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March 22, 1999

Francis Kuk, Ph.D. Director of Audiology Rationale for Binaural Widex Hearing Aid Company, USA Hearing Aid Fittings

Few would disagree that people level, the average increase in loud- People with this degree of hearing with refractory problems in both ness is approximately 6 dB (Hag- loss may not get adequate amplifi- eyes should wear a pair of glasses. gard & Hall, 1982). cation from one single hearing aid Yet, only a percentage of people without risking feedback. Binau- with bilateral hearing loss wear There are many advantages that ral hearing aid use would allow binaurally fit hearing aids. There result from binaural loudness the wearer to enjoy comfortable can be many reasons for the infre- summation. Because is loudness with lower risk of feed- quency of binaural fittings. One of louder when heard in both , back. the main reasons, however, may low intensity that are not be an incomplete understanding perceived in a monaural mode of the real necessity and advan- may become audible when the b. Masking Level tages of binaural fittings. This pa- person is aided binaurally. Be- Difference (MLD) per summarizes the advantages of cause less gain is required from binaural hearing and binaural fit- each hearing aid to reach a com- A binaural system can utilize in- tings, and provides some consider- fortable listening level when the teraural time and intensity differ- ations when deciding between bi- patient is aided binaurally than ences between ears to extract a naural and monaural fittings. when aided monaurally, the signal from the noise background chance of feedback is decreased in order to increase its detectabil- and larger vents may be used (as ity over a monaural situation. appropriate) in a binaural fit. Less This phenomenon is called Mask- Binaural gain from the hearing aid also re- ing Level Difference (MLD, duces the risk of saturation distor- Hirsch, 1948; Licklider, 1948). advantages: tion. In addition, Levitt and The magnitude of MLD varies de- Psychoacoustic Voroba (1980) reported that the pending on the stimuli and com- factors binaural system has better fre- parison conditions. Levitt and Ra- quency and intensity discrimina- biner (1967) reported that the tion ability than the monaural MLD for monosyllabic words can a. Binaural loudness system. These advantages may be be as high as 13 dB. summation responsible for reports of better sound quality and speech under- The occurrence of MLD suggests Binaural loudness summation standing (Balfour and Hawkins, that speech intelligibility in noisy refers to the perceptual increase 1992; Erdman & Sedge, 1981, situations may be enhanced with in the loudness of a sound when it 1986) when the wearers were binaural fittings compared with is presented to both ears simulta- aided binaurally rather than monaural use. This effect has neously as opposed to when it is monaurally. The lower volume been demonstrated in hearing im- presented to one alone may also lead to lower current paired individuals (Harris, 1980; (Reynolds & Stevens, 1951). This drain from each hearing aid, Zelnick, 1970) and for both sym- phenomenon is present in both meaning longer battery life. metrical and asymmetrical hear- normal and hearing impaired peo- ing losses (Markides, 1977). Haw- ple, and its magnitude varies The loudness summation effect kins and Yacullo (1984) showed across individuals. At threshold, from binaural hearing aid use that binaural hearing aid use im- the increase in loudness is approx- may solve some difficult-to-fit proved the signal-to-noise ratio by imately 3 dB. When stimuli are cases. One such case is a severe- about 3-6 dB over that of a monau- presented at a suprathreshold to-profound degree of hearing loss. ral hearing aid condition. Because speech understanding in noise has opening area (i.e., in-the-ear, cause of threshold elevation on the been reported as the major prob- in-the-canal, completely-in-the- unaided side and masking on the lem for hearing impaired people, it canal), provide the hearing im- aided side. is incumbent upon all dispensers paired person better localization to fit binaurally for maximum ability (Westermann & Topholm, Binaural hearing aid use elimi- speech understanding and hear- 1985). Kochkin and Kuk (1997) nates any potential head shadow ing aid satisfaction. also showed that binaurally fit effect. In cases where the noise hearing aid wearers were about source is primarily from one side Real-world hearing aid satisfac- 15% more satisfied with their abil- of the head, the wearer can turn tion surveys also reflected in- ity to tell the direction of sounds the hearing aid off on that side to creased satisfaction from binaural than those wearing a single hear- minimize any masking effect. hearing aid use. Kochkin and Kuk ing aid. (1997) compared consumer satis- faction for binaural amplification Other associated binaural advan- in two groups of hearing aid tages include stereophonic hear- Neurophysiological wearers. One group wore conven- ing, better sound clarity and qual- tional hearing aids (N=835), while ity (Balfour and Hawkins, 1992), necessity the other group wore digitally and better speech understanding for binaural programmable hearing aids and sound quality perception in amplification? (N=2237). Their results showed reverberant environments (Nabe- two interesting findings. First, lek and Robinson, 1982). binaural amplification yielded a. Preservation of higher overall satisfaction than integrity monaural amplification in both d. Head shadow groups of subjects. Secondly, the The integrity of a sensory system group of subjects who wore pro- Head shadow refers to the attenu- depends on external stimulation. grammable hearing aids reported ation of sounds as they travel from Deprivation studies conclusively 8% higher satisfaction for binau- one side of the head to the other. showed that sensory is ral fittings than monaural fittings High frequency sounds are more needed for the structural develop- in noisy situations. This suggests affected because of their shorter ment of sensory neurons as well as that enhanced hearing aid tech- wavelength. The effect can be as their functional connectivity nology may also affect the degree much as 15 dB at 4000 Hz (Sivian (Webster & Webster, 1977). Stud- of binaural preference. & White, 1933). Because vowels ies on children’s development are primarily low frequency showed that appropriate sensory sounds and consonants high fre- stimulation, especially between c. Localization quency sounds, the head shadow the ages of 0 to 2 years, is critical effect can significantly affect the for proper development in later Sounds that are generated in a recognition of consonants. years (Ross, Brackett, & Maxon, sound field, arrive at the two ears 1991). These studies illustrate the at different times. Such difference The consequence of head shadow importance of binaural hearing on in time of arrival gives rise to in- can be substantial for a hearing the proper speech, language, and teraural phase and intensity cues impaired person. Consider the cognitive skills of children, as well which can be used to localize the case when the desired speech sig- as providing the rationale for sound source (Levitt & Voroba, nal is presented to the unaided ear early identification, intervention, 1980) as well as the angle of eleva- of a monaurally aided person with and binaural hearing aid fitting tion of the source. In the median a bilaterally symmetrical hearing for children with hearing loss. plane, it is estimated that normal loss. Because of the elevated hearing individuals can detect a threshold, the desired speech sig- Until recently, it was a little difference in azimuth of as little as nal is not perceived at the unaided known fact that adults may also one degree (Zelnick, 1980). ear. Rather, it has to “cross over” suffer from sensory deprivation if the head to the aided ear for per- their hearing losses are not prop- The ability to localize the sound ception. In the process, the inten- erly managed. In a landmark re- source is important in daily com- sity of the high frequency speech port by Silman, Gelfand, and Sil- munication situations. For exam- sounds is diminished. This may verman (1984), the authors ple, it alerts the listener to the lo- affect intelligibility. Furthermore, demonstrated that hearing im- cation or source of potential because only one ear (i.e., the paired people with bilaterally danger in the environment, e.g., aided ear) perceives the speech symmetrical hearing losses who traffic. Often, the ability to com- signal, information on the direc- were fit monaurally suffered a loss municate in noisy situations re- tion of the sound is lost. In cases of speech recognition ability in the quires the listener to first identify where noise is present on the unaided ear (average loss of the source of the sound. Binau- aided side and speech on the un- 18.5%). Subsequent reports by the rally fit hearing aids, especially aided side, the speech signal origi- same group of researchers those with the microphone inlets nating from the unaided side will (Gelfand, Silman, and Ross, 1987; recessed into the concha-canal not be perceived in either ear be- Silverman and Silman, 1990) and others (Burkey and Arkis, 1993; proved ease of listening with bin- Candidates for Gatehouse, 1989; Hood, 1990) also aural hearing aids (Brooks and showed that such deprivation only Bulmer, 1981; Briskey and Cole, binaural occurred in the unaided ear of a 1983; Erdman and Sedge, 1986; amplification monaurally fitted subject, but not Chung and Stephens, 1986). Zel- in the unaided ears of a non- nick (1985) also reported that The psychoacoustic advantages wearer. hearing impaired people were and neurophysiological necessity more relaxed and experienced less of binaural hearing aid fittings led Results from the deprivation stud- tension in listening situations many clinicians and researchers ies have both fitting and legal im- when aided binaurally compared (e.g., Hawkins, 1986; Skinner, plications. In order to avoid poten- to monaurally. 1988) to suggest that hearing im- tial deterioration of performance paired people with aidable hear- in the unaided ear of a bilateral ing loss in both ears should be con- hearing loss, binaural hearing aid • Preference over monaural sidered candidates for binaural fitting is necessary. Legally, these fitting amplification. This view had been results form a physiological basis questioned in the past. that requires binaural amplifica- Most surveys comparing monau- tion as the fitting standard in ral/binaural amplification favored cases with aidable hearing loss in binaural amplification (e.g., a. Symmetry of hearing both ears. Departure from such Briskey and Cole, 1983; Chung loss practice may constitute malprac- and Stephens, 1986; Erdman and tice and could be subjected to legal Sedge, 1986; Jordan et al, 1967). Some authors (Markides, 1977; actions. Indeed, Lowe (1988) re- The preference for binaural was Davis and Haggard, 1982; Gate- ported a case where an audiologist found regardless of the experience house and Haggard, 1986) have and an otologist were sued by a of the hearing aid wearers, their suggested that binaural amplifi- hearing impaired patient for fail- gender, the symmetry of their cation should be recommended ing to inform her of binaural am- hearing loss, and whether they only to people with symmetrical plification. paid for the hearing aids. hearing loss who have less than 15 dB difference in threshold and less A recent large scale (over 3,000 re- than 8% difference in speech b. Bilateral tinnitus spondents) consumer survey also recognition scores between ears. reduction confirmed these findings. Kochkin Substantial difference in the val- and Kuk (1997) compared con- ues of suprathreshold indices (like One effective approach to treat bi- sumer satisfaction between binau- MCL and LDL), and hearing loss lateral tinnitus in patients with ral and monaural hearing aid configuration (flat in one ear and associated hearing loss is the use wearers in the USA. The results of precipitous in other ear) were also of hearing aids (Brooks and Bul- the survey showed that more bin- cited as contraindications for bin- mer, 1981). Over 50% of hearing aural hearing aid wearers were aural amplification. impaired people reported being satisfied with their hearing aids less affected by their tinnitus than monaural hearing aid wear- On the other hand, some reports when a hearing aid is worn on the ers. The advantages of binaural suggested that symmetry of hear- affected ear. Masking of the tinni- fittings over monaural fittings in- ing loss should not be a factor in tus, and enhancement of hearing cluded the ability to localize considering binaural candidacy ability with hearing aid use are of- sounds, longer battery life, better (Chung & Stephens,1986; Haw- ten quoted as the reasons for the performance in noise, better kins, 1986; Moncur & Dirks, noted tinnitus relief. In cases of bi- sound quality, and higher satis- 1967). This is because the crite- lateral tinnitus but monaural faction in specific listening situa- rion of ear symmetry does not con- hearing aid use, the hearing im- tions like one-on-one, small and sider the ability of the auditory paired person may report suppres- large groups, and outdoors. These system to integrate and process sion of tinnitus on the aided ear, findings confirm the theoretical information received from both but noticeable tinnitus on the un- advantages of binaural amplifica- ears. Clinically, it is frequently ob- aided ear. Binaural hearing aid tion discussed previously. served that a majority of people use may be extremely beneficial with asymmetric hearing loss en- for bilateral tinnitus. joy good (or even better) speech recognition and sound quality judgments when they are aided c. Consumer report binaurally compared to monau- rally, even though one ear has sig- nificantly poorer sound quality • Ease of listening comfort and speech intelligibility scores when it is aided alone. Further- Consumer surveys of binaural more, these patients are able to re- hearing aid wearers also revealed port many of the advantages of that a major advantage is the im- binaural amplification. These comments are possible because of persons showed reduced binaural another study, Kochkin and Kuk the integration of information advantages, there remained sig- (1997) showed that consumers from both ears. nificant benefits for the elderly who were fit binaurally with pro- hearing impaired person when lis- grammable hearing aids per- Similarly, the criterion for binau- tening with both ears instead of ceived their hearing aids to be of ral amplification cannot be deter- one. These reports showed that “higher value” (cost vs. benefit) mined simply based on threshold age, despite the reduced potential than monaurally fit hearing aids. and/or suprathreshold informa- central processing ability seen in If our goal in hearing aid fitting is tion on each ear alone (exception: some elderly people, should not be to improve the patient’s commu- dead ear). Rather, such a decision a limiting factor in binaural fit- nicative ability and their satisfac- must be based on an evaluation of ting. tion towards hearing aids, we binaural fusion in the impaired should not let our perception of , a process that is However, the physical and cogni- the cost of the hearing aid be the dependent on the central auditory tive ability of elderly individuals primary factor that determines processing capability of the hear- to handle their hearing aids may our recommendation for the pa- ing impaired individual. limit their successful use. Fortu- tient. Rather, inform the patients nately, technologies are available of binaural benefits and let them There is not yet a standard proto- today that simplify the use of make the decision. col to determine fusion for the pur- hearing aids. Some digital and pose of binaural amplification. digitally programmable hearing However, one relatively simple aids can automatically adjust clinical procedure may have some their volume and frequency re- merits (Mercola and Wenke-Mer- sponses to adapt to the changing cola, 1985). In this procedure, listening environments. Some speech is presented to both ears of programmable hearing aids also the hearing impaired listener un- come with a remote control that til it is at a comfortable listening obviates the need for external vol- level (levels may be different be- ume adjustment. A detailed evalu- tween ears). Those who can main- ation of the needs of the hearing tain a midline image are consid- impaired person, and the applica- ered potential candidates for tion of proper technology could en- binaural amplification. Those who sure that these individuals enjoy report separate images at each all the benefits provided by binau- ear, despite careful adjustment of ral amplification. presentation levels, would not be good candidates for binaural am- plification. In such cases, the ear c. Cost and other factors with a better speech recognition score and/or better sound quality Other factors are important to will be aided. In the case of a consider in the recommendation of “dead” ear, CROS (Contralateral binaural hearing aid use. Clearly, Routing of Signal), BiCROS (Bi- those with a severe loss must wear lateral Contralateral Routing of two hearing aids to ensure ade- Signal), or Power-CROS are suit- quate output (Day, Browning, able alternatives to monaural Gatehouse, 1988). Children with hearing aids. (See Skinner, 1988 bilateral hearing loss should be for a description of their proper amplified binaurally as early as use.) possible in order to maximize their learning potential. It is grat- ifying to note that such practice is b. Age: central process- widely adopted in many countries ing ability & dexterity (Byrne & Upfold, 1986; Duffy, 1980; Ross, 1980).

Rousch (1985) reviewed several Cost is often cited as a factor reports on the performance of the in binaural recommendation binaural system in different age (Schreurs & Olsen, 1985). How- groups and reported that the bin- ever, Kochkin (1992) showed that aural system was resistant to dis- cost is the least important factor tortion and that there was no dif- in a consumer’s perception of their ference in performance between hearing aid satisfaction. Improved elderly and younger subjects on hearing, especially in multiple lis- binaural tasks. Grose (1996) also tening environments, is the key reported that, although elderly factor governing satisfaction. In A Summary

Binaural amplification is gradu- ally being accepted as the norm in hearing aid fitting in many coun- tries because of its noted perfor- mance advantages and neuro- physiological necessity. Progress in hearing aid technology has helped to realize some of the theo- retical advantages of binaural fit- tings. Such benefits include: bet- ter speech understanding in quiet and in noise, better sound quality, better localization of sounds, avoidance of the head shadow ef- fect, and higher listening comfort in various situations. Binaurally fit hearing aids are also effective in the management of bilateral tinnitus in some patients and in potentially preserving the in- tegrity of the binaural auditory system.

In order for hearing impaired peo- ple to benefit from binaural ampli- fication, it is important that hear- ing healthcare professionals select the appropriate technology and counsel their patients on binaural benefits properly. The hearing impaired person has the right to know why binaural fitting may be more appropriate for his degrees of hearing loss.

Appropriate candidates for binau- ral amplification should include all hearing impaired people with measurable hearing loss in both ears. The overriding criterion is that the central auditory system can integrate auditory images from both ears to form an undis- torted, midline image. Although the ultimate decision maker of binaurality is the hearing im- paired person, the hearing aid dis- pensers play a critical role in de- termining if the patient has the chance to make the monaural/bin- aural decision. Perhaps an adjust- ment period may be allowed so that the hearing impaired person can experience the difference out- side the clinic before a decision on the mode of fitting is made (Hawkins, 1986; Skinner, 1988). References

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Haggard, M. Hall, J. (1982). Forms of bin- Nabelek, A.K. Robinson, P.K. (1982). aural summation and the implications of Monaural and binaural speech perception individual variability for binaural hearing in reverberation for listeners of various aids. Binaural Effects in Normal and Im- ages. J. Acoust. Soc. Am. 71(5):1242-1248. paired Hearing- Scand. Audiol. Suppl (15): 47-63. (P!W0P!5213!101UY) Printed by FB / 7-99 P W0P 5213 101