Electric-acoustic stimulation: For whom, in which ear, and how Teresa YC Ching 1,2 , Paola Incerti 1,2, Kerrie Plant 2,3 1

National Acoustic Laboratories, Sydney, Australia, 2HEARing CRC, Australia, 3Cochlear Ltd., Australia

Keywords: Electric–acoustic stimulation, Speech perception, Localization, Candidacy, Hybrid, Hearing aids, Cochlear implants, Bimodal fitting

Introduction Electric–acoustic stimulation refers to the provision of electrical stimulation via cochlear implantation and acoustic stimulation via hearing-aid fitting. These two forms of stimulation may be provided in contralateral ears or in the same ear. The latter is made possible by recent advances in cochlear implant technology and improved surgical techniques for hearing preservation in cochlear implantation. The underlying rationale for providing electric– acoustic stimulation is to enable binaural hearing, and to maximize the range of sounds that are audible in both ears. Because of the size of the head and the position of the ears, a sound that is located to one side of the head will reach the near ear sooner than when it reaches the far ear. The difference in timing varies as a function of the direction of the sound source, with a maximum possible difference around 700 μs for sounds originating from 90° azimuth (Dillon, 2001; figure 14.2). These time differences are preserved by the auditory system for lowfrequency sounds, because neural impulses are phase-locked to the sound stimulus. Head-shadow effects cause the signal-to-noise ratio at the near ear to be superior to that in the far ear, giving rise to interaural level differences. Because of the size of the head relative to the wavelength of sounds, these interaural level differences are most marked at the high frequencies. When sounds are audible in both ears, the binaural system makes use of the differences in time and level information arriving at the two ears to process sounds more effectively than if auditory information is available in one ear only. These interaural time and level differences are useful for locating the source of sounds. In addition, speech intelligibility can be enhanced by effects relating to head shadow, binaural squelch, and binaural redundancy. Head shadow effects cause the signal-to-noise ratio (SNR) Correspondence to: Teresa YC Ching, National Acoustic Laboratories, Australian Hearing Hub, Macquarie University, 16 University Avenue, Sydney NSW 2109, Australia. Email: [email protected]

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at one ear to be superior to the other when the signal and noise are spatially separated. When one ear is closer to the signal of interest (speech), and the other ear is closer to a dominant noise source, the brain can attend to the ear with a better SNR leading to enhanced speech intelligibility. The brain can also combine the signal and noise arriving at both ears to partially reduce the impact of noise using time/phased differences between ears or binaural squelch. Even when the SNR are equal between ears, receiving two inputs rather than one gives an advantage. This is often referred to as ‘binaural redundancy’. When combining electric and acoustic stimulation, there is an additional advantage relating to ‘complementarity’. Amplified low-frequency residual hearing provides information that is complementary to the high-frequency information available via electrical stimulation. Candidacy for cochlear implantation has been extended from those with bilateral profound hearing loss to those who have residual hearing in the low and mid-frequencies in recent years. With advances in technology, there are now several commercially available systems that integrate acoustic and electrical stimulation thereby allowing stimulation with both modes in the same ear. Improvements in surgical techniques have resulted in successful hearing preservation after cochlear implantation. Increasingly, patients who receive a cochlear implant in one ear or both ears present with useful residual hearing in one or both ears. Effective management of these patients needs to be guided by research evidence. This paper draws on recent research to address three clinical questions: (1) should a hearing aid be worn with a cochlear implant in opposite ears? (2) If a hearing aid and a cochlear implant are worn in opposite ears, and if there is residual hearing in the implanted ear, should a hearing aid be fitted to amplify sounds acoustically in the implanted ear? (3) If electric-acoustic stimulation is provided in the same ear and a hearing aid is worn in the opposite

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Table 1 Terminology Ear 1 E+A EA EA + A EA + E EA + EA A+A

Electric (cochlear implantation) Electric–acoustic Electric–acoustic Electric–acoustic Electric–acoustic Acoustic

Ear 2 Acoustic (hearing aid amplification) Nil Acoustic Electric Electric–acoustic Acoustic

ear, how should these devices be fitted to optimize benefits? As terminology for describing combinations of devices varies across studies, this paper proposes a simplified nomenclature for the purpose of discussion. The term ‘E’ refers to electrical stimulation via cochlear implants, and ‘A’ refers to acoustic stimulation via hearing aids. For each of the two ears, E or A or EA is possible. Combining devices across the two ears is denoted by a plus sign ‘+’. Table 1 gives a summary of device combinations to be discussed further in this paper. 1. E + A vs. E: or, Is combining a cochlear implant and a hearing aid in contralateral ears better than using a cochlear implant alone? There is now much evidence attesting to the benefits of using E + A (for a review, see Ching et al., 2007). Fig. 1A depicts audiograms typical of participants reported in published literature. Even though acoustic hearing in the implanted ear was non-existent after surgery, residual hearing in the non-implanted ear

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was usable with amplification. Drawing on studies in which the participants were fitted with hearing aids that were optimized with their cochlear implants using a systematic procedure (Ching, 2004), there was clear evidence that localization and speech perception with E + A was superior to E alone, for both children and adults (Ching and Incerti, 2012). Although effects of binaural squelch were not observed, there were significant speech perception benefits from complementarity and binaural redundancy when speech and noise were collocated, and additionally head shadow effects when speech and noise were separated. Tests of consonant perception demonstrated that perception of low-frequency voicing and manner information improved when hearing aids were used with cochlear implants (Incerti et al., 2011). The data from these studies suggest that speech perception benefits were possible for hearing thresholds at 500 Hz up to about 100 dB HL (Ching, 2005). 2. EA + A vs. E + A? or, If a patient is already using a cochlear implant with a hearing aid in contralateral ears, should residual hearing in the implanted ear be aided? Fig. 1B depicts audiograms that characterize patients who participated in studies that allowed examination of this question. Plant (2014) evaluated 13 adults who used nucleus systems that integrated electric and acoustic stimulation. Localization errors were significantly reduced when they used EA + A (hybrid in one ear, and a hearing aid in the opposite ear), relative to E + A (implant in one ear and a

Figure 1 Hearing threshold levels of recipients of cochlear implants. Audiograms are generally similar between ears. (A) Audiograms typical of participants in previous studies that compared E + A with E. (B) Audiograms of participants in recent studies that investigated the effects of preserved low-frequency hearing after implantation. In the latter group, low-frequency hearing is preserved after cochlear implantation.

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hearing aid in the opposite ear). Speech perception in noise was improved by about 1 dB when listening in the EA + A mode, compared with E + A. Gifford et al. (2013) evaluated 38 adults using nucleus devices or Med EL devices under the two listening conditions. On average, speech perception in noise and in reverberation improved by about 2 dB when listeners used EA + A compared with E + A. Skarzynski et al. (2014) reported results from 35 adults who used nucleus systems, demonstrating that although there were no significant difference between listening conditions for speech perception in quiet, there was a significant advantage of EA + A for listening to speech in noise. The availability of acoustically amplified lowfrequency information in both ears possibly contributed to auditory streaming and voicing information thereby enhancing speech perception. Plant (2014) found that benefits are possible for people with postimplant low-frequency (250 and 500 Hz) thresholds ranging from 40 to 60 dB HL, and that there was no direct relation between the low-frequency acoustic hearing in the implanted ear and speech perception or localization benefits. It is of interest to examine the contribution of A + A, compared with EA + A. Incerti et al. (2014) showed that horizontal localization of 14 adults was on average equally good in both listening conditions. This is consistent with findings reported by Gifford et al. (2014) on 14 adults. However, there is no doubt that speech perception was superior when EA + A was used, relative to A + A (Gifford et al., 2013; Rader et al., 2013). The evidence to date supports amplification of residual hearing both in the implanted ear and in the non-implanted ear. 3. How should EA and A be fitted? The amplification procedure includes assessment of hearing thresholds, selection of hearing devices, derivation of amplification targets using a prescription, verification with real-ear measurements that target gains are matched, and checking of maximum power output of devices. This applies irrespective of whether the ear to be fitted has a cochlear implant or not. After a stable cochlear implant, MAP has been obtained, a paired-comparison procedure can be used to determine the gain-frequency response that is best for speech intelligibility, and loudness of the hearing aid needs to be adjusted to provide comfortable listening with the cochlear implant (Ching, 2004; Incerti http://www.hearnetlearning.org.au). However, proprietary fitting procedures for devices that provide electric–acoustic stimulation vary in relative acoustic–electric output and in acoustic–electric cross-over frequencies; and no empirically derived procedures are available (Incerti et al., 2013). Preliminary research findings (Incerti et al., 2014) suggest that the relative electric–acoustic output needs to be optimized

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for listening comfort and sound quality, and that the choice of acoustic–electric cross-over frequency makes a difference to listeners’ speech perception ability.

Summary There is much evidence to support the provision of binaural hearing to people who receive a cochlear implant in one ear by fitting a hearing aid in the contralateral ear. Benefits can be maximized by adopting a systematic procedure to optimize the hearing aid with the cochlear implant. There is also accumulating evidence showing that even when a cochlear implant and a hearing aid are used in contralateral ears, there is benefit to amplify residual hearing in the implanted ear. This will maximize the range of sounds audible in both ears, which have been shown to benefit localization and speech perception in noise and reverberation. One validated procedure for optimizing hearing aids with cochlear implant has previously been established (Ching, 2004). This forms the basis for the development of a procedure to optimize the use of acoustic hearing in the implanted and the non-implanted ear through adjustment of the relative gains of electric and acoustic stimulation and the cross-over frequency. Future work will investigate how best to determine these parameters for optimizing performance.

Acknowledgements We acknowledge the financial support of the Commonwealth of Australia through the establishment of the HEARing CRC and the Cooperative Research Centres.

References Ching T.Y.C. 2004. Fitting and evaluating a hearing aid for recipients of a unilateral cochlear implant: the NAL approach. Part 1. Hearing Review, July, 14–22. Ching T.Y.C. 2005. The evidence calls for making binaural-bimodal fittings routine. Hearing Journal, 58(11): 32–41. Ching T.Y.C., Incerti P. 2012. Bimodal fitting or bilateral cochlear implantation? In: Wong L., Hickson L., (eds) Evidence based practice in audiologic intervention. San Diego: Plural Publishing, pp. 213–233. 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. Dillon H. 2001. Hearing aids. Chapter 14. New York: Thieme Publication. Gifford R.H., Dorman M.F., Skarzynski H., Lorens A., Polak M., Driscoll C.L.W., et al. 2013. Cochlear implantation with hearing preservation yields significant benefit for speech recognition in complex listening environments. Ear and Hearing, 34: 413–424. Gifford R.H., Grantham D.W., Sheffield S.W., Davis T.J., Dwyer R., Dorman M.F. 2014. Localization and interaural time difference (ITD) thresholds for cochlear implant recipients with preserved acoustic hearing in the implanted ear. Hearing Research, 312: 28–37. Incerti P., Ching T.Y.C., Cowan R. 2013. A systematic review of electric-acoustic stimulation: device fitting ranges, outcomes & clinical fitting practices. Trends in Amplification, 17: 3–26.

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Incerti P., Ching T.Y.C., Hill A. 2011. Consonant perception by adults with bimodal fitting. Seminars in Hearing, 32: 90–102. Incerti P., Ching T.Y.C., Hou S. 2014. Electric-acoustic fittings: where to from here? Workshop presented at the XXXII World Congress of Audiology, Brisbane, 5–8 May 2014. Plant K. 2014. Effect of contralateral hearing on bimodal outcomes: candidacy considerations. Paper presented at the XXXII World Congress of Audiology, Brisbane, 5–8 May 2014.

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Rader T., Fastl H., Baumann U. 2013. Speech perception with combined electric-acoustic stimulation and bilateral cochlear implants in a multisource noise field. Ear and Hearing, 34: 324–332. Skarzynski H., Lorens A., Matusiak M., Porowski M., Skarzynski P.H., James C.J. 2014. Cochlear implantation with the Nucleus slim straight electrode in subjects with residual low-frequency hearing. Ear and Hearing, 32(2): e33–e43.

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