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2 January 2006 Rationale for Bilateral Cochlear Implantation in Children and Adults of bilateral cochlear implantation in ability.6-9 Binaural hearing is made of a second ear receiving a poorer SNR. appropriate candidates.4 This article will possible by several sub-phenomena: the An improvement of 3dB on average is review research and presentations of head shadow effect, binaural redundancy, possible when compared to just listening import on the issue of bilateral cochlear and the binaural squelch effect. to the ear receiving the better SNR.13 In implantation in children and adults. effect, the SNR can be about 3dB worse The “head shadow effect” is a physical for a binaural listener and still achieve BINAURAL HEARING: phenomenon that is the result of the head the same speech understanding as a SUBJECTIVE IMPREssIONS AND acting as an acoustic barrier to sounds monaural listener via the squelch effect. ObJECTIVE BENEFITS and noise coming from different locations Through these binaural mechanisms in space. The ear furthest from the noise The expected benefits of bilateral (head shadow, redundancy, and source will have a more advantageous cochlear implantation are based on squelch) the binaural listener is able to signal-to-noise ratio (SNR) than the ear known deficits experienced by both understand speech with a more adverse closest to the noise source. This effect 9,14 unilateral cochlear implant users as well SNR than a monaural listener. results in an average of 6.4dB of noise as individuals with unilateral profound attenuation but can be as high as 20dB Sound localization ability is made hearing loss (single sided deafness- for high frequency speech sounds.10,11 This possible by the central auditory system’s SSD). Subjectively, adult patients with may not seem significant but increasing amazing ability to calculate minute normal hearing who suddenly lose all the SNR by that amount can result in differences in the characteristics of hearing in one ear are a good source substantial improvements in speech sound arriving at each ear. Differences of feedback concerning the benefits of intelligibility, in some listening environments in sound intensity, phase, frequency binaural hearing. It is revealing that when by as much as 50%.9,12 Interestingly, in spectrum, and arrival time are calculated adults who have had normal hearing in patients with unilateral hearing loss, the for each ear to determine the origin of both ears their entire life suddenly lose head shadow effect still occurs but in sound.9,15-17 The normal human auditory all hearing in one ear, these individuals their case can be a detriment if speech system can distinguish as little as a 1dB are often devastated by the loss. This originates on the opposite side of the difference in sound intensity between is true even though they have better head from their only hearing ear.11 each ear and .1msec difference in arrival hearing in their remaining normal ear time.15,17 Frequency spectrum differences than is currently possible to achieve Binaural redundancy and squelch are are caused by the variable attenuation with a cochlear implant. They invariably two central auditory processes which, of the head shadow effect and the shape report a dramatic decrease of speech when combined, improve on the distinct of the pinna on differenct frequencies. understanding in the presence of even acoustic signals arriving at each ear. The central auditory system, “knowing” relatively mild background noise as well Binaural redundancy can be thought of these spectral effects, can calculate the as marked impairment in the hearing of as the advantage to be derived from origin of sound in both the horizontal sounds originating on their deaf side. listening with two ears, over one, even and vertical planes. These mechanisms Many of these individuals withdraw when the signal received at each ear is are so accurate that the normal hearing from social and occupational arenas the same.13 For example, if speech and binaural listener has an accuracy down to that present challenging acoustic noise is presented from the same source 1 degree for identifying a sound source.17 environments. They state that in front of the listener binaural hearing “hearing” requires more conscious affords a 1 to 2dB advantage in terms These binaural mechanisms greatly and intentional effort resulting in of SNR, compared with the monaural enhance an individuals ability to fatigue by the end of the day. 5 condition. If however the speech and understand speech in quiet as well as noise signals are from spatially separated in the presence of background noise, in Binaural hearing enables optimal sources, the binaural squelch effect some instances by as much as 60% when performance of the auditory system. The comes into play. When speech and noise compared to the monaural condition.9 binaural advantage results in improved are spatially separated each ear receives It is through these mechanisms that speech understanding in quiet and the signal at a different SNR. Binaural the mature auditory system is able to in noise, as well as sound localization squelch is demonstrated by the addition process the time and intensity cues of 3 the auditory percept arriving at each ear and binaural squelch effects in noise.20 represents the greatest challenge to for optimal hearing in adverse listening These benefits are indicative of central communication in real world conditions. environments such as noisy restaurants, auditory integration of the two different The fact that bilateral cochlear implants classrooms, and group meetings. modes of stimulation, acoustic and are able to provide a recipient with Individuals with unilateral hearing are, electrical. However, such benefits have significant binaural benefit is now for the most part, unable to utilize these not been demonstrated for patients who well documented by numerous adult binaural mechanisms that are important use two microphones; one over each ear studies.25-33 These studies have shown for effective hearing in the real world.9,18 with one cochlear implant and processor that all mechanisms of binaural auditory Even with many years of experience with processing both signals rather than using processing for improved speech unilateral hearing loss central auditory a hearing aid in the non-implanted ear.24 understanding (head shadow, squelch, mechanisms are unable to compensate A bilateral microphone arrangement of and redundancy) can occur in bilateral for one-sided auditory deprivation.9,19 this type was found to have a potentially cochlear implant subjects.26 The head detrimental effect in certain listening shadow effect appears to be the most ADULTS conditions with regard to sound consistently beneficial mechanism localization and could potentially worsen for adult cochlear implant users. The Lessons from Bilateral Hearing Aids the SNR presented to the cochlear binaural advantage of bilateral vs. and Bimodal Stimulation implant. This supports the premise that unilateral cochlear implants for speech two distinct signals, processed by two discrimination in noise increases with The importance of providing people cochlea are required to achieve binaural more adverse SNR.32 When tested in who have hearing loss with binaural gains. It is apparent from the above more favorable listening conditions (SNR hearing has been recognized for many referenced studies of bimodal benefit that of +10dB) speech scores for Cantonese years with regard to hearing aid (HA) the mature auditory system is capable lexical tones improve an average of fitting practices.6,13,15 It is considered the of making use of whatever good quality 15-20% compared to the unilateral “standard of care” to fit patients with auditory input it has available from either condition.32 However when tested in bilateral symmetrical hearing loss with ear. It should now be considered standard more adverse conditions (SNR -10 to bilateral amplification in order to achieve practice to have all unilateral cochlear -15dB) speech scores are immeasurable binaural gains. Also, it is considered implant recipients utilize a hearing aid on in the unilateral condition, but still standard practice to fit patients with their non- implanted ear if the residual average 80% in the bilateral condition in mild to severe unilateral hearing loss hearing in that ear is capable of providing some patients.32 who have normal hearing in their other a binaural advantage. However, in our ear with a hearing aid in their poorer ear Sound localization ability is greatly 11,13 experience many cochlear implant for the same purpose. The perceived enhanced through bilateral cochlear recipients have poor residual hearing in limitations of sound quality through implants when compared to one implant their non-implanted ear and amplification hearing aids is primarily related to the alone.17,27,28 This is achieved primarily either provides no benefit or even amount and quality of residual hearing through detection of interaural intensity interferes with the hearing of the cochlear available to the individual. differences.31 Most bilaterally implanted implant ear. In such cases the only way to subjects cannot benefit from interaural The benefit of a hearing aid used provide effective binaural hearing is with timing differences with the current opposite a cochlear implant in patients bilateral cochlear implants. arrangement of separately programmed with significant residual hearing in speech processors. their non-implanted ear has also been Bilateral Cochlear Implantation 20-23
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