Restoring the High-Frequency Dynamic Visual Acuity with a Vestibular Implant Prototype in Humans.

INTRODUCTION: The vestibular implant could become a clinically useful device in the near future. This study investigated the feasibility of restoring the high-frequency dynamic visual acuity (DVA) with a vestibular implant, using the functional Head Impulse Test (fHIT). METHODS: A 72-year-old female, with bilateral vestibulopathy and fitted with a modified cochlear implant incorporating three vestibular electrodes (MED-EL, Innsbruck, Austria), was available for this study. Electrical stimulation was delivered with the electrode close to the lateral ampullary nerve in the left ear. The high-frequency DVA in the horizontal plane was tested with the fHIT. After training, the patient underwent six trials of fHIT, each with a different setting of the vestibular implant: (1) System OFF before stimulation; (2) System ON, baseline stimulation; (3) System ON, reversed stimulation; (4) System ON, positive stimulation; (5) System OFF, without delay after stimulation offset; and (6) System OFF, 25 min delay after stimulation offset. The percentage of correct fHIT scores for right and left head impulses were compared between trials. RESULTS: Vestibular implant stimulation improved the high-frequency DVA compared to no stimulation. This improvement was significant for "System ON, baseline stimulation" (p = 0.02) and "System ON, positive stimulation" (p < 0.001). fHIT scores changed from 19 to 44% (no stimulation) to maximum 75-94% (System ON, positive stimulation). CONCLUSION: The vestibular implant seems capable of improving the high-frequency DVA. This functional benefit of the vestibular implant illustrates again the feasibility of this device for clinical use in the near future.

Superior Canal Dehiscence Syndrome: Relating Clinical Findings With Vestibular Neural Responses From a Guinea Pig Model.

HYPOTHESIS: In superior canal dehiscence (SCD), fluid displacement of the endolymph activates type I vestibular hair cells in the crista of the affected canal and thus irregular superior canal (SC) neurons in Scarpa's ganglion, which provides the neurophysiological basis for the clinical presentation of SCD. BACKGROUND: Patients with SCD display sound- and vibration-induced vertigo/nystagmus and increased amplitudes of vestibular evoked myogenic potentials. METHODS: Extracellular recordings from n = 25 primary vestibular neurons of 16 female guinea pigs were analyzed. We recorded from the same vestibular neuron before, during and after creating the dehiscence and after closing the dehiscence. Neurobiotin labeling was employed in n = 11 neurons. RESULTS: After SCD, previously unresponsive irregular SC neurons displayed a stimulus-locked increase in discharge during application of air-conducted sound (ACS) or bone-conducted vibration (BCV) for a broad range of frequencies (ACS: 200-4000 Hz; BCV: 500-1500 Hz). This typical response was only observed for irregular SC neurons (n = 19), but not regular SC neurons, or irregular/regular horizontal canal neurons (n = 2 each), and was abolished after closing the dehiscence. Eleven irregular SC neurons responsive to ACS and/or BCV were traced back to calyx synapses in the central crista of the affected superior canal by neurobiotin labeling. CONCLUSIONS: Stimulus-locked activation of irregular SC neurons by ACS and BCV is the neurophysiological basis for sound- and vibration-induced vertigo/nystagmus and increased VEMP amplitudes in SCD. The results of the present study help to improve vestibular diagnostics in patients with suspected SCD.

Neuronal network-based mathematical modeling of perceived verticality in acute unilateral vestibular lesions: from nerve to thalamus and cortex.

Acute unilateral lesions of vestibular graviceptive pathways from the otolith organs and semicircular canals via vestibular nuclei and the thalamus to the parieto-insular vestibular cortex regularly cause deviations of perceived verticality in the frontal roll plane. These tilts are ipsilateral in peripheral and in ponto-medullary lesions and contralateral in ponto-mesencephalic lesions. Unilateral lesions of the vestibular thalamus or cortex cause smaller tilts of the perceived vertical, which may be either ipsilateral or contralateral. Using a neural network model, we previously explained why unilateral vestibular midbrain lesions rarely manifest with rotational vertigo. We here extend this approach, focussing on the direction-specific deviations of perceived verticality in the roll plane caused by acute unilateral vestibular lesions from the labyrinth to the cortex. Traditionally, the effect of unilateral peripheral lesions on perceived verticality has been attributed to a lesion-based bias of the otolith system. We here suggest, on the basis of a comparison of model simulations with patient data, that perceived visual tilt after peripheral lesions is caused by the effect of a torsional semicircular canal bias on the central gravity estimator. We further argue that the change of gravity coding from a peripheral/brainstem vectorial representation in otolith coordinates to a distributed population coding at thalamic and cortical levels can explain why unilateral thalamic and cortical lesions have a variable effect on perceived verticality. Finally, we propose how the population-coding network for gravity direction might implement the elements required for the well-known perceptual underestimation of the subjective visual vertical in tilted body positions.

Toward Optimizing cVEMP: 2,000-Hz Tone Bursts Improve the Detection of Superior Canal Dehiscence.

BACKGROUND: The cervical vestibular evoked myogenic potential (cVEMP) test measures saccular and inferior vestibular nerve function. The cVEMP can be elicited with different frequency stimuli and interpreted using a variety of metrics. Patients with superior semicircular canal dehiscence (SCD) syndrome generally have lower cVEMP thresholds and larger amplitudes, although there is overlap with healthy subjects. The aim of this study was to evaluate which metric and frequency best differentiate healthy ears from SCD ears using cVEMP. METHODS: Twenty-one patients with SCD and 23 age-matched controls were prospectively included and underwent cVEMP testing at 500, 750, 1,000 and 2,000 Hz. Sound level functions were obtained at all frequencies to acquire threshold and to calculate normalized peak-to-peak amplitude (VEMPn) and VEMP inhibition depth (VEMPid). Third window indicator (TWI) metrics were calculated by subtracting the 250-Hz air-bone gap from the ipsilateral cVEMP threshold at each frequency. Ears of SCD patients were divided into three groups based on CT imaging: dehiscent, thin or unaffected. The ears of healthy age-matched control subjects constituted a fourth group. RESULTS: Comparing metrics at all frequencies revealed that 2,000-Hz stimuli were most effective in differentiating SCD from normal ears. ROC analysis indicated that for both 2,000-Hz cVEMP threshold and for 2,000-Hz TWI, 100% specificity could be achieved with a sensitivity of 92.0%. With 2,000-Hz VEMPn and VEMPid at the highest sound level, 100% specificity could be achieved with a sensitivity of 96.0%. CONCLUSION: The best diagnostic accuracy of cVEMP in SCD patients can be achieved with 2,000-Hz tone burst stimuli, regardless of which metric is used.

The N3 potential and the efferent cochlear pathway in profound sensorineural hearing loss.

OBJECTIVE: This study aimed to evaluate the presence of the N3 potential (acoustically evoked short latency negative response) in profound sensorineural hearing loss, its association with the cervical vestibular evoked myogenic potential and the relationship between both potentials and loss of auditory function. METHODS: Otological examinations of 66 ears from 50 patients aged from 4 to 36 years were performed, and the vestibular evoked myogenic potential and auditory brainstem response were measured. RESULTS: The N3 potential was recorded in 36 out of 66 ears (55 per cent) and a vestibular evoked myogenic potential was recorded in 34 (52 per cent). The N3 potential was recorded in 23 out of 34 ears (68 per cent) with a vestibular evoked myogenic potential response and absent in 19 out of 32 ears (59 per cent) without a vestibular evoked myogenic potential response. The presence of an N3 potential was significantly associated with a vestibular evoked myogenic potential response (p = 0.028), but there was no significant difference in the latency or amplitude of the N3 potential in either the presence or absence of a vestibular evoked myogenic potential. CONCLUSION: The presence of an N3 potential in profound sensorineural hearing loss with good or poor vestibular function can be explained by the contribution of the efferent cochlear pathway through olivocochlear fibres that join the inferior vestibular nerve. This theory is supported by its early latency and reversed polarity, which is masked in normal hearing by auditory brainstem response waves.

Noisy vestibular stimulation improves dynamic walking stability in bilateral vestibulopathy.

OBJECTIVE: To examine the effects of imperceptible levels of white noise galvanic vestibular stimulation (nGVS) on dynamic walking stability in patients with bilateral vestibulopathy (BVP). METHODS: Walking performance of 13 patients with confirmed BVP (mean age 50.1 ± 5.5 years) at slow, preferred, and fast speeds was examined during walking with zero-amplitude nGVS (sham trial) and nonzero-amplitude nGVS set to 80% of the individual cutaneous threshold for GVS (nGVS trial). Eight standard gait measures were analyzed: stride time, stride length, base of support, double support time percentage as well as the bilateral phase coordination index, and the coefficient of variation (CV) of stride time, stride length, and base of support. RESULTS: Compared to the sham trial, nGVS improved stride time CV by 26.0% ± 8.4% (p < 0.041), stride length CV by 26.0% ± 7.7% (p < 0.029), base of support CV by 27.8% ± 2.9% (p < 0.037), and phase coordination index by 8.4% ± 8.8% (p < 0.013). The nGVS effects on walking performance were correlated with subjective ratings of walking balance (ρ = 0.79, p < 0.001). Effect of nGVS on walking stability was most pronounced during slow walking. CONCLUSIONS: In patients with BVP, nGVS is effective in improving impaired gait performance, predominantly during slower walking speeds. It primarily targets the variability and bilateral coordination characteristics of the walking pattern, which are linked to dynamic walking stability. nGVS might present an effective treatment option to immediately improve walking performance and reduce the incidence of falls in patients with BVP. CLASSIFICATION OF EVIDENCE: This study provides Class IV evidence that in patients with BVP, an imperceptible level of nGVS improves dynamic walking stability.

Caloric Vestibular Stimulation Reduces Pain and Somatoparaphrenia in a Severe Chronic Central Post-Stroke Pain Patient: A Case Study.

Central post-stroke pain is a neuropathic syndrome characterized by intolerable contralesional pain and, in rare cases, somatic delusions. To date, there is limited evidence for the effective treatments of this disease. Here we used caloric vestibular stimulation to reduce pain and somatoparaphrenia in a 57-year-old woman suffering from central post-stroke pain. Resting-state functional magnetic resonance imaging was used to assess the neurological effects of this treatment. Following vestibular stimulation we observed impressive improvements in motor skills, pain, and somatic delusions. In the functional connectivity study before the vestibular stimulation, we observed differences in the patient's left thalamus functional connectivity, with respect to the thalamus connectivity of a control group (N = 20), in the bilateral cingulate cortex and left insula. After the caloric stimulation, the left thalamus functional connectivity with these regions, which are known to be involved in the cortical response to pain, disappeared as in the control group. The beneficial use of vestibular stimulation in the reduction of pain and somatic delusion in a CPSP patient is now documented by behavioral and imaging data. This evidence can be applied to theoretical models of pain and body delusions.

Influence of the stimulus parameters of galvanic vestibular stimulation on unilateral spatial neglect.

Galvanic vestibular stimulation (GVS) stimulates the vestibular system electrically with low-amplitude direct current through surface electrodes applied to the left and right mastoids. The effects of GVS on unilateral spatial neglect (USN) in poststroke patients were recently reported, but the influence of the current intensity and application duration of GVS on USN has not been sufficiently investigated. Here we explored the influence of these stimulus parameters on USN. We recruited seven patients with right-hemisphere stroke and left-sided USN (four female) for this single-blind, sham-controlled cross-over trial. Their scores on the line cancellation test were measured under three stimulation conditions [left-cathodal/right-anodal GVS (L-GVS), right-cathodal/left-anodal GVS, and sham] at three time points (before the start of GVS, 10 min after the start of GVS, and 20 min after the start of GVS). The GVS intensity was set below the sensory threshold and differed among the patients (0.4-2.0 mA). The cancellation scores were significantly increased after 10 and 20 min L-GVS, with a greater increase observed after the latter (P<0.0001). The other stimulus conditions had no significant effect. There was a significant positive correlation between the change in the increase in the cancellation score with L-GVS and the total charge (r=0.81, P=0.0004). The effect of GVS on USN may depend on its application duration, current intensity, and polarity.

The role of cervical and ocular vestibular evoked myogenic potentials in the assessment of patients with vestibular schwannomas.

OBJECTIVES: To investigate the clinical utility of VEMPs in patients suffering from unilateral vestibular schwannoma (VS) and to determine the optimal stimulation parameter (air conducted sound, bone conducted vibration) for evaluating the function of the vestibular nerve. METHODS: Data were obtained in 63 patients with non-operated VS, and 20 patients operated on VS. Vestibular function was assessed by caloric, cervical and ocular VEMP testing. 37/63 patients with conclusive ACS ocular VEMPs responses were studied separately. RESULTS: In the 63 non-operated VS patients, cVEMPs were abnormal in 65.1% of patients in response to AC STB and in 49.2% of patients to AC clicks. In the 37/63 patients with positive responses from the unaffected side, oVEMPs were abnormal in 75.7% of patients with ACS, in 67.6% with AFz and in 56.8% with mastoid BCV stimulation. In 16% of the patients, VEMPs were the only abnormal test (normal caloric and normal hearing). Among the 26 patients who did not show oVEMP responses on either side with ACS, oVEMPs responses could be obtained with AFz (50%) and with mastoid stimulation (89%). CONCLUSIONS: The VEMP test demonstrated significant clinical value as it yielded the only abnormal test results in some patients suffering from a unilateral vestibular schwannoma. For oVEMPs, we suggest that ACS stimulation should be the initial test. In patients who responded to ACS and who had normal responses, BCV was not required. In patients with abnormal responses on the affected side using ACS, BCV at AFz should be used to confirm abnormal function of the superior vestibular nerve. In patients who exhibited no responses on either side to ACS, BCV was the only approach allowing assessment of the function of the superior vestibular nerve. We favor using AFz stimulation first because it is easier to perform in clinical practice than mastoid stimulation.

International guidelines for the clinical application of cervical vestibular evoked myogenic potentials: an expert consensus report.

BACKGROUND: Cervical vestibular evoked myogenic potentials (cVEMPs) are electromyogram responses evoked by high-level acoustic stimuli recorded from the tonically contracting sternocleidomastoid (SCM) muscle, and have been accepted as a measure of saccular and inferior vestibular nerve function. As more laboratories are publishing cVEMP data, there is a wider range of recording methods and interpretation, which may be confusing and limit comparisons across laboratories. OBJECTIVE: To recommend minimum requirements and guidelines for the recording and interpretation of cVEMPs in the clinic and for diagnostic purposes. MATERIAL AND METHODS: We have avoided proposing a single methodology, as clinical use of cVEMPs is evolving and questions still exist about its underlying physiology and its measurement. The development of guidelines by a panel of international experts may provide direction for accurate recording and interpretation. RESULTS: cVEMPs can be evoked using air-conducted (AC) sound or bone conducted (BC) vibration. The technical demands of galvanic stimulation have limited its application. For AC stimulation, the most effective frequencies are between 400 and 800 Hz below safe peak intensity levels (e.g. 140 dB peak SPL). The highpass filter should be between 5 and 30 Hz, the lowpass filter between 1000 and 3000 Hz, and the amplifier gain between 2500 and 5000. The number of sweeps averaged should be between 100 and 250 per run. Raw amplitude correction by the level of background SCM activity narrows the range of normal values. There are few publications in children with consistent results. CONCLUSION: The present recommendations outline basic terminology and standard methods. Because research is ongoing, new methodologies may be included in future guidelines.

The role of cervical and ocular vestibular-evoked myogenic potentials in the follow-up of vestibular neuritis.

This study evaluates the recovery of vestibular nerve function after vestibular neuritis (VN) by vestibular-evoked myogenic potentials (VEMPs). Twenty-six patients with the diagnosis of VN were included. All patients underwent ocular VEMP (oVEMP) and cervical VEMP (cVEMP) recordings, at 6 days and 6 months from the onset of the symptoms. Of the 26 patients, 14 showed improvement on oVEMP at month 6 (group 1), and 12 showed no change or worsening on oVEMP at 6 months (group 2). At the same time, there was no change in the amplitudes of the cVEMP on either healthy or affected sides in both groups. Inability to perform the Fukuda test, and chronic white matter supratentorial lesions present on brain magnetic resonance imaging (MRI) were more frequent in patients with worse outcome on oVEMP (P = 0.044 and 0.045, respectively). Although involvement of the inferior branch of the vestibular nerve was not associated with oVEMP outcome, oVEMP latencies (N10 and P13) were associated with improvement or worsening in oVEMP amplitudes, showing that prolonged latencies correlate with 6-month improvement in oVEMP amplitudes (Pearson correlation -0.472, P = 0.041 and -0.580, P = 0.009, respectively). This study identified clinical, MRI and neurophysiological predictors of recovery in patients with superior VN, and offers additional insight into, and better understanding of, the role of VEMP in diagnosis and prognosis of patients with VN. Further studies are needed to validate this diagnostic procedure and to assess its clinical usefulness in VN management.

The effect of repeated sessions of galvanic vestibular stimulation on target cancellation in visuo-spatial neglect: preliminary evidence from two cases.

OBJECTIVE: In recent years it has emerged that the attentional disorder of visuo-spatial neglect can be overcome via artificial stimulation of the balance system. One means of achieving this is via galvanic vestibular stimulation (GVS), a simple procedure in which tiny, electrical currents are discharged to the part of the scalp overlying the vestibular nerves. Attempts to remediate neglect with GVS have utilized only a single session of stimulation and, although this can induce spontaneous recovery, symptoms resurface soon after stimulation. This study assessed whether repeated sessions induce longer carry-over. METHODS: Two individuals diagnosed with neglect post-stroke received 5 days of sub-sensory, left anodal GVS. Performance was assessed via the letter and star cancellation tasks of the Behavioural Inattention Test on four occasions; 3 days before the start of stimulation, on the first and last day of stimulation and 3-days after stimulation. RESULTS: Analyses of variance indicated that both participants missed significantly fewer targets in both tasks on the fifth day of stimulation compared to baseline. More so, this improvement was still evident at follow-up 3 days later. CONCLUSION: The results strengthen the need for a larger, sham-controlled trial to establish whether repeated GVS provides lasting relief from neglect.

Ocular and cervical vestibular evoked myogenic potentials in multiple sclerosis patients.

OBJECTIVE: Vestibular evoked myogenic potentials (VEMPs) are thought to provide useful information about brainstem functions, as the neural pathways of both ocular and cervical VEMPs pass through the brainstem. The aim of this study was to investigate the clinical value of ocular and cervical VEMP tests in the evaluation of brainstem involvement in multiple sclerosis (MS) patients and to assess their relation with clinical and cranial MRI findings. METHODS: Ocular and cervical VEMPs were recorded in 62 MS patients and 35 age and sex matched healthy volunteers. The latencies, amplitude asymmetry ratios of both VEMP responses and abnormality ratios (prolonged latencies and absent responses) were compared between the MS patients and the control group and among the groups of MS patients. RESULTS: oVEMP mean n1 and p1 latencies and cVEMP mean p13 latency were significantly prolonged in MS patients. Although the abnormality ratios of both VEMPs were higher in patients with brainstem clinical or MRI lesions, the correlation was not statistically significant. Both ocular and cervical VEMP latencies were significantly correlated with expanded disability status scale. CONCLUSIONS: Although there is no significant correlation with clinical or MRI findings, MS patients show high frequency of abnormality in VEMP tests, especially in oVEMP tests. SIGNIFICANCE: VEMP tests may be useful as an adjunct test in the evaluation of brainstem dysfunction in MS patients.

Minor adverse effects of galvanic vestibular stimulation in persons with stroke and healthy individuals.

OBJECTIVE: Galvanic vestibular stimulation (GVS) induces polarity-specific activations in the vestibular nerves and upstream in the vestibular and parietotemporal cortices as well as sub-cortical regions. This makes it an attractive technique for cognitive neuromodulation. However, systematic studies regarding adverse effects of GVS are unavailable. Thus, this study assessed adverse effects during and after sub-sensory GVS (mean: 0.6 mA) and GVS with 1.5 mA. METHODS: Two hundred and fifty-five GVS sessions delivered to 55 persons with stroke and 30 healthy individuals were analysed using a 34-item-questionnaire including potential symptoms and rating scales for adverse effects. RESULTS: The most frequent symptoms during and after GVS were slight itching (mean: 10.2%) and tingling (mean: 10.7%) underneath the electrodes. Healthy individuals and persons with stroke did not differ in their incidence and rated intensity of adverse effects, nor did persons with or without unilateral spatial neglect. Adverse effects were found more frequently with GVS with 1.5 mA as with sub-sensory GVS. Participants were unable to differentiate real from sham conditions during sub-sensory GVS. Importantly, neither seizures nor vertigo or nausea were observed. CONCLUSION: Sub-sensory GVS and GVS with 1.5 mA induce very few and mild adverse effects in healthy and persons with stroke and are safe when safety guidelines are followed.

[Air conducted ocular VEMP: II. First clinical investigations]. / Luftleitungsinduzierte oculäre VEMP: II. Erste klinische Untersuchungen.

BACKGROUND: Vestibular-evoked myogenic potentials (VEMP) are widely used to assess vestibular function. Air conducted (AC) cervical VEMP (cVEMP) reflect sacculus and inferior vestibular nerve function. Ocular VEMP (oVEMP) however has been hardly examined up to now. In recent studies it has been assumed that AC oVEMP probably reflects superior vestibular nerve function. The aim of this pilot study was to evaluate clinical application of the AC oVEMP. MATERIAL AND METHODS: AC oVEMP were recorded in patients with peripheral vestibular disorders (n=21). In addition thermal irritation and head impulse test were performed and AC cVEMP were recorded. For intense AC-sound stimulation tone bursts (500 Hz) with 100 dB nHL were used. RESULTS: In peripheral vestibular disorders AC oVEMP and AC cVEMP could be classified into: • type 1 (inferior vestibular neuritis) with loss of AC oVEMP but normal AC cVEMP, • type 2, probable type of superior vestibular neuritis, showing present AC cVEMP but loss of AC oVEMP, • type 3, probable complete vestibular neuritis, without AC oVEMP and AC cVEMP. CONCLUSIONS: AC oVEMP may be used as an appropriate test for clinical investigation in patients with vestibular disorders. AC oVEMP is an additional, essential test for assessing otolith function beside AC cVEMP. Further vestibular test are necessary for precise clinical interpretation.

Identifying the affected branches of vestibular nerve in vestibular neuritis.

CONCLUSION: The inner ear monitoring system is useful for identifying the affected branches of the vestibular nerve in cases of vestibular neuritis, providing insight about the interval for the relief of vertigo. OBJECTIVE: This study conducted an inner ear monitoring system including audiometry, and caloric, ocular vestibular evoked myogenic potential (oVEMP), and cervical VEMP (cVEMP) tests in cases of vestibular neuritis for assessing the affected branches of the vestibular nerve. METHODS: Twenty patients with vestibular neuritis underwent caloric, oVEMP, and cVEMP tests. Type I indicates that one of the three tests is abnormal; type II indicates that two test results are abnormal; and type III indicates that no test result is normal. RESULTS: All patients had normal hearing, bilaterally. Nineteen (95%) of 20 patients had abnormal caloric responses, 11 patients (55%) had abnormal oVEMPs, and 5 patients (25%) had abnormal cVEMPs. Restated, of all 20 patients, 8 patients were classified as type I, 9 were type II, and 3 were type III. The mean intervals between presentation and relief of vertigo were significantly different among the three types. One year after treatment, caloric, oVEMP, and cVEMP tests returned to normal responses in three (60%) of five patients.

Effects of vestibular prosthesis electrode implantation and stimulation on hearing in rhesus monkeys.

To investigate the effects of vestibular prosthesis electrode implantation and activation on hearing in rhesus monkeys, we measured auditory brainstem responses (ABR) and distortion product otoacoustic emissions (DPOAE) in four rhesus monkeys before and after unilateral implantation of vestibular prosthesis electrodes in each of 3 left semicircular canals (SCC). Each of the 3 left SCCs were implanted with electrodes via a transmastoid approach. Right ears, which served as controls, were not surgically manipulated. Hearing tests were conducted before implantation (BI) and then 4 weeks post-implantation both without electrical stimulation (NS) and with electrical stimulation (S). During the latter condition, prosthetic electrical stimuli encoding 3 dimensions of head angular velocity were delivered to the 3 ampullary branches of the left vestibular nerve via each of 3 electrode pairs of a multichannel vestibular prosthesis. Electrical stimuli comprised charge-balanced biphasic pulses at a baseline rate of 94 pulses/s, with pulse frequency modulated from 48 to 222 pulses/s by head angular velocity. ABR hearing thresholds to clicks and tone pips at 1, 2, and 4 kHz increased by 5-10 dB from BI to NS and increased another ∼5 dB from NS to S in implanted ears. No significant change was seen in right ears. DPOAE amplitudes decreased by 2-14 dB from BI to NS in implanted ears. There was a slight but insignificant decrease of DPOAE amplitude and a corresponding increase of DPOAE/Noise floor ratio between NS and S in implanted ears. Vestibular prosthesis electrode implantation and activation have small but measurable effects on hearing in rhesus monkeys. Coupled with the clinical observation that patients with cochlear implants only rarely exhibit signs of vestibular injury or spurious vestibular nerve stimulation, these results suggest that although implantation and activation of multichannel vestibular prosthesis electrodes in human will carry a risk of hearing loss, that loss is not likely to be severe.

Effects of biphasic current pulse frequency, amplitude, duration, and interphase gap on eye movement responses to prosthetic electrical stimulation of the vestibular nerve.

An implantable prosthesis that stimulates vestibular nerve branches to restore sensation of head rotation and vision-stabilizing reflexes could benefit individuals disabled by bilateral loss of vestibular (inner ear balance) function. We developed a prosthesis that partly restores normal function in animals by delivering pulse frequency modulated (PFM) biphasic current pulses via electrodes implanted in semicircular canals. Because the optimal stimulus encoding strategy is not yet known, we investigated effects of varying biphasic current pulse frequency, amplitude, duration, and interphase gap on vestibulo-ocular reflex (VOR) eye movements in chinchillas. Increasing pulse frequency increased response amplitude while maintaining a relatively constant axis of rotation. Increasing pulse amplitude (range 0- 325 µA) also increased response amplitude but spuriously shifted eye movement axis, probably due to current spread beyond the target nerve. Shorter pulse durations (range 28- 340 µs) required less charge to elicit a given response amplitude and caused less axis shift than longer durations. Varying interphase gap (range 25- 175 µs) had no significant effect. While specific values reported herein depend on microanatomy and electrode location in each case, we conclude that PFM with short duration biphasic pulses should form the foundation for further optimization of stimulus encoding strategies for vestibular prostheses intended to restore sensation of head rotation.