Psychophysical and physiological measures of electrical-field interaction in cochlear implants

Neural recovery function of the auditory nerve in cochlear implant surgery: Comparison between different regions of the cochlea

INTRODUCTION

Cochlear implants allow measures of neural function, through Neural response telemetry (NRT) and Auditory nerve recovery function (REC). These help in programming the speech processor and understanding the auditory system. However, not many studies have evaluated and compared these in different regions of the cochlea.

OBJECTIVE

Comparing NRT and REC in different regions of the cochlea.

METHODS

Cross-sectional, descriptive and prospective. NRT and REC (through the function of T0 - absolute refractory period, A - amplitude and TAU - time constant of the relative refractory period parameters) were evaluated, in three groups according to the stimulated electrode of the cochlea: apical, medial and basal.

RESULTS

26 adult patients were evaluated, 2 bilateral, totalling 28 ears. Data analysis showed no statistically significant difference between NRT between medial and basal but showed between apical and medial and apical and basal. For T0, there was a significant difference between medial and basal; for A, there was a significant difference between apical and basal and also medial and basal; and for TAU, there was no significant difference.

CONCLUSION

There was a statistically significant difference in NRT and REC when compared between different regions of the cochlea.

The Perception of Ramped Pulse Shapes in Cochlear Implant Users

The electric stimulation provided by current cochlear implants (CI) is not power efficient. One underlying problem is the poor efficiency by which information from electric pulses is transformed into auditory nerve responses. A novel stimulation paradigm using ramped pulse shapes has recently been proposed to remedy this inefficiency. The primary motivation is a better biophysical fit to spiral ganglion neurons with ramped pulses compared to the rectangular pulses used in most contemporary CIs. Here, we tested the hypotheses that ramped pulses provide more efficient stimulation compared to rectangular pulses and that a rising ramp is more efficient than a declining ramp. Rectangular, rising ramped and declining ramped pulse shapes were compared in terms of charge efficiency and discriminability, and threshold variability in seven CI listeners. The tasks included single-channel threshold detection, loudness-balancing, discrimination of pulse shapes, and threshold measurement across the electrode array. Results showed that reduced charge, but increased peak current amplitudes, was required at threshold and most comfortable levels with ramped pulses relative to rectangular pulses. Furthermore, only one subject could reliably discriminate between equally-loud ramped and rectangular pulses, suggesting variations in neural activation patterns between pulse shapes in that participant. No significant difference was found between rising and declining ramped pulses across all tests. In summary, the present findings show some benefits of charge efficiency with ramped pulses relative to rectangular pulses, that the direction of a ramped slope is of less importance, and that most participants could not perceive a difference between pulse shapes.

Intra-Cochlear Current Spread Correlates with Speech Perception in Experienced Adult Cochlear Implant Users

Broader intra-cochlear current spread (ICCS) implies higher cochlear implant (CI) channel interactions. This study aimed to investigate the relationship between ICCS and speech intelligibility in experienced CI users. Using voltage matrices collected for impedance measurements, an individual exponential spread coefficient (ESC) was computed. Speech audiometry was performed to determine the intelligibility at 40 dB Sound Pressure Level (SPL) and the 50% speech reception threshold: I40 and SRT50 respectively. Correlations between ESC and either I40 or SRT50 were assessed. A total of 36 adults (mean age: 50 years) with more than 11 months (mean: 34 months) of CI experience were included. In the 21 subjects for whom all electrodes were active, ESC was moderately correlated with both I40 (r = −0.557, p = 0.009) and SRT50 (r = 0.569, p = 0.007). The results indicate that speech perception performance is negatively affected by the ICCS. Estimates of current spread at the closest vicinity of CI electrodes and prior to any activation of auditory neurons are indispensable to better characterize the relationship between CI stimulation and auditory perception in cochlear implantees.

Intraoperative transimpedance and spread of excitation profile correlations with a lateral-wall cochlear implant electrode array

A limiting factor of cochlear implant technology is the spread of electrode-generated intracochlear electrical field (EF) leading to spread of neural excitation (SOE). In this study, we investigated the relation of the spread of the intracochlear EF, assessed via transimpedance matrix (TIM), and SOE. A total of 43 consecutive patients (ages 0.7-82 years; 31.0 ± 25.7 years, mean ± SD) implanted with a Cochlear Nucleus CI522 or CI622 cochlear implant with Slim Straight electrode array (altogether 51 ears) were included in the study. Cochlear nerve was visualized for all patients in preoperative imaging and there were no cochlear anomalies in the study sample. The stimulated electrodes were in the basal, middle, and apical parts of the electrode array (electrode numbers 6, 11, and 19, respectively). The stimulation level was 210 CL on average for the TIM measurement and always 230 CL for the SOE measurement. Approximately 90% of the individual TIM and SOE profiles correlated with each other (p < .05; r = 0.61-0.99). Also, the widths of the TIM and SOE peaks, computed at 50% of the maximum height, exhibited a weak correlation (r = 0.39, p = .007). The 50% widths of TIM and SOE were the same only in the apical part of the electrode array; in the basal part SOE was wider than TIM, and in the middle part TIM was wider than SOE (p < .01 and p = .048, respectively). Within each measurement, TIM 50% widths were different between all three parts of the electrode array, while for SOE, only the basal electrode differed from the middle electrode. Finally, the size of the cochlea and the 50% widths of TIM and SOE had the strongest correlation in the middle part of the electrode array (r = -0.63, and -0.37, respectively). Our results suggest that there is a correlation between the spread of intracochlear EF and neural SOE at least in the apical part of the electrode array used in this study, and that larger cochleae are associated with more focused TIM and SOE.

Prediction of the Functional Status of the Cochlear Nerve in Individual Cochlear Implant Users Using Machine Learning and Electrophysiological Measures

OBJECTIVES

This study aimed to create an objective predictive model for assessing the functional status of the cochlear nerve (CN) in individual cochlear implant (CI) users.

DESIGN

Study participants included 23 children with cochlear nerve deficiency (CND), 29 children with normal-sized CNs (NSCNs), and 20 adults with various etiologies of hearing loss. Eight participants were bilateral CI users and were tested in both ears. As a result, a total of 80 ears were tested in this study. All participants used Cochlear Nucleus CIs in their test ears. For each participant, the CN refractory recovery function and input/output (I/O) function were measured using electrophysiological measures of the electrically evoked compound action potential (eCAP) at three electrode sites across the electrode array. Refractory recovery time constants were estimated using statistical modeling with an exponential decay function. Slopes of I/O functions were estimated using linear regression. The eCAP parameters used as input variables in the predictive model were absolute refractory recovery time estimated based on the refractory recovery function, eCAP threshold, slope of the eCAP I/O function, and negative-peak (i.e., N1) latency. The output variable of the predictive model was CN index, an indicator for the functional status of the CN. Predictive models were created by performing linear regression, support vector machine regression, and logistic regression with eCAP parameters from children with CND and the children with NSCNs. One-way analysis of variance with post hoc analysis with Tukey's honest significant difference criterion was used to compare study variables among study groups.

RESULTS

All three machine learning algorithms created two distinct distributions of CN indices for children with CND and children with NSCNs. Variations in CN index when calculated using different machine learning techniques were observed for adult CI users. Regardless of these variations, CN indices calculated using all three techniques in adult CI users were significantly correlated with Consonant-Nucleus-Consonant word and AzBio sentence scores measured in quiet. The five oldest CI users had smaller CN indices than the five youngest CI users in this study.

CONCLUSIONS

The functional status of the CN for individual CI users was estimated by our newly developed analytical models. Model predictions of CN function for individual adult CI users were positively and significantly correlated with speech perception performance. The models presented in this study may be useful for understanding and/or predicting CI outcomes for individual patients.

Examination and Comparison of Electrically Evoked Compound Action Potentials and Electrically Evoked Auditory Brainstem Response Results of Children with Cochlear Implantation without Inner Ear Anomaly

Objective

To investigate the relationship between electrically evoked compound action potentials (ECAP) and electrically evoked auditory brainstem responses (EABR) in children with cochlear implants (CI) without inner ear anomalies.

Methods

Sixteen children between the ages of two and six years who were CI users participated in the study. ECAP thresholds were recorded from one electrode in the basal, medial, and apical regions of the cochlear implant. EABRs were recorded from electrodes whose ECAP thresholds were determined. The latency-intensity functions, amplitude and morphological analyzes of the eIII and eV waves at 200 and 180 current unit (CU) excitation levels were performed. The data obtained were analyzed statistically.

Results

ECAP thresholds were found to be 171.5±11.38, 169.69±20.32 and 160.81±20.03 CU at the basal, medial and apical electrodes, respectively. EABR thresholds were also found to be 169.69±12.17, 165.62±16.41 and 160±15.49 CU in basal, medial and apical electrodes, respectively. There was a strong positive correlation between ECAP and EABR thresholds in apical, medial and basal electrodes (p<0.05). EABR threshold levels were not significantly different between basal, medial and apical region electrodes (p>0.05), and ECAP threshold values were significantly different between apical and basal region electrodes (p=0.002). When the significance values of EABR eV wave latencies were analyzed in terms of electrode region, the difference between basal and apical regions was found to be significant (p=0.03).

Conclusion

Consistency was found between ECAP and EABR recordings. However, it was concluded that one could not be preferred over the other because the data quality of the two tests was different. In future studies, ECAP and EABR recordings may be recommended by selecting more electrodes for stimulation.

Auditory enhancement and the role of spectral resolution in normal-hearing listeners and cochlear-implant users

Detection of a target tone in a simultaneous multi-tone masker can be improved by preceding the stimulus with the masker alone. The mechanisms underlying this auditory enhancement effect may enable the efficient detection of new acoustic events and may help to produce perceptual constancy under varying acoustic conditions. Previous work in cochlear-implant (CI) users has suggested reduced or absent enhancement, due perhaps to poor spatial resolution in the cochlea. This study used a supra-threshold enhancement paradigm that in normal-hearing listeners results in large enhancement effects, exceeding 20 dB. Results from vocoder simulations using normal-hearing listeners showed that near-normal enhancement was observed if the simulated spread of excitation was limited to spectral slopes no shallower than 24 dB/oct. No significant enhancement was observed on average in CI users with their clinical monopolar stimulation strategy. The variability in enhancement between CI users, and between electrodes in a single CI user, could not be explained by the spread of excitation, as estimated from auditory nerve evoked potentials. Enhancement remained small, but did reach statistical significance, under the narrower partial-tripolar stimulation strategy. The results suggest that enhancement may be at least partially restored by improvements in the spatial resolution of current CIs.

Effects of stimulus level and rate on psychophysical thresholds for interleaved pulse trains in cochlear implants

This study examined channel interactions using interleaved pulse trains to assess masking and potential facilitative effects in cochlear-implant recipients using clinically relevant stimuli. Psychophysical thresholds were measured for two adjacent mid-array electrodes; one served as the masker and the other as the probe. Two rates representative of those found in present-day strategies were tested: 1700 and 3400 pulses per second per channel. Four masker levels ranging from sub-threshold to loud-but-comfortable were tested. It was hypothesized that low-level maskers would produce facilitative effects, shifting to masking effects at high levels, and that faster rates would yield smaller masking effects due to greater stochastic neural firing patterns. Twenty-nine ears with Cochlear or Advanced Bionics devices were tested. High-level maskers produced more masking than low-level maskers, as expected. Facilitation was not observed for sub-threshold or threshold-level maskers in most cases. High masker levels yielded reduced probe thresholds for two Advanced Bionics subjects. This was partly eliminated with a longer temporal offset between each masker-probe pulse pair, as was used with Cochlear subjects. These findings support the use of temporal gaps between stimulation of subsequent electrodes to reduce channel interactions.

Initial Results With Image-guided Cochlear Implant Programming in Children

HYPOTHESIS

Image-guided cochlear implant (CI) programming can improve hearing outcomes for pediatric CI recipients.

BACKGROUND

CIs have been highly successful for children with severe-to-profound hearing loss, offering potential for mainstreamed education and auditory-oral communication. Despite this, a significant number of recipients still experience poor speech understanding, language delay, and, even among the best performers, restoration to normal auditory fidelity is rare. Although significant research efforts have been devoted to improving stimulation strategies, few developments have led to significant hearing improvement over the past two decades. Recently introduced techniques for image-guided CI programming (IGCIP) permit creating patient-customized CI programs by making it possible, for the first time, to estimate the position of implanted CI electrodes relative to the nerves they stimulate using CT images. This approach permits identification of electrodes with high levels of stimulation overlap and to deactivate them from a patient's map. Previous studies have shown that IGCIP can significantly improve hearing outcomes for adults with CIs.

METHODS

The IGCIP technique was tested for 21 ears of 18 pediatric CI recipients. Participants had long-term experience with their CI (5 mo to 13 yr) and ranged in age from 5 to 17 years old. Speech understanding was assessed after approximately 4 weeks of experience with the IGCIP map.

RESULTS

Using a two-tailed Wilcoxon signed-rank test, statistically significant improvement (p < 0.05) was observed for word and sentence recognition in quiet and noise, as well as pediatric self-reported quality-of-life (QOL) measures.

CONCLUSION

Our results indicate that image guidance significantly improves hearing and QOL outcomes for pediatric CI recipients.

The use of auditory and visual context in speech perception by listeners with normal hearing and listeners with cochlear implants

There is a wide range of acoustic and visual variability across different talkers and different speaking contexts. Listeners with normal hearing (NH) accommodate that variability in ways that facilitate efficient perception, but it is not known whether listeners with cochlear implants (CIs) can do the same. In this study, listeners with NH and listeners with CIs were tested for accommodation to auditory and visual phonetic contexts created by gender-driven speech differences as well as vowel coarticulation and lip rounding in both consonants and vowels. Accommodation was measured as the shifting of perceptual boundaries between /s/ and /∫/ sounds in various contexts, as modeled by mixed-effects logistic regression. Owing to the spectral contrasts thought to underlie these context effects, CI listeners were predicted to perform poorly, but showed considerable success. Listeners with CIs not only showed sensitivity to auditory cues to gender, they were also able to use visual cues to gender (i.e., faces) as a supplement or proxy for information in the acoustic domain, in a pattern that was not observed for listeners with NH. Spectrally-degraded stimuli heard by listeners with NH generally did not elicit strong context effects, underscoring the limitations of noise vocoders and/or the importance of experience with electric hearing. Visual cues for consonant lip rounding and vowel lip rounding were perceived in a manner consistent with coarticulation and were generally used more heavily by listeners with CIs. Results suggest that listeners with CIs are able to accommodate various sources of acoustic variability either by attending to appropriate acoustic cues or by inferring them via the visual signal.

Masking patterns for monopolar and phantom electrode stimulation in cochlear implants

Phantom electrode (PE) stimulation consists of out-of-phase stimulation of two electrodes. When presented at the apex of the electrode array, phantom stimulation is known to produce a lower pitch sensation than monopolar (MP) stimulation on the most apical electrode. The ratio of the current between the primary electrode (PEL) and the compensating electrode (CEL) is represented by the coefficient σ, which ranges from 0 (monopolar) to 1 (full bipolar). The exact mechanism by which PE stimulation produces a lower pitch sensation is unclear. In the present study, unmasked and masked thresholds were obtained using a forward masking paradigm to estimate the spread of current for MP and PE stimulation. Masked thresholds were measured for two phantom electrode configurations (1) PEL = 4, CEL = 5 (lower pitch phantom) and (2) PEL = 4, CEL = 3 (higher pitch phantom). The unmasked thresholds were subtracted from the masked thresholds to obtain masking patterns which were normalized to their peak. The masking patterns reveal (1) differences in the spread of excitation that are consistent with the direction of pitch shift produced by PE stimulation, and (2) narrower spread of electrical excitation for PE stimulation relative to MP stimulation.

Differences between electrically evoked compound action potential (ECAP) and behavioral measures in children with cochlear implants operated in the school age vs. operated in the first years of life

OBJECTIVE

The aim of this study was to identify the differences in the NRT measures, behavioral measures, and their relationship between the group of congenitally deaf children operated in the first years of life and the group of children operated in the school age.

METHODS

The study included 40 congenitally deaf children with cochlear implants divided into two groups. Group 1 was composed of 20 children (mean age at operation 2.3 years, range 1.4-4.6 years) and Group 2 was composed of 20 children (mean age at operation 11.3 years, range 7.0-17.1 years). The ECAP was recorded using the Nucleus 24 neural response telemetry (NRT) system. In each child, the responses were evoked by the apical, middle and basal electrodes. The analyzed parameters were: the ECAP threshold (T-NRT), N1P2 amplitude, N1 latency, slope of the amplitude growth function, response morphology, threshold (T-) level, maximum comfort (C-) level, dynamic range (DR), T-NRT as a percentage of the map DR, the correlation between the T-NRT and the T- and C-levels. The recordings of parameters were performed two years after implantations.

RESULTS

The T-NRT, DR, T-NRT as a percentage of the map DR and the correlation between T-NRT and C-levels were significantly different between both groups of children. There were no statistically significant differences between the groups with respect to the amplitude, latency, slope and morphology recorded using the same electrodes. However, intragroup differences regarding NRT measures and behavioral measures with respect to the position of stimulating electrode were more prominent in Group 2 than in the Group 1.

CONCLUSIONS

Results of this study have also found a great variability of NRT and MAP measures within and across patients in both groups of children, but it was still more pronounced in the group of school children. NRT profile across electrodes follows MAP profiles better in the Group 1 then in the Group 2. Overall findings of NRT and MAP measures are not consistent and unambiguous as we expected, but still suggest potential differences between results in children operated in first years of life, and those operated in school age.

Preliminary results of the relationship between the binaural interaction component of the electrically evoked auditory brainstem response and interaural pitch comparisons in bilateral cochlear implant recipients

OBJECTIVE

: The purpose of this study was to investigate the relationship between electrophysiologic measures of the binaural interaction component (BIC) of the electrically evoked auditory brainstem response and psychophysical measures of interaural pitch comparisons in Nucleus bilateral cochlear implant users.

DESIGN

: Data were collected for 10 postlingually deafened adult cochlear implant users. Each subject conducted an interaural pitch-comparison task using a biphasic pulse train with a pulse rate of 1000 pulses per second (pps) at high stimulation levels. Stimuli were presented in a two-interval, two-alternative forced-choice procedure with roving current variations. A subgroup of four subjects repeated the task at low stimulation levels. BICs were measured using loudness balanced, biphasic current pulses presented at a rate of 19.9 pps for each subject by pairing the electrode 12 (out of 22 intracochlear electrodes) in the right ear with each of 11 electrodes spaced across the electrode array in the left ear. The BIC was measured at high stimulation levels in 10 subjects and at low stimulation levels in 7 subjects. Because of differences in stimulation rate used in BIC measures and interaural pitch comparisons, the actual stimulation levels were different in these two measures. The relationship between BIC responses and results of interaural pitch comparisons was evaluated for each of the individual subjects and at the group level. Evaluation was carried out separately for results obtained at high and low stimulation levels.

RESULTS

: There was no significant correlation between results of BIC measures and interaural pitch comparisons on either the individual or group levels. Lower stimulation level did not improve the relationship between these two measures.

CONCLUSIONS

: No significant correlations between psychophysical measures of interaural pitch comparisons and electrophysiologic measures of the BIC of the electrically evoked auditory brainstem response were found. The lack of correlation may be attributed to methods used to quantify the data, small number of subjects retested at low stimulation levels, and central processing components involved in the interaural pitch-comparison task.

Temporal response properties of the auditory nerve: data from human cochlear-implant recipients

The primary goal of this study was to characterize the variability in auditory-nerve temporal response patterns obtained with the electrically evoked compound action potential (ECAP) within and across a relatively large group of cochlear-implant recipients. ECAPs were recorded in response to each of 21 pulses in a pulse train for five rates (900, 1200, 1800, 2400, and 3500 pps) and three cochlear regions (basal, middle, and apical). An alternating amplitude pattern was typically observed across the pulse train for slower rates, reflecting refractory properties of individual nerve fibers. For faster rates, the alternation ceased and overall amplitudes were substantially lower relative to the first pulse in the train, reflecting cross-fiber desynchronization. The following specific parameters were examined: (1) the rate at which the alternating pattern ceased (termed stochastic rate), (2) the alternation depth and the rate at which the maximum alternation occurred, and (3) the average normalized ECAP amplitude across the pulse train (measure of overall adaptation/desynchronization). Data from 29 ears showed that stochastic rates for the group spanned the entire range of rates tested. The majority of subjects (79%) had different stochastic rates across the three cochlear regions. The stochastic rate occurred most frequently at 2400 pps for basal and middle electrodes, and at 3500 pps for apical electrodes. Stimulus level was significantly correlated with stochastic rate, where higher levels yielded faster stochastic rates. The maximum alternation depth averaged 19% of the amplitude for the first pulse. Maximum alternation occurred most often at 1800 pps for basal and apical electrodes, and at 1200 pps for middle electrodes. These differences suggest some independence between alternation depth and stochastic rate. Finally, the overall amount of adaptation or desynchronization ranged from 63% (for 900 pps) to 23% (for 3500 pps) of the amplitude for the first pulse. Differences in temporal response properties across the cochlea within subjects may have implications for developing new speech-processing strategies that employ varied rates across the array.

Electrically evoked compound action potential measures for virtual channels versus physical electrodes

OBJECTIVES

The number of distinct pitch percepts for cochlear implant (CI) listeners is somewhat limited by the number of physical electrodes in the array. Newer-generation CIs have the capability to potentially increase this number by stimulating areas of the cochlea between the physical electrodes. Currently, this is achieved by electrically coupling adjacent electrodes or by simultaneously activating two electrodes with independent current sources (i.e., current steering). Presumably, either type of dual-electrode stimulation will generate neural excitation patterns that are intermediate to those generated by either physical electrode alone (henceforth termed virtual channel). However, it is not clear whether virtual-channel stimulation yields neural recruitment patterns with similar shapes and rates of growth as compared with each physical electrode alone. The purpose of this study was to compare basic electrically evoked compound action potential (ECAP) measures for physical electrodes and virtual channels to determine whether properties of the respective excitation patterns were similar.

DESIGN

Data were collected for 12 adult CI recipients (six Nucleus Freedom CI24RE, two Advanced Bionics HiResolution 90K, and four Advanced Bionics CII). ECAP responses were measured for a set of three adjacent physical electrodes and two corresponding intermediate virtual channels (e.g., physical electrodes 4, 5, and 6 and virtual channels 4 + 5 and 5 + 6) at three positions along the electrode array (basal, middle, and apical). Virtual channels for Nucleus subjects were produced via electrical coupling of adjacent electrode pairs (dual-electrode mode). For Advanced Bionics subjects, virtual channels were produced via simultaneous, in-phase stimulation of adjacent electrode pairs with 50% of the total current delivered to each electrode in the pair. Specific ECAP measures were as follows: (1) threshold and slope of the input/output functions, (2) amplitude for a masker-probe interval of 1500 μsecs (measure of refractory recovery), and (3) relative location of spread of excitation (SOE) functions among virtual channels and adjacent physical electrodes. Measures for virtual channels were compared with those for the flanking physical electrodes using a multivariate analysis of variance.

RESULTS

There were no statistically significant differences between physical electrodes and virtual channels for ECAP thresholds, slope of the input/output function, or refractory recovery. On average, SOE functions for the virtual channels were spatially located approximately halfway between SOE functions for the adjacent physical electrodes.

CONCLUSIONS

Results from this study suggest that virtual channels produce neural recruitment patterns with properties similar to those elicited by the adjacent physical electrodes.

Comparing spatial tuning curves, spectral ripple resolution, and speech perception in cochlear implant users

Spectral ripple discrimination thresholds were measured in 15 cochlear-implant users with broadband (350-5600 Hz) and octave-band noise stimuli. The results were compared with spatial tuning curve (STC) bandwidths previously obtained from the same subjects. Spatial tuning curve bandwidths did not correlate significantly with broadband spectral ripple discrimination thresholds but did correlate significantly with ripple discrimination thresholds when the rippled noise was confined to an octave-wide passband, centered on the STC's probe electrode frequency allocation. Ripple discrimination thresholds were also measured for octave-band stimuli in four contiguous octaves, with center frequencies from 500 Hz to 4000 Hz. Substantial variations in thresholds with center frequency were found in individuals, but no general trends of increasing or decreasing resolution from apex to base were observed in the pooled data. Neither ripple nor STC measures correlated consistently with speech measures in noise and quiet in the sample of subjects in this study. Overall, the results suggest that spectral ripple discrimination measures provide a reasonable measure of spectral resolution that correlates well with more direct, but more time-consuming, measures of spectral resolution, but that such measures do not always provide a clear and robust predictor of performance in speech perception tasks.

Spatial tuning curves from apical, middle, and basal electrodes in cochlear implant users

Forward-masked psychophysical spatial tuning curves (fmSTCs) were measured in 15 cochlear-implant subjects, 10 using monopolar stimulation and 5 using bipolar stimulation. In each subject, fmSTCs were measured at several probe levels on an apical, middle, and basal electrode using a fixed-level probe stimulus and variable-level maskers. Tuning curve slopes and bandwidths did not change significantly with probe level for electrodes located in the apical, middle, or basal region although a few subjects exhibited dramatic changes in tuning at the extremes of the probe level range. Average tuning curve slopes and bandwidths did not vary significantly across electrode regions. Spatial tuning curves were symmetrical and similar in width across the three electrode regions. However, several subjects demonstrated large changes in slope and/or bandwidth across the three electrode regions, indicating poorer tuning in localized regions of the array. Cochlear-implant users exhibited bandwidths that were approximately five times wider than normal-hearing acoustic listeners but were in the same range as acoustic listeners with moderate cochlear hearing loss. No significant correlations were found between spatial tuning parameters and speech recognition; although a weak relation was seen between middle electrode tuning and transmitted information for vowel second formant frequency.

Characterization of the electrically evoked compound action potential of the vestibular nerve

OBJECTIVE

We recorded intraoperative and postoperative electrically evoked compound action potentials (ECAPs) in rhesus monkeys implanted with a vestibular neurostimulator. The objectives were to correlate the generation of slow-phase nystagmus or eye twitches induced by electrical stimulation of the implanted semicircular canal with the presence or absence of the vestibular ECAP responses and to assess the effectiveness of ECAP monitoring during surgery to guide surgical insertion of electrode arrays into the canals.

DESIGN

Four rhesus monkeys (a total of 7 canals) were implanted with a vestibular neurostimulator modified from the Nucleus Freedom cochlear implant. ECAP recordings were obtained during surgery or at various intervals after surgery using the Neural Response Telemetry feature of the clinical Custom Sound EP software. Eye movements during electrical stimulation of individual canals were recorded with a scleral search coil system in the same animals.

RESULTS

Measurable vestibular ECAPs were observed intraoperatively or postoperatively in 3 implanted animals. Robust and sustained ECAPs were obtained in 3 monkeys at the test intervals of 0, 7, or greater than 100 days after implantation surgery. In all 3 animals, stimulation with electrical pulse trains produced measurable eye movements in a direction consistent with the vestibulo-ocular reflex from the implanted semicircular canal. In contrast, electrically evoked eye movements could not be measured in 3 of the 7 implanted canals, none of which produced distinct vestibular ECAPs. In 2 animals, ECAP waveforms were systematically monitored during surgery, and the procedure proved crucial to the success of vestibular implantation.

CONCLUSION

Vestibular ECAPs exhibit similar morphology and growth characteristics to cochlear ECAPs from human cochlear implant patients. The ECAP measure is well correlated with the functional activation of eye movements by electrical stimulation after implantation surgery. The intraoperative ECAP recording technique is an efficient tool to guide the placement of electrode array into the semicircular canals.

Effect of stimulus and recording parameters on spatial spread of excitation and masking patterns obtained with the electrically evoked compound action potential in cochlear implants

OBJECTIVES

Spread of excitation within the cochlea in response to electrical stimulation can be measured with the electrically evoked compound action potential (ECAP). Different spread of excitation measurement techniques have been reported in the literature. One method uses a fixed stimulus location while varying the recording electrode along the length of the implanted array. This results in a relatively coarse estimate of spatial spread (SS) along the cochlea. Another method uses a forward-masking paradigm to evaluate the relative overlap of stimulated neural populations between electrodes. Both the probe and recording electrodes are fixed in location while a masker stimulus is systematically applied across electrodes. This method, which yields a more precise estimate of spatial excitation patterns, is termed spatial masking (SM). Five experiments were conducted to examine potential effects of stimulus and/or recording parameters on SS and SM patterns. Experiment 1 examined whether SS patterns were systematically broader than SM patterns across electrodes and subjects. Experiments 2 and 3 evaluated the effects of stimulus level on SS and SM patterns, respectively, to determine whether increased stimulus level systematically resulted in broader patterns. Experiment 4 evaluated whether recording electrode location affected SM patterns, and Experiment 5 evaluated whether SM patterns varied significantly across repeated trials within a test session.

DESIGN

Data were collected for 27 ears in 26 adult and teenage subjects (N = 6 ears with Advanced Bionics CII, N = 8 ears with Advanced Bionics HiRes 90K, N = 10 ears with Nucleus 24R[CS], N = 3 ears with Nucleus 24RE[CA] Freedom). A standard forward-masking subtraction paradigm was used for all ECAP measures. For SS patterns, the masker and probe were fixed on the same electrode at the same level while the recording electrode varied across the remaining electrodes in the array. For SM patterns, the probe and recording locations were fixed while the masker location varied across all electrodes except the recording electrode.

RESULTS

In experiment 1, SS patterns were broader than SM patterns. Subjects with Advanced Bionics devices exhibited relatively broad patterns for both measures, whereas Nucleus subjects typically exhibited narrower SM functions relative to SS functions. In experiments 2 and 3, there was a significant effect of stimulus level on the spread of both SS and SM patterns in roughly one-third of measures in each experiment. In experiment 4, there was a significant effect of recording electrode location on the width/spread of SM patterns for only 11.5% of comparisons. In experiment 5, there were no significant differences in SM amplitudes across repeated trials for 94% of comparisons, which suggests that ECAP measures are highly robust within a test session.

CONCLUSIONS

Results showed that SS functions were generally broader than SM functions, which suggests that SS measures reflect volume conduction of the ECAP response along the length of the cochlea. Differences in the spread of SM functions across devices are likely due to differences in modiolar proximity between the respective electrode array designs. Stimulus level had a more significant effect on the spread of SM functions than recording electrode location. Finally, ECAP measures were shown to be highly stable across repeated measurements within a test session; however, repeatability was not assessed across sessions or over extended time intervals.

A relation between electrode discrimination and amplitude modulation detection by cochlear implant listeners

The objective of this study was to examine the relation between measures of spectral and temporal resolutions in cochlear implant listeners at a particular electrode location. The hypothesis was that a common underlying factor, such as the health of local groups of neurons, might partially determine patients' sensitivity to both spectral and temporal cues at specific tonotopic locations. Participants were adult cochlear implant listeners. A significant correlation was found between electrode discrimination measured at soft levels (20% and 30% of the dynamic range) and modulation sensitivity at those levels, for stimulation in bipolar mode and a 100 Hz modulation rate. Correlations between the two measures were weaker under monopolar stimulation, or when the modulation rate was 10 Hz. At a higher stimulation level (40% of the dynamic range), no significant correlations between these measures were observed. It is hypothesized that the more restricted excitation pattern at lower levels and/or with a narrower stimulation mode allows the measurement of locally driven sensitivity to spectral and temporal cues, particularly under more challenging listening conditions. Thus, psychophysical measures obtained under conditions that evoke a narrower excitation pattern may serve as a useful indicator of the functional health of local neural populations.