Fluctuations in Vestibular Afferent Excitability in Menière's Disease.

OBJECTIVE: To determine if Menière's disease is associated with fluctuations in afferent excitability in four human subjects previously implanted with vestibular stimulators. STUDY DESIGN: Longitudinal repeated measures. SETTING: Tertiary referral center, human vestibular research laboratory. PATIENTS: Four human subjects with previously uncontrolled Menière's disease unilaterally implanted in each semicircular canal with a vestibular stimulator. One subject had only two canals implanted. INTERVENTION(S): Repeated measures of electrically-evoked slow phase eye velocity and vestibular electrically-evoked compound action potentials (vECAP) over 2 to 4 years. MAIN OUTCOME MEASURE(S): Slow phase eye velocity and N1-P1 vECAP amplitudes as a function of time. RESULTS: There were statistically significant fluctuations in electrically evoked slow phase eye velocity over time in at least one semicircular canal of each subject. vECAP N1-P1 amplitudes measured at similar time intervals and stimulus intensities seem to show somewhat correlated fluctuations. One of the subjects had a single Menière's attack during this time period. The others did not. CONCLUSIONS: In these four subjects originally diagnosed with Menière's disease, there was fluctuating electrical excitability of the ampullar nerve of at least one canal in each subject. These fluctuations occurred without active symptoms of Menière's disease.

Results From a Second-Generation Vestibular Implant in Human Subjects: Diagnosis May Impact Electrical Sensitivity of Vestibular Afferents.

OBJECTIVE: Auditory and vestibular outcomes after placement of a vestibular-cochlear implant in subjects with varying causes of vestibular loss. STUDY DESIGN: Prospective case study. SETTING: Tertiary referral center. PATIENTS: Three human subjects received a vestibular-cochlear implant. Subject 1 had sudden hearing and vestibular loss 10 years before implantation. Subjects 2 and 3 had bilateral Menière's disease with resolution of acute attacks. All subjects had severe-profound deafness in the implanted ear and bilateral vestibular loss. INTERVENTION: Vestibular-cochlear implant with electrode positions confirmed by CT. MAIN OUTCOME MEASURES: Electrically-evoked vestibular and cochlear compound action potentials (ECAPs), speech perception, and electrically-evoked slow-phase eye velocities. RESULTS: Subject 1 had no vestibular ECAP, but normal cochlear ECAPs and cochlear implant function. She had minimal eye-movement with vestibular stimulation. Subject 2 had vestibular ECAPs. This subject had the largest eye velocities from electrical stimulation that we have seen in humans, exceeding 100 degrees per second. Her cochlear implant functions normally. Subject 3 had vestibular and cochlear ECAPs, and robust eye-movements and cochlear implant function. CONCLUSION: The etiology of vestibular loss appears to have a profound impact on sensitivity of vestibular afferents in distinction to cochlear afferents. If this dichotomy is common, it may limit the application of vestibular implants to diagnoses with preserved sensitivity of vestibular afferents. We speculate it is due to differences in topographic organization of Scarpa's versus the spiral ganglion. In two subjects, the second-generation device can produce higher velocity eye movements than seen in the four subjects receiving the first-generation device.

Mandarin tone recognition in English speakers with normal hearing and with cochlear implants.

Objective: Mandarin-speaking cochlear implant users have difficulty perceiving tonal changes in speech with current signal processing strategies. The purpose of this study was to evaluate whether English-speaking cochlear implant and normal hearing listeners can be trained to recognise closed-set Mandarin tones. The validity of using native-English speakers to evaluate Mandarin tone perception in cochlear implants was tested.Design: Two groups of native-English speaking participants were evaluated. All listeners were given training rounds and evaluation rounds in which their tonal identification was tested. The normal-hearing group was also tested with acoustic simulations of the traditional Continuous Interleaved Sampling (CIS) strategy.Study sample: Ten normal-hearing English speakers and seven cochlear implant listeners participated.Results: The normal-hearing group correctly identified unprocessed tones at 87% and CIS-processed tones at 58% on average. The cochlear implant listeners achieved 56% correct identification on average.Conclusions: This level of performance for native English speaking CI users was comparable to previous studies using native Mandarin-speaking CI listeners, which showed a mean of 59% in 19 CI users.

Hybrid Music Perception Outcomes: Implications for Melody and Timbre Recognition in Cochlear Implant Recipients.

OBJECTIVE: To examine whether or not electric-acoustic music perception outcomes, observed in a recent Hybrid L24 clinical trial, were related to the availability of low-frequency acoustic cues not present in the electric domain. STUDY DESIGN: Prospective, repeated-measures, within-subject design. SETTING: Academic research hospital. SUBJECTS: Nine normally hearing individuals. INTERVENTION: Simulated electric-acoustic hearing in normally hearing individuals. MAIN OUTCOMES MEASURES: Acutely measured melody and timbre recognition scores from the University of Washington Clinical Assessment of Music Perception (CAMP) test. RESULTS: Melody recognition scores were consistently better for listening conditions that included low-frequency acoustic information. Mean scores for both acoustic (73.5%, S.D. = 15.5%) and electric-acoustic (67.9%, S.D. = 21.2%) conditions were significantly better (p < 0.001) than electric alone (39.2%, S.D. = 18.1%). This was not the case for timbre recognition for which scores were more variable across simulated listening modes with no significant differences found in mean scores across electric (36.1%, S.D. = 17.7%), acoustic (38.0%, S.D. = 20.4%), and electric-acoustic (40.7%, S.D. = 19.7%) conditions (p > 0.05). CONCLUSION: Recipients of hybrid cochlear implants demonstrate music perception abilities superior to those observed in traditional cochlear implant recipients. Results from the present study support the notion that electric-acoustic stimulation confers advantages related to the availability of low-frequency acoustic hearing, most particularly for melody recognition. However, timbre recognition remains more limited for both hybrid and traditional cochlear implant users. Opportunities remain for new coding strategies to improve timbre perception.

The Dynamics of Prosthetically Elicited Vestibulo-Ocular Reflex Function Across Frequency and Context in the Rhesus Monkey.

Electrical vestibular neurostimulation may be a viable tool for modulating vestibular afferent input to restore vestibular function following injury or disease. To do this, such stimulators must provide afferent input that can be readily interpreted by the central nervous system to accurately represent head motion to drive reflexive behavior. Since vestibular afferents have different galvanic sensitivity, and different natural sensitivities to head rotational velocity and acceleration, and electrical stimulation produces aphysiological synchronous activation of multiple afferents, it is difficult to assign a priori an appropriate transformation between head velocity and acceleration and the properties of the electrical stimulus used to drive vestibular reflex function, i.e., biphasic pulse rate or pulse current amplitude. In order to empirically explore the nature of the transformation between vestibular prosthetic stimulation and vestibular reflex behavior, in Rhesus macaque monkeys we parametrically varied the pulse rate and current amplitude of constant rate and current amplitude pulse trains, and the modulation frequency of sinusoidally modulated pulse trains that were pulse frequency modulated (FM) or current amplitude modulated (AM). In addition, we examined the effects of differential eye position and head position on the observed eye movement responses. We conclude that there is a strong and idiosyncratic, from canal to canal, effect of modulation frequency on the observed eye velocities that are elicited by stimulation. In addition, there is a strong effect of initial eye position and initial head position on the observed responses. These are superimposed on the relationships between pulse frequency or current amplitude and eye velocity that have been shown previously.

Effects of age and hearing mechanism on spectral resolution in normal hearing and cochlear-implanted listeners.

Spectral resolution limits speech perception with a cochlear implant (CI) in post-lingually deaf adults. However, the development of spectral resolution in pre-lingually deaf implanted children is not well understood. Acoustic spectral resolution was measured as a function of age (school-age versus adult) in CI and normal-hearing (NH) participants using spectral ripple discrimination (SRD). A 3-alternative forced-choice task was used to obtain SRD thresholds at five ripple depths. Effects of age and hearing method on SRD and spectral modulation transfer function (SMTF) slope (reflecting frequency resolution) and x-intercept (reflecting across-channel intensity resolution) were examined. Correlations between SRD, SMTF parameters, age, and speech perception in noise were studied. Better SRD in NH than CI participants was observed at all depths. SRD thresholds and SMTF slope correlated with speech perception in CI users. When adjusted for floor performance, x-intercept did not correlate with SMTF slope or speech perception. Age and x-intercept correlations were positive and significant in NH but not CI children suggesting that across-channel intensity resolution matures during school-age in NH children. No evidence for maturation of spectral resolution beyond early school-age in pre-lingually deaf implanted CI users was found in the present study.

Loss of Afferent Vestibular Input Produces Central Adaptation and Increased Gain of Vestibular Prosthetic Stimulation.

Implanted vestibular neurostimulators are effective in driving slow phase eye movements in monkeys and humans. Furthermore, increases in slow phase velocity and electrically evoked compound action potential (vECAP) amplitudes occur with increasing current amplitude of electrical stimulation. In intact monkeys, protracted intermittent stimulation continues to produce robust behavioral responses and preserved vECAPs. In lesioned monkeys, shorter duration studies show preserved but with somewhat lower or higher velocity behavioral responses. It has been proposed that such changes are due to central adaptive changes in the electrically elicited vestibulo-ocular reflex (VOR). It is equally possible that these differences are due to changes in the vestibular periphery in response to activation of the vestibular efferent system. In order to investigate the site of adaptive change in response to electrical stimulation, we performed transtympanic gentamicin perfusions to induce rapid changes in vestibular input in monkeys with long-standing stably functioning vestibular neurostimulators, disambiguating the effects of implantation from the effects of ototoxic lesion. Gentamicin injection was effective in producing a large reduction in natural VOR only when it was performed in the non-implanted ear, suggesting that the implanted ear contributed little to the natural rotational response before injection. Injection of the implanted ear produced a reduction in the vECAP responses in that ear, suggesting that the intact hair cells in the non-functional ipsilateral ear were successfully lesioned by gentamicin, reducing the efficacy of stimulation in that ear. Despite this, injection of both ears produced central plastic changes that resulted in a dramatically increased slow phase velocity nystagmus elicited by electrical stimulation. These results suggest that loss of vestibular afferent activity, and a concurrent loss of electrically elicited vestibular input, produces an increase in the efficacy of a vestibular neurostimulator by eliciting centrally adapted behavioral responses without concurrent adaptive increase of galvanic afferent activation in the periphery.

Vestibular implantation and longitudinal electrical stimulation of the semicircular canal afferents in human subjects.

Animal experiments and limited data in humans suggest that electrical stimulation of the vestibular end organs could be used to treat loss of vestibular function. In this paper we demonstrate that canal-specific two-dimensionally (2D) measured eye velocities are elicited from intermittent brief 2 s biphasic pulse electrical stimulation in four human subjects implanted with a vestibular prosthesis. The 2D measured direction of the slow phase eye movements changed with the canal stimulated. Increasing pulse current over a 0-400 µA range typically produced a monotonic increase in slow phase eye velocity. The responses decremented or in some cases fluctuated over time in most implanted canals but could be partially restored by changing the return path of the stimulation current. Implantation of the device in Meniere's patients produced hearing and vestibular loss in the implanted ear. Electrical stimulation was well tolerated, producing no sensation of pain, nausea, or auditory percept with stimulation that elicited robust eye movements. There were changes in slow phase eye velocity with current and over time, and changes in electrically evoked compound action potentials produced by stimulation and recorded with the implanted device. Perceived rotation in subjects was consistent with the slow phase eye movements in direction and scaled with stimulation current in magnitude. These results suggest that electrical stimulation of the vestibular end organ in human subjects provided controlled vestibular inputs over time, but in Meniere's patients this apparently came at the cost of hearing and vestibular function in the implanted ear.

Longitudinal performance of an implantable vestibular prosthesis.

Loss of vestibular function may be treatable with an implantable vestibular prosthesis that stimulates semicircular canal afferents with biphasic pulse trains. Several studies have demonstrated short-term activation of the vestibulo-ocular reflex (VOR) with electrical stimulation. Fewer long-term studies have been restricted to small numbers of animals and stimulation designed to produce adaptive changes in the electrically elicited response. This study is the first large consecutive series of implanted rhesus macaque to be studied longitudinally using brief stimuli designed to limit adaptive changes in response, so that the efficacy of electrical activation can be studied over time, across surgeries, canals and animals. The implantation of a vestibular prosthesis in animals with intact vestibular end organs produces variable responses to electrical stimulation across canals and animals, which change in threshold for electrical activation of eye movements and in elicited slow phase velocities over time. These thresholds are consistently lower, and the slow phase velocities higher, than those obtained in human subjects. The changes do not appear to be correlated with changes in electrode impedance. The variability in response suggests that empirically derived transfer functions may be required to optimize the response of individual canals to a vestibular prosthesis, and that this function may need to be remapped over time. This article is part of a Special Issue entitled .

Optimal combination of neural temporal envelope and fine structure cues to explain speech identification in background noise.

The dichotomy between acoustic temporal envelope (ENV) and fine structure (TFS) cues has stimulated numerous studies over the past decade to understand the relative role of acoustic ENV and TFS in human speech perception. Such acoustic temporal speech cues produce distinct neural discharge patterns at the level of the auditory nerve, yet little is known about the central neural mechanisms underlying the dichotomy in speech perception between neural ENV and TFS cues. We explored the question of how the peripheral auditory system encodes neural ENV and TFS cues in steady or fluctuating background noise, and how the central auditory system combines these forms of neural information for speech identification. We sought to address this question by (1) measuring sentence identification in background noise for human subjects as a function of the degree of available acoustic TFS information and (2) examining the optimal combination of neural ENV and TFS cues to explain human speech perception performance using computational models of the peripheral auditory system and central neural observers. Speech-identification performance by human subjects decreased as the acoustic TFS information was degraded in the speech signals. The model predictions best matched human performance when a greater emphasis was placed on neural ENV coding rather than neural TFS. However, neural TFS cues were necessary to account for the full effect of background-noise modulations on human speech-identification performance.

Prosthetic implantation of the human vestibular system.

HYPOTHESIS: A functional vestibular prosthesis can be implanted in human such that electrical stimulation of each semicircular canal produces canal-specific eye movements while preserving vestibular and auditory function. BACKGROUND: A number of vestibular disorders could be treated with prosthetic stimulation of the vestibular end organs. We have previously demonstrated in rhesus monkeys that a vestibular neurostimulator, based on the Nucleus Freedom cochlear implant, can produce canal-specific electrically evoked eye movements while preserving auditory and vestibular function. An investigational device exemption has been obtained from the FDA to study the feasibility of treating uncontrolled Ménière's disease with the device. METHODS: The UW/Nucleus vestibular implant was implanted in the perilymphatic space adjacent to the three semicircular canal ampullae of a human subject with uncontrolled Ménière's disease. Preoperative and postoperative vestibular and auditory function was assessed. Electrically evoked eye movements were measured at 2 time points postoperatively. RESULTS: Implantation of all semicircular canals was technically feasible. Horizontal canal and auditory function were largely, but not totally, lost. Electrode stimulation in 2 of 3 canals resulted in canal-appropriate eye movements. Over time, stimulation thresholds increased. CONCLUSION: Prosthetic implantation of the semicircular canals in humans is technically feasible. Electrical stimulation resulted in canal-specific eye movements, although thresholds increased over time. Preservation of native auditory and vestibular function, previously observed in animals, was not demonstrated in a single subject with advanced Ménière's disease.

Postural responses to electrical stimulation of the vestibular end organs in human subjects.

A multichannel vestibular prosthesis that delivers electrical stimulation to the perilymph of individual semicircular canals is a potential new treatment modality for patients with vestibular deficiencies. Most research in this field has evaluated the efficacy of this approach by its ability to reproduce eye movements in response to head rotations. Our group has developed such a device and implanted it in four human subjects with intractable unilateral Meniere's disease. This allows us to evaluate individual semicircular canal contribution to the control of balance and posture in human subjects. In this report, we demonstrate that electrical stimulation trains delivered to the perilymph of individual semicircular canals elicit postural responses specific to the particular canal stimulated, with some current spread to adjacent end organs. Modulation of stimulation current modulates the amplitude of the postural response. However, eye movements elicited by the same electrical stimuli were not consistent with postural responses in magnitude or direction in all subjects. Taken together, these findings support the feasibility of a vestibular prosthesis for the control of balance and illustrate new challenges for the development of this technology.

Improved perception of music with a harmonic based algorithm for cochlear implants.

The lack of fine structure information in conventional cochlear implant (CI) encoding strategies presumably contributes to the generally poor music perception with CIs. To improve CI users' music perception, a harmonic-single-sideband-encoder (HSSE) strategy was developed , which explicitly tracks the harmonics of a single musical source and transforms them into modulators conveying both amplitude and temporal fine structure cues to electrodes. To investigate its effectiveness, vocoder simulations of HSSE and the conventional continuous-interleaved-sampling (CIS) strategy were implemented. Using these vocoders, five normal-hearing subjects' melody and timbre recognition performance were evaluated: a significant benefit of HSSE to both melody (p < 0.002) and timbre (p < 0.026) recognition was found. Additionally, HSSE was acutely tested in eight CI subjects. On timbre recognition, a significant advantage of HSSE over the subjects' clinical strategy was demonstrated: the largest improvement was 35% and the mean 17% (p < 0.013). On melody recognition, two subjects showed 20% improvement with HSSE; however, the mean improvement of 7% across subjects was not significant (p > 0.090). To quantify the temporal cues delivered to the auditory nerve, the neural spike patterns evoked by HSSE and CIS for one melody stimulus were simulated using an auditory nerve model. Quantitative analysis demonstrated that HSSE can convey temporal pitch cues better than CIS. The results suggest that HSSE is a promising strategy to enhance music perception with CIs.

An experimental vestibular neural prosthesis: design and preliminary results with rhesus monkeys stimulated with modulated pulses.

A vestibular neural prosthesis was designed on the basis of a cochlear implant for treatment of Meniere's disease and other vestibular disorders. Computer control software was developed to generate patterned pulse stimuli for exploring optimal parameters to activate the vestibular nerve. Two rhesus monkeys were implanted with the prototype vestibular prosthesis and they were behaviorally evaluated post implantation surgery. Horizontal and vertical eye movement responses to patterned electrical pulse stimulations were collected on both monkeys. Pulse amplitude modulated (PAM) and pulse rate modulated (PRM) trains were applied to the lateral canal of each implanted animal. Robust slow-phase nystagmus responses following the PAM or PRM modulation pattern were observed in both implanted monkeys in the direction consistent with the activation of the implanted canal. Both PAM and PRM pulse trains can elicit a significant amount of in-phase modulated eye velocity changes and they could potentially be used for efficiently coding head rotational signals in future vestibular neural prostheses.

Maximizing the spectral and temporal benefits of two clinically used sound processing strategies for cochlear implants.

Previous work showed that the Fidelity120 processing strategy provides better spectral sensitivity, while the HiResolution processing strategy can deliver more detailed temporal information for Advanced Bionics cochlear implant users. The goal of this study was to develop a new sound processing strategy by maximizing the spectral benefit of Fidelity120 and the temporal benefit of HiResolution to improve both aspects of hearing. Using acoustic simulations of Fidelity120 and HiResolution strategies, a dual-processing strategy was created by combining Fidelity120 in the low frequency channels and HiResolution in the high frequency channels. Compared to Fidelity120, the dual processing provided an improvement in performance for Schroeder-phase discrimination at 200 Hz and temporal modulation detection at 200 Hz with the cost of a slightly decreased performance for spectral-ripple discrimination relative to Fidelity120. Spectral-ripple discrimination was better with the dual processing than with HiResolution. However, no benefit for speech perception in noise was found for the dual-processing strategy over Fidelity 120 or HiResolution in our preliminary tests. Some other more optimal combination of Fidelity120 and HiResolution may be required to maximize the spectral and temporal benefits to yield improved speech perception.

Improved perception of speech in noise and Mandarin tones with acoustic simulations of harmonic coding for cochlear implants.

Harmonic and temporal fine structure (TFS) information are important cues for speech perception in noise and music perception. However, due to the inherently coarse spectral and temporal resolution in electric hearing, the question of how to deliver harmonic and TFS information to cochlear implant (CI) users remains unresolved. A harmonic-single-sideband-encoder [(HSSE); Nie et al. (2008). Proceedings of IEEE International Conference on Acoustics, Speech, and Signal Processing; Lie et al., (2010). Proceedings of IEEE International Conference on Acoustics, Speech, and Signal Processing] strategy has been proposed that explicitly tracks the harmonics in speech and transforms them into modulators conveying both amplitude modulation and fundamental frequency information. For unvoiced speech, HSSE transforms the TFS into a slowly varying yet still noise-like signal. To investigate its potential, four- and eight-channel vocoder simulations of HSSE and the continuous-interleaved-sampling (CIS) strategy were implemented, respectively. Using these vocoders, five normal-hearing subjects' speech recognition performance was evaluated under different masking conditions; another five normal-hearing subjects' Mandarin tone identification performance was also evaluated. Additionally, the neural discharge patterns evoked by HSSE- and CIS-encoded Mandarin tone stimuli were simulated using an auditory nerve model. All subjects scored significantly higher with HSSE than with CIS vocoders. The modeling analysis demonstrated that HSSE can convey temporal pitch cues better than CIS. Overall, the results suggest that HSSE is a promising strategy to enhance speech perception with CIs.

The ability of cochlear implant users to use temporal envelope cues recovered from speech frequency modulation.

Previous studies have demonstrated that normal-hearing listeners can understand speech using the recovered "temporal envelopes," i.e., amplitude modulation (AM) cues from frequency modulation (FM). This study evaluated this mechanism in cochlear implant (CI) users for consonant identification. Stimuli containing only FM cues were created using 1, 2, 4, and 8-band FM-vocoders to determine if consonant identification performance would improve as the recovered AM cues become more available. A consistent improvement was observed as the band number decreased from 8 to 1, supporting the hypothesis that (1) the CI sound processor generates recovered AM cues from broadband FM, and (2) CI users can use the recovered AM cues to recognize speech. The correlation between the intact and the recovered AM components at the output of the sound processor was also generally higher when the band number was low, supporting the consonant identification results. Moreover, CI subjects who were better at using recovered AM cues from broadband FM cues showed better identification performance with intact (unprocessed) speech stimuli. This suggests that speech perception performance variability in CI users may be partly caused by differences in their ability to use AM cues recovered from FM speech cues.

Implantation of the semicircular canals with preservation of hearing and rotational sensitivity: a vestibular neurostimulator suitable for clinical research.

HYPOTHESIS: It is possible to implant a stimulating electrode array in the semicircular canals without damaging rotational sensitivity or hearing. The electrodes will evoke robust and precisely controlled eye movements. BACKGROUND: A number of groups are attempting to develop a neural prosthesis to ameliorate abnormal vestibular function. Animal studies demonstrate that electrodes near the canal ampullae can produce electrically evoked eye movements. The target condition of these studies is typically bilateral vestibular hypofunction. Such a device could potentially be more widely useful clinically and would have a simpler roadmap to regulatory approval if it produced minimal or no damage to the native vestibular and auditory systems. METHODS: An electrode array was designed for insertion into the bony semicircular canal adjacent to the membranous canal. It was designed to be sufficiently narrow so as to not compress the membranous canal. The arrays were manufactured by Cochlear, Ltd., and linked to a Nucleus Freedom receiver/stimulator. Seven behaviorally trained rhesus macaques had arrays placed in 2 semicircular canals using a transmastoid approach and "soft surgical" procedures borrowed from Hybrid cochlear implant surgery. Postoperative vestibulo-ocular reflex was measured in a rotary chair. Click-evoked auditory brainstem responses were also measured in the 7 animals using the contralateral ear as a control. RESULTS: All animals had minimal postoperative vestibular signs and were eating within hours of surgery. Of 6 animals tested, all had normal postoperative sinusoidal gain. Of 7 animals, 6 had symmetric postoperative velocity step responses toward and away from the implanted ear. The 1 animal with significantly asymmetric velocity step responses also had a significant sensorineural hearing loss. One control animal that underwent canal plugging had substantial loss of the velocity step response toward the canal-plugged ear. In 5 animals, intraoperative electrically evoked vestibular compound action potential recordings facilitated electrode placement. Postoperatively, electrically evoked eye movements were obtained from electrodes associated with an electrically evoked vestibular compound action potential wave form. Hearing was largely preserved in 6 animals and lost in 1 animal. CONCLUSION: It is possible to implant the vestibular system with prosthetic stimulating electrodes without loss of rotational sensitivity or hearing. Because electrically evoked eye movements can be reliably obtained with the assistance of intraoperative electrophysiology, it is appropriate to consider treatment of a variety of vestibular disorders using prosthetic electrical stimulation. Based on these findings, and others, a feasibility study for the treatment of human subjects with disabling Ménière's disease has begun.

Auditory outcomes following implantation and electrical stimulation of the semicircular canals.

We measured auditory brainstem responses (ABRs) in eight Rhesus monkeys after implantation of electrodes in the semicircular canals of one ear, using a multi-channel vestibular prosthesis based on cochlear implant technology. In five animals, click-evoked ABR thresholds in the implanted ear were within 10 dB of thresholds in the non-implanted control ear. Threshold differences in the remaining three animals varied from 18 to 69 dB, indicating mild to severe hearing losses. Click- and tone-evoked ABRs measured in a subset of animals before and after implantation revealed a comparable pattern of threshold changes. Thresholds obtained five months or more after implantation--a period in which the prosthesis regularly delivered electrical stimulation to achieve functional activation of the vestibular system--improved in three animals with no or mild initial hearing loss and increased in a fourth with a moderate hearing loss. These results suggest that, although there is a risk of hearing loss with unilateral vestibular implantation to treat balance disorders, the surgery can be performed in a manner that preserves hearing over an extended period of functional stimulation.