.

Conditions that in NH listeners exploit binaural interactions to provide perceived spatial release from masking will also be tested.

Implications and future directions are summarized in Chapter 5. Combined with the existing literature, the data presented here will have large-scale implications regarding the efficacy of CI as a treatment option for SSD and how the CI should be fit in SSD listeners.

Chapter 2: Mechanisms of Localization and Speech Perception with Collocated and Spatially Separated Noise and Speech Maskers Under Single-Sided Deafness with a Cochlear Implant

Sections reprinted from: Dirks, C., Nelson, P. B., Sladen, D. P., & Oxenham, A. J. (2019). Mechanisms of Localization and Speech Perception with Colocated and Spatially Separated Noise and Speech Maskers Under Single-Sided Deafness with a Cochlear Implant. Ear and hearing, 40(6), 1293-1306.

Introduction Single-sided deafness (SSD) refers to a condition in which people have normal or near-normal hearing in one ear and little or no residual hearing in the other ear. It is estimated that approximately 200 new cases of SSD per million people are diagnosed each year (Baguley et al., 2006), implying about 50,000 new cases per year in the US adult population alone. Despite having normal hearing in one ear, people with SSD find it challenging to localize sounds and to understand speech in noisy environments, especially when sound originates on the side of the deaf or “poorer” ear (Bess, Tharpe, & Gibler, 1986; Lieu, 2004;

Sargent, Herrmann, Hollenbeak, & Bankaitis, 2001).

Most sound localization is mediated through binaural auditory input.

Localization is typically achieved in normal-hearing (NH) listeners by combining information from interaural time differences (ITDs), interaural level differences

(ILDs), and monaural high-frequency spectral cues produced by the head, torso, and pinnae (Blauert, 1997). For broadband sounds under anechoic conditions, the low-frequency (<1500 Hz) ITDs conveyed via the temporal fine structure

(TFS)—rapid oscillations in the stimulus waveform—dominate localization in the

horizontal plane (Kistler & Wightman, 1992). However, when low-frequency cues are not available, localization can be achieved via ILD cues and by ITD cues in the temporal envelope fluctuations of high-frequency complex sounds

(Macpherson & Middlebrooks, 2002). Most people with SSD experience substantial deficits in localizing sounds in the horizontal plane (Newton, 1983;

Slattery & Middlebrooks, 1994). The magnitude of their localization errors are generally greatest when sounds originate on the side of the poorer ear, and sounds presented on the side of the poorer ear tend to be localized to the better- ear side (Angell & Fite, 1901; Gatehouse, 1976; Gatehouse & Cox, 1972;

Jongkees & Van der Veer, 1957). Van Wanrooij and van Opstal (2004) found that people with SSD can use monaural level and spectral cues to localize sound somewhat when the sound originates from the better ear side, but that accuracy degrades in unfamiliar listening environments.

The loss of function of one ear also affects the ability of people with SSD to understand speech in noisy environments in at least three ways. First, the head shadow effect at high frequencies means that the signal to masker ratio

(SMR) will generally be better on one side than the other when the speech and masker come from different locations. If the speech is located on the side of the impaired ear, the SMR at the normal ear will be reduced (Zurek, 1993). Second, the loss of one ear results in an inability to use binaural interactions, including summation of information from the two ears and interaural timing and phase differences, to produce binaural masking release (Durlach, 1963). Third, poorer localization may lead to a reduced ability to use perceived spatial differences

between sources to assist in the segregation of the target speech from an interfering masker, especially when the target speech and masker are similar, leading to “informational masking” (Brungart, Simpson, Ericson, & Scott, 2001;

Freyman et al., 1999).

In principle, people with SSD could take advantage of better-ear effects by orienting their head to ensure that the target speech is always on the side of the normal ear and/or that the masker is on the side of the impaired ear. In practice, however, such orientation may not always be possible. To address this problem, audiologists may fit people with SSD with devices that transmit signals from the impaired ear to the normal ear using contralateral routing of signals hearing aids or bone-anchored hearing aids. However, these devices suffer from some limitations. First, the devices may decrease the SMR when speech originates from the normal-ear side and the masker originates from the impaired-ear side, thereby reducing the better-ear advantage. Second, they do not restore hearing in the poorer ear and so do not provide any usable binaural cues for sound localization or spatial masking release (Agterberg et al., 2019). These limitations may explain why many people with SSD reject such devices (Lin et al., 2006;

Pennings, Gulliver, & Morris, 2011).

Recently, some people with SSD have received a cochlear implant (CI) in their poorer ear. Many have sought a CI for tinnitus relief (Arndt et al., 2017;

Firszt et al., 2017; Mertens et al., 2016; Ramos Macías et al., 2018), while others have sought this treatment option for hearing restoration (Sladen et al., 2018;

Sladen, Carlson, et al., 2017; Sladen, Frisch, et al., 2017). Research suggests

that localization accuracy for fixed single-source sounds improves after implantation and is superior to performance with rerouting devices (Arndt et al.,

2011; Blasco & Redleaf, 2014; Hansen, Gantz, & Dunn, 2013; Hassepass et al.,

2013; Kitterick, Smith, & Lucas, 2016; Stelzig, Jacob, & Mueller, 2011; Van Zon,

Peters, Stegeman, Smit, & Grolman, 2015), but that motion perception (both perceived direction and degree angle of movement) with the CI remains impaired

(Litovsky et al., 2019). The effects on speech perception in noise are less clear, with a few studies suggesting a small benefit (Blasco & Redleaf, 2014; Cabral

Junior, Pinna, Alves, Malerbi, & Bento, 2014; Kamal, Robinson, & Diaz, 2012;

Sladen, Frisch, et al., 2017; Vlastarakos, Nazos, Tavoulari, & Nikolopoulos,

2014) but other reviews and studies failing to identify significant improvements

(Kitterick et al., 2016; Litovsky et al., 2019; Van Zon et al., 2015). A recent FDA clinical trial followed people with SSD with a CI (SSD+CI) for 1 year and found that binaural benefits consistently appear in localization and spatialized speech in noise tests (with the masker originating from the 0° azimuth or 90° on the NH side) at 1 mo but asymptote after 3 mo (Buss, Dillon, Rooth, King, Deres,

Buchman, Pillsbury, Brown, et al., 2018).

Little is known about the specific cues used by SSD+CI listeners to localize sound and potentially improve speech understanding in spatial environments. Preliminary evidence suggests that SSD+CI listeners primarily rely on ILD information to localize sound (Dorman et al., 2015), similar to what has been observed with bilateral CI users (Seeber & Fastl, 2008), whose ITD sensitivity is generally poor (Aronoff et al., 2010; Long, Carlyon, Litovsky, &

Downs, 2006; Noel & Eddington, 2013; Richard J. M. Van Hoesel, Jones, &

Litovsky, 2009). However, the study by Dorman et al. used lowpass filtered, highpass filtered, and unfiltered noise, which did not allow them to isolate each binaural mechanism and estimate its unique contribution to sound source localization ability in SSD+CI listeners. For instance, low-pass filtered noise has both TFS and temporal envelope cues that could be used for localization, and high-pass filtered noise has both level and temporal-envelope cues, at least in terms of onset and offset (Dorman et al., 2015). In a study involving bimodal hearing (people with a CI in one ear and a hearing aid in the nonimplanted ear), four of eight listeners were able to detect ITDs of 91 to 341 μsec between acoustically and electrically presented pulse trains (Francart et al., 2009). A more recent study from that group shows that, when frequency-specific delays are added to channels, ITD sensitivity can be not only measured in SSD+CI listeners but also used to roughly determine the cochlear region that a given electrode stimulates (Francart et al., 2018). These findings suggest that some ITD sensitivity may also be found in SSD+CI listeners, although it is not known whether this sensitivity is useful for sound localization or speech perception in spatially separated noise.

The aim of this study was to explore the mechanisms underlying localization and speech perception in spatially separated maskers in SSD+CI listeners. The first experiment extended the study by Dorman et al. (2015); it involved the localization of different sounds that were designed to emphasize different spatial cues, including ILDs, ITDs based on TFS cues, and ITDs based

on temporal-envelope cues. The second experiment measured speech intelligibility under different conditions of masker-target spatial separation.

Different types of maskers were used to emphasize either energetic masking or informational masking, where listeners are more likely to confuse the target voice with voices in the masker. It was hypothesized that if the CI improves localization then it may also partially restore the benefits of spatial hearing, particularly in situations where spatial masking release relies on a release from informational masking due to a perceived spatial separation between the target and masker.

Age-matched NH listeners completed each of these experiments in the sound field and under headphones. Performance in the sound field was measured as a reference for the most benefit that listeners with SSD could potentially receive with a CI. In addition, vocoder simulations were performed under headphones with nonindividualized head-related transfer functions

(HRTFs), recorded under anechoic conditions, to determine whether acoustic hearing in one ear and a simulation of a CI in the other produce the same pattern of results observed in actual SSD+CI listeners.

Experiment I: Localization

Materials and Methods

Listeners Five SSD+CI listeners participated. Two of the SSD+CI listeners used MED-

EL implants with “Fine-Structure Processing” (FS4-p in SSD-12 and FS4 in SSD-

16) enabled in the four most apical electrodes of their clinical maps. This type of processing is designed to maintain some information about the TFS in the timing

of the electrical pulses presented to the most apical electrodes. The other three listeners used Cochlear devices. See Table 2.1 for further details concerning the

SSD+CI listeners. Pure-tone air conduction thresholds at all audiometric (octave) frequencies between 250 and 8 kHz were greater than 70 dB HL in the CI ear and less than 30 dB HL, on average, at 0.5, 1, 2, and 4 kHz in the unimplanted ear (Figure 2.1). Five age-matched control listeners were also enrolled (Table

2.1). The control listeners had NH bilaterally, defined as pure-tone thresholds of

20 dB HL or less at all audiometric frequencies between 250 and 8 kHz.

Figure 2.1 Audiograms of acoustic ear in listeners with single-sided deafness with a cochlear implant (SSD+CI). Numbers represent individual listeners with SSD (Table 1). Numbers in red and blue colors represent hearing thresholds in listeners’ nonimplanted right and left ears, respectively. The black solid line represents average hearing thresholds in the acoustic ear across listeners. Symbols that fall in the gray- shaded region represent hearing thresholds that fall outside the normal range.

SSD SSD+CI NH Implant Etiology Duration Duration CI

Subject age control side of SSD of implant manufacturer

ID age (mos) use (mos) and model

15 33 30 Left Meningitis 7 6 Cochlear

Nucleus CI

422

12 39 36 Left Unknown 18 18 MED-EL Flex

24; FS4-p

16 40 36 Right Unknown 11 16 MED-EL Flex

28; FS4

3 50 55 Left Otosclerosis 44 11 Cochlear

Nucleus CI

422

1 62 62 Right Onset 24 7 Cochlear

following Nucleus

hysterectomy Hybrid L24

4 52 49 Left Unknown 38 12 Cochlear

Nucleus CI

522

10 53 55 Right Unknown 16 9 MED-EL Flex

11 50 49 Right Acoustic 70 36 MED-EL Flex

Neuroma 28

Table 2.1 SSD+CI listener demographics.

Stimuli Listeners’ ability to localize sounds was tested with several stimuli, designed to convey different binaural cues (Tables 2.2 and 2.3). The low- frequency stimuli were designed to provide ITD cues (either in the TFS only, or in both the TFS and envelope) but little or no ILD cues; the high-frequency stimuli were designed to provide ILD cues, and in some cases also ITD cues in the

temporal envelope. Low- and high-pass filtering of the speech was achieved with fourth-order Butterworth filters. The nonspeech stimuli were 300 msec in duration, including 100-msec onset and offset ramps to reduce the potential envelope cues associated with onset and offset. The average duration of each word from the NU-6 words was 463 msec (SD 72 msec).

Freq Stimulus Acronym Basic Details Cues Available

Content Name

Low Unmodulated ULT 200-Hz pure tone ITDs in TFS

Tone

Low Modulated MLT 200-Hz pure tone; amplitude ITDs in TFS, ITD

Tone modulated by 40-Hz sinusoid, envelope information

100% modulation depth via amplitude

modulations

Low Speech LP NU-6 words lowpass-filtered at ITDs from TFS and

640 Hz temporal envelope

High Unmodulated UHC Inharmonic complex tone; pure ILDs, little to no ITD

Complex tone frequencies depended on information in TFS, little

subject group to no ITD information

SSD+CI participants:, used from temporal envelope

channel CFs above 1500 Hz in due to lack of temporal

individual current maps (Table modulation or temporal

3) interactions between

NH listeners: used CFs of the the components

standard MED-EL clinical map

above 1500 Hz (Table 3)

High Modulated MHC Same as high-frequency ILDs, ITDs in temporal

Complex inharmonic complex tone; envelope via 40-Hz

amplitude-modulated by 40-Hz amplitude modulation

sinusoid, 100% modulation

depth

High Speech HP NU-6 words highpass-filtered ILDs, ITDs from high-

at 1500 Hz frequency temporal

envelope

Broadband Speech U NU-6 words All natural binaural cues

(ILDs, ITDs in TFS,

ITDs in stimulus

envelope)

Table 2.2 Localization stimulus details. ITD represents interaural timing differences, ILD represents interaural level differences, and TFS represents temporal fine structure.

Subject PT1 PT2 PT3 PT4 PT5

SSD+CI 15 2184 2869 3805 4991 6486

SSD+CI 12 1977 2713 3858 5238 7335

SSD+CI 16 2227 3064 4085 5656 7352

SSD+CI 3 2184 2869 3805 4991 6486

SSD+CI 1 2184 2869 3805 4991 6486

NH 2207 2994 4045 5450 7331

Table 2.3 Pure tone components used for modulated and unmodulated inharmonic complex tone. For the SSD+CI participants (listed in order of ascending age), the tone frequencies within the complex were selected to coincide with the center frequencies above 1500 Hz of their individual current maps. All sounds were presented at a nominal root-mean-square (RMS) level of

60 dB SPL, as measured at the location corresponding to the position of the listener’s head. This level was sufficiently high to be in the middle of the dynamic range of most SSD+CI listeners’ programmed maps and sufficiently low to fall well below the acoustic detection thresholds of the SSD+CI listeners’ implanted ear. The level of all stimuli was roved by ±10 dB around the nominal level of 60

dB SPL on every presentation to reduce the reliability of monaural loudness cues

(Van Wanrooij & Van Opstal, 2005).

To avoid the detection of low-frequency distortion products potentially generated by the high-frequency stimuli, a threshold-equalizing noise (TEN;

Moore, Huss, Vickers, Glasberg, & Alciintara, 2000) was added to all the highpass filtered nonspeech conditions and presented from all speakers in the horizontal plane (19 total) simultaneously, with the same (correlated) noise presented from each speaker. At the listener’s position in the sound booth, the level of the TEN in the equivalent rectangular bandwidth of the auditory filter around 1 kHz was 10 dB below the level per component of the modulated and unmodulated high complex stimuli and was roved in the same way as the stimuli.

This level was chosen because it was sufficiently intense to mask any low- frequency distortion products but not so intense as to mask the target stimuli themselves. The low- and high-pass cutoff frequencies of the TEN were 20 and

1500 Hz, respectively. The TEN was gated on and off using 100-msec raised- cosine ramps 1 sec before and after stimulus onset and offset, respectively, for a total duration of 2.3 sec. No masking noise was played under the speech conditions.

Procedure

Sound Field Presentation The SSD+CI and NH listeners were tested individually in a large (3.05 m x

3.96 m x 2.59 m) sound-attenuating chamber with 10-cm foam on all walls to reduce reverberation. Speakers (Anthony Gallo Acoustics: A’Diva ti) were located

along the horizontal plane approximately level with the listener’s head at a spacing of 10° from −90° to +90° azimuth. The speakers were placed along the walls of the chamber, with distances from the listener’s head ranging from 1.4 to

2.4 m and were equalized (in terms of time delay, level, and spectrum) to match their outputs at the point corresponding to the center of the listener’s head to that of the frontal speaker (0°), which was 1.8 m from the listener’s head. On each trial a stimulus was presented from one of the speakers with equal a priori probability, and the listener was asked to indicate from which one of the 19 speakers they heard the sound source via a virtual button on a computer screen with a display of the speaker arrangement.

Each type of stimulus was played 5 times from each of the 19 speakers in the front horizontal plane for a total of 95 trials per stimulus and listening condition (monaural or binaural). One stimulus type was presented per block.

The location of the stimulus varied from trial to trial and the presentation order was determined randomly at the beginning of each block. One block of each stimulus type was tested before any other was repeated. The SSD listeners performed this task with the acoustic-hearing ear only (i.e., with the CI turned off) and with the combination of the acoustic-hearing ear and a CI. The order of listening condition and general stimulus category (tonal or speech stimuli) was counterbalanced across listeners. Within each category, the stimulus presentation order was counterbalanced using a Latin squares design.

The NH listeners completed this task with and without masking noise in one ear to simulate SSD. Three types of noise maskers were used: an octave

band noise centered at 300 Hz for low-frequency pure-tone stimuli, a band-pass filtered TEN with cutoff frequencies of 1700 and 8400 Hz for the high-frequency complex-tone stimuli, and noise with the same long-term average spectrum as the unfiltered NU-6 words for the speech stimuli. The masking noise was presented via an Etymotic Research ER1 insert earphone to the ear corresponding to the poorer ear of each NH listener’s age-matched SSD listener and was presented at a fixed level of 75 dB SPL, which exceeded the maximum possible stimulus level (70 dB SPL) by 5 dB but still avoided loudness discomfort.

Headphone and Vocoder Presentation The five NH listeners also completed a simulation of the localization task under headphones. After nonindividualized (KEMAR) HRTFs (Gardner & Martin, 1995) were applied to the stimuli, they were presented either unprocessed or via a tone-excited envelope vocoder to simulate aspects of CI processing (Dorman,

Loizou, Fitzke, & Tu, 1998; Whitmal, Poissant, Freyman, & Helfer, 2007). Under one condition, the unprocessed sounds were presented to one ear (monaural condition, simulating SSD); under the other condition, one ear was presented with the unprocessed sounds, while the other ear was presented with the vocoded sounds (simulating SSD+CI).

A tone-excited envelope vocoder was used with 16 frequency subbands, equally spaced on a logarithmic scale with center frequencies between 333 and

6665 Hz, as shown in Table 2.4. This spacing corresponds to the standard clinical map for Advanced Bionics devices and has been used in several previous vocoding studies (Oxenham & Kreft, 2014, 2016). The temporal envelope from

each subband was extracted using a Hilbert transform, and then the resulting envelope was low-pass filtered with a fourth-order Butterworth filter and a cutoff frequency of 50 Hz. Although this cutoff frequency is much lower than those applied in most current CI speech processors, 50 Hz was used to reduce the possibility that vocoding produced spectrally resolved components via amplitude modulation.

Chan 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 nel

CF 33 45 54 64 76 90 107 127 151 180 214 254 302 359 426 666

3 5 0 2 2 6 6 8 8 3 2 4 2 0 4 5

Table 2.4 Center frequencies for 16 sub-bands of Advanced Bionics cochlear implant. These sub-bands were used for the tone vocoder simulation of the speech understanding in noise task. The effects of current spread were incorporated as described by Oxenham and Kreft (2014) by including the summation of the intensity envelopes from neighboring channels with an attenuation corresponding to 12 dB/octave. Finally, the resultant temporal envelopes were used to modulate pure tones at the center frequencies of each subband before the modulated carriers were summed and presented to the listeners.

Listeners in the vocoder simulations were seated in a small sound- attenuating chamber and stimuli were presented at a nominal level of 60 dB SPL via Sennheiser HD650 headphones. All other aspects of the experiment, including the stimuli, the response screen, and the task, were the same as under the sound field conditions. The listeners always completed this task after completing the sound field task.

Data Analysis Three measures were used to quantify the accuracy of sound localization. The first was the RMS error, defined as the square root of the mean squared difference in degrees between the actual location of the sound source and the reported sound location. This measure provides a combined estimate of systematic bias and response variability, as has been used in many previous studies (Macpherson & Middlebrooks, 2002; Middlebrooks, 1992). The second measure was the systematic localization bias, derived by calculating the mean

(signed) difference between the actual and perceived angles for each source.

This measure separates the variability from the systematic response bias to determine whether the responses were biased toward any given direction.

Finally, to evaluate whether errors were location dependent, the results from each location were evaluated separately using confusion matrices.

Statistical analysis was performed using repeated-measures analyses of variance (ANOVA). Three separate analyses were performed for RMS error and bias. First, the nonspeech conditions were organized according to whether they included high frequencies (high-frequency complex tones) or not (low-frequency pure tones) and whether they contained amplitude modulations or not. The difference in performance between monaural and binaural conditions served as the dependent variable. This was done for SSD+CI listeners only. By analyzing the nonspeech conditions separately, it was possible to isolate each binaural hearing mechanism using similar stimuli and thus estimate the contribution of each mechanism to localization performance.

The next two analyses compared performance in the SSD+CI and vocoder groups, as these two groups were tested under the most comparable conditions.

A repeated-measures ANOVA was performed for monaural performance alone with listener group as the between-subjects variable and stimulus (including both speech and nonspeech stimuli) as a within-subjects factor. Finally, a repeated- measures ANOVA was performed on the difference in performance between monaural and binaural conditions with stimulus type (speech and nonspeech) as a within-subjects factor and group as the between-subjects variable. In those cases where within-subjects effects did not meet Mauchly test of sphericity,

Greenhouse-Geisser corrections were used.

Results Figure 2.2 shows the individual and average performance of the five SSD+CI listeners in terms of localization error. Dark blue bars represent performance without the CI and light blue bars represent performance with the CI. The first analysis was performed to determine whether and how the addition of a CI affected localization errors, based on the spectral and temporal cues available in the stimulus. To simplify the analysis, the nonspeech conditions were organized according to whether they included only high frequencies (providing access to

ILDs; unmodulated high complex tone, and modulated high complex tone) or only low frequencies (providing access to ITDs in the TFS; unmodulated low tone and modulated low tone) and whether they provided ITD cues in the temporal envelope by containing amplitude modulations (modulated low tone and modulated high complex tone) or not (unmodulated low tone and unmodulated

high complex tone). The dependent variable was the difference in performance

(RMS error) between monaural and binaural conditions. The ANOVA revealed a

2 significant main effect of frequency content [F(1,4) = 17.7, p = 0.014, ηp =

2 0.815], but no effect of amplitude modulation [F(1,4) = 0.76, p = 0.43, ηp = 0.16],

2 and no interaction [F(1,4) = 0.84, p = 0.41, ηp = 0.17]. Paired t tests on the difference between monaural and binaural RMS errors separated by frequency content (low versus high) and averaged across modulation type (with or without modulation in the temporal envelope) showed that the RMS error decreased significantly with the addition of the CI under the high-frequency tone conditions

[decrease in error 26°; t(4) = 4.41, p = 0.012, d = 1.47], whereas the change in error was not significant under the low-frequency tone conditions [increase in error 12°; t(4) = −2.19, p = 0.093, d = −0.96]. In summary, binaural localization errors were smaller than unilateral (acoustic-hearing ear only) localization errors when high-frequency information was present. The addition of the CI did not improve performance under conditions relying on low-frequency TFS, and no further improvement was found for either the low- or high-frequency stimuli when temporal-envelope ITD information was added via amplitude modulations.

Therefore, the CI improved performance only when SSD+CI listeners had access to high-frequency ILDs.

Figure 2.2 Localization error [root-mean-square (RMS) error in degrees] for listeners with single-sided deafness with a cochlear implant (SSD+CI) in the soundfield, organized by stimulus group. Stimulus type appears on the x axis: unmodulated low tone (ULT), modulated low tone (MLT), low-pass filtered speech (LP), unmodulated high complex tone (UHC), modulated low tone (MLT), low-pass filtered speech (LP), unmodulated high complex tone (UHC), modulated high complex tone (MHC), high-pass filtered speech (HP), unfiltered speech (U). RMS error in degrees appears on the y axis. Dark blue bars represent performance with the CI turned off and removed. Light blue bars represent performance with the CI turned on. Error bars in the final panel represent ±1 SE. The data from the speech conditions (low-pass filtered speech, high-pass filtered speech, and unfiltered speech in Figure 2.2) appear consistent with this interpretation: the average RMS error was 8° higher, 30° lower, and 17° lower under the binaural condition compared with the monaural condition in the lowpass, high-pass, and unfiltered stimulus conditions, respectively. However, these trends failed to reach statistical significance, as there was no significant effect of speech condition (low-pass filtered speech, high-pass filtered speech, or unfiltered speech) on the difference in RMS error between the monaural and binaural conditions [F(1.04, 4.17) = 5.76, p = 0.071].

Figure 2.3 replots the mean data from the SSD+CI group from Figure 2.2

(Figure 2.3B), along with the mean data from the NH group listening in the sound field (Figure 2.3A) and the NH group listening over headphones with the vocoded

stimuli (Figure 2.3C). The intent of including NH listeners in the sound field was to provide a baseline measure for the maximum (though unrealistic) amount of benefit SSD listeners could obtain with the CI activated. The vocoder simulation, on the other hand, was designed to approximate localization with and without a

CI using nonindividualized HRTFs. Therefore, only the SSD+CI and vocoder simulation groups were compared in the statistical analysis, which included all speech and nonspeech stimuli.

Figure 2.3 Localization error (root-mean-square in degrees) organized by stimulus group for normal-hearing (NH) listeners in the sound field (first panel), single-sided deafness (SSD) listeners in the sound field (second panel), and NH listeners in the vocoder simulations (third panel). Stimulus type appears on the x axis: unmodulated low tone (ULT), modulated low tone (MLT), low-pass filtered speech (LP), unmodulated high complex tone (UHC), modulated high complex tone (MHC), high-pass filtered speech (HP), unfiltered speech (U). Dark blue bars represent the monaural condition. Light blue bars represent the binaural condition. For the monaural condition, one ear was masked for NH listeners in the sound field, and the cochlear implant (CI) was removed for SSD listeners, and no sound was delivered to one ear in the NH vocoder simulation. For the binaural condition, masking noise was removed for NH listeners in the sound field, CI was replaced and turned on for SSD listeners, and vocoded stimuli were delivered to one ear with unprocessed sound presented to the other ear in the NH listeners under headphones. Improvement in localization error (binaural condition subtracted from monaural condition) for each stimulus group appears in a box below the x axis. Error bars represent ±1 SE. A repeated-measures ANOVA on the difference between the monaural and binaural RMS localization error, with group (SSD+CI or vocoder simulation) as a between-subjects factor, showed a significant main effect of group [F(1,8) =

2 12.30, p = 0.008, ηp = 0.61], with the average improvement under the binaural conditions being 17° larger for the NH vocoder group than for the CI+SSD group

(p = 0.023). There was also a significant main effect of stimulus type [F(6,48) =

2 18.70, p < 0.001, ηp = 0.70]. The boxes at the bottom on Figure 2.3 demonstrate this effect by showing the difference in monaural and binaural localization errors across stimuli in the low-frequency, high-frequency, and broadband stimulus groups. Although the interaction between group and stimulus type was

2 statistically significant [F(6,48) = 2.91, p = 0.017, ηp = 0.27], none of the individual contrasts reached significance.

Figure 2.4 shows the localization bias for the individual SSD+CI listeners, along with the average data. Under the low frequency conditions, where localization performance was very poor, adding the CI led to a bias toward the side of the CI. Under the high-frequency conditions, where performance improved with the addition of the CI, the bias was reduced and tended toward zero. Figure 2.5 shows the mean localization bias for the SSD+CI and NH groups

(both with and without the vocoder). The trend of hearing sounds toward the

(simulated) CI was not observed with the vocoded NH group.

Figure 2.4 Localization bias in degrees for listeners with single-sided deafness with a cochlear implant (SSD+CI) in the sound field, organized by stimulus group. Stimulus type appears on the x axis: unmodulated low tone (ULT), modulated low tone (MLT), low-pass filtered speech (LP), unmodulated high complex tone (UHC), modulated high complex tone (MHC), high-pass filtered speech (HP), and unfiltered speech (U). Dark blue bars represent the monaural condition in which the CI was removed. Light blue bars represent the binaural condition in which the CI was attached and turned on. Error bars in the final panel represent ±1 SE.

Figure 2.5 Localization bias in degrees organized by stimulus group for normal-hearing (NH) listeners in the sound field (first panel), single-sided deafness (SSD) listeners in the sound field (second panel) and NH listeners under vocoder simulation (third panel). Stimulus type appears on the x axis: unmodulated low tone (ULT), modulated low tone (MLT), unmodulated high complex tone (UHC), modulated high complex tone (MHC), low-pass filtered speech (LP), high-pass filtered speech (HP), unfiltered speech (U). Dark blue bars represent the monaural condition. Light blue bars represent the binaural condition. For the monaural condition, one ear was masked for NH listeners in the sound field, the cochlear implant (CI) was removed for SSD listeners, and no sound was delivered to one ear in the NH vocoder simulation. For the binaural condition, masking noise was removed for NH listeners in the sound field, the CI was turned on for SSD listeners, and vocoded stimuli were delivered to one ear for NH listeners under headphones. Change in localization bias (binaural condition subtracted from monaural condition) for each stimulus group appears in a box below the x axis. Error bars represent ±1 SE.

Figure 2.6 confirms the impressions provided by the localization accuracy and bias in Figures 2.2–2.5, by plotting actual location against perceived location for SSD+CI listeners when the CI is turned on. A confusion matrix for NH listeners’ binaural localization performance in the sound field for the unfiltered word stimulus is included in the bottom right panel for comparison. Clearly, SSD listeners’ responses were more variable than those of the NH listeners and tended to fall along the diagonal only for the stimuli that included some high- frequency content. For low-frequency stimuli, SSD listeners tended to perceive sounds as originating on the side of their implant. The NH listeners, on the other hand, showed highly accurate and precise responses when listening with two ears, as expected. Actual Location in Degrees in Location Actual

Perceived Location in Degrees (+ = acoustic-hearing ear side, - = CI ear side)

Figure 2.6 Confusion matrices for each stimulus type in listeners with single-sided deafness with a cochlear implant (SSD+CI) in the binaural listening condition. The confusion matrix for normal-hearing (NH) listeners in the unfiltered binaural listening conditions was included (lower right-hand plot) for comparison. For each matrix, ordinate values represent the actual stimulus location. Abscissa values correspond to position that listeners perceived the stimulus to be. Negative values represent degrees on the poorer ear (CI) side and positive values represent degrees on the acoustic ear side. Warmer colors correspond to higher response rates for a given actual/perceived location combination across listeners; cooler colors correspond to lower response rates. Stimulus type appears in white text in the upper right-hand corner of each plot. Stimulus types are labeled as follows: unmodulated low tone (ULT), modulated low tone (MLT), unmodulated high complex tone (UHC), modulated high complex tone (MHC), low-pass filtered speech (LP), high-pass filtered speech (HP), and unfiltered speech (U).

Discussion In general, it appears that SSD listeners were able to integrate information from the CI and their NH ears to evaluate ILDs (Figure 2.2). Without the CI, SSD listeners tended to perceive high-frequency stimuli on the acoustic ear side and low-frequency stimuli as occurring from 0° azimuth. When the CI was switched on, localization errors were ~30° for stimuli with high-frequency energy and slightly biased toward the acoustic ear. Stimuli limited to low-frequency energy were biased entirely toward one ear or the other, depending on the listener

(Figure 2.4), and localization did not improve with the addition of the CI. The results are consistent with those of Dorman et al. (2015), which also suggested that SSD+CI listeners rely primarily on ILD cues to localize sound.

Our SSD listeners’ postimplant localization ability is similar to that found in previous studies using broadband stimuli such as words (Firszt, Holden, Reeder,

Waltzman, & Arndt, 2012), sentences (Arndt et al., 2011, 2017; Hassepass,

Schild, et al., 2013), noise (M. T. Dillon et al., 2017; Jacob, Stelzig, Nopp, &

Schleich, 2011; Mertens, De Bodt, & Van de Heyning, 2017), and environmental sounds (Hansen et al., 2013). In general, these studies have shown that CI listeners with significant residual monaural hearing localize sound better with the addition of a CI than when they are unaided or aided with devices that route signals to the acoustic ear.

In summary, the results confirm previous findings that SSD+CI listeners can use ILDs to some extent. The current study also extends previous work by showing that ITDs in either the TFS or temporal envelope do not contribute to

SSD+CI listeners’ localization abilities. None of the SSD+CI listeners were able

to use to low-frequency TFS information, including the two listeners (SSD-12 and

-16) who had “Fine-Structure Processing” enabled in the four most apical channels of their clinical maps. It is possible that some of the differences observed between NH listeners in the vocoder simulation and SSD+CI listeners are due to the somewhat unnatural physical cues produced by nonindividualized

HRTFs in the NH group; HRTFs were measured in a room setup that was different from the one used in this experiment. Furthermore, NH listeners are not accustomed to listening to vocoded stimuli in conjunction with normal acoustic stimuli and so may experience less perceptual fusion than SSD+CI listeners, who have had time to adjust to the stimulation method. Overall, therefore, it appears that acute presentation of vocoder simulations of a localization task in NH listeners will not necessarily provide accurate predictions of performance by actual SSD+CI listeners.

Experiment 2: Speech recognition with spatially colocated and separated noise and speech maskers

Materials and Methods

Listeners Three additional SSD+CI listeners and three additional NH age-matched controls were tested in experiment 2, along with the listeners from experiment 1, for a total of eight listeners in each group. Listener 4 (who only participated in this experiment) used a hearing aid in the contralateral ear during testing, as he uses the hearing aid in his daily life. Given the amount of hearing loss in this listener’s acoustic-hearing ear, he technically should be classified as bimodal (as opposed

to SSD). This listener reports that he relies mostly on his hearing aid for speech understanding and uses his CI for sound localization. As in experiment 1, the NH listeners completed the task both under sound field and headphone (vocoded) conditions.

Stimuli and Procedure Harvard IEEE sentence lists spoken by a single female talker were used as the target speech (Rothauser et al., 1969). These materials provided sufficient lists to test all the conditions under consideration without repetition in SSD+CI listeners and without repeating each sentence more than once in NH listeners (because

NH listeners completed both sound field and headphone versions of the experiment). The target speech was always presented at an RMS level of 55 dB

SPL, and the masker level was varied to achieve the desired SMR. The masker was either time-reversed two-talker female babble, generated by concatenating random segments from the same IEEE materials as the target speech, or

Gaussian noise, spectrally shaped to match the long-term average spectrum of the target sentences. These two maskers were chosen to produce high and low contributions of informational masking, respectively (Balakrishnan & Freyman,

2008; Freyman, Balakrishnan, & Helfer, 2004). The SSD+CI listeners were tested under two listening conditions: with the acoustic-hearing ear only and with the combination of the acoustic-hearing ear and the CI. Under the sound field conditions, the NH control listeners were tested with unilateral masking noise to simulate unilateral listening conditions, and without unilateral masking to allow full use of binaural cues.

Under the headphone conditions, the NH control listeners were tested either unilaterally or bilaterally with the HRTF-filtered sound to one ear and the

HRTF-filtered and then vocoded sound to the other ear. The speech and masker were presented at five different SMRs. The SMRs were selected individually based on the results from pilot testing in each combination of masker type, listening status (CI on or off), and spatial configuration that was tested in the main experiment (12 total for SSD+CI listeners, 24 total for NH listeners). The pilot testing involved an adaptive one up one down procedure, where the SMR was increased if the listener correctly reported less than four of five keywords and was decreased otherwise. The SMRs tested in the main experiment corresponded to +3, 0, −3, −6, −9 dB, relative to the SMR obtained from the pilot adaptive procedure for each listener. In the main experiment, one SMR was tested for per list, with each list containing 10 sentences. Listeners entered their responses using a computer keyboard and were encouraged to guess as many words as possible. The proportion of keywords (out of five per sentence) correctly identified in each sentence was used to assess performance.

Performance was measured in three spatial configurations: speech and masker co-occurring from 0° azimuth (S0N0), speech at 0° and masker at 60° on the (actual or simulated) CI side (S0NCI), and speech at 0° and masker at 60° on the acoustic ear side (S0NNH). An angle of 60° was chosen because this angle has been found to produce the largest head shadow effects (Culling et al., 2012).

For each SSD listener, 60 conditions were tested (2 masker types, 3 spatial configurations, and 5 SMRs with and without the CI turned on). For each NH

listener, 60 conditions were tested in the sound field (2 masker types, 3 spatial configurations, 5 SMRs, and 2 listening modes) and then the 60 conditions were repeated under headphones for a total of 120 conditions. One sentence list was used per condition and SMR (i.e., five lists per condition when combining across

SMRs).

Data Analysis A speech reception threshold (SRT) was determined for each listener under each condition by logistic regression using maximum-likelihood estimation. The following equation was used to predict the SRT, defined as the SMR at which listeners correctly report 50% of the words in the sentences:

1 �� = 1 + �() where PC is the proportion of correct responses at a given SMR, SRT represents the SMR at PC = 50%, and s represents the slope of the function where s is 1/4

Å~ slope at PC = 50% (Smits, Kapteyn, & Houtgast, 2004). SRT and s were treated as free parameters. A starting set of parameters (the mean SMR used in the experiment and 1 for the value of s) were then used within a minimization routine to find the parameters that produced the largest log-likelihood ratio.

Statistical analysis was performed using repeated-measures ANOVAs with one between-subjects factor (group; SSD+CI sound field and NH vocoder simulation only) and three within-subjects factors (monaural/binaural listening, masker type, and masker location). The dependent variable was SRT in dB. In cases where within-subjects effects did not meet Mauchly test of sphericity,

Greenhouse-Geisser corrections were used. Follow-up analyses were completed using paired t tests.

Results Speech recognition results from the individual SSD listeners (represented by the listener identification number from Table 2.1) are plotted along with the mean data in Figure 2.7; listeners with some hearing loss in the acoustic ear are represented by numbers in gray and listeners with clinically normal hearing (no audiometric thresholds greater than 20 dB HL) are represented by numbers in black. Lower SRTs correspond to better speech understanding. The data show some of the expected effects of spatial separation: for both the noise and two- talker maskers, SRTs were generally lowest (best) when the masker was presented to the side of the CI (S0NCI), and highest (worst) when the masker was presented to the side of the acoustic ear (S0NNH).

Figure 2.7 Speech recognition thresholds for listeners with single-sided deafness with a cochlear implant (SSD+CI). Speech and noise location appear on the x axis and speech reception threshold (SRT) in dB appears on the y axis. Blue bars represent conditions where the masker was speech-shaped noise (SSN). Red bars represent conditions where the masker was time-reversed two-talker babble (TTB). Darker shaded bars represent the monaural condition. Lighter shaded bars represent the binaural condition. Individual data appear as numbers where the numbers correspond to listener identification number from Table 2.1. Numbers in purple color represent listeners with hearing loss and numbers in black color represent listeners with normal hearing. Error bars in the final panel represent ±1 SE.

A repeated-measures ANOVA using the SSD listeners alone, with SRT as the dependent variable and factors of CI status (on or off), masker type (noise or two-talker interferer), and masker position (CI side, middle, acoustic side)

2 revealed no main effect of CI status (on or off) [F(1,7) = 2.24, p = 0.18, ηp =

0.24], but a significant interaction between CI status and masker type [F(1,7) =

2 7.81, p = 0.03, ηp = 0.53], reflecting a trend for more of an effect of CI status in the two-talker babble than in the noise. A significant main effect of masker type

2 was observed [F(1,7) = 451, p < 0.001, ηp = 0.99], showing that two-talker, same-gender babble (average SRT: −0.47 dB SMR) was a more effective masker than speech-shaped noise (average SRT: −10.8 dB SMR). The main effect of masker position was statistically significant [F(2,14) = 53.35, p < 0.001,

2 ηp = 0.88], showing that SRT decreased on average as the masker moved from the acoustic ear to CI ear side. In addition, the interaction between masker type

2 and masker position was statistically significant [F(1.17,8.22) = 10.7, p = 0.01, ηp

= 0.60], showing that SRT increased at a faster rate in two-talker babble than stationary noise when the noise source moved from the CI side to the acoustic ear side. The interaction between CI status and masker position just failed to

2 reach significance [F(2,14) = 3.40, p = 0.06, ηp = 0.33]. Finally, the three-way interaction between CI status, masker type, and masker position failed to reach

2 significance [F(2,8) = 1.33, p = 0.32, ηp = 0.25].

On average, trends toward benefit provided by the CI (compare dark and light shaded bars; without CI and with CI listening conditions, respectively) were observed under all spatially separated conditions, except when noise (blue bars)

originated on the CI side. Notably, the CI made a difference in the most challenging listening situation: when babble originated on the acoustic-hearing side (S0NNH), seven of eight listeners showed an improvement in SRT, with an average improvement of 3.5 dB. Although the main effect of CI status and the interaction between CI status and masker position failed to reach significance, it was planned a priori to compare performance with and without the CI turned on when the maskers originated on the acoustic-hearing side, as this configuration was expected to most clearly demonstrate any observable benefit, based on the improved SMR at the CI. A paired t test (monaural versus binaural performance) revealed a statistically significant binaural benefit when two-talker babble [t(7) =

2.98, p = 0.02] but not speech-shaped noise [t(7) = 1.21, p = 0.26] originated on the acoustic-hearing side. This was the only case where the difference between the monaural and binaural SRT reached statistical significance.

Figure 2.8 replots the mean data from the CI listeners and compares them with the mean data from the NH listeners either under sound field conditions (first panel) or over headphones with vocoder simulations of CI processing (third panel). Performance of the NH listeners with one ear (dark blue and red bars) was generally quite similar to that observed with the SSD+CI listeners (second panel). The NH listeners in the sound field gained a substantial benefit from having both ears available, as expected. In speech-shaped noise, the average

SRT improved by 4.0 dB with the addition of a second ear. The benefit was much greater with the two-talker babble, with an average improvement of 9.7 dB in

SRT with the addition of a second ear. This finding is consistent with those of

Freyman et al. (2001), showing that release from informational masking tends to be larger than predicted by classic binaural interactions. As with the SSD+CI listeners, the benefit of the vocoded ear compared with the no-vocoder condition was minimal under some conditions (e.g., S0NCI in SSN; addition of the vocoder increased average SRT threshold by 2.3 dB) and more substantial in others (e.g.,

S0NNH in SSN; addition of vocoder decreased average SRT threshold by 5.8 dB).

Figure 2.8 Speech recognition thresholds for normal-hearing (NH) listeners in the sound field (first panel), NH listeners in the vocoder simulation (second panel), and listeners single-sided deafness with cochlear implant (SSD+CI) in the sound field (third panel) groups, respectively. Speech and noise location appear on the x axis and speech reception threshold (SRT) in dB appears on the y axis. Blue bars represent conditions where the masker was speech-shaped noise (SSN). Red bars represent conditions where the masker was time-reversed two-talker babble (TTB). Darker shaded bars represent the monaural condition. Lighter shaded bars represent the binaural condition. Error bars represent ±1 SE. A repeated-measures ANOVA with SRT as the dependent variable, listener group (SSD group and NH vocoder group only) as a between-subjects factor, and listening conditions (monaural/binaural), masker type (noise or two- talker interferer), and masker position (CI side, middle, acoustic ear side) as

within-subjects factors revealed that all main effects were statistically significant.

2 The main effect of group [F(1,14) = 24.79, p < 0.001, ηp = 0.64] showed that

SRTs averaged across masker type and position were better (lower) for NH listeners under vocoder simulation conditions (SRT = −10 dB SMR) than SSD listeners in the sound field (SRT = −5.6 dB SMR). The main effect of listening

2 condition (monaural/binaural) [F(1,14) = 9.94, p = 0.007, ηp = 0.42] showed that listening with two ears (SRT = −8.6 dB SMR) was better than listening with one ear (SRT = −7.0 dB SMR) overall. The main effect of masker type [F(1,14) =

2 209.33, p < 0.001, ηp = 0.94] showed that babble (informational masking) was a more effective masker than stationary speech-shaped noise (energetic masking) across groups, listening condition, and noise direction (SRT = −3.0 dB SMR and

SRT = −13 dB SMR, respectively). Finally, as expected based on the head shadow effect, the main effect of masker location [F(2,28) = 151.80, p < 0.001,

2 ηp = 0.92] was highly significant, with SRT improving on average as the masker moved from the acoustic to the (real or simulated) CI ear side.

The two-way interaction between listening condition and masker direction

2 reached statistical significance [F(2,28) = 15.61, p < 0.001, ηp = 0.53]. Averaging across groups and masker type, performance was essentially the same for monaural and binaural listening when the masker originated at 0° azimuth (−5.64 dB SMR monaural, −6.68 dB SMR binaural) or on the (simulated) CI side (−12.04 dB SMR monaural, −12.35 dB SMR binaural), suggesting no binaural benefit in these configurations. However, there was a trend toward improvement in SRT across groups from monaural (−3.32 dB SMR) to binaural (−6.79 dB SMR)

listening when the masker originated on the acoustic-hearing side, as would be expected based on the improved effective SMR at the CI ear under these conditions.

A similar pattern was observed in the significant two-way interaction

2 between masker type and masker direction [F(2,28) = 3.54, p = 0.043, ηp =

0.20], perhaps because the difference in SRT between the two maskers appears somewhat greater for the S0NCI conditions than under the other two spatial conditions.

Finally, the three-way interaction between noise type, noise direction, and

2 group [F(2,28) = 3.58, p = 0.041, ηp = 0.20] was statistically significant. This arose because the differences between groups (SSD+CI and NH vocoder simulation) were greater in some masker locations than others, depending on the masker type. All other two and three-way interactions were not statistically significant.

Discussion The purpose of the second experiment was to investigate whether and how SSD+CI listeners use their CI when listening to speech in noise under various spatial conditions. Marginal trends toward a benefit of the CI were observed in all spatial configurations, particularly when listening to speech in two- talker babble. The only statistically significant benefit of the CI, however, was observed when two-talker babble was presented to the acoustic ear side.

Overall, the speech understanding in noise results seem to suggest that the addition of a CI does not restore binaural masking release under spatially

separated conditions but rather allows the listener to use the ear with the better signal to noise ratio (SNR), based on the head shadow effect.

In terms of binaural summation, the addition of the CI did not significantly improve the SRT when the target and masker were collocated, consistent with earlier studies (Arndt et al., 2011; Buechner et al., 2010; Firszt, Holden, Reeder,

Waltzman, et al., 2012; Mertens et al., 2017; Sladen, Frisch, et al., 2017; Stelzig et al., 2011; Vermeire & Van De Heyning, 2009), and consistent with the fact that speech perception via the CI is considerably worse than via a NH ear. To our knowledge, only two studies in postlingually deafened adults have demonstrated a small but significant improvement with a CI under the S0N0 condition where two- or four-talker babble was the interfering masker (Arndt et al., 2017; Távora-

Vieira, Marino, Krishnaswamy, Kuthbutheen, & Rajan, 2013). In addition, Sladen et al. (2017) found a small, yet significant improvement for speech in noise when speech was at 0° and speech maskers were coming from eight speakers located

360° around the listener.

Our results did not mirror those reported by Bernstein et al. (2016), who found that the CI significantly improved sentence recognition in SSD listeners when target speech was presented to the NH ear and one or two same-gender interfering talkers were presented diotically. The difference may be related to the specific configuration used by Bernstein et al., with the monaural target and dichotic maskers, which does not reflect fully realistic listening conditions, and the coordinate response measure, which has a closed set of words and can produce very low SRTs. Under our most challenging condition (S0NNH with the

two-talker babble), the difference in SRT was in the right direction, but the size of the difference was small (3.5 dB) and did not reach statistical significance.

Bernstein et al. (2015) observed a similar trend in vocoder simulations of

SSD+CI; unprocessed speech and one or two same-gender (informational) speech maskers presented one ear and vocoded masker(s) presented alone to the contralateral ear facilitated the separation between speech and masker(s).

Likewise, the addition of the vocoded contralateral ear over silence had no effect when the competing maskers were stationary noise.

Although a larger cohort will be needed to address this question more conclusively, other studies with larger sample sizes have shown that a CI supplies a benefit in speech understanding when listening to speech in noise.

Vermeire and Van de Heyning (2009), Mertens et al. (2017), Arndt et al. (2017), and Sladen et al. (2018) have all reported that listeners with normal to moderate hearing loss in the acoustic-hearing ear can experience some improvements in performance with the addition of a CI. It is likely that the benefit of the CI increases with increasing degrees of hearing loss in the acoustic ear.

Results from the NH listeners were as expected, based on earlier studies: listening with two ears provided more speech understanding than listening with one ear alone when the target and masker were spatially separated, and the benefit was greater for the two-talker babble than for the speech-shaped noise masker. In general, the NH listeners with the vocoder showed a pattern of performance that was similar to that found with the SSD+CI listeners, with a trend toward a small improvement in performance with the addition of the vocoded ear.

General Discussion Overall, the results from our experiments confirm that a CI partially restores spatial awareness in listeners with SSD. In addition, the results from the various stimulus conditions were consistent in showing that localization was possible via high-frequency ILD cues but was not improved by ITD cues in either the temporal envelope or fine structure.

Because most CIs do not transmit TFS, it was reasonable to assume that low-frequency fine-structure ITD cues were not available to SSD+CI listeners.

The lack of usable temporal fine-structure information was confirmed in all our listeners. Thus, our study provides no evidence suggesting that SSD+CI listeners are sensitive to TFS-based ITDs in the low frequencies, even with processing schemes that explicitly code such information, such as the two listeners (SSD-12 and -16) using fine-structure processing in the four most apical channels of their

MED-EL CIs. In this respect, our results are consistent with those of Magnusson

(2011), who also found no benefits associated with the FSP strategy. However, a larger sample size would be needed to confirm this finding.

In the absence of fine-structure ITD cues, it was possible that envelope

ITDs could have been salient for SSD+CI listeners. In a sample size of five,

Dorman et al. (2015) showed that SSD+CI listeners were able to localize lowpass noise above chance. Moreover, work by Stakhovskaya et al. (2016) suggested that SSD+CI listeners may be sensitive to interaural timing differences in the onset of electric and (band-pass filtered) acoustic pulse trains. However,

our results suggest that SSD+CI listeners were not able to utilize ITD cues in the temporal envelope to improve sound localization.

In terms of speech perception, it was hypothesized that the benefit of binaural listening would be most apparent in cases involving informational masking, where a perceived spatial separation between the target and masker can lead to improved speech perception (Brungart et al., 2001; Freyman et al.,

1999). In seven of eight SSD+CI listeners, a benefit of the CI when the speech and two-talker masker were spatially separated, but only when the noise masker originated on the acoustic ear side, suggesting a benefit based on improved

SMR at the CI ear, rather than a use of binaural cues. Furthermore, the degree of benefit from the head shadow effect aligns with the SNR at the CI side. That is, the better the SNR, the larger the benefit. Fortunately, under no conditions did the addition of the CI lead to poorer performance overall.

Overall, the results of this study suggest that, although the CI allows listeners with SSD to compare the relative intensity level across ears to localize sound, SSD+CI listeners are not able to utilize binaural timing cues when listening to speech in noise under spatialized conditions. It is possible that other changes to the CI programming (such as better mapping to encourage binaural fusion between the ears) may allow SSD+CI listeners to truly tap into binaural processes.

Conclusions A CI can partially restore SSD listeners’ ability to localize sound, based on high-frequency ILD cues. The SSD+CI listeners were not able to make use of

ITD cues in either TFS or the temporal envelope to improve performance. The addition of the CI did not produce a significant overall improvement in speech perception in noise or two-talker babble, when considering all conditions.

However, some improvement was observed in specific cases, when the noise was presented on the side of the acoustic ear, thereby improving the speech-to- masker ratio at the CI. This effect was particularly evident in the cases of a two- talker babble. In no cases did the CI interfere with localization or speech understanding.

Chapter 3: Sensitivity to binaural temporal- envelope beats with single-sided deafness and a cochlear implant as a measure of tonotopic match

Sections reprinted from: Dirks, C., Nelson, P. B., Sladen, D. P., Winn. M. B., & Oxenham, A. J. (under revision with JASA). Mechanisms of Localization and Speech Perception with Colocated and Spatially Separated Noise and Speech Maskers Under Single-Sided Deafness with a Cochlear Implant.

Introduction Cochlear implantation has become a viable treatment option for people with unilateral hearing loss and near-normal hearing in the contralateral ear, or single- sided deafness (SSD). The addition of a cochlear implant (CI) improves localization acuity for transient broadband sounds from fixed locations in space

(Litovsky et al., 2019), although the improvement is generally based on interaural level differences rather than interaural timing differences (ITDs) in the stimulus envelope or fine structure (C. Dirks et al., 2019). The addition of the CI can also improve speech perception in spatial listening environments, although the benefits appear to be primarily due to head-shadow or better-ear effects rather than binaural interactions between the acoustic-hearing and CI ears (Williges et al., 2019).

The lack of interactions between the acoustic-hearing ear and the CI may reflect fundamental differences in the nature of stimulation or may simply reflect a mismatch between the tonotopic maps of two ears. Based on a review of insertion angles via x-rays in 661 CI users (Landsberger et al., 2015), Wess et al.

(2017) estimated an astonishingly wide range of frequency-to-place mismatch

across CI users, with the 95% confidence interval ranging from -0.6 and 12 units of estimated human equivalent rectangular bandwidth (ERB), or -0.5 to 11 mm.

Vocoder simulations of frequency-to-place mismatch in cases of SSD with a CI

(SSD+CI) have demonstrated systematic declines in performance on spatial hearing tasks with increasing interaural frequency mismatch (Wess et al., 2017).

Pitch matching has been used to assess tonotopic mismatches between the CI and acoustic-hearing ears (L. A. J. Reiss et al., 2015) but the results can be influenced by various non-sensory biases (Carlyon et al., 2010) and may also change over time, suggesting perceptual plasticity in pitch judgments (L. A. J.

Reiss et al., 2015). Binaural interactions are thought to reflect processes earlier in the auditory pathways, which may be less susceptible to neural plasticity (Hu &

Dietz, 2015). Tests of binaural ITD sensitivity in SSD+CI patients have been proposed as a method to determine a tonotopic match (J. G. W. Bernstein et al.,

2018; Francart et al., 2018). However, these methods require the determination of a baseline ITD to correct for any frequency-dependent interaural latency differences between the acoustic and electric ear (S. Zirn et al., 2015). To overcome this challenge, we propose the use of continuously varying dynamic envelope ITDs, which eliminate the need to account for any baseline latency differences between the ears.

Binaural beats refer to the sensation produced when two low-frequency tones of slightly different frequency are presented to opposite ears. Due to the difference in frequency, the interaural phase relationship in the temporal fine structure between the tones changes over time, cycling through all phase

relationships with a repetition rate that corresponds to the difference in frequency between the two tones, and leading to the perception of “binaural beats”

(Licklider, Webster, & Hedlun, 1950; Perrott & Musicant, 1977). Binaural sensitivity to phase differences between pure tones does not extend beyond 1.5 kHz (Brughera, Dunai, & Hartmann, 2013) and CIs remove temporal fine structure during processing. Therefore, to test binaural sensitivity in SSD+CI patients, we used small differences in the rate of acoustic or electric pulses presented to the two ears, thus producing binaural temporal-envelope beats

(McFadden & Pasanen, 1975). We hypothesized that binaural interactions would be most salient when the place of stimulation in the CI and acoustic-hearing ear are best matched. We first determined whether SSD+CI listeners could detect binaural temporal-envelope beats using broadband acoustic pulse trains. We then bandpass filtered the acoustic pulse trains to test whether sensitivity varied as a function of the relative place of stimulation between ears. The results showed that the SSD+CI listeners were sensitive to binaural disparities in ways that were tonotopically tuned, suggesting that the method could be used to create a CI map that optimizes the tonotopic match between the acoustic and electric ears, and hence, binaural sensitivity.

Methods

Participants Nine listeners took part in the experiments. They all had adult-onset severe-to-profound sensorineural hearing loss and a CI in one ear (CI ear). Most had normal or near-normal hearing in the non-implanted acoustic-hearing ear

across audiometric frequencies; S1 and S3 had moderate high-frequency hearing loss in the acoustic-hearing ear. The listeners’ audiograms appear in

Figure 3.1. Eight of the participants used MED-EL devices and one used a

Cochlear device (see Table 3.1 for details). All experimental protocols were approved by the Institutional Review Board of the University of Minnesota and all listeners provided informed written consent prior to participation and were paid for their time.

S1 20 S2 S3 40 S4 S5 60 S6 S7

in Acoustic-Hearing Ear 80 S8 S9 Hearing Threshold Level (dB HL) 0.25 0.5 1 2 48 Frequency (kHz)

Figure 3.1 Audiograms of the acoustic-hearing ear in participants with unilateral hearing loss and a CI in the contralateral ear. Line color and type differentiate individual subjects. In general, listeners were tested at octave frequencies between 250 and 8000 Hz; a subset of listeners were also tested at 750 and 1500 Hz. Symbols that fall in the gray shaded region represent hearing thresholds outside the range of clinically normal hearing (> 20 dB hearing level, HL).

Subje Age Etiology Duration Impla Duration of CI Electrodes ct of HL nted CI Listening Tested (apical Code Prior to Ear Experience to basal) Activatio n S1 53 Unknown 1 yr, 4 R 1 yr, 8 mos MED-EL 6, 8, 10 mos Synchrony, Flex 28, Rondo 1, FS4 S2 37 Unknown >10 years R 5 yrs, 5 mos MED-EL 5, 7, 9 Concert, Flex 24, Rondo 1, FSP S3 68 Otoschlerosi 7 mos R 2 yrs, 5 mos MED-EL 5, 7, 9 s Synchrony, Flex

28, Rondo 1, FS4-p S4 65 Onset 2 yrs R 3 yrs, 4 mos MED-EL 4, 7, 9 following Synchrony, Flex hysterectom 28, Rondo 1, y FS4-p S5 44 Unknown 10 mos R 4 yrs, 9 mos MED-EL 5, 7, 9 Concerto, Flex 28, Rondo 1, FS4 S6 43 Unknown 1 yr, 4 L 4 yrs, 6 mos MED-EL 5, 7, 9 mos Concerto, Flex 24, Rondo 1, FS4-p S7 51 Acoustic 5 yrs 10 R 3 yrs, 10 mos MED-EL 5, 7, 9 neuroma mos Concerto, Flex 28, Sonnet 1, FS4 S8 39 Meningitis 6 mos L 4 yrs, 3 mos Cochlear 13, 9, 5 Nucleus CI422, CP900 S9 35 Acoustic 4 mos L 2 yrs MED-EL 5, 7, 9 neuroma Synchrony, Flex 28, Sonnet 1, FS4 Table 3.1 Listener demographics.

Broadband stimuli and procedure Two sets of broadband acoustic pulse trains were used in a two- alternative forced-choice (2AFC) task. In one interval, the pulse trains had the same rate in the acoustic- and electric-hearing ears. In the other interval, the pulse rate in one ear was 1 Hz higher than the pulse rate in the other ear, resulting in an ITD between individual pulses that cycled through all possible phases at a repetition rate equal to the difference in pulse rate – 1 Hz. The listeners’ task was to select the interval with the dynamic ITD. The dynamic nature of the ITD makes any inherent latency differences between the two ears, and hence any baseline ITD, irrelevant. To further minimize any effects of differences in baseline ITDs, the starting phase relationship between the pulses in each ear was selected at random with uniform distribution in each of the two intervals in each trial.

Stimuli were generated in Matlab (The Mathworks, Natick, MA), converted to an analog signal via an E22 soundcard (Lynx Studio Technology, Costa Mesa,

CA) at 24-bit resolution, and delivered to the acoustic-hearing ear via insert earphone (3M E-A-RTONE GOLD 3A insert earphones) and to the CI ear via direct audio input. The pulse trains were created by producing harmonic complex tones with a fundamental frequency of around 50 Hz and all harmonics up to 16 kHz added in sine phase, and then bandpass filtering the tones with a 4-octave passband from 500 to 8000 Hz and attenuation rates of 48 dB/octave outside the passband. On a given trial, the F0 in one ear was between 47 and 53 Hz

(selected at random with uniform distribution) and the F0 in the other ear was either the same or was higher by 1 Hz. The ear with the higher pulse rate was chosen at random on every trial. The pulse trains were 2 s in total duration, including 100-ms raised-cosine onset and offset ramps in each interval, to allow for two full cycles of the 1-Hz beat. The overall root-mean-square level of each stimulus in the acoustic ear was initially set to 50 dB SPL (pulse trains had an

17.8-dB crest factor), with the exception of S6 who had significant high-frequency hearing loss above 2 kHz, and who required the stimulus in the acoustic-hearing ear to be 60 dB SPL to reach a comfortable loudness. The electrical level presented to the CI was the same as that used to generate 50 dB SPL in the acoustic-hearing ear.

The use of a roved F0 (between 47 and 53 Hz), the randomization of the ear receiving the higher F0, and the very small (1-Hz) F0 difference between the two ears prevented listeners from performing the task based on monaural cues.

Instead, listeners had to be able to make binaural comparisons. Successful completion of the task therefore implies sensitivity to binaural interactions. A low

(50-Hz) pulse rate was chosen to ensure that the listeners were sensitive to potential ITD cues in the stimulus; higher pulse rates lead to less salient temporal-envelope fluctuations in the acoustic-hearing ear (Kohlrausch, Fassel,

& Dau, 2000) and less rate and ITD sensitivity in the CI ear (Kan & Litovsky,

2015). Finally, the stimuli included relatively long-duration onset and offset ramps to avoid stimulus onset dominance (Rakerd & Hartmann, 1986).

Listeners completed at least 2 blocks of 20 trials each on the broadband task before moving to the narrowband task. Listeners were permitted to move to the narrowband task once they had achieved at least 80% correct on at least one block of the broadband task.

Narrowband stimuli and procedure To measure potential frequency selectivity for binaural temporal-envelope beats, the stimuli were filtered in the spectral domain using linear-phase triangular filters with 48 dB/octave slopes on either side of the center frequency

(CF). In the acoustic-hearing ear, the CF of the triangular filter was equal to one the following values in each trial: 500, 700, 1000, 1400, 2000, 2800, 4000, 5600, or 8000 Hz. In the CI ear, the filter CF equal to the CF of a CI channel allocated to one of three electrodes along the array; an apical, medial, and basal electrode were tested in each listener. Since electrodes were not directly stimulated, it is possible that electrodes adjacent to the target electrode were also stimulated somewhat, but at a relatively low current level. Stimuli were otherwise identical to

those used in the broadband task (e.g., duration, fundamental-frequency rove range and corresponding pulse rate.)

Within one block, the CI stimulus was fixed and acoustic ear stimuli were presented at different CFs several times in random order. Each narrowband stimulus in the acoustic-hearing ear was presented at least 90 times for a total of

810 trials per CI stimulus (9 CFs x 90 trials each). Occasionally, more blocks (30 trials per CF x 9 CFs = 270 total trials per block) were collected to strengthen confidence in the frequency at which best binaural temporal envelope beat sensitivity was achieved. The CI stimulus was fixed for several blocks of trials.

Unlike in the broadband task, each acoustic-ear stimulus was balanced in loudness relative to the fixed CI stimulus. This occurred at the beginning of every new session and every time a new CI stimulus was used. On each trial, the CI stimulus was presented first followed by a 20 ms silent gap and then the acoustic-ear stimulus. Listeners adjusted the intensity of the acoustic-ear stimulus in three step sizes (5 dB, 2 dB, and 1 dB) without time or repetition limitations until that stimulus was judged to be equal in loudness to the CI stimulus. Responses were recorded and used to individually set the levels of the acoustic-ear stimuli. Across listeners and electrodes, this produced an average level at the acoustic-hearing ear of 49.9, 53.0, and 53.5 dB SPL at the apical, medial, and basal electrode, respectively.

For loudness-balancing purposes alone, the stimuli had a fixed 50 Hz pulse rate and were 500 ms in duration including 50-ms onset and offset ramps.

The level of the narrowband stimulus delivered to the CI ear via direct audio input

was fixed at an electrical level that produced a 50 dB SPL output when presented via the earphones, which was reported by all the listeners to be soft but audible.

The starting level of the stimulus in the acoustic-hearing ear was randomly selected from a range 15 dB above and below 50 dB SPL. Responses were limited to this range as well. S6, the listener with significant high-frequency hearing loss, was the only person who could not achieve appropriate loudness balancing within the provided range. The size of the presentation and answer range was increased for acoustic-ear stimuli only. Although the range was increased for all frequency comparisons in the acoustic-hearing ear, the only comparisons that exceeded the standard range were matches above 4 kHz. The average level needed to produce a loudness match across electrodes and test session for S6 at 5600 and 8000 Hz was 66.9 and 76.7 dB SPL, respectively.

Results All nine listeners were able to achieve at least 80% correct performance on the broadband binaural beats task. Performance on the last two blocks (n=40 trials) ranged from 75-100% correct (93% mean, 8% standard deviation). Some performed well immediately, whereas others required more practice, but most were able to achieve good performance within 10-20 minutes.

The results from the narrowband stimuli are shown in Fig. 3.2 for the individual listeners, with each panel showing performance (proportion correct in the 2AFC procedure) plotted as a function of the CF in the acoustic ear. Data from basal, medial, and apical electrodes are shown in the upper, middle, and lower row of panels, respectively. Data from each individual listener are shown in

each column of panels. The vertical gray bar in each panel represents the CF of the stimulated electrode, as determined by each listener’s current clinical map. A tonotopic match between the two ears would therefore appear as a peak in the performance function coincident with the vertical gray bar. Data points denoted by X’s represent performance that is not significantly different from chance, based on binomial distribution after Bonferroni correction, where α=0.0056 (p =

0.05 ÷ 9 acoustic frequencies per test electrode). The diameter of the circle at the point of best performance reflects the extent to which performance at that point is significantly higher (α=0.00625; p=0.05 ÷ 8 comparison points) than that at other points, providing a measure of the frequency selectivity of function.

Figure 3.2 Sensitivity to binaural temporal-envelope beats for basal, medial, and apical electrodes (rows 1, 2, and 3, respectively) in nine listeners (columns 1-9) with a unilateral CI and acoustic hearing in the contralateral ear. The proportion of trials in which the “beating” interval was correctly identified in a two- interval forced-choice task is shown as a function of the center frequency of the stimulus delivered to the acoustic-hearing ear. Filled circles indicate conditions where performance was significantly above chance; X’s appear in cases where performance was not significantly different from chance (top right legend). Circles with a yellow outline denote the acoustic frequency at which best binaural temporal envelope beat identification (or “peak” performance) was achieved. The radius of the yellow circle (bottom right legend) indicates the proportion of non-peak comparisons that were significantly different (lower) than “peak” performance. The vertical gray bar in each panel is located at the center frequency of the electrode tested in the CI ear. Differences in location between the gray bars and yellow circles indicates a potential tonotopic mismatch between a listener’s current clinical map and their acoustic-hearing ear. For all but two listeners (S7 and S8), binaural temporal-envelope beats were most salient at lower frequencies for apical electrodes and at higher

frequencies for more basal electrodes. Across listeners and electrodes, the mean difference (±1 SD) between the center frequency assigned to a given electrode and its frequency of best binaural temporal envelope beat sensitivity was -

0.84±3.19 mm (range: -7.29 to 9.06 mm) or -0.18±0.70 octaves (range: -1.61 to

1.95 octaves), where negative numbers represent an apical shift in the clinical frequency allocation table relative to the estimated tonotopic place.

A linear mixed effects model with a fixed effect of cochlear place (apical, medial, or basal) and random intercept effects for each listener partially confirmed this observation, using the medial electrode as the reference place in the model. Specifically, the basal position was estimated to yield an acoustic frequency match (3733 Hz) that was significantly higher than that of the medial position (2111 Hz; interaction estimate for basal position was +1622 Hz; s.e. =

577 Hz, t = 2.81, p = 0.013). The estimated best-match frequency for the apical position (1555 Hz) was not statistically different from the best-match frequency for the medial position (p = 0.35).

A few interesting patterns emerged at the level of single electrodes. First, some electrodes produced clear sharp peaks in performance at the (presumably) best frequency match in the acoustic-hearing ear (e.g., S7, basal electrode), whereas others produced more broadly tuned peaks (e.g., S4, all electrodes).

Second, some nonmonotonic relationships were observed. For instance, in S3 at the basal electrode, secondary peaks can be observed that appear to be remote from the expected tonotopic location. It may be that such smaller peaks represent cross-turn stimulation. Finally, some listeners showed relatively good

matches between the assigned frequency and the best match based on binaural- beat detection (e.g., S9, all electrodes), as reflected by a match between the vertical gray bar and the yellow circle, while others showed systemic deviations between the two (e.g. S6 and S7).

Discussion All SSD+CI listeners in this study were sensitive to binaural temporal- envelope beats. These results are consistent with previous studies demonstrating envelope ITD sensitivity in SSD+CI listeners (J. G. W. Bernstein et al., 2018; Francart et al., 2018). The mean difference (±1 SD) between the clinical frequency allocation and frequency of best binaural temporal envelope beat sensitivity was estimated to be -0.84±3.19 mm or -0.18±0.70 octaves across electrodes (n=27). This finding is largely consistent with x-ray estimates of electrode position in Landsberger et al.’s (Landsberger et al., 2015) large sample of CI users (n=661); they observed a 0.24±0.86 octave shift above the standard frequency allocation table. However, our estimate is smaller than that found by

Bernstein et al. (2018), who reported a 1.15±0.87 octave shift. The results also extend previous findings by showing that dynamic temporal-envelope ITDs can be used to measure sensitivity to binaural interactions in a way that does not require any knowledge of, or correction for, inherent latency differences between the acoustic- and electric-hearing ear. Finally, it should be noted that the neural mechanisms underlying the SSD+CI patients’ detection of the binaural beats is not known; they could be responding to dynamically varying interaural timing cues (McFadden & Pasanen, 1975) or short-term interaural intensity differences

that arise as the stimulus envelopes move in and out of phase (L. R. Bernstein &

Trahiotis, 1996).

Most importantly, this study demonstrates that sensitivity to dynamic interaural differences can be used to probe the tonotopic alignment between the acoustic and electric ear. SSD+CI simulations suggest that frequency alignment will improve binaural integration, but only when temporal misalignments in the CI ear are also corrected (Wess et al., 2017). Zirn et al. (2015) measured a ~1.5 ms difference in wave V latency above 1 kHz between ears in SSD+CI listeners in which the CI was leading. When temporal delays are added to high-frequency CI channels, sound localization appears to improve (Seebacher et al., 2019; Stefan

Zirn, Angermeier, Arndt, Aschendorff, & Wesarg, 2019) Whether such realignments improve spatial hearing performance in actual SSD+CI patients remains to be tested. Sheffield et al. (2019) showed that substantial low- frequency information could be removed from the CI ear via sequential deactivation of apical electrodes in SSD+CI patients without eliminating head shadow benefit. Bernstein et al. (2018) provide an extensive discussion of the clinical implications of improved frequency-to-place mapping.

One possible limitation of our approach (like other ITD-based and pitch- matching approaches) is that, in some cases, data from three electrodes may reveal patterns about the underlying electrode-to-neural interface that are not sufficient to change the CI frequency allocation table in a way that potentially reduces the frequency-to-electrode mismatch. This is certainly the case for subjects S7 and S8 who, at three separate remotely spaced electrodes,

demonstrated the greatest sensitivity for binaural temporal envelope beats at roughly the same acoustic frequency. This could also be the case for S4, who performed best in the binaural temporal-envelope beats task at one center frequency in the acoustic-hearing ear but demonstrated good sensitivity at acoustic frequencies that were more than an octave away from the “best” match.

Another limitation is that some patients, such children, may not have the attentional capacity to perform the task. In these cases, CI frequency allocation tables could be based not only on the results of these binaural tests, but also (or instead) by using post-operative CT scans or insertion depth information from surgical notes in conjunction with published frequency-to-place maps to potentially reduce frequency-to-electrode mismatch (Landsberger et al., 2015).

Conclusions SSD+CI patients are sensitive to continuously varying ITDs in the temporal envelope of pulse trains. Sensitivity to these binaural temporal- envelope beats depends on the stimulated electrode and spectrum of the stimulus in the acoustic-hearing ear, in most cases showing a distinct peak in performance that may reflect the best tonotopic match between the ears. This technique could be used to map CIs, with the aim of reducing interaural place of stimulation mismatches between the two ears in SSD+CI listeners.

Chapter 4: Spatial hearing and speech perception in cochlear-implant users with unilateral hearing loss using maps designed to optimize binaural interactions

Introduction Cochlear implantation is becoming an increasingly common treatment for single- sided deafness (SSD), a condition where people have normal hearing (NH) or mild hearing loss in one ear and profound hearing loss in the other ear. Unlike the more common treatment option – contralateral routing of signal (CROS) devices – a cochlear implant (CI) can alleviate incapacitating tinnitus (Arndt et al.,

2011; Gartrell et al., 2014; Mertens et al., 2016; Van de Heyning et al., 2008) and restore sound awareness and speech perception via the deafened ear (Firszt et al., 2018; Sladen et al., 2018; Sladen, Frisch, et al., 2017). Moreover, CIs have the potential to restore spatial hearing in people with unilateral hearing loss. It is well known that the addition of a CI improves localization accuracy for static sounds (Buss, Dillon, Rooth, King, Deres, Buchman, Pillsbury, & Brown, 2018;

Firszt et al., 2018) and motion perception (Litovsky et al., 2019) in people with

SSD. Furthermore, the addition of a CI has been shown to improve speech recognition thresholds in noise, particularly when noise is presented from the side of the non-implanted ear (C. Dirks et al., 2019; Williges et al., 2019).

Despite the advantages over traditional treatment options, spatial hearing outcomes for listeners with SSD and a CI (SSD+CI) are less than optimal. In terms of sound localization, the addition of a CI restores sensitivity to interaural

level differences but not interaural timing differences in the stimulus envelope or fine structure (C. Dirks et al., 2019; Dorman et al., 2015). Localization accuracy is poor for sounds that occur on the CI side and, regardless of frequency content, perceived sound location is often strongly biased toward the NH ear. Localization accuracy is generally better for sounds occurring on the NH side but, relative to two NH ears, overall accuracy is still substantially reduced. Finally, in a preliminary report on motion perception in four SSD+CI listeners, Litovsky et al.

(2019) reported that three could reliably detect the presence of motion but none could discriminate the direction of the motion.

In terms of speech perception in noise, a CI provides SSD patients with a way to take advantage of better signal-to-noise ratios on the side of the implanted ear, but there is no evidence that SSD+CI can make use of binaural interactions to improve performance (C. Dirks et al., 2019; Williges et al., 2019).

Thus, the benefit of a CI is most consistently measured when noise is presented from the side of the NH ear, giving the CI ear the better signal-to-masker ratio

(SMR); in such cases the improvement in SMR at threshold (e.g, where 50% of words are correctly reported) has been reported to be on the order of 2-5 dB

(Arndt et al., 2011; J. G. W. Bernstein, Schuchman, & Rivera, 2017; Firszt,

Holden, Reeder, Cowdrey, & King, 2012; Vermeire & Van De Heyning, 2009). In contrast, there are few reports of improved speech intelligibility with a CI when the NH ear has the same or better SMR (Arndt et al., 2011). Even in cases where spatial masking release in NH listeners is due to perceived spatial separation between the target and masker, rather than traditional binaural cues (e.g.,

Freyman et al. 1999), SSD+CI listeners tend not to show robust improvements due to the presence of the CI (C. Dirks et al., 2019). Overall, it seems that speech-perception results from SSD+CI listeners can be best understood by assuming that the CI ear operates independently of the NH ear.

Despite differences in stimulation mode, SSD+CI listeners have been shown to be sensitive to binaural interactions. For instance, Francart et al. (2018) measured ITD detection thresholds in SSD+CI listeners of between 200 and

1600 µs using fixed-rate band-limited acoustic pulse trains in the NH ear and direct pulsatile stimulation of three different electrodes in the CI ear. Bernstein et al. (2018) and Dirks et al. (under review) extended these findings to show that

SSD+CI listeners are sensitive to dynamic temporal envelope ITDs in a frequency-dependent manner. Presumably, sensitivity was optimal when the regions of stimulation overlapped across ears (Batra & Yin, 2004; Joris et al.,

1998; Kan, Litovsky, & Goupell, 2015). SSD+CI listeners can use binaural interactions to improve speech intelligibility in noise in certain situations (J. G. W.

Bernstein et al., 2016; Ma et al., 2016) but not others (J. G. W. Bernstein,

Stakhovskaya, Jensen, & Goupell, 2019).

There are a variety of factors that may account for the modest spatial hearing benefits and lack of binaural interactions observed in SSD+CI listeners including differences in latency, dynamic range, and spectral resolution. One modifiable factor is the frequency-to-place alignment between the electrode array and underlying tonotopic positioning. Frequency mismatch has been shown to reduce perceptual fusion, a “prerequisite” for binaural hearing (Aronoff,

Shayman, Prasad, Suneel, & Stelmach, 2015) and limit the amount of contralateral unmasking observed in vocoder simulations of SSD+CI (Wess et al., 2017). The NH ear is approximately 35 mm in length (depending on age and gender) and processes sound frequencies between 20-20000 Hz. CI electrode arrays vary in length but, on average, are inserted between 15-20 mm into the cochlea, which corresponds to a tonotopic place of 500-1500 Hz. The range of frequencies an implant processes and delivers to the electrode array, however, is much smaller than the NH ear, between 250-8000 Hz. This often creates a mismatch between the stimulus information the electrode array delivers to the spiral ganglion cells and the tonotopic regions that are stimulated. Wess et al. estimated that the difference is 3.6-5.4 mm or 4-6 ERB based on a study done in more than 800 subjects in a study by Landsberger et al. (2017).

In this study, frequency allocation tables were adjusted in nine people with unilateral hearing loss and a CI to settings that were designed to reduce frequency-to-place mismatch. Listeners wore these maps outside the laboratory as default settings for at least 6 months before returning to their default clinical map settings. Subjective and objective measures of spatial hearing were measured at 2-month intervals during the experimental test phase as well as before and after the experimental phase using the clinical map. This study asked three main questions. First, how does performance change acutely with the experimental map relative to the clinical map settings? Second, how does performance with the experimental map change over time? Third, does a extended listening experience with frequency-aligned maps produce different

spatial hearing outcomes than standard clinical maps? In the sound localization task, we hypothesized that frequency-matched settings may provide listeners with improved access to ITDs in the stimulus envelope, improving listeners’ localization abilities, particularly in conditions where high-frequency interaural level differences (ILDs) were not salient. In terms of speech understanding in noise, it was expected that speech recognition thresholds may decrease

(improve) if the frequency allocation tables were better aligned with the underlying tonotopic placement, and thus provide better tonotopic matches between the two ears, potentially providing some benefits of binaural interaction between the CI and the acoustic ear that exceed simple head-shadow effects.

General methods

Participants Nine listeners took part in the experiments. The same nine listeners participated in a previous experiment (Dirks et al., under review) where the electrode-to- neural interface in terms of frequency was probed using dynamic ITD detection.

They all had adult-onset severe-to-profound sensorineural hearing loss and a CI in one ear (CI ear). Most had normal or near-normal hearing in the non-implanted ear (acoustic-hearing ear) across audiometric frequencies; S1 and S3 had moderate high-frequency hearing loss in the acoustic-hearing ear. The listeners’ audiograms appear in Figure 4.1. Eight of the participants used MED-EL devices and one used a Cochlear device; see Table 4.1 for details.

S1 20 S2 S3 40 S4 S5 60 S6 S7

in Acoustic-Hearing Ear 80 S8 S9 Hearing Threshold Level (dB HL) 0.25 0.5 1 2 48 Frequency (kHz)

Figure 4.1 Audiograms of the acoustic-hearing ear in participants with unilateral hearing loss and a CI in the contralateral ear. Line color and type differentiate individual subjects. In general, listeners were tested at octave frequencies between 250 and 8000 Hz; a subset of listeners were also tested at 750 and 1500 Hz. Symbols that fall in the gray shaded region represent hearing thresholds outside the range of clinically normal hearing (> 20 dB hearing level, HL).

S IE Etiology DoHL A DoCI CI Manufacturer Str CM FSP EM FSP

g and Model ate Enabled;Ch Enabled;

e gy annel Channel

Numbers Numbers

S R Unknown 1 yr, 4 5 1 yr, 8 MED-EL FS4 Yes; 4 Yes; 4 most

1 mos 3 mos Synchrony, Flex apical

28, Rondo 1 channels

S R Unknown >10 3 5 yrs, MED-EL Concert, FS Yes; 2 No

2 yrs 7 5 mos Flex 24, Rondo 1 P

S R Otoschlerosis 7 mos 6 2 yrs, MED-EL FS4 Yes; 4 Yes; 4 most

3 8 5 mos Synchrony, Flex -p apical

28, Rondo 1 channels

S R Onset 2 yrs 6 3 yrs, MED-EL FS4 Yes; 4 No

4 following 5 4 mos Synchrony, Flex -p

hysterectomy 28, Rondo 1

S R Unknown 10 4 4 yrs, MED-EL Concert, FS4 Yes; 4 Yes; 3 most

5 mos 4 9 mos Flex 28, Rondo 1 apical

channels

S L Unknown 1 yr, 4 4 4 yrs, MED-EL Concert, FS4 Yes; 1, 3, & Yes; 1 & 3

6 mos 3 6 mos Flex 24, Rondo 1 -p 4

S R Acoustic 5 yrs, 5 4 yrs, MED-EL Concert, FS4 Yes; 4 Yes; 1 & 2

7 Neuroma 10 2 11 Flex 28, Sonnet 1

mos mos

S L Meningitis 6 mos 3 4 yrs, Cochlear Nuclear AC NA NA

8 9 3 mos CI422, CP900 E

S L Acoustic 4 mos 3 2 yrs, MED-EL FS4 Yes; 4 most Yes; 4 most

9 Neuroma 6 3 mos Synchrony, Flex apical apical

28, Sonnet 1 channels channels

Table 4.1 SSD+CI listener demographics. S stands for subject. IE stands for implanted ear. DoHL stands for duration of hearing loss at activation. Age corresponds to a subject’s age at test session 2. DoCI stands for duration of CI listening experience at test test session 2. CM stands for clinical map. EM stands for experimental map.

Mapping process Dirks et al. (under review) estimated the tonotopic alignment of individual electrode arrays using a two-interval forced-choice dynamic ITD detection task using acoustics pulse trains at a rate of about 50 Hz. In this task, listeners were asked to select the “moving” or “beating” interval. The stimuli were loudness- balanced, narrowband-filtered harmonic complex tones added in sine-phase to create acoustic pulse trains that were 2 seconds in duration. Long-duration onset and offset ramps were applied to the tones and the starting phase of the tones were randomized to reduce sensitivity to onset ITD cues. In one interval, the fundamental frequencies of the complex tones (and hence, the acoustic pulse rates) were identical; the temporal relationship between pulses across ears was fixed. In the other interval, the fundamental frequencies differed by 1 Hz. Here,

the inter-pulse relationship across ears systematically changed over time such that pulses cycled between left- and right-leading. In NH listeners, this creates the percept of binaural temporal-envelope beats, or a sound that fluctuates between the ears at a rate of 1 Hz. Stimuli were delivered to the acoustic-hearing ear via insert earphone and to the CI ear via direct audio insert.

Binaural beat sensitivity was measured for three electrodes in each listener, each of whom participated in this study, while using his or her clinical map. The center frequency of the narrowband stimulus in the CI ear was fixed at the center frequency of an apical, middle, or basal electrode according to the clinical map frequency allocation table. The center frequency of the narrowband stimulus in the acoustic-hearing ear was varied in roughly half-octave intervals from 500-8000 Hz. ITD sensitivity is thought to be optimal when overlapping tonotopic regions are stimulated across ears (Batra & Yin, 2004; J. G. W.

Bernstein et al., 2018; Francart et al., 2018; Joris et al., 1998; Kan et al., 2015).

Therefore, it was hypothesized that sensitivity to binaural temporal envelope beats would peak at the point at which electrical and acoustic stimuli were activating the same region in the two cochleae.

All listeners demonstrated frequency selectivity for binaural temporal- envelope beats. Seven of the nine listeners demonstrated tonotopicity, such that the acoustic frequency corresponding to best beat sensitivity increased as electrode position moved from apex to base. These data were sufficient to create maps that should have reduced the frequency-to-place mismatch between the ears. In these listeners, the best match for a given electrode was converted from

frequency to millimeters relative to the most apical point in the cochlea (Donald

D. Greenwood, 1961):

mm = log10((f/165.4+0.88))*35/2.1 where f represents the best frequency match in Hz. A linear model was then fitted to the frequency matches in mm as a function of electrode number. The resulting model was then used to estimate the tonotopic place of each electrode and the midpoint between electrodes in mm then converted back to Hz:

f = 165.4*(10.^(2.1*mm/35)-0.88)

For the two listeners who did not demonstrate tonotopicity (S7 and S8), insertion depth data was gathered from surgical notes and combined with specific information about each electrode array according to the manufacturer specifications.

For MED-EL CI listeners, the midpoint values were used to define the upper and lower bound of each channel in the experimental map frequency allocation table. For the Cochlear CI listener, the center frequency of each electrode was defined and the Cochlear software determined the upper and lower limits of each channel. When the estimated tonotopic place of the most basal frequency channels exceeded the CI processing range, channels that fell within the processing limits were fitted as they would be normally and the remaining frequency range was split geometrically between remaining channels on a logarithmic scale. A comparison of clinical and experimental frequency allocation tables appears in Figure 4.2.

S1 S2 S3 S4 S5 S6 S7 S8 S9

● ● ● ● ● ● 8000 ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 4000 ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 2000 ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 1000 ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 500 ● ● ● ● ● ● ● ● ● ● ● ● ● ● Electrode CF ● ● ● ● ● ● ● ● ● ● 250 ● ● ● ● ●

● ● ● ● ● ● ● ● ●

Clinical Clinical Clinical Clinical Clinical Clinical Clinical Clinical Clinical

Experimental Experimental Experimental Experimental Experimental Experimental Experimental Experimental Experimental Map Type

ElectrodeNumber 4 8 1216

Figure 4.2 Comparison of channel center frequencies in clinical and experimental map frequency allocation tables. Each panel represents a single subject. Electrode CF is plotted along the y-axis. Data on the left and right side of each panel corresponds to clinical and experimental map CFs, respectively. Darker shades of blue represent lower electrode numbers. MED-EL channel numbers increase with increasing frequency; Cochlear channel numbers decrease with increasing frequency. Occasionally, the default volume was increased or lowered when switching from one map to the other (only at acute remapping test sessions) according to listener comfort and preference. All other signal processing features (number of active channels, signal processing strategy, dynamic range, MAP law) were identical to the clinical map. From this point on, the clinical and experimental frequency allocation table settings will be referred to as the “clinical map” and

“experimental map.”

Testing protocol The experiment was performed over seven sessions per participant, each lasting approximately 3 hours. In the first session, listeners used their clinical maps;

performance here represented 6 or more months of listening experience with clinical settings. The next four sessions were completed using the experimental map; listeners were tested acutely after experimental map fitting and then again after 2, 4, and 6 months of listening experience. During this time period, listeners wore the experimental map full-time. The final two sessions were completed with the clinical map; listeners were tested acutely and again after one month of full- time listening experience with the original clinical map.

To help listeners acclimatize to the experimental map immediately after fitting, each participant listened to a narration of “The Raven” by Edgar Allen Poe in a simulated restaurant environment (Nelson, Perry, Gregan, & VanTasell,

2018). This process only occurred in the second session, before any formal testing of the experimental map. Listeners were provided a visual script of the poem and instructed to actively read along (silently) with the story. The SMR was fixed at +10 dB and the passage was approximately 15 minutes long. Since listeners had some experience with the clinical settings, a short period (~15 minutes) of listening experience was provided through informal conversations with the first author at the sixth session, when switching back to the clinical map.

Sound booth SSD+CI listeners were tested individually in a large (3 m by 4 m by 2.5 m) sound- attenuating chamber with 10 cm foam on all walls to reduce reverberation.

Speakers (Anthony Gallo Acoustics: A'Diva ti) were located along the horizontal plane approximately level with the participant’s head at a spacing of 10° from -

90° to +90° azimuth. The speakers were placed along the walls of the chamber,

with distances from the listener’s head ranging from 1.4 to 2.4 m, and were equalized (in terms of time delay, level, and spectrum) to match their outputs at the point corresponding to the center of the listener’s head.

Data analysis For each experiment three statistical analyses were performed per listening condition. The first analysis involved two-tailed paired-samples t-tests between performance in sessions 1 and 2 to determine whether acute experience with the experimental maps was different than extended listening experience with the clinical maps. The second analysis involved one-way repeated-measures

ANOVAs with a within-subjects factor of test session between sessions 2 and 5.

A priori, it was decided to test for a linear contrast across test sessions to determine whether performance improved, stayed the same, or declined over time with the experimental map. The third analysis again involved two-tailed paired t-tests between session 5 (the last with the experimental map) and session 7 (after one month of listening experience with the clinical map at the end of the study) to test whether any improvement found by session 5 was indeed due to the experimental map or simply due to the practice of having performed the tests multiple times. Listeners with incomplete data sets for a given analysis were excluded from that analysis. Additional analyses were completed for speech understanding in noise data and are described in the Data

Analysis section of Experiment II.

Experiment I: Localization

Rationale Previous studies have shown that ILD sensitivity is at least partially restored with the addition of a CI in SSD listeners but that SSD+CI listeners are typically not sensitive to ITDs in either the temporal fine structure or envelope. In animals, it is known that ITD sensitivity is optimal when timing information from neural fibers with the same characteristic frequency coincide in the brainstem (Joris et al.,

1998). Similarly, ITD sensitivity is best in bilateral and bimodal CI users when interaural stimuli are matched based on pitch, fusion, and ILD sensitivity (J. G.

W. Bernstein et al., 2018; Francart et al., 2009, 2011, 2018; Kan et al., 2015).

Sensitivity to ILDs also improves as frequency-place mismatch between the two ears decreases (Goupell & Stakhovskaya, 2018; Goupell, Stoelb, Kan, &

Litovsky, 2013). Therefore, it was hypothesized that individualizing SSD+CI listeners frequency allocation tables based on ITD sensitivity would improve access to binaural hearing cues, particularly ITD-based binaural hearing mechanisms. These improvements in processing spatial cues should result in improved localization performance with the experimental maps, relative to the clinical maps.

Stimuli Listeners’ ability to localize sounds was tested with several stimuli, designed to emphasize different binaural cues. Lowpass-filtered words with a 640-Hz cutoff frequency were used to provide primarily ITD cues (in both the temporal fine structure and envelope) with reduced ILD cues. Highpass-filtered words with a

100

1500-Hz cutoff frequency were used to provide ILD cues and temporal-envelope

ITD cues. Unfiltered speech provided access to all binaural localization cues.

Low- and high-pass filtering of the speech was achieved with 4th-order

Butterworth filters. The average duration of each word from the NU-6 words was

463 ms (SD 72 ms).

All sounds were presented at a nominal root-mean-square (rms) level of

60 dB SPL, as measured at the location corresponding to the position of the listener’s head. This level was sufficiently high to be in the middle of the dynamic range of most SSD+CI participants’ programmed maps, and sufficiently low to fall well below the acoustic detection thresholds of the SSD+CI participants’ implanted ear with the CI turned off. The levels of all stimuli were roved by ±10 dB around the nominal level of 60 dB SPL on every presentation to reduce the reliability of monaural loudness cues (Van Wanrooij & Van Opstal, 2005).

Procedure Listeners were tested binaurally with the CI attached and turned on. On each trial a stimulus was presented from one of the speakers with equal a priori probability, and the participant was asked to indicate from which one of the 19 speakers they heard the sound source via a virtual button on a computer screen with a display of the speaker arrangement.

The location of the stimulus varied from trial to trial and the presentation order

101

was determined randomly at the beginning of each block. One block of each stimulus type was tested before any other was repeated. Stimulus presentation order was randomized across listeners and test sessions using a Latin-squares design.

Results Rms error, defined as the square root of the mean squared difference in degrees between the actual location of the sound source and the reported sound location, was used to quantify localization acuity. Localization acuity across sessions is plotted in Figure 4.3, where lower rms error represents better spatial acuity. Data are split into panels according to stimulus type. rms error in degrees appears on the y-axis and test session appears on the x-axis; regions shaded in gray correspond to the time period when listeners used the experimental (frequency- matched) map. In these conditions, chance performance (random selected location with uniform distribution) corresponds to approximately 60° rms error.

Consistent with Dirks et al. (2019) and Dorman et al. (Dorman et al., 2015), rms error was highest (worse) for lowpass-filtered words and lower (better) for highpass-filtered and unfiltered words. Averaged across map type, test session, and listeners who completed the entire study, rms error was 74° for lowpass- filtered words and 27° for both highpass-filtered and unfiltered words. The high rms error in the lowpass-filtered conditions exceeds the error expected based on chance performance. The reason can be seen in the confusion matrices shown in Figure 4.4. It can be seen that on average the participants judged the words to emanate from the CI side. In contrast, performance lies roughly along the

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diagonal for highpass and unfiltered words between -50° and +50°. Perceived location appears to be biased toward center for extreme values on the NH side in the both conditions. For extreme values on the CI side, responses appear to be more strongly biased toward the far CI side in the highpass condition and variable in the unfiltered condition.

Lowpass Highpass Unfiltered

● ●● 100 ● ● 100 ● ● Subject ● ● ● ● ● ●● ● ● ● ● ● ● S1 80 ● 80 ● ● ●● ● S2 ● ● ● ● ● ● ● ●● ● S3 ● ● ● ● ● ● ●● ● ●● ● ● ● 60 ●● ●● ● ● ● 60 ● ● ● ● ● S4 ● ● ● ● ● S5 ● ● ● ● ● ● ● ● ● ● ● ● S6 40 ● ● ● ● ● 40 ● ● ● ● ● ● ● ● ● ● ●● ● ● ●●● S7 ● ● ●● ● ● ● ● ●● ● ● ●● ● ●● ●● ● ● ● ● ● ●● ●● ● ●● ● ● ●● ● ● ● ●● ● ● ●●● ● S8 ● ● ●● ● ● ● ● ● 20 ●● ● ● 20 ●

rms error (degrees) ● ● ● S9

2MO4MO6MO PEC 2MO4MO6MO PEC 2MO4MO6MO PEC CLIN PEAC CLIN PEAC CLIN PEAC ACUTE ACUTE ACUTE Map/test session

Figure 4.3 Rms error as a function of time. Data are separated into panels based on stimulus type: lowpass words, highpass words, and unfiltered words. Test session appears on the x-axis, where “CLIN” represents 6+ months of listening experience with the clinical maps, “ACUTE” represents acute listening experience with the experimental maps. “2MO,” “4MO,” and “6MO” represent 2, 4, and 6 months of listening experience with the experimental maps. “PEAC” represents post-experimental map acute listening experience with the clinical maps. “PEC” represents post-experimental map listening experience with the clinical map. Line and point color represent data from one listener. Gray shaded regions represent sessions performed under experimental map settings.

Lowpass Highpass Unfiltered 100

50 Frequency 60 0 40 20 0 −50 (positive = CI side) (positive

−100 Perceived location in degrees Perceived 0 0 0 50 50 50 −50 100 −50 100 −50 100 −100 −100 −100 Source location in degrees (positive = CI side)

Figure 4.4 Confusion matrices for localization task data pooled across subject and test session. Data are separated into panels based on stimulus type. Veridical source location appears on the x-axis and perceived (response) location appears on the y-axis. Darker shades of blue represent higher response frequency.

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For the lowpass-filtered words, map type did not appear to affect localization acuity. Performance remained about the same over time for each listener except S8 who showed a large increase in rms error when switching from the experimental map back to the clinical map, but partially recovered in the final session. A paired t-test between rms error at the clinical (72°) and acute experimental (70°) test sessions revealed no statistically significant difference

[t(8)=0.983, p=0.354]. A one-way repeated-measures ANOVA revealed no main effect of test session [F(3,12)=0.669, p=0.587] and hence no linear trend in rms error over time [F(1,4)=0.697, p=0.451]. A final paired t-test between rms error at the six-month experimental map test session (73°) and one month after returning to the clinical map (73°) revealed no statistically significant difference [t(5)=-

0.110, p=0.916].

For highpass-filtered words, different patterns of behavior were observed between participants. Depending on the participant, rms error remained stable

(S7), worsened over time with the experimental map (S1), improved immediately with the experimental map (S8), and declined initially when switching to the experimental settings and returned to clinical levels with experience (S3).

Overall, however, there were no significant changes across session, as in the lowpass-filtered conditions. A paired t-test between the clinical (28°) and acute experimental (30°) test sessions was not significant [t(8)=0.818, p=0.437]. A one- way repeated-measures ANOVA revealed neither a main effect of test session

[F(3,12)=0.202, p=0.893] nor evidence of a linear trend in rms error over time

[F(1,4)=0.040, p=0.851]. Finally, a paired t-test between the six-month

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experimental map test session (31°) and one month after returning to the clinical map (31°) revealed no statistically significant difference [t(5)=-0.021, p=0.984].

Finally, localization acuity and behavior patterns over time in the unfiltered word condition were similar to the highpass word condition. No statistically significant outcomes were observed in the unfiltered word condition [clinical (33°) vs. acute experimental (33°): t(8)=0.516, p=0.620; main effect of test session:

F(3,12)=0.525, p=0.673; linear contrast of rms error over time: F(1,4)=0.692, p=0.452; 6-month experimental map (31°) vs 1-month return to clinical map

(33°): t(5)=-0.784, p=0.468].

Discussion The results of this experiment do not support the original hypothesis. On average, individualized frequency allocation tables (the experimental map) had no effect on localization acuity in five SSD+CI listeners initially, over time, or after switching back to original settings. These results held true whether listeners had access to ITDs in the stimulus temporal envelope and fine structure (lowpass- filtered words), ILDs and envelope ITDs (highpass-temporal words), or all binaural localization cues (unfiltered words).

There are several possible explanations for why individualized frequency allocation tables appeared to provide no benefit over standard clinical settings.

First, it is possible that the natural limit of localization acuity under SSD+CI was reached in our listeners before switching to the experimental map settings. Ausili et al. (2019) performed real-time vocoder simulations of SSD+CI in NH listeners.

They observed response variability (rms error) for broadband and highpass white

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noise bursts in the horizontal plane of around 30° when stimuli originated on the

CI side and ~10° when the stimuli originated on the NH side and directly in front of the listener. This averages to ~15° across spatial locations under optimal conditions; certain technical aspects, such as dynamic range compression and temporal misalignment, were not simulated and are expected to affect localization acuity.

In terms of ILD sensitivity, frequency remapping was not expected to affect the number of discriminable intensity steps in the CI ear. T-levels, C-levels, and compression parameters were identical across the clinical and experimental maps. Moreover, the stimuli we used to assess ILD sensitivity were broad in terms of spectrum. For this type of stimulus, overall loudness differences between the ears (which would not be systematically affected by the remapping) may be sufficient to localize sounds.

It is unclear why temporal envelope ITD sensitivity did not improve with the experimental map. It may be that the temporal misalignment between the neural signals reaching the acoustic and the implanted ear was sufficiently large to negate any sensitivity to differences in stimulus envelope ITDs. Although

Francart et al. (2018) found ITD thresholds of around 200 μs for SSD+CI listeners, such thresholds were obtained after compensating for any inherent delay differences. The baseline ITD between the NH and CI ear in the 1000 Hz region has been found to be close to zero for the MED-EL MAESTRO platform

(S. Zirn et al., 2015); however, in lower and higher regions the difference is greater and these baseline differences may hamper listeners’ ability to use

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envelope ITDs as a reliable localization cue. As expected (C. Dirks et al., 2019;

Dorman et al., 2015), we found no evidence suggesting that listeners can use

ITDs from temporal fine structure information.

Experiment II: Speech recognition with spatially collocated and separated noise and speech maskers

Rationale Spatial hearing cues are important not only for localizing sound but also for separating sounds into different streams. Frequency-to-place alignment increases binaural fusion, which is critical to parsing complex sounds into streams and, therefore, the ability to use spatial cues to perceptually segregate those streams further. A second experiment conducted to see whether the experimental map improves speech intelligibility in competing maskers.

Stimuli and listening configurations English-language International Matrix sentence lists spoken by a single female talker were used as the target speech (Hagerman, 1982). These sentences have a fixed grammatical structure of five words beginning with a proper noun followed by a verb, number, color, and noun. Each word in the sentence was part of a closed set of 10 words, giving a total of 100,000 possible sentences. A total of

720 unique sentences were used in this experiment.

Speech intelligibility was measured in the presence of two different types of maskers. The first was a babble masker. To generate the babble, all sentences spoken by the talker were concatenated. The concatenated stimulus was then time-reversed. On each trial, two segments with random starting points

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were selected from the concatenated, time-reversed stimulus. These segments were equal in duration to the target speech segment plus 2 seconds (1 second buffer before and after the target stimulus). The second was a speech-shaped noise (SSN) masker generated from the long-term average speech spectrum of the target speech material. Under SSN, the masker was one independent sample of SSN and presented to only one speaker.

In all cases, the target speech fixed in position at 0° azimuth and in level at 65 dB SPL. The masker was scaled to the appropriate level and then presented from one of three locations. The masker occurred at either 0° azimuth

(S0M0), 60° away from front on the NH side (S0MNH) , or 60° away from front on the CI side (S0MCI). This produced three separate spatial configurations. These conditions – where a masker was presented from one speaker – will be referred to as the main conditions. An angle of 60° was chosen because this angle has been found to produce the largest head-shadow effects (Culling et al., 2012).

Two additional conditions were tested to see whether SSD+CI listeners could benefit from the perception of spatial separation based on the precedence effect. The precedence effect does not produce spatial release from masking when static noise maskers used, but can produce substantial masking release in the case of speech maskers that are easily confusable with the target (Freyman et al., 1999). Therefore, the final two conditions were only tested using the babble masker. In these conditions, the masker that collocated with the target was preceded by a copy of the masker presented from ±60° azimuth 4 ms earlier. Corrections were not applied to the maskers to account for the level

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summation at the position of the listeners’ head. In other words, the effective

SMR with the addition of the second masker was ~3 dB higher at the position of the listener’s head. In NH listeners, this creates the perception of a single masker that originates at the location of the first masker and is thus spatially separated from the target. In all conditions, the masker babble was switched on 1 s prior to the target and continued after the target sentence ended for another second.

Procedure Speech reception thresholds (SRTs) were measured adaptively over the course of 20 trials. The target sentence was fixed at a level of 65 dB SPL. The starting

SMR was 0 dB. During the first six trials, the SMR was changed according to the following rules: 5 correct words decreased SMR 3 dB, 4 correct words decreased

SMR 2 dB, 3 correct words decreased SMR 1 dB, 2 correct word increased SMR

1 dB, 1 correct word increased SMR 2 dB, and 0 correct words increased SMR 3 dB. For the remaining 14 trials, the SMR changed according to the following rules: 5 correct words decreased SMR 2 dB, 4 correct words decreased SMR 1 dB, 3 correct words decreased SMR 0 dB, 2 correct word increased SMR 0 dB, 1 correct word increased SMR 1 dB, and 0 correct words increased SMR 2 dB.

SRT was defined as the average of all SMRs from the last 9 trials, not an average of reversals. In total, 8 SRTs were measured (2 frequency alignments x

2 temporal alignments x 2 listening configurations).

Within a session, listening conditions were blocked according to masker type and spatial configuration was determined randomly. Masker presentation order was approximately counterbalanced across subjects. 20 sentences were

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presented per condition. In total, 320 sentences were presented at each test session (8 spatial configurations x 2 repetitions x 20 sentences per condition).

Data analysis In addition to the analyses described under General Methods, two additional tests were performed. The first tested for within-subjects' main effects of and interactions between map type (clinical or experimental), masker type, and spatial configuration in the main conditions (maskers from single speaker) on the outcome variable – SRT –using a repeated-measures ANOVA. The second tested for effects of map type (clinical or experimental) and spatial configuration on SRT in the “precedence effect” conditions, again using a repeated-measures

ANOVA. The precedence effect conditions were compared to the single babble masker condition where speech and masker were collocated from the front; any improvement in the precedence conditions relative to the collocated condition would indicate binaural integration that occurs to produce spatial release from masking, despite the presence of an additional masker and no better-ear advantage. In each case, SRTs at the 6-month (fifth) testing session were used to represent experimental map performance and SRTs at the final test session were used to represent clinical map performance. Although these test sessions were different in terms of listening experience, they were chosen to reduce learning effects, a well-known limitation of matrix tests (Dietz et al., 2014). In cases where Mauchly’s test indicated a violation of the assumption of sphericity,

Greenhouse-Geiser corrections were applied.

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Results SRTs measured over time are plotted in Figure 4.5. Overall performance appears to better (lower SRTs) when either masker is directed toward the CI ear and worse (higher SRTs) when directed toward the acoustic-hearing ear. Broadly speaking, SRTs appear to stay the same or improve over time even after switching back to the clinical maps at the end of the study. Performance varies considerably across subjects when the babble masker occurs on the acoustic- hearing ear side compared to other conditions. S3, the subject with the most hearing loss in the acoustic-hearing ear, consistently performs more poorly than other listeners. This listener reports that the CI ear is often his “better” ear and relies on it for speech perception. This report may explain why thresholds are substantially higher than other listeners when maskers are directed toward the CI ear.

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Babble SSN

0 Acoustic − hearing 0 ● ● ● ● ● ● ● ● ● ● ● −5 ● ● ● −5 ● ● ● ●● ● ● ● ● ● ● ● side ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● −10 ● ● ● ● ● ● ● −10 ● ● ●● ● ● ● ● ● ● ●● ● ●● ●● ● ● ● ●● ●● ● ● ●● ● ● ● ● ● ● ● ● −15 ● ● −15 ● ● ● ● −20 −20

● ● Subject 0 ● ● 0 ● ● ● ● ● ●● ● ●● S1 ● ● ● ● −5 ● ● ●● ● ● ● −5 ● ● ●● ● ● ● ● S2 ●● ● ● ●● ● ● ● ● ● ●● ● ● ● ● ● ● ● ●● ● ● ● ● 0° S3 ● ● ● ● ● ●● ● −10 ● ●● ● ●● ●● ● ●●● −10 ● ● ●●● ● ● ● ●● ●● S4 ● ● ●● ● ● ● ● ● ● −15 −15 S5 ● S6 −20 −20 ● S7 ● S8 ● S9 ● 0 ● ● ● 0 ● ● −5 −5 CI side ● ● Speech recognition threshold (dB) ● ● −10 ● ● −10 ● ● ● ● ● ● ● ● ● ● ● −15 ● ●● ● ● ● ● ● ● −15 ● ●● ●● ● ● ● ● ● ●● ● ● ● ● ●● ● ● ● ● ●● ●● ● ● ●●● ● ● ● ● ● ● ● ●● ● ●● ● ● ● ● ● ● ● ● ● ● −20 ● ● ● ● −20 ● ● ● ● ● ●

2MO4MO6MO PEC 2MO4MO6MO PEC CLIN PEAC CLIN PEAC ACUTE ACUTE Map/test session

Figure 4.5 Speech recognition threshold as a function of test session separated into columns based on masker type and rows based on spatial configuration. Test session appears on the x-axis, where “CLIN” represents 6+ months of listening experience with the clinical maps, “ACUTE” represents acute listening experience with the experimental maps. “2MO,” “4MO,” and “6MO” represent 2, 4, and 6 months of listening experience with the experimental maps. “PEAC” represents post-experimental map acute listening experience with clinical maps. “PEC” represents post-experimental map listening experience with the clinical maps. Line and point color represent data from one listener. Gray shaded regions represent sessions performed under experimental map settings. Statistical analyses confirm the visual impressions in the main conditions, or those where the masker was presented from one speaker. Table 4.2 displays results from two-tailed paired t-tests comparing mean SRTs in the first (6+

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months clinical map) and second (acute experimental map) test sessions. None of the conditions reached statistical significance after correcting for multiple comparisons (α = 0.05/6 comparisons = 0.0083). Similarly, Table 4.3 shows that the one-way repeated-measures ANOVAs with test session (sessions 2-5 using the experimental map only) as the within-subjects variable revealed no main effect of test session on SRT in any of the main conditions. A priori, it was determined to test for a linear contrast across test sessions. None of the conditions reached statistical significance after correcting for multiple comparisons. Finally, Table 4.4 displays results from two-tailed paired t-tests comparing mean SRTs in the fifth (6+ months experimental map) and seventh (1- month after returning to clinical map) test sessions. None of the tests indicated statistically significant differences.

A second repeated-measures ANOVA was completed on SRTs measured in the main test conditions. This analysis had three within-subjects variables: map type (6-months listening experience with the experimental map, 1-month listening experience with the clinical map at the end of the study), noise type

(SSN or babble), and spatial configuration (S0M0, S0MNH, and S0MCI).

Consistent with the final paired t-test, the main effect of map type was not statistically significant [F(1,5)=0.408, p=0.551]. However, the main effects of masker type [F(1,5)=12.374, p=0.017] and spatial configuration [F(1.110,

5.551)=23.875, p=0.003] were highly significant. In terms of masker type, average SRT was lower (better) in speech-shaped noise (-13.5 dB) than in two- talker babble (-10.2 dB). In terms of spatial configuration, SRT increased as the

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masker moved from the CI side (-16.3 dB) to the NH side (-10.4 dB) to the front (-

8.7 dB). The two-way interactions between map type and masker type

[F(1,5)=1.612, p=0.260] as well as map type and spatial configuration

[F(2,10)=1.018, p=0.396] did not reach statistical significance. However, the two- way interaction between masker type and spatial configuration was statistically significant [F(2,10)=5.543, p=0.024]. SRTs were nearly 5 dB lower in SSN than in babble when the masker originated from the front (speech-shaped noise: -11.0 dB, babble: -6.4 dB) and NH side (SSN: -12.3 dB, babble: -8.5 dB). SRTs were similar across masker types when the maskers originated on the CI side

(speech-shaped noise: -17.1 dB, babble: -15.6). The three-way interaction was not statistically significant [F(2,10)=0.201, p=0.821].

Masker Spatial Configuration 6MO+ CM Mean Acute EM T-statistic (8 p-value

Type (dB) Mean (dB) degrees of

freedom)

SSN S0, M0 -10.4 -10.5 0.311 0.764

SSN S0, MNH -11.5 -12 0.86 0.415

SSN S0, MCI -16.8 -16.5 -0.693 0.508

Babble S0, M0 -4.5 -5.3 2.695 0.027

Babble S0, MNH -7.7 -8.5 1.245 0.248

Babble S0, MCI -14.6 -15.5 1.65 0.138

Babble S0, M1NH, M20+4ms -1.9 -3.7 1.968 0.085

Babble S0, M1CI, M20+4ms -4.5 -5.3 1.606 0.147

Table 4.2 Means, t-statistics, and p-values for two-tailed paired t-tests between SRT means in the first and second test sessions. The first test session corresponds to performance with 6+ months of listening experience with the clinical map (CM). The second test session corresponds to performance when listeners were tested acutely with the experimental map (EM). Precedence effect conditions are separated from the main conditions by a double line.

Masker Spatial Configuration Main Effect of Test Session Coefficient Contrast: Linear Trend

Type

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SSN S0, M0 F(3,12)=1.733, p=0.213 F(1,4)=2.052, p=0.225

SSN S0, MNH F(3,12)=1.057, p=0.403 F(1,4)=1.372, p=0.306

SSN S0, MCI F(3,12)=1.685, p=0.223 F(1,4)=2.721, p=0.174

Babble S0, M0 F(3,12)=3.068, p=0.069 F(1,4)=6.181, p=0.068

Babble S0, MNH F(3,12)=1.008, p=0.423 F(1,4)=1.339, p=0.312

Babble S0, MCI F(3,12)=3.111, p=0.067 F(1,4)=9.869, p=0.035

Babble S0, M1NH, M20+4ms F(3,12)=3.216, p=0.061 F(1,4)=13.244, p=0.022

Babble S0, M1CI, M20+4ms F(3,12)=2.456, p=0.113 F(1,4)=3.359, p=0.141

Table 4.3 Test statistics for one-way repeated-measures ANOVAs with a within-subjects variable of test sessions 2-5 using the experimental map only. Precedence effect conditions are separated from the main conditions by a double line.

Masker Spatial 6MO EM 1MO CM T-statistic (5 p-value

Type Configuration Mean (dB) Mean (dB) degrees of

freedom)

SSN S0, M0 -10.6 -11.4 2.274 0.072

SSN S0, MNH -12.5 -12.2 -0.386 0.715

SSN S0, MCI -16.8 -17.3 1.124 0.312

Babble S0, M0 -6.4 -6.4 0.2 0.849

Babble S0, MNH -8.7 -8.4 -0.435 0.682

Babble S0, MCI -15.5 -15.7 0.632 0.555

Babble S0, M1NH, -4.1 -3.7 -0.657 0.54

M20+4ms

Babble S0, M1CI, -5.9 -7.7 2.384 0.063

M20+4ms

Table 4.4 Means, t-statistics, and p-values for two-tailed paired t-tests between SRT means in the fifth and seventh test sessions. The fifth test session corresponds to performance with 6+ months of listening experience with the experimental map (EM). The seventh session test session corresponds to performance after 1 month of listening experience with the clinical map at the end of the study (CM). Precedence effect conditions are separated from the main conditions by a double line. SRTs measured over time under conditions of the precedence effect, or conditions where maskers were presented from two speakers, are plotted in the left and right panels of Figure 4.6. The closest comparison condition – where only

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one copy of the babble masker occurred at 0° azimuth -- appears in the middle panel. Visual inspection suggests that SRTs improve (decrease) over time for nearly every listener in every condition. Only one listener – S3 – appears to benefit from the experimental map under these conditions; SRTs remain at higher (poorer) levels after switching back to the clinical map in the conditions that are intended to produce the perception of spatial separations. Despite these visual trends, none of the t-tests returned statistically significant results. Only one repeated-measures ANOVA returned a statistically significant result after correcting for multiple comparisons (α = 0.05/2 comparisons = 0.025): the linear contrast for the precedence effect condition where the first copy of the masker occurred on the NH side.

Acoustic−hearing 0° CI side side ● ● ● Subject ● ● ● ● ● ● ● ● ● ● ● ● ● S1 0 ● ● ● ● ● ● 0 ●● ● ● ● ● ● ● ● ● S2 ● ● ●● ● ● ● ● ● ● ● ● ● ● ● ● ●● ● ●● ● ● ● ● ● ● S3 ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●● ● −5 ● ● ● ●● ● ● ● ● ● ● ● ● −5 ● ● ● ● ● ● ● ● ● S4 ● ● ● ● ● ● ● ● ●● ●● ● ●● ● ● ● ● ● ● ● ● ● ● ● S5 ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● S6 −10 ● ● ● ● −10 ● ● S7 ● ● threshold (dB) ● ● ● ● S8 ● ● S9

Speech recognition −15 −15

2MO4MO6MO PEC 2MO4MO6MO PEC 2MO4MO6MO PEC CLIN PEAC CLIN PEAC CLIN PEAC ACUTE ACUTE ACUTE Map/test session

Figure 4.6 Speech recognition threshold measured in babble alone as a function of test session separated into panels based on test condition. The center panel represents the comparison condition where target speech and a single babble masker were collocated at 0° azimuth. Panels on the left and right represent conditions that elicit the precedence effect in NH listeners where the first copy of the masker occurred on the NH and CI side, respectively, and a second copy of the masker was presented 4 ms later from the 0° azimuth. Test session appears on the x-axis, where “CLIN” represents 6+ months of listening experience with the clinical maps, “ACUTE” represents acute listening experience with the experimental maps. “2MO,” “4MO,” and “6MO” represent 2, 4, and 6 months of listening experience with the experimental maps. “PEAC” represents post-experimental map acute listening experience with the clinical maps. “PEC” represents post- experimental map listening experience with the clinical map. Line and point color represent data from one listener. Gray shaded regions represent sessions performed under experimental map settings.

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A final repeated-measures ANOVA was completed on SRTs measured in the precedence effect conditions at the fifth (experimental map) and seventh

(clinical map) test sessions. No main effect of map type [F(1,5)=1.486, p=0.277] was observed. However, a significant main effect of spatial configuration was found [F(1.105, 5.526)=9.789, p=0.021]. Pairwise comparisons showed a statistically significant difference (p=0.001) of 2.5 dB in SRT between the collocated single masker condition (-6.4 dB) and the two-masker condition when the first masker occurred on the NH side (-3.9 dB). In contrast, no difference

(p=1.000) was observed between the collocated single masker condition and the two-masker condition when the first masker occurred on the CI side (-6.8 dB).

The interaction between map type and spatial configuration just missed statistical significance [F(1.126,5.632)=4.664, p=0.075].

Discussion Data from this experiment are broadly consistent with the localization experiment.

In a small sample of subjects, there appear to be trends toward improvement in

SRT across test sessions. However, statistical analyses on the main and precedence effect conditions generally find no support for differences in SRT between the clinical and experimental maps. Switching to the experimental map acutely (first set of t-test in Table 4.2) had no significant effect on SRT despite substantial shifts in frequency allocation table for some subjects (e.g., S4, S7 and

S8). SRTs appear to improve (decrease) on average between test sessions 2 and 5, suggesting acclimatization or learning over time, but statistical analyses

(Table 4.3) did not reveal effects of test session or evidence of a linear trends

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across test sessions. Finally, it was hypothesized that SRTs would be better with the experimental map compared to the clinical map after extended listening experience, in which case SRTs were expected to increase from session 5

(experimental map) to session 7 (clinical map). The final set of t-tests (Table 4.4) found no difference between these test sessions for neither the main nor precedence effect conditions. This again suggests that switching back to the clinical map at the end of the study had no effect on SRT, despite substantial some substantial changes in frequency allocation table.

Consistent with our previous study, there were statistically significant effects of masker type and spatial configuration on SRTs obtained with the experimental map at the 6-month test session and the clinical map in the last test session. As expected, the two-talker time-reversed babble was a more effective masker than SSN, particularly when the masker was collocated with target speech and when the masker originated on the acoustic-hearing side. Effects of spatial configuration were also apparent in the two-talker masker conditions designed to elicit the precedence effect. Interestingly, SRT increased ~3 dB, corresponding to a doubling in sound intensity, when a second masker was added on the NH side but not when directed toward the CI ear when compared to the single-masker collocated condition.

General discussion Evidence of binaural interactions in natural listening situations is largely absent in the CI literature. Restoring binaural interactions may not be critical for people who rely on the CI for speech understanding; however, it may be an appropriate

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primary objective for an emerging CI population: people with SSD or significant residual hearing in the non-implanted ear. Data from bilateral CI listeners and

SSD+CI simulations shows that binaural fusion and integration largely depends on the frequency-to-place match, or the alignment between the electrode array and its tonotopic position (Aronoff et al., 2015; Ma et al., 2016; Oh et al., 2019;

Wess et al., 2017; Wess, Spencer, & Bernstein, 2016). Depending on the method, frequency-to-place mismatch can be estimated quickly using CT scans and generic tonotopic maps or more accurately by exploiting ITD sensitivity.

Frequency alignment can also be easily manipulated in clinical software unlike other limiting factors such as dynamic range compression and spectral resolution. In our small sample of SSD+CI listeners, frequency-realignment alone does not appear to be sufficient to restore binaural interactions. In tasks of localization and spatialized speech perception in noise, performance was highly variable across subjects but, on average, improvement over time appeared to be due to learning rather than binaural sensory integration.

There are several possible explanations for the lack of a frequency- remapping effect. First, the clinical map may have already provided satisfactory frequency-to-place matches. In our previous study, mean difference (±1 SD) between the clinical frequency allocation and frequency of best binaural temporal envelope beat sensitivity was less than 1 mm (-0.84±3.19 mm) or 1/5 of an octave (-0.18±0.70 octaves) across tested electrodes (n=27). Eight of the nine listeners in this study used MED-EL devices, six of whom were fit with the Flex

28 electrode array. This array, in particular, is the second longest array on the

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market, second to MED-EL’s Classic Standard and FlexSOFT arrays which can be inserted up to 31.5 mm into the cochlea. Landsberger et al. (Landsberger et al., 2015) used x-ray images to measure insertion depth in 92 CI ears and found that, compared to Advanced Bionics HiFocus 1J and Cochlear Contour, MED-EL electrode arrays (Standard and Flex28) were inserted the furthest. Theoretically, this should lead to a smaller frequency-to-place mismatch at least for certain frequency regions.

Wess et al. (2017) directly manipulated spectral alignment in vocoder simulations of SSD+CI to measure perceptual release from masking using

Bernstein et al.’s (2016) contralateral unmasking paradigm. In particular, they compared speech intelligibility under monaural and binaural listening when a speech+masker mixture was presented unprocessed to one ear and a vocoded copy of the masker alone was added to the other ear. Manipulating spectral shift alone, contralateral unmasking was preserved for shifts smaller than ±2 ERBs, reduced but present between ±2 and 4 ERBs, and completely eliminated above

±7 ERBs. Based on these results, the difference between the experimental and clinical maps in our study should have produced a relatively similar results in terms of spatial release from masking assuming 0.9 mm / ERB (Moore, 1986).

Even if the clinical frequency allocation tables were not perfectly aligned with the underlying tonotopy, spread of electrical current in the CI ear will allow

SSD+CI listeners to be resilient to some amount of frequency mismatch, at least in certain situations. Bernstein et al. (2018) showed in eight SSD+CI listeners that sensitivity to dynamic ITDs (pulse train bursts with systematically varying

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onset ITDs) was possible up to 3.2 mm or 0.67 octaves above and below the estimated frequency match for a given electrode, on average. Similarly, some

SSD+CI listeners in our previous study demonstrated broad “tuning” for binaural temporal envelope beats (Dirks et al., under review). Wess et al. (2017) corroborated these findings in their SSD+CI simulations of perceptual release from masking. NH listeners were more resilient to spectral shifting with fewer spectral channels when the target was in the acoustic-hearing ear. Poor spectral resolution is not helpful, however, when the target is only presented to the CI ear.

More spectral channels would be advantageous in this case to not only support speech perception but also make the target salient; high-quality maskers in the

NH are difficult to ignore even at favorable SMRs (J. G. W. Bernstein et al.,

2019).

Another major technological limitation is the latency difference between

NH and CI ears. In the NH ear, sound travels through the outer, middle, and inner ear before neural transmitters are released, stimulating the auditory nerve.

Group delay is roughly constant across the spectrum at the outer and middle ear while the inner ear imposes the traveling wave delay; low frequencies arrive at the auditory nerve later than higher frequencies (Ruggero & Temchin, 2007). CIs bypass these processes entirely; they detect sound outside the pinna, manipulate it in the external processors and then deliver the processed signals to the internal receivers/stimulators. Signal processing primarily accounts for delay in the CI ear and varies across manufacturers. CIs are delayed relative to the NH ear ~9-11 ms for Advanced Bionics devices and ~10.5-12.5 ms for Cochlear

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devices (Wess et al., 2017). MED-EL device latencies are more closely aligned with NH ears but vary slightly in a frequency dependent manner. The NH leads the CI ear by ~1 ms above 1 kHz above which the CI leads the NH ear (S. Zirn et al., 2015).

Fortunately, SSD+CI simulations suggest that delays of this magnitude may have a small effect on spatial release from masking. Wess et al. (2017) showed that delays within this range reduced contralateral unmasking (or binaural benefit) ~10 percentage points, on average, but still afforded performance ~20 percentage points above monaural listening. Contralateral unmasking was completely eliminated for delays beyond ~50 ms. Interestingly, misalignments in the temporal and spectral domains reduced contralateral unmasking but not in an additive fashion. Listening with two ears was still better than one even under typical amounts of temporal and spectral shifting albeit to a small degree. Temporal realignment has already been implemented in SSD+CI, showing ~10° improvement in broadband sound localization when a 1ms delay is added to the CI ear above 1 kHz (Seebacher et al., 2019). The same study found a trend toward improved speech intelligibility under this condition when speech and noise were collocated but the effect was not statistically significant.

In addition to temporal alignment, there are several other factors that could affect NH and CI ear integration but are beyond the control of researchers and clinicians. As mentioned earlier, spectral spread can improve detection of interaural temporal envelope correlations, but this may only helpful when target speech reaches the acoustic-hearing ear. Dynamic range is also reduced in the

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CI ear compared to the NH ear; compressing sounds within that dynamic range can introduce envelope distortion that may help or hurt speech intelligibility in spatial listening situations (Wess & Bernstein, 2019). Moreover, pitch perception can change dramatically in the implanted ear over time (L. A. J. Reiss et al.,

2015, 2007). This may be related to a recent finding in SSD+CI patients showing that binaural benefits continue to emerge out to 18 months (Gartrell et al., 2014).

Neural survival and duration of deafness prior to implantation are also closely linked to auditory outcomes and may relate to the ability to integrate sound across ears. Like typical CI listeners, SSD listeners achieve better speech intelligibility with short durations of deafness (Cohen & Svirsky, 2019). Finally, age and the amount of residual hearing in the acoustic-hearing ear may affect spatial hearing outcomes. In spite of a short duration of deafness in the implanted ear, the oldest listener in this study had the most high-frequency hearing loss and performed substantially worse than all other subjects in both experiments.

Future studies should directly manipulate the degree of spectral mismatch to determine how precise frequency can and should be matched. Manipulations of frequency mismatch should also be combined with temporal realignment to see whether corrections to these two major dimensions improves speech perception outcomes. Finally, future studies should implement individualized frequency allocation tables upon activation to see whether binaural hearing outcomes are different than those who were initially fit with standard clinical frequency allocation tables. Combined these data will have implications for

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whether binaural hearing should be a primary objective for SSD+CI listeners, the precision with which those parameters should be fit, and the methods that should be used to employ frequency-matching.

Conclusion Since they have one NH ear, SSD+CI listeners may benefit more from a fitting approach that optimizes binaural interactions and fusion between the acoustic and implanted ear, as opposed to speech perception with the CI alone. Data from these experiments show that shifting the frequency allocation table to presumably reduce frequency-to-place mismatch does not degrade performance, but also provides no clear benefit in spatial hearing or speech perception, relative to standard clinical frequency allocation settings. It is possible that other differences between acoustic and electric hearing (e.g., the baseline timing relationship between the two ears and amplitude compression characteristics) must be compensated for in order to take advantage of improved frequency-to- place alignment between the two ears.

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Chapter 5: Conclusions

General Discussion Over the past 10 years, there has been an international uptick in treating single- sided deafness (SSD) and asymmetric hearing loss with a cochlear implant (CI).

In the US, however, cochlear implantation is generally an “off-label” treatment option for SSD; only one company – MED-EL – has received FDA approval for

SSD implantation, including those with some hearing loss in the better-hearing ear. Implantation typically happens after trial periods with alternative, more cost- effective treatment options – contralateral routing of signal devices (CROS) – none of which restore hearing to the poorer ear. Two US-based clinical investigations of cochlear implantation in SSD are active at present; a third appears to have been completed (“Search of: cochlear implant | single-sided deafness - List Results - ClinicalTrials.gov,” n.d.). These trends lead to one important question: can cochlear implantation in cases of SSD be justified?

Based on the available evidence, Kitterick et al.’s (2016) metanalysis showed that CIs tended to provide statistically superior spatial hearing and quality of life outcomes compared to unaided, bone-anchored hearing aid, and air-conduction hearing aid conditions. Recent evidence suggests that outcomes persist up to 10 years after activation (Arndt et al., 2017; Távora-Vieira, Rajan,

Van De Heyning, & Mertens, 2019). The benefit of cochlear implantation is most obvious in localization tests, where binaural hearing is important. Unaided or aided with a CROS device, sounds are lateralized to the better-hearing ear side.

Monaural level and spectral cues can be learned, allowing the listener to localize

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sound, but these cues are fragile and interrupted using CROS devices. Aided with a CI, localization in the horizontal plane is partially restored; listeners can accurately and reliably perceive sounds as emanating from the CI side as well as the acoustic-hearing side.

In terms of speech perception in noise, CI and CROS devices restore audibility of sounds occurring on the poorer ear side. They diverge in the way that they deliver sound to the listener. CIs electrically stimulate the auditory nerve on the poorer ear side whereas CROS devices route sound to the better ear.

Mechanistically, this makes listening with a CI in noise advantageous. CROS devices help when target sounds originate on the poorer ear side but interfere when directed toward the better ear. In contrast, the addition of CI gives listeners access to another “better ear,” or the ear with the better signal-to-noise ratio, when the target originates on the poorer ear side. The listener can ignore information from the acoustic-hearing ear and attend to the CI ear. Like CROS devices, CIs have the potential to cause central interfere in cases where the target is on the better ear side but data from Chapter 2 shows that, in realistic listening situations, this is not an issue. Speech intelligibility is about the same with or without the CI when target speech is on the better ear side.

Listeners with two ears can use a combination of monaural and binaural mechanisms to spatialize their listening environment. Prior literature indicated that that SSD+CI listeners could take advantage of monaural mechanisms such as the “better ear” effect and its companion – the head shadow effect – to localize sound and improve speech intelligibility in noise. It was less clear,

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however, whether SSD+CI listeners could integrate sound across the acoustic- and electric-hearing ears to access binaural mechanisms. Moreover, it was unclear whether masker type (energetic or informational) affected SSD+CI listeners in the same way as normal-hearing (NH) listeners. Informational maskers tend to produce greater spatial release from masking than energetic maskers (Freyman et al., 1999).

Chapter 2 systematically evaluated the use and relative contributions of monaural and binaural hearing mechanisms in localization and speech intelligibility in noise tasks. Stimuli in the localization task were designed to isolate interaural level differences (ILDs) and interaural timing differences (ITDs) in the stimulus envelope and fine structure and assess horizontal localization acuitty under different combinations of these cues. Results showed that SSD+CI listeners were sensitive to ILDs but not ITDs in the stimulus envelope or fine structure signal, even in listeners using signal processing strategies that conveyed fine structure information in the apical channels. Speech intelligibility was measured in a variety of spatial configurations under two types of maskers with and without the CI attached. The addition of the CI improved speech recognition thresholds (SRTs) when maskers were directed toward the CI ear but had no effect when directed toward the acoustic-hearing ear. This pattern of results suggested that SSD+CI listeners could benefit from monaural head- shadow or “better ear” effects but not binaural mechanisms like squelch and summation. The latter mechanisms are related to the correlation between signals across ears, where highly correlated information improves the ability of the brain

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to estimate different components of the signals. Binaural benefit was similar for the energetic and informational masker.

Compared to NH listeners, the magnitude of binaural benefit in SSD+CI listeners was modest at best. Furthermore, evidence of binaural integration was absent. This raised two further questions. Are SSD+CI listeners capable of true binaural hearing? If so, what factors limit binaural integration of acoustic and electric ears? Chapter 3 showed that SSD+CI listeners are indeed sensitive to binaural interactions, echoing other recent findings (J. G. W. Bernstein et al.,

2018; Francart et al., 2018). Broadband pulse trains were delivered to the acoustic and electric-hearing ears and, when the pulse rate in the acoustic- hearing ear was slightly different than in the electric-hearing ear, SSD+CI listeners could detect binaural temporal envelope beats. When the pulse trains were bandlimited, listeners demonstrated frequency-selectivity for binaural temporal envelope beat sensitivity.

It is known that ITD sensitivity is optimal when the same frequency regions are stimulated across ears (J. G. W. Bernstein et al., 2018; Francart et al., 2018;

Kan et al., 2015). Moreover, ITD sensitivity is relatively immutable in response to a frequency mismatch between the frequency information delivered to an electrode and its tonotopic place of stimulation (Kan & Litovsky, 2015).

Therefore, it was inferred that the narrowband acoustic stimulus that produced the best binaural temporal envelope beat detection for a fixed narrowband electric stimulus reflected the underlying frequency-to-place match of electrode assigned to that frequency information. Given that SSD listeners have one NH

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ear which they likely rely on for speech perception, it has been hypothesized that a CI should be fit to optimize binaural hearing outcomes. The final question this thesis sought to address was whether and how maps designed to optimize binaural interactions in the frequency domain affect SSD+CI spatial hearing ability relative to clinical maps with standard non-individualized frequency allocation table settings.

Narrowband binaural temporal envelope beat sensitivity data collected in

Chapter 3 was used to create maps that, in theory, should improve binaural integration. Localization acuity and speech perception in noise tasks was measured over time using the experimental (frequency-to-place matched) map and compared to performance under the clinical map. Emerging evidence suggests that, on average, there are no differences in spatial hearing outcomes between experimental and clinical maps. Root-mean-square error remained the same for lowpass, highpass, and unfiltered stimuli across map type and test sessions. Speech recognition thresholds appeared to decrease (improve) with listening experience for the experimental map but statistical analyses found no evidence of such trends in any spatial configuration. After correcting for multiple comparisons, no differences were observed between the experimental and clinical maps in terms of speech recognition thresholds at the beginning or end of the experiment.

Does this pattern of results suggest that frequency-to-place matching clinically futile in SSD+CI listeners? Not necessarily. It is possible that the frequency-to-place alignment was acceptable in the clinical map settings. Eight of

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the nine listeners use MED-EL CIs and electrode arrays which tend to have the deepest insertion depths (and therefore better frequency-to-place alignment) compared to other manufacturers (Landsberger et al., 2015). Future studies should systematically manipulate frequency-to-place alignment in SSD+CI listeners and measure speech and spatial hearing outcomes. Similiarly, SSD listeners implanted with other devices should be included in frequency-to-place remapping studies.

MED-EL devices are also well-matched to the NH ear in terms of temporal latency. Frequencies below ~1000 Hz arrive at the auditory nerve 1 ms sooner in the NH compared to the CI ear; the opposite is true ~1000 Hz (S. Zirn et al.,

2015). While this may not be adequate to support temporal envelope ITD sensitivity (as observed in Chapter 4), it can support binaural fusion and, therefore, good speech intelligibility in noise. In contrast, Cochlear and Advanced

Bionics devices have significantly longer processing delays -- up to 12 ms (Wess et al., 2017). Systematic latency manipulations have already been tested in

SSD+CI listeners, showing that millisecond delays at the CI ear can have dramatic effects on localization accuracy of broadband sounds in MED-EL

SSD+CI listeners (Seebacher et al., 2019). Future studies should test different combinations of frequency and temporal alignment to determine the extent to which these factors affect hearing outcomes and, more broadly, whether enhancing binaural interactions should be the primary clinical objective for people with unilateral hearing loss that are fit with a CI.

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Future Directions It is still possible that ITD sensitivity can be partially restored in SSD+CI listeners by adjusting the processing latency on the CI side to better match the

NH ear. The human head produces a maximum ITD of +/- 600 microseconds when a sound source is located 90 degrees to the right or left of 0 degrees azimuth. Zirn et al. (2015) showed a frequency-dependent difference in Wave V latency between acoustic- and electric-hearing ears on the order of 1.5 ms in

MED-EL CI users. Interaural latency differences are even more pronounced in

Cochlear and Advanced Bionics devices, whose processing schemes cause delays at the CI ear of 10.5-12.5 ms and 9-11 ms, respectively (Wess et al.,

2017).

Mossop and Culling (1997) measured ITD sensitivity in NH listeners as a function of baseline temporal offset (or ITD) between ears. The stimuli were broadband sounds in which phase was manipulated (as opposed to onset time) to create a perceptual lag in one ear. They found that, with a baseline delay of

700 microseconds or more, listeners were only sensitive to ITDs beyond 400 microseconds. In other words, baseline ITDs that exceeded natural ITD limits imposed by the head rendered listeners insensitive to most physiologically relevant, naturally occurring ITDs. NH listeners were generally to larger ITDs

(0.5-10 ms) with baseline ITDs up to 10 ms but only when frequency information below ~700 Hz was present.

If the primary objective for SSD+CI listeners is to optimize binaural integration, latency should be matched across ears. Correcting temporal

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misalignment improves sound source localization acutely but not speech intelligibility when the speech and noise are collocated (Seebacher et al., 2019).

Moreover, the improvement in spatial release from masking due to binaural integration, known as binaural squelch or contrast (Dieudonné & Francart,

2019a), is only observed when ITDs are available (Bronkhorst & Plomp, 1988).

ITDs, however, are disrupted in CI listeners. In SSD+CI vocoder simulations, timing and frequency needed to be aligned in order for binaural integration to occur (Wess et al., 2017). It is possible, therefore, that differences in acoustic and electric processing latencies, or temporal (mis)alignment, masked the expected benefit of frequency-to-place matching explored in Chapter 4. A future direction of this research is to manipulate frequency and temporal alignment and

(1) see whether evidence of binaural integration emerges in real-world listening situations and (2) the degree to which each dimension needs to be aligned in order to observe binaural integration.

Another area for future consideration is the apparent perceptual quality reports from CI+SSD patients. Even though the results in Chapter 4 showed no difference between clinical and experimental maps, some subjects reported perceptual differences between the maps. The Speech, Spatial, and Qualities of

Hearing Questionnaire (SSQ) was administered at each test session to try to capture these experiences. The questionnaire is split into sections that evaluate hearing ability in terms of speech understanding, spatial hearing, and sound quality. Listeners rate their experience on a Likert scale where 1 represents poor hearing ability and 10 represents excellent hearing ability.

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Speech Spatial Qualities

● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● Subject ● ● ● ● ● ● 7.5 ● ●●● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●● ●● ● ● ● ● ● ● ● ● ● ● S1 ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●● ● ● ● ● S2 ●● ● ● ●● ● ● ● ● ● S3 ● ● ● ● ● ● ● ● ● ● ● S4 5.0 ● ● ● ● ● ● ● ● ● ● ● ● S5 ● ● ● ● ● ● ● S6 ● ● ● ● ● ● ● ● ● ● ● ● S7 ● ● ● ● ● ● ● ● ● ● S8 ● ● ● ● ● SSQ Score (0 − 10) 2.5 ● S9 ● ● ●

2MO4MO6MO CPE 2MO4MO6MO CPE 2MO4MO6MO CPE Acute PEAC Acute PEAC Acute PEAC Clinical Clinical Clinical Map/Testing Time Point

Table 5.1 Self-perceived hearing ability on the Speech, Spatial, and Qualities of Hearing Questionnaire as a function of test session. Higher number represent better hearing ability. Data are separated into panels based on section (auditory domain): speech understanding, spatial hearing, and sound quality. Test session appears on the x-axis, where “CLIN” represents 6+ months of listening experience with the clinical maps, “ACUTE” represents acute listening experience with the experimental maps. “2MO,” “4MO,” and “6MO” represent 2, 4, and 6 months of listening experience with the experimental maps. “PEAC” represents post- experimental map acute listening experience with the clinical maps. “PEC” represents post-experimental map listening experience with the clinical map. Line and point color represent data from one listener. Gray shaded regions represent sessions performed under experimental map settings. Overall, questionnaire responses were consistent with objective outcomes measured in Chapter 4, suggesting that SSD+CI listeners perceive no difference between the clinical and experimental maps. It is equally likely, however, that the

SSQ is not sensitive to perceptual differences between the clinical and experimental maps. Moulin et al. (2019) administered the full SSQ (including various short forms) to people with NH and sensorineural hearing loss who did and did not use hearing aids. Separate receiver-operator curves were computed for hearing aid users and non-users. Curves were essentially identical across populations and questionnaire portion (full SSQ or subscale) and, more

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importantly, sensitivity-to-specificity trade-off ratios were poor. Roughly, the best criterion could correctly identify a person with hearing loss 80% of the time and a person with normal-hearing 50% of the time. Interestingly, the spatial subscale produced systematically lower accuracy (~0.70 area under the curve) than any of the other subscales.

Hence, another future direction is to develop tests to measure real-world functional outcomes in SSD+CI listeners and, more generally, people who receive treatment for hearing loss. Typical speech perception tests demonstrate improvement in the perception of individual words and phrases given treatment but often lack predictive validity in terms of real-world outcomes or benefit (Cox,

Johnson, & Xu, 2014; Wu, 2010). At best, speech intelligibility tests are moderately correlated with real-world outcomes (H. Dillon, 2012). Other measures are more efficient and informative than measuring word or sentence recognition and noise as well. The Speech Intelligibility Index and Speech

Transmission Index, for instance, provides fairly accurate estimates of aided word and sentence recognition scores given a listener’s hearing thresholds alone

(although unaided word recognition can improve predictions). Moreover, the benefit of a CI depends on the test conditions and the overall benefit of treatment is not simply the average of speech intelligibility improvement across test conditions. Finally, traditional speech intelligibility tests do not account for the nature of the auditory task and the motivation of the listener to engage in communication (Beechey, Buchholz, & Keidser, 2019; Winn, Wendt, Koelewijn, &

Kuchinsky, 2018). Most clinical and research speech tests are trial-based and

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ask the listener to repeat back what they heard, whether it be words or sentences, without requiring understanding and often without repetition. Ideally, a functional outcome measure could be developed that will be sensitive enough to discriminate between treatment options and fitting approaches in ways that better predict real-world outcomes.

Overall, it is clear giving a CI to people with unilateral hearing loss can improve overall hearing health compared to alternative treatment options (e.g., contralateral-routing-of-signal devices), whether the outcome measure is tinnitus severity, speech intelligibility in poorer ear alone, localization, or speech understanding in noise. It remains unclear, however, whether this population should be fit differently than typical CI listeners who rely on the CI for speech understanding. Investigations into clinically modifiable factors, such as frequency and temporal alignment, will shed light on this question and contribute to a larger body of literature investigating the cost-effectiveness of CI for unilateral hearing loss.

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Bibliography Adel, Y., Nagel, S., Weissgerber, T., Baumann, U., & Macherey, O. (2019). Pitch

matching in cochlear implant users with single-sided deafness: effects of

electrode position and acoustic stimulus type. Frontiers in Neuroscience,

13(November), 1119. https://doi.org/10.3389/fnins.2019.01119

Agterberg, M. J. H., Hol, M. K. S., Van Wanrooij, M. M., Van Opstal, A. J., & Snik,

A. F. M. (2014). Single-sided deafness and directional hearing: contribution

of spectral cues and high-frequency hearing loss in the hearing ear.

Frontiers in Neuroscience, 8, 188. https://doi.org/10.3389/fnins.2014.00188

Agterberg, M. J. H., Snik, A. F. M., Van de Goor, R. M. G., Hol, M. K. S., & Van

Opstal, A. J. (2019). Sound-localization performance of patients with single-

sided deafness is not improved when listening with a bone-conduction

device. Hearing Research, 372, 62–28.

https://doi.org/10.1016/J.HEARES.2018.04.007

Andéol, G., Macpherson, E. A., & Sabin, A. T. (2013). Sound localization in noise

and sensitivity to spectral shape. Hearing Research, 304, 20–27.

https://doi.org/10.1016/j.heares.2013.06.001

Angell, J. R., & Fite, W. (1901). The monaural localization of sound.

Psychological Review, 8(3), 225–246. https://doi.org/10.1037/h0073690

Arehart, K. H., Kates, J. M., Anderson, M. C., & Harvey, L. O. (2007). Effects of

noise and distortion on speech quality judgments in normal-hearing and

hearing-impaired listeners. The Journal of the Acoustical Society of America,

122(2), 1150–1164. https://doi.org/10.1121/1.2754061

136

Arndt, S., Aschendorff, A., Laszig, R., Beck, R., Schild, C., Kroeger, S., …

Wesarg, T. (2011). Comparison of pseudobinaural hearing to real binaural

hearing rehabilitation after cochlear implantation in patients with unilateral

deafness and tinnitus. Otology and Neurotology, 32(1), 39–47.

https://doi.org/10.1097/MAO.0b013e3181fcf271

Arndt, S., Laszig, R., Aschendorff, A., Hassepass, F., Beck, R., & Wesarg, T.

(2017). Cochlear implant treatment of patients with single-sided deafness or

asymmetric hearing loss. HNO, 65(2), S98–S108.

https://doi.org/10.1007/s00106-016-0297-5

Aronoff, J. M., Shayman, C., Prasad, A., Suneel, D., & Stelmach, J. (2015).

Unilateral spectral and temporal compression reduces binaural fusion for

normal hearing listeners with cochlear implant simulations. Hearing

Research, 320, 24–29. https://doi.org/10.1016/j.heares.2014.12.005

Aronoff, J. M., Yoon, Y.-S., Freed, D. J., Vermiglio, A. J., Pal, I., & Soli, S. D.

(2010). The use of interaural time and level difference cues by bilateral

cochlear implant users. The Journal of the Acoustical Society of America,

127(3), EL87–EL92. https://doi.org/10.1121/1.3298451

Ausili, S. A., Backus, B., Agterberg, M. J. H., van Opstal, A. J., & van Wanrooij,

M. M. (2019). Sound localization in real-time vocoded cochlear-implant

simulations with normal-hearing listeners. Trends in Hearing, 23, 1–18.

https://doi.org/10.1177/2331216519847332

Baguley, D. M., Bird, J., Humphriss, R. L., & Prevost, A. T. (2006). The evidence

base for the application of contralateral bone anchored hearing aids in

137

acquired unilateral sensorineural hearing loss in adults. Clinical

Otolaryngology, 31(1), 6–14. https://doi.org/10.1111/j.1749-

4486.2006.01137.x

Balakrishnan, U., & Freyman, R. L. (2008). Speech detection in spatial and

nonspatial speech maskers. The Journal of the Acoustical Society of

America, 123(5), 2680–2691. https://doi.org/10.1121/1.2902176

Başkent, D., & Shannon, R. V. (2006). Frequency transposition around dead

regions simulated with a noiseband vocoder. The Journal of the Acoustical

Society of America, 119(2), 1156. https://doi.org/10.1121/1.2151825

Batra, R., & Yin, T. C. T. (2004). Cross correlation by neurons of the medial

superior olive: A reexamination. Journal of the Association for Research in

Otolaryngology, 5(3), 238–252. https://doi.org/10.1007/s10162-004-4027-4

Batteau, D. W. (1967). The role of the pinna in human localization. Proceedings

of the Royal Society of London. Series B, Biological Sciences, 168(1011),

158–180.

Beechey, T., Buchholz, J. M., & Keidser, G. (2019). Eliciting naturalistic

conversations: A method for assessing communication ability, subjective

experience, and the impacts of noise and hearing impairment. Journal of

Speech, Language, and Hearing Research, 62(2), 470–484.

https://doi.org/10.1044/2018_JSLHR-H-18-0107

Bernstein, J. G. W., Goupell, M. J., Schuchman, G. I., Rivera, A. L., & Brungart,

D. S. (2016). Having two ears facilitates the perceptual separation of

concurrent talkers for bilateral and single-sided deaf cochlear implantees.

138

Ear and Hearing, 37(February), 289–302.

https://doi.org/10.1097/AUD.0000000000000284

Bernstein, J. G. W., Iyer, N., & Brungart, D. S. (2015). Release from informational

masking in a monaural competing-speech task with vocoded copies of the

maskers presented contralaterally. The Journal of the Acoustical Society of

America, 137(2), 702–713. https://doi.org/10.1121/1.4906167

Bernstein, J. G. W., Schuchman, G. I., & Rivera, A. L. (2017). Head shadow and

binaural squelch for unilaterally deaf cochlear implantees. Otology and

Neurotology, 38(7), e195–e202.

https://doi.org/10.1097/MAO.0000000000001469

Bernstein, J. G. W., Stakhovskaya, O. A., Jensen, K. K., & Goupell, M. J. (2019).

Acoustic hearing can interfere with single-sided deafness cochlear-implant

speech perception. Ear and Hearing, 1.

https://doi.org/10.1097/aud.0000000000000805

Bernstein, J. G. W., Stakhovskaya, O. A., Schuchman, G. I., Jensen, K. K., &

Goupell, M. J. (2018). Interaural time-difference discrimination as a measure

of place of stimulation for cochlear-implant users with single-sided deafness.

Trends in Hearing, 22, 1–19. https://doi.org/10.1177/2331216518765514

Bernstein, L. R., & Trahiotis, C. (1996). Binaural beats at high frequencies:

Listeners’ use of envelope-based interaural temporal and intensitive

disparities. The Journal of the Acoustical Society of America, 99(3), 1670–

1679. https://doi.org/10.1121/1.414689

Bess, F. H., Tharpe, A. M., & Gibler, A. M. (1986). Auditory performance of

139

children with unilateral sensorineural hearing loss. Ear and Hearing, 7(1),

20–26. https://doi.org/10.1097/00003446-198602000-00005

Blasco, M. A., & Redleaf, M. I. (2014). Cochlear implantation in unilateral sudden

deafness improves tinnitus and speech comprehension: meta-analysis and

systematic review. Otology & Neurotology, 35(8), 1426–1432.

https://doi.org/10.1097/MAO.0000000000000431

Blauert, J. (1997). Spatial Hearing: The Psychophysics of Human Sound

Localization. MIT Press. https://doi.org/10.1121/1.392109

Boggess, W. J., Baker, J. E., & Balkany, T. J. (1989). Loss of residual hearing

after cochlear implantation. Laryngoscope, 99(10 Pt 1), 1002–1005.

Bronkhorst, A. W., & Plomp, R. (1988). The effect of head-induced interaural time

and level differences on speech intelligibility in noise. Journal of the

Acoustical Society of America, 83(4), 1508–1516.

https://doi.org/10.1121/1.395906

Brown, A. D., Stecker, G. C., & Tollin, D. J. (2015). The precedence effect in

sound localization. Journal of the Association for Research in

Otolaryngology, 16(1), 1–28. https://doi.org/10.1007/s10162-014-0496-2

Brughera, A., Dunai, L., & Hartmann, W. M. (2013). Human interaural time

difference thresholds for sine tones: The high-frequency limit. The Journal of

the Acoustical Society of America, 133(5), 2839–2855.

https://doi.org/10.1121/1.4795778

Brungart, D. S., Simpson, B. D., Ericson, M. A., & Scott, K. R. (2001).

Informational and energetic masking effects in the perception of multiple

140

simultaneous talkers. The Journal of the Acoustical Society of America,

110(5), 2527–2538. https://doi.org/10.1121/1.1408946

Buechner, A., Brendel, M., Lesinski-Schiedat, A., Wenzel, G., Frohne-Buechner,

C., Jaeger, B., & Lenarz, T. (2010). Cochlear implantation in unilateral deaf

subjects associated with ipsilateral tinnitus. Otology and Neurotology, 31(9),

1381–1385. https://doi.org/10.1097/MAO.0b013e3181e3d353

Buss, E., Dillon, M. T., Rooth, M. A., King, E. R., Deres, E. J., Buchman, C. A., …

Brown, K. D. (2018). Effects of cochlear implantation on binaural hearing in

adults with unilateral hearing loss. Trends in Hearing, 22, 1–15.

https://doi.org/10.1177/2331216518771173

Buss, E., Dillon, M. T., Rooth, M. A., King, E. R., Deres, E. J., Buchman, C. A., …

Brown, K. D. (2018). Effects of cochlear implantation on binaural hearing in

adults With unilateral hearing loss. Trends in Hearing, 22,

2331216518771173. https://doi.org/10.1177/2331216518771173

Cabral Junior, F., Pinna, M. H., Alves, R. D., Malerbi, A. F. D. S., & Bento, R. F.

(2014). Cochlear implantation and single-sided deafness: A systematic

review of the literature. International Archives of Otorhinolaryngology, 20(1),

69–75. https://doi.org/10.1055/s-0035-1559586

Carlyon, R. P., MacHerey, O., Frijns, J. H. M., Axon, P. R., Kalkman, R. K.,

Boyle, P., … Dauman, R. (2010). Pitch comparisons between electrical

stimulation of a cochlear implant and acoustic stimuli presented to a normal-

hearing contralateral ear. Journal of the Association for Research in

Otolaryngology, 11(4), 625–640. https://doi.org/10.1007/s10162-010-0222-7

141

Ching, T. Y. C., van Wanrooy, E., & Dillon, H. (2007). Binaural-bimodal fitting or

bilateral implantation for managing severe to profound deafness: a review.

Trends in Amplification, 11(3), 161–192.

https://doi.org/10.1177/1084713807304357

Cochlear Implant: Why Choose Us? | University of Iowa Hospitals & Clinics.

(n.d.). Retrieved March 6, 2020, from https://uihc.org/cochlear-implant-why-

choose-us

Cohen, S. M., & Svirsky, M. A. (2019). Duration of unilateral auditory deprivation

is associated with reduced speech perception after cochlear implantation: A

single-sided deafness study. Cochlear Implants International, 20(2), 51–56.

https://doi.org/10.1080/14670100.2018.1550469

Cox, R. M., Johnson, J. A., & Xu, J. (2014). Impact of advanced hearing aid

technology on speech understanding for older listeners with mild to

moderate, adult-onset, sensorineural hearing Loss. Gerontology, 60(6), 557–

568. https://doi.org/10.1159/000362547

Culling, J. F., Jelfs, S. A., Talbert, A. S., Grange, J. S., & Backhouse, S. S.

(2012). The benefit of bilateral versus unilateral cochlear implantation to

speech intelligibility in noise. Ear and Hearing, 33(6), 673–682.

https://doi.org/10.1097/AUD.0b013e3182587356

Dieudonné, B., & Francart, T. (2019a). Redundant information is sometimes

more beneficial than spatial information to understand speech in noise. Ear

and Hearing, 40(3), 545–554.

https://doi.org/10.1097/AUD.0000000000000660

142

Dieudonné, B., & Francart, T. (2019b). The quest for binaural cue processing in

cochlear implant users’ speech perception. In Conference on Implantable

Auditory Prostheses. Lake Tahoe, CA.

Dillon, H. (2012). Hearing Aids (2nd ed.). New York: Thieme Publishers New

York.

Dillon, M. T., Buss, E., Anderson, M. L., King, E. R., Deres, E. J., Buchman, C.

A., … Pillsbury, H. C. (2017). Cochlear implantation in cases of unilateral

hearing loss: Initial localization abilities. Ear and Hearing, 38(5), 611–619.

https://doi.org/10.1097/AUD.0000000000000430

Dirks, C. E., Nelson, P. B., Winn, M. B., & Oxenham, A. J. (n.d.). Sensitivity to

binaural temporal-envelope beats with single-sided deafness and a cochlear

implant as a measure of tonotopic match.

Dirks, C., Nelson, P. B., Sladen, D. P., & Oxenham, A. J. (2019). Mechanisms of

localization and speech perception with colocated and spatially separated

noise and speech maskers under single-sided deafness with a cochlear

implant. Ear and Hearing, 40(6), 1293–1306.

https://doi.org/10.1097/AUD.0000000000000708

Djourno, A., & Eyries, C. (1957). Auditory prosthesis by means of a distant

electrical stimulation of the sensory nerve with the use of an indwelt coiling.

La Presse Medicale, 65(63), 1417.

Djourno, A., Eyries, C., & Vallancien, B. (1957a). Electric excitation of the

cochlear nerve in man by induction at a distance with the aid of micro-coil

included in the fixture. Comptes Rendus Des Seances de La Societe de

143

Biologie et de Ses Filiales, 151(3), 423–425.

Djourno, A., Eyries, C., & Vallancien, B. (1957b). Preliminary attempts of

electrical excitation of the auditory nerve in man, by permanently inserted

micro-apparatus. Bulletin de l’Academie Nationale de Medecine, 141(21–

23), 481–483.

Dorman, M. F., Loizou, P. C., Fitzke, J., & Tu, Z. (1998). The recognition of

sentences in noise by normal-hearing listeners using simulations of

cochlear-implant signal processors with 6-20 channels. The Journal of the

Acoustical Society of America, 104(6), 3583–3585.

https://doi.org/10.1121/1.423940

Dorman, M. F., Zeitler, D. M., Cook, S. J., Loiselle, L., Yost, W. A., Wanna, G. B.,

& Gifford, R. H. R. H. (2015). Interaural level difference cues determine

sound source localization by single-sided deaf patients fit with a cochlear

implant. Audiology & Neurotology, 20(3), 183–188.

https://doi.org/10.1159/000375394.

Durlach, N. I. (1963). Equalization and cancellation theory of binaural masking-

level differences. The Journal of the Acoustical Society of America, 35(8),

1206–1218. https://doi.org/10.1121/1.1918675

Faber, H. T., Kievit, H., De Wolf, M. J. F., Cremers, C. W. R. J., Snik, A. F. M., &

Hol, M. K. S. (2012). Analysis of factors predicting the success of the bone

conduction device headband trial in patients with single-sided deafness.

Archives of Otolaryngology - Head and Neck Surgery, 138(12), 1129–1135.

https://doi.org/10.1001/jamaoto.2013.754

144

Firszt, J. B., Holden, L. K., Reeder, R. M., Cowdrey, L. M., & King, S. M. (2012).

Cochlear implantation in adults with asymmetric hearing loss. Ear and

Hearing, 33(4), 521–533.

https://doi.org/10.1097/AUD.0b013e31824b9dfc.Cochlear

Firszt, J. B., Holden, L. K., Reeder, R. M., Waltzman, S. B., & Arndt, S. (2012).

Auditory abilities after cochlear implantation in adults with unilateral

deafness: A pilot study. Otology & Neurotology, 33(8), 1339–1346.

https://doi.org/10.1097/MAO.0b013e318268d52d

Firszt, J. B., Reeder, R. M., & Holden, L. K. (2017). Unilateral hearing loss:

Understanding speech recognition and localization variability — implications

for cochlear implant candidacy. Ear and Hearing, 38(2), 159–173.

https://doi.org/10.1097/AUD.0000000000000380

Firszt, J. B., Reeder, R. M., Holden, L. K., & Dwyer, N. Y. (2018). Results in adult

cochlear implant recipients with varied asymmetric hearing. Ear and

Hearing, 39(5), 845. https://doi.org/10.1097/AUD.0000000000000548

Francart, T., Brokx, J., & Wouters, J. (2009). Sensitivity to interaural time

differences with combined cochlear implant and acoustic stimulation. Journal

of the Association for Research in Otolaryngology, 10(1), 131–141.

https://doi.org/10.1007/s10162-008-0145-8

Francart, T., Lenssen, A., & Wouters, J. (2011). Sensitivity of bimodal listeners to

interaural time differences with modulated single- and multiple-channel

stimuli. Audiology and Neurotology, 16(2), 82–92.

https://doi.org/10.1159/000313329

145

Francart, T., & McDermott, H. J. (2013). Psychophysics, fitting, and signal

processing for combined hearing aid and cochlear implant stimulation. Ear

and Hearing, 34(6), 685–700.

https://doi.org/10.1097/AUD.0b013e31829d14cb

Francart, T., Wiebe, K., & Wesarg, T. (2018). Interaural time difference

perception with a cochlear implant and a normal ear. Journal of the

Association for Research in Otolaryngology, 19(6), 703–715.

https://doi.org/10.1007/s10162-018-00697-w

Freyman, R. L., Balakrishnan, U., & Helfer, K. S. (2001). Spatial release from

informational masking in speech recognition. The Journal of the Acoustical

Society of America, 109(5), 2112–2122. https://doi.org/10.1121/1.1354984

Freyman, R. L., Balakrishnan, U., & Helfer, K. S. (2004). Effect of number of

masking talkers and auditory priming on informational masking in speech

recognition. The Journal of the Acoustical Society of America, 115(5), 2246–

2256. https://doi.org/10.1121/1.1689343

Freyman, R. L., Helfer, K. S., McCall, D. D., & Clifton, R. K. (1999). The role of

perceived spatial separation in the unmasking of speech. The Journal of the

Acoustical Society of America, 106(6), 3578–3588.

https://doi.org/10.1121/1.428211

Friesen, L. M., Shannon, R. V., Baskent, D., & Wang, X. (2001). Speech

recognition in noise as a function of the number of spectral channels:

Comparison of acoustic hearing and cochlear implants. The Journal of the

Acoustical Society of America, 110(2), 1150–1163.

146

https://doi.org/10.1121/1.1381538

Gardner, W. G., & Martin, K. D. (1995). HRTF measurements of a KEMAR. The

Journal of the Acoustical Society of America, 97(6), 3907–3908.

https://doi.org/10.1121/1.412407

Gartrell, B. C., Jones, H. G., Kan, A., Buhr-Lawler, M., Gubbels, S. P., &

Litovsky, R. Y. (2014). Investigating long-term effects of cochlear

implantation in single-sided deafness: A best practice model for longitudinal

assessment of spatial hearing abilities and tinnitus handicap. Otology &

Neurotology, 35(9), 1525–1532.

https://doi.org/10.1097/MAO.0000000000000437

Gatehouse, R. W. (1976). Further research in localization of sound by completely

monaural subjects. Journal of Auditory Research, 16(4), 265–273.

Gatehouse, R. W., & Cox, W. (1972). Localization of sound by completely

monaural deaf subjects. Journal of Auditory Research, 12(2), 179–183.

Gersuni, G. V., & Volokhov, A. A. (1936). On the electrical excitability of the

auditory organ on the effect of alternating currents on the normal auditory

apparatus. Journal of Experimental Psychology, 19(3), 370–382.

https://doi.org/10.1037/h0060771

Gifford, R. H., Dorman, M. F., Shallop, J. K., & Sydlowski, S. A. (2010). Evidence

for the expansion of adult cochlear implant candidacy. Ear and Hearing,

31(2), 186–194. https://doi.org/10.1097/AUD.0b013e3181c6b831

Goossens, H. H. L. M., & Van Opstal, A. J. (1999). Influence of head position on

the spatial representation of acoustic targets. Journal of Neurophysiology,

147

81(6), 2720–2736. https://doi.org/10.1152/jn.1999.81.6.2720

Goupell, M. J., Cosentino, S., Stakhovskaya, O. A., & Bernstein, J. G. W. (2019).

Interaural pitch-discrimination range effects for bilateral and single-sided-

deafness cochlear-implant users. Journal of the Association for Research in

Otolaryngology, 20(2), 187–203. https://doi.org/10.1007/s10162-018-00707-

Goupell, M. J., & Stakhovskaya, O. A. (2018). Across-channel interaural-level-

difference processing demonstrates frequency dependence. The Journal of

the Acoustical Society of America, 143(2), 645–658.

https://doi.org/10.1121/1.5021552

Goupell, M. J., Stoelb, C., Kan, A., & Litovsky, R. Y. (2013). Effect of mismatched

place-of-stimulation on the salience of binaural cues in conditions that

simulate bilateral cochlear-implant listening. The Journal of the Acoustical

Society of America, 133(4), 2272–2287. https://doi.org/10.1121/1.4792936

Greenwood, D. D. (1990). A cochlear frequency-position function for several

species--29 years later. The Journal of the Acoustical Society of America,

87(6), 2592–2605. https://doi.org/10.1121/1.399052

Greenwood, Donald D. (1961). Critical bandwidth and the frequency coordinates

of the basilar membrane. The Journal of the Acoustical Society of America,

33(10), 1344–1356. https://doi.org/10.1121/1.1908437

Hagerman, B. (1982). Sentences for testing speech intelligibility in noise.

Scandinavian Audiology, 11(2), 79–87.

https://doi.org/10.3109/01050398209076203

148

Hansen, M. R., Gantz, B. J., & Dunn, C. (2013). Outcomes following cochlear

implantation for patients with single-sided deafness, including those with

recalcitrant Ménière’s disease. Otology & Neurotology, 34(9), 1681–1687.

https://doi.org/10.1097/MAO.0000000000000102

Hassepass, F., Aschendorff, A., Wesarg, T., Kröger, S., Laszig, R., Beck, R. L.,

… Arndt, S. (2013). Unilateral deafness in children: audiologic and

subjective assessment of hearing ability after cochlear implantation. Otology

& Neurotology, 34(1), 53–60.

https://doi.org/10.1097/MAO.0b013e31827850f0

Hassepass, F., Schild, C., Aschendorff, A., Laszig, R., Maier, W., Beck, R., …

Arndt, S. (2013). Clinical outcome after cochlear implantation in patients with

unilateral hearing loss due to labyrinthitis ossificans. Otology & Neurotology,

34(7), 1278–1283. https://doi.org/10.1097/MAO.0b013e3182937ad4

Haywood, N. R., Undurraga, J. A., Marquardt, T., & McAlpine, D. (2015). A

comparison of two objective measures of binaural processing: The interaural

phase modulation following response and the binaural interaction

component. Trends in Hearing, 19, 1–17.

https://doi.org/10.1177/2331216515619039

He, S., Brown, C. J., & Abbas, P. J. (2010). Effects of stimulation level and

electrode pairing on the binaural interaction component of the electrically

evoked auditory brain stem response. Ear and Hearing, 31(4), 457–470.

https://doi.org/10.1097/AUD.0b013e3181d5d9bf

Hofman, P. M., & Van Opstal, A. J. (2003). Binaural weighting of pinna cues in

149

human sound localization. Experimental Brain Research, 148(4), 458–470.

https://doi.org/10.1007/s00221-002-1320-5

Hofman, Paul M., Van Riswick, J. G. A., & Van Opstal, A. J. (1998). Relearning

sound localization with new ears. Nature Neuroscience, 1(5), 417–421.

https://doi.org/10.1038/1633

Hu, H., & Dietz, M. (2015). Comparison of interaural electrode pairing of bilateral

cochlear implants. Trends in Hearing, 19, 1–22.

https://doi.org/10.1177/2331216515617143

Hughes, M. L., Neff, D. L., Simmons, J. L., & Moeller, M. P. (2014). Performance

outcomes for borderline cochlear implant recipients with substantial

preoperative residual hearing. Otology & Neurotology, 35(8), 1373–1384.

https://doi.org/10.1097/MAO.0000000000000367

Jacob, R., Stelzig, Y., Nopp, P., & Schleich, P. (2011). Audiologische ergebnisse

mit cochlear implant bei einseitiger taubheit. HNO, 59(5), 453–460.

https://doi.org/10.1007/s00106-011-2321-0

Jones, R. C., Stevens, S. S., & Lurie, M. H. (1940). Three mechanisms of

hearing by electrical stimulation. Journal of the Acoustical Society of

America, 12(2), 281–290. https://doi.org/10.1121/1.1916103

Jongkees, L. B. W., & Van der Veer, R. A. V. D. (1957). Directional hearing

capacity in hearing disorders. Acta Oto-Laryngologica, 48(5–6), 465–474.

https://doi.org/10.3109/00016485709126908

Joris, P. X., Smith, P. H., & Yin, T. C. . (1998). Coincidence detection in the

auditory system. Neuron, 21(6), 1235–1238. https://doi.org/10.1016/s0896-

150

6273(00)80643-1

Kamal, S. M., Robinson, A. D., & Diaz, R. C. (2012). Cochlear implantation in

single-sided deafness for enhancement of sound localization and speech

perception. Current Opinion in Otolaryngology & Head and Neck Surgery,

20(5), 393–397. https://doi.org/10.1097/MOO.0b013e328357a613

Kan, A., & Litovsky, R. Y. (2015). Binaural hearing with electical stimulation.

Hearing Research, 322, 127–137.

https://doi.org/10.1016/j.heares.2014.08.005

Kan, A., Litovsky, R. Y., & Goupell, M. J. (2015). Effects of interaural pitch

matching and auditory image centering on binaural sensitivity in cochlear

implant users. Ear and Hearing, 36(3), e62–e68.

https://doi.org/10.1097/AUD.0000000000000135

Kan, A., Stoelb, C., Litovsky, R. Y., & Goupell, M. J. (2013). Effect of mismatched

place-of-stimulation on binaural fusion and lateralization in bilateral cochlear-

implant users. The Journal of the Acoustical Society of America, 134(4),

2923–2936. https://doi.org/10.1121/1.4820889

Kandler, K., Clause, A., & Noh, J. (2009). Tonotopic reogranization of developing

auditory brainstem circuits. Nature Neuroscience, 12(6), 711–717.

https://doi.org/10.1038/nn.2332

Kistler, D. J., & Wightman, F. L. (1992). A model of head-related transfer

functions based on principal components analysis and minimum-phase

reconstruction. The Journal of the Acoustical Society of America, 91(3),

1637–1647. https://doi.org/10.1121/1.402444

151

Kitterick, P. T., Smith, S. N., & Lucas, L. (2016). Hearing instruments for

unilateral severe-to-profound sensorineural hearing loss in adults: A

systematic review and meta-analysis. Ear and Hearing, 37(5), 495–507.

https://doi.org/10.1097/AUD.0000000000000313

Koehnke, J., Culotta, C. P., Hawley, M. L., & Colburn, H. S. (1995). Effects of

reference interaural time and intensity differences on binaural performance

in listeners with normal and impaired hearing. Ear and Hearing, 16(4), 331–

353. https://doi.org/10.1097/00003446-199508000-00001

Kohlrausch, A., Fassel, R., & Dau, T. (2000). The influence of carrier level and

frequency on modulation and beat-detection thresholds for sinusoidal

carriers. The Journal of the Acoustical Society of America, 108(2), 723–734.

https://doi.org/10.1121/1.429605

Landsberger, D. M., Svrakic, M., Roland, J. T., Svirsky, M., Svrakic, S., Roland,

J. T., & Svirsky, M. (2015). The relationship between insertion angles,

default frequency allocations, and spiral ganglion place pitch in cochlear

implants. Ear and Hearing, 36(5), e207-13.

https://doi.org/10.1097/AUD.0000000000000163

Lehnhardt, E. (1993). Intracochlear placement of cochlear implant electrodes in

soft surgery technique. HNO, 41(7), 356–359.

Lenssen, A., Francart, T., Brokx, J., & Wouters, J. (2011). Bimodal listeners are

not sensitive to interaural time differences in unmodulated low-frequency

stimuli (L). The Journal of the Acoustical Society of America, 129(6), 3457–

3460. https://doi.org/10.1121/1.3557051

152

Licklider, J. C. R., Webster, J. C., & Hedlun, J. M. (1950). On the frequency limits

of binaural beats. The Journal of the Acoustical Society of America, 22(4),

468–473. https://doi.org/10.1121/1.1906629

Lieu, J. E. C. (2004). Speech-language and educational consequences of

unilateral hearing loss in children. Archives of Otolaryngology--Head & Neck

Surgery, 130(5), 524–530. https://doi.org/10.1001/archotol.130.5.524

Lin, L.-M., Bowditch, S., Anderson, M. J., May, B., Cox, K. M., & Niparko, J. K.

(2006). Amplification in the rehabilitation of unilateral deafness: speech in

noise and directional hearing effects with bone-anchored hearing and

contralateral routing of signal amplification. Otology & Neurotology, 27(2),

172–182. https://doi.org/10.1097/01.mao.0000196421.30275.73

Litovsky, R. Y., Moua, K., Godar, S., Kan, A., Misurelli, S. M., & Lee, D. J. (2019).

Restoration of spatial hearing in adult cochlear implant users with single-

sided deafness. Hearing Research, 372, 69–79.

https://doi.org/10.1016/j.heares.2018.04.004

Long, C. J., Carlyon, R. P., Litovsky, R. Y., & Downs, D. H. (2006). Binaural

unmasking with bilateral cochlear implants. Journal of the Association for

Research in Otolaryngology, 7(4), 352–360. https://doi.org/10.1007/s10162-

006-0049-4

Long, C. J., Eddington, D. K., Colburn, H. S., & Rabinowitz, W. M. (2003).

Binaural sensitivity as a function of interaural electrode position with a

bilateral cochlear implant user. The Journal of the Acoustical Society of

America, 114(3), 1565–1574. https://doi.org/10.1121/1.1603765

153

Ma, N., Morris, S., & Kitterick, P. T. (2016). Benefits to speech perception in

noise from the binaural integration of electric and acoustic signals in

simulated unilateral deafness. Ear and Hearing, 37(3), 248–259.

https://doi.org/10.1097/AUD.0000000000000252

Macpherson, E. A., & Middlebrooks, J. C. (2002). Listener weighting of cues for

lateral angle: the duplex theory of sound localization revisited. The Journal of

the Acoustical Society of America, 111(5), 2219–2236.

https://doi.org/10.1121/1.1471898

Magnusson, L. (2011). Comparison of the fine structure processing (FSP)

strategy and the CIS strategy used in the MED-EL cochlear implant system:

speech intelligibility and music sound quality. International Journal of

Audiology, 50(4), 279–287. https://doi.org/10.3109/14992027.2010.537378

Majdak, P., Baumgartner, R., & Laback, B. (2014). Acoustic and non-acoustic

factors in modeling listener-specific performance of sagittal-plane sound

localization. Frontiers in Psychology, 5(APR), 1–10.

https://doi.org/10.3389/fpsyg.2014.00319

McFadden, D., & Pasanen, E. G. (1975). Binaural beats at high frequencies.

Science, 190(4212), 394–396. https://doi.org/10.1126/science.1179219

Mehta, A. H., & Oxenham, A. J. (2017). Vocoder simulations explain complex

pitch perception limitations experienced by cochlear implant users. Journal

of the Association for Research in Otolaryngology, 18(6), 789–802.

https://doi.org/10.1007/s10162-017-0632-x

Mertens, G., De Bodt, M., & Van de Heyning, P. (2016). Cochlear implantation as

154

a long-term treatment for ipsilateral incapacitating tinnitus in subjects with

unilateral hearing loss up to 10 years. Hearing Research, 331, 1–6.

https://doi.org/10.1016/j.heares.2015.09.016

Mertens, G., De Bodt, M., & Van de Heyning, P. (2017). Evaluation of long-term

cochlear implant use in subjects with acquired unilateral profound hearing

loss: focus on binaural auditory outcomes. Ear and Hearing, 38(1), 117–125.

https://doi.org/10.1097/AUD.0000000000000359

Middlebrooks, J. C. (1992). Narrow-band sound localization related to external

ear acoustics. The Journal of the Acoustical Society of America, 92(5),

2607–2624. https://doi.org/10.1121/1.404400

Middlebrooks, J. C., & Green, D. M. (1991). Sound localization by human

listeners. Annual Review of Psychology, 42, 135–159.

Mills, A. W. (1958). On the minimum audible angle. Journal of the Acoustical

Society of America, 30(4), 237–246. https://doi.org/10.1121/1.1909553

Moore, B. C. J. (1986). Parallels betwen frequency selectivity measured

psychophysically and in cochlear mechanics. Scandinavian Audiology

Supplement, 25, 139–152.

Moore, B. C. J., Huss, M., Vickers, D. A., Glasberg, B. R., & Alciintara, J. I.

(2000). A test for the diagnosis of dead regions in the cochlea. British

Journal of Audiology, 34(4), 205–224.

https://doi.org/10.3109/03005364000000131

Mossop, J. E., & Culling, J. F. (1997). Lateralization for large interaural delays.

British Journal of Audiology, 31(2), 99. https://doi.org/10.1121/1.424369

155

Moulin, A., Vergne, J., Gallego, S., & Micheyl, C. (2019). A new Speech, Spatial,

and Qualities of Hearing Scale short-form: Factor, cluster, and comparative

analyses. Ear and Hearing, 40(4), 938–950.

https://doi.org/10.1097/AUD.0000000000000675

Nelson, P. B., Perry, T. T., Gregan, M., & VanTasell, D. (2018). Self-adjusted

amplification parameters produce large between-subject variability and

preserve speech intelligibility. Trends in Hearing, 22, 1–13.

https://doi.org/10.1177/2331216518798264

Newton, V. E. (1983). Sound localisation in children with a severe unilateral

hearing loss. Audiology, 22(2), 189–198.

https://doi.org/10.3109/00206098309072782

Niparko, J. K. (2009). Cochlear implants: Principles & practices (2nd ed.).

Wolters Kluwer Health/Lippincott Williams & Wilkins.

Niparko, J. K., Cox, K. M., & Lustig, L. R. (2003). Comparison of the bone

anchored hearing aid implantable hearing device with contralateral routing of

offside signal amplification in the rehabilitation of unilateral deafness.

Otology and Neurotology, 24(1), 73–78. https://doi.org/10.1097/00129492-

200301000-00015

Noble, J. H., Labadie, R. F., Gifford, R. H., & Dawant, B. M. (2013). Image-

guidance enables new methods for customizing cochlear implant stimulation

strategies. IEEE Transactions of Neural Systems and Rehabilitation

Engineering, 21(5), 820–829. https://doi.org/10.1109/TNSRE. 2013.2253333

Noel, V. A., & Eddington, D. K. (2013). Sensitivity of bilateral cochlear implant

156

users to fine-structure and envelope interaural time differences. The Journal

of the Acoustical Society of America, 133(4), 2314–2328.

https://doi.org/10.1121/1.4794372

Oh, Y., Hartling, C. L., Srinivasan, N. K., Eddolls, M., Diedesch, A. C., Gallun, F.

J., & Reiss, L. A. J. (2019). Broad binaural fusion impairs segregation of

speech based on voice pitch differences in a ‘ cocktail party ’ environment.

BioRxiv, 1–39.

Oh, Y., Reiss, L., Srinivasan, N., Jakien, K., Diedesch, A., Gallun, F. J., &

Hartling, C. (2017). Voice gender release from masking in cochlear implant

users is correlated with binaural pitch fusion. The Journal of the Acoustical

Society of America, 141(5), 3816–3816. https://doi.org/10.1121/1.4988444

Oxenham, A. J., Fligor, B. J., Mason, C. R., & Kidd, G. (2003). Informational

masking and musical training. The Journal of the Acoustical Society of

America, 114(3), 1543–1549. https://doi.org/10.1121/1.1598197

Oxenham, A. J., & Kreft, H. A. (2014). Speech perception in tones and noise via

cochlear implants reveals influence of spectral resolution on temporal

processing. Trends in Hearing, 18, 1–14.

https://doi.org/10.1177/2331216514553783

Oxenham, A. J., & Kreft, H. A. (2016). Speech masking in normal and impaired

hearing: Interactions between frequency selectivity and inherent temporal

fluctuations in noise. In Physiology, Psychoacoustics and Cognition in

Normal and Impaired Hearing (pp. 125–132). Springer, Cham.

https://doi.org/10.1007/978-3-319-25474-6_14

157

Pedley, A. J., & Kitterick, P. T. (2017). Contralateral routing of signals disrupts

monaural level and spectral cues to sound localisation on the horizontal

plane. Hearing Research. https://doi.org/10.1016/j.heares.2017.06.007

Pennings, R. J. E., Gulliver, M., & Morris, D. P. (2011). The importance of an

extended preoperative trial of BAHA in unilateral sensorineural hearing loss:

A prospective cohort study. Clinical Otolaryngology, 36(5), 442–449.

https://doi.org/10.1111/j.1749-4486.2011.02388.x

Perrott, D. R., & Musicant, A. D. (1977). Rotating tones and binaural beats. The

Journal of the Acoustical Society of America, 61(5), 1288–1292.

https://doi.org/10.1121/1.381430

Rakerd, B., & Hartmann, W. M. (1986). Localization of sound in rooms, III: Onset

and duration effects. The Journal of the Acoustical Society of America,

80(6), 1695–1706. https://doi.org/10.1121/1.394282

Ramos Macías, A., Falcón-González, J. C., Manrique Rodríguez, M., Morera

Pérez, C., García-Ibáñez, L., Cenjor Español, C., … Killian, M. (2018). One-

year results for patients with unilateral hearing loss and accompanying

severe tinnitus and hyperacusis treated with a cochlear implant. Audiology &

Neurotology, 23(1), 8–19. https://doi.org/10.1159/000488755

Reiss, L. A. J., Fowler, J. R., Hartling, C. L., & Oh, Y. (2018). Binaural pitch

fusion in bilateral cochlear implant users. Ear and Hearing, 39(2), 390–397.

https://doi.org/10.1097/AUD.0000000000000497

Reiss, L. A. J., Ito, R. A., Eggleston, J. L., Laio, S. J., Becker, J. E., Lakin, C. M.,

… McMenomey, S. O. (2015). Pitch adaptation patterns in bimodal cochlear

158

implant users: Over time and after experience. Ear and Hearing, 36(2), e23–

e34. https://doi.org/10.1097/AUD.0000000000000114

Reiss, L. A. J. J., Eggleston, J. L., Walker, E. P., & Oh, Y. (2016). Two ears are

not always better than one: mandatory vowel fusion across spectrally

mismatched ears in hearing-impaired listeners. Journal of the Association for

Research in Otolaryngology, 17(4), 341–356.

https://doi.org/10.1007/s10162-016-0570-z

Reiss, L. A. J., Lowder, M. W., Karsten, S. A., Turner, C. W., & Gantz, B. J.

(2011). Effects of extreme tonotopic mismatches between bilateral cochlear

implants on electric pitch perception: A case study. Ear and Hearing, 32(4),

536–540. https://doi.org/10.1097/AUD.0b013e31820c81b0

Reiss, L. A. J., Turner, C. W., Erenberg, S. R., & Gantz, B. J. (2007). Changes in

pitch with a cochlear implant over time. Journal of the Association for

Research in Otolaryngology, 8(2), 241–257. https://doi.org/10.1007/s10162-

007-0077-8

Reiss, L., Simmons, S., Anderson, L., & Molis, M. R. (2018). Effects of broad

binaural fusion and hearing loss on dichotic concurrent vowel identification.

The Journal of the Acoustical Society of America, 143(3), 1942–1942.

https://doi.org/10.1121/1.5036358

Risoud, M., Hanson, J.-N., Gauvrit, F., Renard, C., Lemesre, P.-E., Bonne, N.-X.,

& Vincent, C. (2018). Sound source localization. European Annals of

Otorhinolaryngology, Head and Neck Diseases, 135, 259–264.

https://doi.org/10.1016/j.anorl.2018.04.009

159

Rothauser, E. H., Chapman, W. D., Guttman, N., Nordby, K. S., Silbiger, H. R.,

Urbanek, G. E., & Weinstock, M. (1969). I.E.E.E. recommended practice for

speech quality measurements. IEEE Transactions on Audio and

Electroacoustics, 17(297), 227–246.

https://doi.org/10.1109/TAU.1969.1162058

Ruggero, M. A., & Temchin, A. N. (2007). Similarity of traveling-wave delays in

the hearing organs of humans and other tetrapods. Journal of the

Association for Research in Otolaryngology, 8(2), 153–166.

https://doi.org/10.1007/s10162-007-0081-z

Sargent, E. W., Herrmann, B., Hollenbeak, C. S., & Bankaitis, A. E. (2001). The

minimum speech test battery in profound unilateral hearing loss. Otology &

Neurotology, 22(4), 480–486. https://doi.org/10.1097/00129492-200107000-

00012

Schatzer, R., Vermeire, K., Visser, D., Krenmayr, A., Kals, M., Voormolen, M., …

Zierhofer, C. (2014). Electric-acoustic pitch comparisons in single-sided-deaf

cochlear implant users: Frequency-place functions and rate pitch. Hearing

Research, 309(2014), 26–35. https://doi.org/10.1016/j.heares.2013.11.003

Search of: cochlear implant | single-sided deafness - List Results -

ClinicalTrials.gov. (n.d.). Retrieved February 19, 2020, from

https://clinicaltrials.gov/ct2/results?cond=single-

sided+deafness&term=cochlear+implant&cntry=&state=&city=&dist=

Seebacher, J., Franke-Trieger, A., Weichbold, V., Zorowka, P., & Stephan, K.

(2019). Improved interaural timing of acoustic nerve stimulation affects

160

sound localization in single-sided deaf cochlear implant users. Hearing

Research, 371, 19–27. https://doi.org/10.1016/j.heares.2018.10.015

Seeber, B. U., & Fastl, H. (2008). Localization cues with bilateral cochlear

implants. The Journal of the Acoustical Society of America, 123(2), 1030–

1042. https://doi.org/10.1121/1.2821965

Sheffield, S. W., Goupell, M. J., Spencer, N. J., Stakhovskaya, O. A., &

Bernstein, J. G. W. (2019). Binaural optimization of cochlear implants:

discarding frequency content without sacrificing head-shadow benefit. Ear

and Hearing. https://doi.org/10.1097/AUD.0000000000000784

Simpson, A., Hersbach, A. A., & McDermott, H. J. (2005). Improvements in

speech perception with an experimental nonlinear frequency compression

hearing device. International Journal of Audiology, 44(5), 281–292.

https://doi.org/10.1080/14992020500060636

Sladen, D. P., Carlson, M. L., Dowling, B. P., Olund, A. P., DeJong, M. D.,

Breneman, A. W., … Driscoll, C. L. (2018). Cochlear implantation in adults

with asymmetric hearing loss: Speech recognition in quiet and in noise, and

health related quality of life. Otology & Neurotology, 39(5), 576–581.

https://doi.org/10.1097/MAO.0000000000001763

Sladen, D. P., Carlson, M. L., Dowling, B. P., Olund, A. P., Teece, K., DeJong, M.

D., … Driscoll, C. L. (2017). Early outcomes after cochlear implantation for

adults and children with unilateral hearing loss. The Laryngoscope, 127(7),

1683–1688. https://doi.org/10.1002/lary.26337

Sladen, D. P., Frisch, C. D., Carlson, M. L., Driscoll, C. L. W., Torres, J. H., &

161

Zeitler, D. M. (2017). Cochlear implantation for single-sided deafness: A

multicenter study. The Laryngoscope, 127(1), 223–228.

https://doi.org/10.1002/lary.26102

Slattery, W. H., & Middlebrooks, J. C. (1994). Monaural sound localization: acute

versus chronic unilateral impairment. Hearing Research, 75(1), 38–46.

https://doi.org/10.1016/0378-5955(94)90053-1

Smith, Z. M., Delgutte, B., & Oxenham, A. J. (2002). Chimaeric sounds reveal

dichotomies in auditory perception. Nature, 416(6876), 87–90.

https://doi.org/10.1038/416087a

Smits, C., Kapteyn, T. S., & Houtgast, T. (2004). Development and validation of

an automatic speech-in-noise screening test by telephone. International

Journal of Audiology, 43(1), 15–28.

https://doi.org/10.1080/14992020400050004

Stakhovskaya, O., Schuchman, G. I., Goupell, M. J., & Bernstein, J. G. (2016).

Interaural-time-difference discrimination as a measure of place of stimulation

for cochlear-implant listeners with single-sided deafness. The Journal of the

Acoustical Society of America, 140(4), 3157–3157.

https://doi.org/10.1121/1.4969909

Stakhovskaya, O., Sridhar, D., Bonham, B. H., & Leake, P. A. (2007). Frequency

map for the human cochlear spiral ganglion: Implications for cochlear

implants. Journal of the Association for Research in Otolaryngology, 8(2),

220–233. https://doi.org/10.1007/s10162-007-0076-9

Stelzig, Y., Jacob, R., & Mueller, J. (2011). Preliminary speech recognition

162

results after cochlear implantation in patients with unilateral hearing loss: a

case series. Journal of Medical Case Reports, 5(1), 343.

https://doi.org/10.1186/1752-1947-5-343

Stevens, S. S. (1937). On hearing by electrical stimulation. Journal of the

Acoustical Society of America, 8(3), 191–195.

https://doi.org/10.1121/1.1915894

Stevens, S. S., & Jones, R. C. (1939). The mechanism of hearing by electrical

stimulation. The Journal of the Acoustical Society of America, 10(4), 261–

269. https://doi.org/10.1121/1.1915984

Svirsky, M. A., Silveira, A., Neuburger, H., Teoh, S. W., & Suárez, H. (2004).

Long-term auditory adaptation to a modified peripheral frequency map. Acta

Oto-Laryngologica, 124(4), 381–386.

https://doi.org/10.1080/00016480310000593

Svirsky, Mario A., Talavage, T. M., Sinha, S., Neuburger, H., & Azadpour, M.

(2015). Gradual adaptation to auditory frequency mismatch. Hearing

Research, 322(646), 163–170. https://doi.org/10.1016/j.heares.2014.10.008

Tan, C. T., & Moore, B. C. J. (2008). Perception of nonlinear distortion by

hearing-impaired people. International Journal of Audiology, 47(5), 246–256.

https://doi.org/10.1080/14992020801945493

Távora-Vieira, D., Marino, R., Krishnaswamy, J., Kuthbutheen, J., & Rajan, G. P.

(2013). Cochlear implantation for unilateral deafness with and without

tinnitus: a case series. The Laryngoscope, 123(5), 1251–1255.

https://doi.org/10.1002/lary.23764

163

Távora-Vieira, D., Rajan, G. P., Van De Heyning, P., & Mertens, G. (2019).

Evaluating the long-term hearing outcomes of cochlear implant users with

single-sided deafness. Otology and Neurotology, 40(6), E575–E580.

https://doi.org/10.1097/MAO.0000000000002235

Undurraga, J. A., Haywood, N. R., Marquardt, T., & McAlpine, D. (2016). Neural

representation of interaural time differences in humans—an objective

measure that matches behavioural performance. Journal of the Association

for Research in Otolaryngology, 17(6), 591–607.

https://doi.org/10.1007/s10162-016-0584-6

Van de Heyning, P., Vermeire, K., Diebl, M., Nopp, P., Anderson, I., & De Ridder,

D. (2008). Incapacitating unilateral tinnitus in single-sided deafness treated

by cochlear implantation. Annals of Otology, Rhinology and Laryngology,

117(9), 645–652. https://doi.org/10.1177/000348940811700903 van Hoesel, R. J. M., & Clark, G. M. (1997). Psychophysical studies with two

binaural cochlear implant subjects. The Journal of the Acoustical Society of

America, 102(1), 495–507. https://doi.org/10.1121/1.419611

Van Hoesel, Richard J. M., Jones, G. L., & Litovsky, R. Y. (2009). Interaural time-

delay sensitivity in bilateral cochlear implant users: Effects of pulse rate,

modulation rate, and place of stimulation. Journal of the Association for

Research in Otolaryngology, 10(4), 557–567.

https://doi.org/10.1007/s10162-009-0175-x van Hoesel, Richard J. M., & Tyler, R. S. (2003). Speech perception, localization,

and lateralization with bilateral cochlear implants. The Journal of the

164

Acoustical Society of America, 113(3), 1617–1630.

https://doi.org/10.1121/1.1539520

Van Hoesel, Richard J.M. (2012, June 1). Contrasting benefits from contralateral

implants and hearing aids in cochlear implant users. Hearing Research.

Elsevier. https://doi.org/10.1016/j.heares.2011.11.014

Van Wanrooij, M. M., & Van Opstal, A. J. (2004). Contribution of head shadow

and pinna cues to chronic monaural sound localization. The Journal of

Neuroscience, 24(17), 4163–4171.

https://doi.org/10.1523/JNEUROSCI.0048-04.2004

Van Wanrooij, M. M., & Van Opstal, A. J. (2005). Relearning sound localization

with a new ear. The Journal of Neuroscience, 25(22), 5413–5424.

https://doi.org/10.1523/JNEUROSCI.0850-05.2005

Van Zon, A., Peters, J. P. M., Stegeman, I., Smit, A. L., & Grolman, W. (2015).

Cochlear implantation for patients with single-sided deafness or

asymmetrical hearing loss: a systematic review of the evidence. Otology &

Neurotology, 36(2), 209–219.

https://doi.org/10.1097/MAO.0000000000000681

Vermeire, K., Nobbe, A., Schleich, P., Nopp, P., Voormolen, M. H., & Van de

Heyning, P. H. (2008). Neural tonotopy in cochlear implants: An evaluation

in unilateral cochlear implant patients with unilateral deafness and tinnitus.

Hearing Research, 245(1–2), 98–106.

https://doi.org/10.1016/j.heares.2008.09.003

Vermeire, K., & Van De Heyning, P. (2009). Binaural hearing after cochlear

165

implantation in subjects with unilateral sensorineural deafness and tinnitus.

Audiology and Neurotology, 14(3), 163–171.

https://doi.org/10.1159/000171478

Vlastarakos, P. V., Nazos, K., Tavoulari, E.-F., & Nikolopoulos, T. P. (2014).

Cochlear implantation for single-sided deafness: the outcomes. An

evidence-based approach. European Archives of Oto-Rhino-Laryngology,

271(8), 2119–2126. https://doi.org/10.1007/s00405-013-2746-z

Wendrich, A. W., Kroese, T. E., Peters, J. P. M., Cattani, G. M., & Grolman, W.

M. (2017). Systematic review on the trial period for bone conduction devices

in single-sided deafness: Rates and reasons for rejection. Otology &

Neurotology, 38(5), 632–641.

https://doi.org/10.1097/MAO.0000000000001405

Wess, J. M., & Bernstein, J. G. W. (2019). The effect of nonlinear amplitude

growth on the speech perception benefits provided by a single-sided

vocoder. Journal of Speech, Language, and Hearing Research, 62(3), 745–

757. https://doi.org/10.1044/2018_JSLHR-H-18-0001

Wess, J. M., Brungart, D. S., & Bernstein, J. G. (2017). The effect of interaural

mismatches on contralateral unmasking with single-sided vocoders. Ear and

Hearing, 38(3), 374–386. https://doi.org/10.1097/AUD.0000000000000374

Wess, J. M., Spencer, N. J., & Bernstein, J. G. (2016). Interaural spectral

mismatch and binaural fusion in simulated cochlear-implant listening for

single sided deafness. The Journal of the Acoustical Society of America,

140(4), 3157–3158. https://doi.org/10.1121/1.4969911

166

Whitmal, N. A., Poissant, S. F., Freyman, R. L., & Helfer, K. S. (2007). Speech

intelligibility in cochlear implant simulations: Effects of carrier type, interfering

noise, and subject experience. The Journal of the Acoustical Society of

America, 122(4), 2376–2388. https://doi.org/10.1121/1.2773993

Williges, B., Wesarg, T., Jung, L., Geven, L. I., Radeloff, A., Jü Rgens, T., &

Jürgens, T. (2019). Spatial speech-in-noise performance in bimodal and

single-sided deaf cochlear implant users. Trends in Hearing, 23, 1–16.

https://doi.org/10.1177/2331216519858311

Winn, M. B., Wendt, D., Koelewijn, T., & Kuchinsky, S. E. (2018). Best practices

and advice for using pupillometry to measure listening effort: An introduction

for those who want to get started. Trends in Hearing, 22, 1–32.

https://doi.org/10.1177/2331216518800869

Wu, Y. H. (2010). Effect of age on directional microphone hearing aid benefit and

preference. Journal of the American Academy of Audiology, 21(2), 78–89.

https://doi.org/10.3766/jaaa.21.2.3

Yamasoba, T., Lin, F. R., Someya, S., Kashio, A., Sakamoto, T., & Kondo, K.

(2013). Current concepts in age-related hearing loss: Epidemiology and

mechanistic pathways. Hearing Research, 303, 30–38.

https://doi.org/10.1016/j.heares.2013.01.021

Yawn, R. J., O’Connell, B. P., Dwyer, R. T., Sunderhaus, L. W., Reynolds, S.,

Haynes, D. S., … Gifford, R. H. R. H. (2018). Bilateral cochlear implantation

versus bimodal hearing in patients with functional residual hearing: A within-

subjects comparison of audiologic performance and quality of life. Otology &

167

Neurotology, 39(4), 422–427.

https://doi.org/10.1097/MAO.0000000000001750

Yu, F., Li, H., Zhou, X., Tang, X. L., Galvin, J. J., Fu, Q. J., & Yuan, W. (2018).

Effects of training on lateralization for simulations of cochlear implants and

single-sided deafness. Frontiers in Human Neuroscience, 12(July), 1–10.

https://doi.org/10.3389/fnhum.2018.00287

Zahorik, P., Brungart, D. S., & Bronkhorst, A. W. (2005). Auditory distance

perception in humans: A summary of past and present research. Acta

Acustica United with Acustica, 91(3), 409–420.

Zeng, F.-G., Rebscher, S., Harrison, W. V., Sun, X., & Feng, H. (2008). Cochlear

implants: System design, integration, and evaluation. IEEE Reviews in

Biomedical Engineering, 1, 115–142.

https://doi.org/10.1109/RBME.2008.2008250

Zirn, S., Arndt, S., Aschendorff, A., & Wesarg, T. (2015). Interaural stimulation

timing in single sided deaf cochlear implant users. Hearing Research, 328,

148–156. https://doi.org/10.1016/j.heares.2015.08.010

Zirn, Stefan, Angermeier, J., Arndt, S., Aschendorff, A., & Wesarg, T. (2019).

Reducing the device delay mismatch can improve sound localization in

bimodal cochlear implant or hearing-aid users. Trends in Hearing, 23, 1–13.

https://doi.org/10.1177/2331216519843876

Zurek, P. M. (1993). Binaural advantages and directional effects in speech

intelligibility. In G. A. Studebaker & I. Hochberg (Eds.), Acoustical Factors

Affecting Hearing Aid Performance (2nd ed., pp. 255–275). Boston: Allyn

168

and Bacon.

169