Across-site patterns of modulation detection in listeners with cochlear implants

Effect of reducing electrical stimulation rate on hearing performance of Nucleus® cochlear implant recipients

OBJECTIVE

To evaluate whether a 500 pulses per second per channel (pps/ch) rate would provide non-inferior hearing performance compared to the 900 pps/ch rate in the Advanced Combination Encoder (ACE™) sound coding strategy.

DESIGN

A repeated measures single-subject design was employed, wherein each subject served as their own control. All except one subject used 900 pps/ch at enrolment. After three weeks of using the alternative rate program, both programs were loaded into the sound processor for two more weeks of take-home use. Subjective performance, preference, words in quiet, sentences in babble, music quality, and fundamental frequency (F0) discrimination were assessed using a balanced design.

STUDY SAMPLE

Data from 18 subjects were analysed, with complete datasets available for 17 subjects.

RESULTS

Non-inferior performance on all clinical measures was shown for the lower rate program. Subjects' preference ratings were comparable for the programs, with 53% reporting no difference overall. When a preference was expressed, the 900 pps/ch condition was preferred more often.

CONCLUSION

Reducing the stimulation rate from 900 pps/ch to 500 pps/ch did not compromise the hearing outcomes evaluated in this study. A lower pulse rate in future cochlear implants could reduce power consumption, allowing for smaller batteries and processors.

Relationships Between the Auditory Nerve Sensitivity to Amplitude Modulation, Perceptual Amplitude Modulation Rate Discrimination Sensitivity, and Speech Perception Performance in Postlingually Deafened Adult Cochlear Implant Users

OBJECTIVE

This study assessed the relationships between the salience of amplitude modulation (AM) cues encoded at the auditory nerve (AN), perceptual sensitivity to changes in AM rate (i.e., AM rate discrimination threshold, AMRDT), and speech perception scores in postlingually deafened adult cochlear implant (CI) users.

DESIGN

Study participants were 18 postlingually deafened adults with Cochlear Nucleus devices, including five bilaterally implanted patients. For each of 23 implanted ears, neural encoding of AM cues at 20 Hz at the AN was evaluated at seven electrode locations across the electrode array using electrophysiological measures of the electrically evoked compound action potential (eCAP). The salience of AM neural encoding was quantified by the Modulated Response Amplitude Ratio (MRAR). Psychophysical measures of AMRDT for 20 Hz modulation were evaluated in 16 ears using a three-alternative, forced-choice procedure, targeting 79.4% correct on the psychometric function. AMRDT was measured at up to five electrode locations for each test ear, including the electrode pair that showed the largest difference in the MRAR. Consonant-Nucleus-Consonant (CNC) word scores presented in quiet and in speech-shaped noise at a signal to noise ratio (SNR) of +10 dB were measured in all 23 implanted ears. Simulation tests were used to assess the variations in correlation results when using the MRAR and AMRDT measured at only one electrode location in each participant to correlate with CNC word scores. Linear Mixed Models (LMMs) were used to evaluate the relationship between MRARs/AMRDTs measured at individual electrode locations and CNC word scores. Spearman Rank correlation tests were used to evaluate the strength of association between CNC word scores measured in quiet and in noise with (1) the variances in MRARs and AMRDTs, and (2) the averaged MRAR or AMRDT across multiple electrodes tested for each participant.

RESULTS

There was no association between the MRAR and AMRDT. Using the MRAR and AMRDT measured at only one, randomly selected electrode location to assess their associations with CNC word scores could lead to opposite conclusions. Both the results of LMMs and Spearman Rank correlation tests showed that CNC word scores measured in quiet or at 10 dB SNR were not significantly correlated with the MRAR or AMRDT. In addition, the results of Spearman Rank correlation tests showed that the variances in MRARs and AMRDTs were not significantly correlated with CNC word scores measured in quiet or in noise.

CONCLUSIONS

The difference in AN sensitivity to AM cues is not the primary factor accounting for the variation in AMRDTs measured at different stimulation sites within individual CI users. The AN sensitivity to AM per se may not be a crucial factor for CNC word perception in quiet or at 10 dB SNR in postlingually deafened adult CI users. Using electrophysiological or psychophysical results measured at only one electrode location to correlate with speech perception scores in CI users can lead to inaccurate, if not wrong, conclusions.

Relationship between electrode position and temporal modulation sensitivity in cochlear implant users: Are close electrodes always better?

Temporal modulation sensitivity has been studied extensively for cochlear implant (CI) users due to its strong correlation to speech recognition outcomes. Previous studies reported that temporal modulation detection thresholds (MDTs) vary across the tonotopic axis and attributed this variation to patchy neural survival. However, correlates of neural health identified in animal models depend on electrode position in humans. Nonetheless, the relationship between MDT and electrode location has not been explored. We tested 13 ears for the effect of distance on modulation sensitivity, specifically targeting the question of whether electrodes closer to the modiolus are universally beneficial. Participants in this study were postlingually deafened and users of Cochlear Nucleus CIs. The distance of each electrode from the medial wall (MW) of the cochlea and mid-modiolar axis (MMA) was measured from scans obtained using computerized tomography (CT) imaging. The distance measures were correlated with slopes of spatial tuning curves measured on selected electrodes to investigate if electrode position accounts, at least in part, for the width of neural excitation. In accordance with previous findings, electrode position explained 24% of the variance in slopes of the spatial tuning curves. All functioning electrodes were also measured for MDTs. Five ears showed a positive correlation between MDTs and at least one distance measure across the array; 6 ears showed negative correlations and the remaining two ears showed no relationship. The ears showing positive MDT-distance correlations, thus benefiting from electrodes being close to the neural elements, were those who performed better on the two speech recognition measures, i.e., speech reception thresholds (SRTs) and recognition of the AzBio sentences. These results could suggest that ears able to take advantage of the proximal placement of electrodes are likely to have better speech recognition outcomes. Previous histological studies of humans demonstrated that speech recognition is correlated with spiral ganglion cell counts. Alternatively, ears with good speech recognition outcomes may have good overall neural health, which is a precondition for close electrodes to produce spatially confined neural excitation patterns that facilitate modulation sensitivity. These findings suggest that the methods to reduce channel interaction, e.g., perimodiolar electrode array or current focusing, may only be beneficial for a subgroup of CI users. Additionally, it suggests that estimating neural survival preoperatively is important for choosing the most appropriate electrode array type (perimodiolar vs. lateral wall) for optimal implant function.

Estimating health of the implanted cochlea using psychophysical strength-duration functions and electrode configuration

It is generally believed that the efficacy of cochlear implants is partly dependent on the condition of the stimulated neural population. Cochlear pathology is likely to affect the manner in which neurons respond to electrical stimulation, potentially resulting in differences in perception of electrical stimuli across cochlear implant recipients and across the electrode array in individual cochlear implant users. Several psychophysical and electrophysiological measures have been shown to predict cochlear health in animals and were used to assess conditions near individual stimulation sites in humans. In this study, we examined the relationship between psychophysical strength-duration functions and spiral ganglion neuron density in two groups of guinea pigs with cochlear implants who had minimally-overlapping cochlear health profiles. One group was implanted in a hearing ear (N = 10) and the other group was deafened by cochlear perfusion of neomycin, inoculated with an adeno-associated viral vector with an Ntf3-gene insert (AAV.Ntf3) and implanted (N = 14). Psychophysically measured strength-duration functions for both monopolar and tripolar electrode configurations were then compared for the two treatment groups. Results were also compared to their histological outcomes. Overall, there were considerable differences between the two treatment groups in terms of their psychophysical performance as well as the relation between their functional performance and histological data. Animals in the neomycin-deafened, neurotrophin-treated, and implanted group (NNI) exhibited steeper strength-duration function slopes; slopes were positively correlated with SGN density (steeper slopes in animals that had higher SGN densities). In comparison, the implanted hearing (IH) group had shallower slopes and there was no relation between slopes and spiral ganglion density. Across all animals, slopes were negatively correlated with ensemble spontaneous activity levels (shallower slopes with higher ensemble spontaneous activity levels). We hypothesize that differences in strength-duration function slopes between the two treatment groups were related to the condition of the inner hair cells, which generate spontaneous activity that could affect the across-fiber synchrony and/or the size of the population of neural elements responding to electrical stimulation. In addition, it is likely that spiral ganglion neuron peripheral processes were present in the IH group, which could affect membrane properties of the stimulated neurons. Results suggest that the two treatment groups exhibited distinct patterns of variation in conditions near the stimulating electrodes that altered detection thresholds. Overall, the results of this study suggest a complex relationship between psychophysical detection thresholds for cochlear implant stimulation and nerve survival in the implanted cochlea. This relationship seems to depend on the characteristics of the electrical stimulus, the electrode configuration, and other biological features of the implanted cochlea such as the condition of the inner hair cells and the peripheral processes.

Neural auditory processing of parameterized speech envelopes

Speech perception depends highly on the neural processing of the speech envelope. Several auditory processing deficits are hypothesized to result in a reduction in fidelity of the neural representation of the speech envelope across the auditory pathway. Furthermore, this reduction in fidelity is associated with supra-threshold speech processing deficits. Investigating the mechanisms that affect the neural encoding of the speech envelope can be of great value to gain insight in the different mechanisms that account for this reduced neural representation, and to develop stimulation strategies for hearing prosthesis that aim to restore it. In this perspective, we discuss the importance of neural assessment of phase-locking to the speech envelope from an audiological view and introduce the Temporal Envelope Speech Tracking (TEMPEST) stimulus framework which enables the electrophysiological assessment of envelope processing across the auditory pathway in a systematic and standardized way. We postulate that this framework can be used to gain insight in the salience of speech-like temporal envelopes in the neural code and to evaluate the effectiveness of stimulation strategies that aim to restore temporal processing across the auditory pathway with auditory prostheses.

Neural Modulation Transmission Is a Marker for Speech Perception in Noise in Cochlear Implant Users

OBJECTIVES

Cochlear implants (CIs) restore functional hearing in persons with a severe hearing impairment. Despite being one of the most successful bionic prosthesis, performance with CI (in particular speech understanding in noise) varies considerably across its users. The ability of the auditory pathway to encode temporal envelope modulations (TEMs) and the effect of degenerative processes associated with hearing loss on TEM encoding is assumed to be one of the reasons underlying the large intersubject differences in CI performance. The objective of the present study was to investigate how TEM encoding of the stimulated neural ensembles of human CI recipients is related to speech perception in noise (SPIN).

DESIGN

We used electroencephalography as a noninvasive electrophysiological measure to assess TEM encoding in the auditory pathway of CI users by means of the 40-Hz electrically evoked auditory steady state response (EASSR). Nine CI users with a wide range of SPIN outcome were included in the present study. TEM encoding was assessed for each stimulation electrode of each subject and new metrics; the CI neural modulation transmission difference (CIMTD) and the CI neural modulation transmission index (CIMTI) were developed to quantify the amount of variability in TEM encoding across the stimulated neural ensembles of the CI electrode array.

RESULTS

EASSR patterns varied across the CI electrode array and subjects. We found a strong correlation (r = 0.89, p = 0.001) between the SPIN outcomes and the variability in EASSR amplitudes across the array as assessed with CIMTD/CIMTI.

CONCLUSIONS

The results of the present study show that the 40-Hz EASSR can be used to objectively assess the neural encoding of TEMs in human CI recipients. Overall reduced or largely variable TEM encoding of the neural ensembles across the electrode array, as quantified with the CIMTD/CIMTI, is highly correlated with speech perception in noise outcome with a CI.

Measuring implanted patient response to tone pips

Background

An electrical potential not previously reported—electrical cochlear response (ECR)—observed only in implanted patients is described. Its amplitude and growth slope are a measurement of the stimulation achieved by a tone pip on the auditory nerve. The stimulation and recording system constructed for this purpose, the features of this potential obtained in a group of 43 children, and its possible clinical use are described. The ECR is obtained by averaging the EEG epochs acquired each time the cochlear implant (CI) processes a tone pip of known frequency and intensity when the patient is sleeping and using the CI in everyday mode. The ECR is sensitive to tone pip intensity level, microphone sensitivity, sound processor gain, dynamic range of electrical current, and responsiveness to electrical current of the auditory nerve portion involved with the electrode under test. It allows individual evaluation of intracochlear electrodes by choosing, one at the time, the central frequency of the electrode as the test tone pip frequency, so the ECR measurement due to a variable intensity tone pip allows to establish the suitability of the dynamic range of the electrode current.

Results

There is a difference in ECR measurements when patients are grouped based on their auditory behavior. The ECR slope and amplitude for the Sensitive group is 0.2 μV/dB_HL and 10 μV at 50 dB_HL compared with 0.04 μV/dB_HL and 3 μV at 50dB_HL for the Inconsistent group. The clinical cases show that adjusting the dynamic range of current based on the ECR improved the patient’s auditory behavior.

Conclusions

ECR can be recorded regardless of the artifact due to the electromyographic activity of the patient and the functioning of the CI. Its amplitude and growth slope versus the intensity of the stimulus differs between electrodes. The relationship between minimum ECR detection intensity level and auditory threshold suggests the possibility of estimating patient auditory thresholds this way. ECR does not depend on the subject’s age, cooperation, or health status. It can be obtained at any time after implant surgery and the test procedure is the same regardless of device manufacturer.

Electrophysiological assessment of temporal envelope processing in cochlear implant users

Cochlear-implant (CI) users rely on temporal envelope modulations (TEMs) to understand speech, and clinical outcomes depend on the accuracy with which these TEMs are encoded by the electrically-stimulated neural ensembles. Non-invasive EEG measures of this encoding could help clinicians identify and disable electrodes that evoke poor neural responses so as to improve CI outcomes. However, recording EEG during CI stimulation reveals huge stimulation artifacts that are up to orders of magnitude larger than the neural response. Here we used a custom-built EEG system having an exceptionally high sample rate to accurately measure the artefact, which we then removed using linear interpolation so as to reveal the neural response during continuous electrical stimulation. In ten adult CI users, we measured the 40-Hz electrically evoked auditory steady-state response (eASSR) and electrically evoked auditory change complex (eACC) to amplitude-modulated 900-pulses-per-second pulse trains, stimulated in monopolar mode (i.e. the clinical default), and at different modulation depths. We successfully measured artifact-free 40-Hz eASSRs and eACCs. Moreover, we found that the 40-Hz eASSR, in contrast to the eACC, showed substantial responses even at shallow modulation depths. We argue that the 40-Hz eASSR is a clinically feasible objective measure to assess TEM encoding in CI users.

Temporal Modulation Detection Depends on Sharpness of Spatial Tuning

Prior research has shown that in electrical hearing, cochlear implant (CI) users' speech recognition performance is related in part to their ability to detect temporal modulation (i.e., modulation sensitivity). Previous studies have also shown better speech recognition when selectively stimulating sites with good modulation sensitivity rather than all stimulation sites. Site selection based on channel interaction measures, such as those using imaging or psychophysical estimates of spread of neural excitation, has also been shown to improve speech recognition. This led to the question of whether temporal modulation sensitivity and spatial selectivity of neural excitation are two related variables. In the present study, CI users' modulation sensitivity was compared for sites with relatively broad or narrow neural excitation patterns. This was achieved by measuring temporal modulation detection thresholds (MDTs) at stimulation sites that were significantly different in their sharpness of the psychophysical spatial tuning curves (PTCs) and measuring MDTs at the same sites in monopolar (MP) and bipolar (BP) stimulation modes. Nine postlingually deafened subjects implanted with Cochlear Nucleus® device took part in the study. Results showed a significant correlation between the sharpness of PTCs and MDTs, indicating that modulation detection benefits from a more spatially restricted neural activation pattern. There was a significant interaction between stimulation site and mode. That is, using BP stimulation only improved MDTs at stimulation sites with broad PTCs but had no effect or sometimes a detrimental effect on MDTs at stimulation sites with sharp PTCs. This interaction could suggest that a criterion number of nerve fibers is needed to achieve optimal temporal resolution, and, to achieve optimized speech recognition outcomes, individualized selection of site-specific current focusing strategies may be necessary. These results also suggest that the removal of stimulation sites measured with poor MDTs might improve both temporal and spectral resolution.

Rate modulation detection thresholds for cochlear implant users

The perception of temporal amplitude modulations is critical for speech understanding by cochlear implant (CI) users. The present study compared the ability of CI users to detect sinusoidal modulations of the electrical stimulation rate and current level, at different presentation levels (80% and 40% of the dynamic range) and modulation frequencies (10 and 100 Hz). Rate modulation detection thresholds (RMDTs) and amplitude modulation detection thresholds (AMDTs) were measured and compared to assess whether there was a perceptual advantage to either modulation method. Both RMDTs and AMDTs improved with increasing presentation level and decreasing modulation frequency. RMDTs and AMDTs were correlated, indicating that a common processing mechanism may underlie the perception of rate modulation and amplitude modulation, or that some subject-dependent factors affect both types of modulation detection.

Assessment of Spectral and Temporal Resolution in Cochlear Implant Users Using Psychoacoustic Discrimination and Speech Cue Categorization

OBJECTIVES

This study was conducted to measure auditory perception by cochlear implant users in the spectral and temporal domains, using tests of either categorization (using speech-based cues) or discrimination (using conventional psychoacoustic tests). The authors hypothesized that traditional nonlinguistic tests assessing spectral and temporal auditory resolution would correspond to speech-based measures assessing specific aspects of phonetic categorization assumed to depend on spectral and temporal auditory resolution. The authors further hypothesized that speech-based categorization performance would ultimately be a superior predictor of speech recognition performance, because of the fundamental nature of speech recognition as categorization.

DESIGN

Nineteen cochlear implant listeners and 10 listeners with normal hearing participated in a suite of tasks that included spectral ripple discrimination, temporal modulation detection, and syllable categorization, which was split into a spectral cue-based task (targeting the /ba/-/da/ contrast) and a timing cue-based task (targeting the /b/-/p/ and /d/-/t/ contrasts). Speech sounds were manipulated to contain specific spectral or temporal modulations (formant transitions or voice onset time, respectively) that could be categorized. Categorization responses were quantified using logistic regression to assess perceptual sensitivity to acoustic phonetic cues. Word recognition testing was also conducted for cochlear implant listeners.

RESULTS

Cochlear implant users were generally less successful at utilizing both spectral and temporal cues for categorization compared with listeners with normal hearing. For the cochlear implant listener group, spectral ripple discrimination was significantly correlated with the categorization of formant transitions; both were correlated with better word recognition. Temporal modulation detection using 100- and 10-Hz-modulated noise was not correlated either with the cochlear implant subjects' categorization of voice onset time or with word recognition. Word recognition was correlated more closely with categorization of the controlled speech cues than with performance on the psychophysical discrimination tasks.

CONCLUSIONS

When evaluating people with cochlear implants, controlled speech-based stimuli are feasible to use in tests of auditory cue categorization, to complement traditional measures of auditory discrimination. Stimuli based on specific speech cues correspond to counterpart nonlinguistic measures of discrimination, but potentially show better correspondence with speech perception more generally. The ubiquity of the spectral (formant transition) and temporal (voice onset time) stimulus dimensions across languages highlights the potential to use this testing approach even in cases where English is not the native language.

The effect of presentation level and stimulation rate on speech perception and modulation detection for cochlear implant users

In order to improve speech understanding for cochlear implant users, it is important to maximize the transmission of temporal information. The combined effects of stimulation rate and presentation level on temporal information transfer and speech understanding remain unclear. The present study systematically varied presentation level (60, 50, and 40 dBA) and stimulation rate [500 and 2400 pulses per second per electrode (pps)] in order to observe how the effect of rate on speech understanding changes for different presentation levels. Speech recognition in quiet and noise, and acoustic amplitude modulation detection thresholds (AMDTs) were measured with acoustic stimuli presented to speech processors via direct audio input (DAI). With the 500 pps processor, results showed significantly better performance for consonant-vowel nucleus-consonant words in quiet, and a reduced effect of noise on sentence recognition. However, no rate or level effect was found for AMDTs, perhaps partly because of amplitude compression in the sound processor. AMDTs were found to be strongly correlated with the effect of noise on sentence perception at low levels. These results indicate that AMDTs, at least when measured with the CP910 Freedom speech processor via DAI, explain between-subject variance of speech understanding, but do not explain within-subject variance for different rates and levels.

Recovery from forward masking in cochlear implant listeners depends on stimulation mode, level, and electrode location

Psychophysical recovery from forward masking was measured in adult cochlear implant users of Cochlear^TM and Advanced Bionics^TM devices, in monopolar and in focused (bipolar and tripolar) stimulation modes, at four electrode sites across the arrays, and at two levels (loudness balanced across modes and electrodes). Results indicated a steeper psychophysical recovery from forward masking in monopolar over bipolar and tripolar modes, modified by differential effects of electrode and level. The interactions between factors varied somewhat across devices. It is speculated that psychophysical recovery from forward masking may be driven by different populations of neurons in the different modes, with a broader stimulation pattern resulting in a greater likelihood of response by healthier and/or faster-recovering neurons within the stimulated population. If a more rapid recovery from prior stimulation reflects responses of neurons not necessarily close to the activating site, the spectral pattern of the incoming acoustic signal may be distorted. These results have implications for speech processor implementations using different degrees of focusing of the electric field. The primary differences in the shape of the recovery function were observed in the earlier portion (between 2 and 45 ms) of recovery, which is significant in terms of the speech envelope.

Effect of current focusing on the sensitivity of inferior colliculus neurons to amplitude-modulated stimulation

In multichannel cochlear implants (CIs), current is delivered to specific electrodes along the cochlea in the form of amplitude-modulated pulse trains, to convey temporal and spectral cues. Our previous studies have shown that focused multipolar (FMP) and tripolar (TP) stimulation produce more restricted neural activation and reduced channel interactions in the inferior colliculus (IC) compared with traditional monopolar (MP) stimulation, suggesting that focusing of stimulation could produce better transmission of spectral information. The present study explored the capability of IC neurons to detect modulated CI stimulation with FMP and TP stimulation compared with MP stimulation. The study examined multiunit responses of IC neurons in acutely deafened guinea pigs by systematically varying the stimulation configuration, modulation depth, and stimulation level. Stimuli were sinusoidal amplitude-modulated pulse trains (carrier rate of 120 pulses/s). Modulation sensitivity was quantified by measuring modulation detection thresholds (MDTs), defined as the lowest modulation depth required to differentiate the response of a modulated stimulus from an unmodulated one. Whereas MP stimulation showed significantly lower MDTs than FMP and TP stimulation (P values <0.05) at stimulation ≤2 dB above threshold, all stimulation configurations were found to have similar modulation sensitivities at 4 dB above threshold. There was no difference found in modulation sensitivity between FMP and TP stimulation. The present study demonstrates that current focusing techniques such as FMP and TP can adequately convey amplitude modulation and are comparable to MP stimulation, especially at higher stimulation levels, although there may be some trade-off between spectral and temporal fidelity with current focusing stimulation.

Importance of cochlear health for implant function

Amazing progress has been made in providing useful hearing to hearing-impaired individuals using cochlear implants, but challenges remain. One such challenge is understanding the effects of partial degeneration of the auditory nerve, the target of cochlear implant stimulation. Here we review studies from our human and animal laboratories aimed at characterizing the health of the implanted cochlea and the auditory nerve. We use the data on cochlear and neural health to guide rehabilitation strategies. The data also motivate the development of tissue-engineering procedures to preserve or build a healthy cochlea and improve performance obtained by cochlear implant recipients or eventually replace the need for a cochlear implant. This article is part of a Special Issue entitled .

Relationship between multipulse integration and speech recognition with cochlear implants

Comparisons of performance with cochlear implants and postmortem conditions in the cochlea in humans have shown mixed results. The limitations in those studies favor the use of within-subject designs and non-invasive measures to estimate cochlear conditions. One non-invasive correlate of cochlear health is multipulse integration, established in an animal model. The present study used this measure to relate neural health in human cochlear implant users to their speech recognition performance. The multipulse-integration slopes were derived based on psychophysical detection thresholds measured for two pulse rates (80 and 640 pulses per second). A within-subject design was used in eight subjects with bilateral implants where the direction and magnitude of ear differences in the multipulse-integration slopes were compared with those of the speech-recognition results. The speech measures included speech reception threshold for sentences and phoneme recognition in noise. The magnitude of ear difference in the integration slopes was significantly correlated with the magnitude of ear difference in speech reception thresholds, consonant recognition in noise, and transmission of place of articulation of consonants. These results suggest that multipulse integration predicts speech recognition in noise and perception of features that use dynamic spectral cues.

Effects of site-specific level adjustments on speech recognition with cochlear implants

OBJECTIVES

Modulation detection thresholds (MDTs) vary across stimulation sites in a cochlear implant (CI) electrode array in a manner that is subject and ear specific. Previous studies have demonstrated that speech recognition with a CI can be improved by site-selection strategies, where selected stimulation sites with poor modulation sensitivity are removed from a subject's processor MAP. Limitations of site-selection strategies are that they can compromise spectral resolution and distort frequency-place mapping because the frequencies assigned to the removed sites are usually reallocated to other sites, and site bandwidths are broadened. The objective of the present study was to test an alternative approach for rehabilitation that aimed at improving the across-site mean MDTs by adjusting stimulation parameters at the poorly performing sites. On the basis of previous findings that modulation detection contributes to speech recognition and improves significantly with stimulus level, the authors hypothesized that modulation sensitivity at the poor sites could be improved by artificially increasing stimulation levels at those sites in the speech processor, which then would lead to improved speech recognition.

DESIGN

Nine postlingually deafened ears implanted with Nucleus CIs were evaluated for MDTs, absolute-detection threshold levels (T levels), and the maximum loudness levels (C levels) on each of the available stimulation sites. For each ear, the minimum stimulation level settings in the speech-processor MAP were raised by 5%, and alternatively by 10%, of the dynamic range (DR) from true thresholds on five stimulation sites with the poorest MDTs. For comparison, a 5% level raise was applied globally to all stimulation sites. The C levels were fixed during these level manipulations. MDTs at the five poorest stimulation sites were compared at 20% DR before and after the level adjustments. Speech-reception thresholds (SRTs), that is, signal to noise ratios required for 50% correct speech recognition, were evaluated for these MAPs using CUNY sentences. The site-specific level-adjusted MAPs were compared with the global-level-adjusted MAP and the MAP without level adjustment. The effects on speech recognition of adjusting the minimal stimulation level settings on the five poorest stimulation sites were also compared with effects of removing these sites from the speech-processor MAP.

RESULTS

The 5% level increase on the five electrodes with the worst MDTs resulted in an improvement in the group mean SRT of 2.36 dB SNR relative to the MAP without level adjustment. The magnitude of level increase that resulted in the greatest SRT improvement for individuals varied across ears. MDTs measured at 20% DR significantly improved on the poor sites after the level adjustment that resulted in the best SRT for that ear was applied. Increasing the minimal stimulation levels on all stimulation sites or removing sites selected for rehabilitation, the parsimonious approaches, did not improve SRTs.

CONCLUSIONS

The site-specific adjustments of the T level settings improved modulation sensitivity at low levels and significantly improved subjects' SRTs. Thus, this site-rehabilitation strategy was an effective alternative to site-selection strategies for improving speech recognition in CI users.

Using temporal modulation sensitivity to select stimulation sites for processor MAPs in cochlear implant listeners

Previous studies in our laboratory showed that temporal acuity as assessed by modulation detection thresholds (MDTs) varied across activation sites and that this site-to-site variability was subject specific. Using two 10-channel MAPs, the previous experiments showed that processor MAPs that had better across-site mean (ASM) MDTs yielded better speech recognition than MAPs with poorer ASM MDTs tested in the same subject. The current study extends our earlier work on developing more optimal-fitting strategies to test the feasibility of using a site-selection approach in the clinical domain. This study examined the hypothesis that revising the clinical speech processor MAP for cochlear implant (CI) recipients by turning off selected sites that have poorer temporal acuity and reallocating frequencies to the remaining electrodes would lead to improved speech recognition. Twelve CI recipients participated in the experiments. We found that site selection procedure based on MDTs in the presence of a masker resulted in improved performance on consonant recognition and recognition of sentences in noise. In contrast, vowel recognition was poorer with the experimental MAP than with the clinical MAP, possibly due to reduced spectral resolution when sites were removed from the experimental MAP. Overall, these results suggest a promising path for improving recipient outcomes using personalized processor-fitting strategies based on a psychophysical measure of temporal acuity.

Psychophysically based site selection coupled with dichotic stimulation improves speech recognition in noise with bilateral cochlear implants

The ability to perceive important features of electrical stimulation varies across stimulation sites within a multichannel implant. The aim of this study was to optimize speech processor MAPs for bilateral implant users by identifying and removing sites with poor psychophysical performance. The psychophysical assessment involved amplitude-modulation detection with and without a masker, and a channel interaction measure quantified as the elevation in modulation detection thresholds in the presence of the masker. Three experimental MAPs were created on an individual-subject basis using data from one of the three psychophysical measures. These experimental MAPs improved the mean psychophysical acuity across the electrode array and provided additional advantages such as increasing spatial separations between electrodes and/or preserving frequency resolution. All 8 subjects showed improved speech recognition in noise with one or more experimental MAPs over their everyday-use clinical MAP. For most subjects, phoneme and sentence recognition in noise were significantly improved by a dichotic experimental MAP that provided better mean psychophysical acuity, a balanced distribution of selected stimulation sites, and preserved frequency resolution. The site-selection strategies serve as useful tools for evaluating the importance of psychophysical acuities needed for good speech recognition in implant users.

Across-site patterns of modulation detection: relation to speech recognition

The aim of this study was to identify across-site patterns of modulation detection thresholds (MDTs) in subjects with cochlear implants and to determine if removal of sites with the poorest MDTs from speech processor programs would result in improved speech recognition. Five hundred millisecond trains of symmetric-biphasic pulses were modulated sinusoidally at 10 Hz and presented at a rate of 900 pps using monopolar stimulation. Subjects were asked to discriminate a modulated pulse train from an unmodulated pulse train for all electrodes in quiet and in the presence of an interleaved unmodulated masker presented on the adjacent site. Across-site patterns of masked MDTs were then used to construct two 10-channel MAPs such that one MAP consisted of sites with the best masked MDTs and the other MAP consisted of sites with the worst masked MDTs. Subjects' speech recognition skills were compared when they used these two different MAPs. Results showed that MDTs were variable across sites and were elevated in the presence of a masker by various amounts across sites. Better speech recognition was observed when the processor MAP consisted of sites with best masked MDTs, suggesting that temporal modulation sensitivity has important contributions to speech recognition with a cochlear implant.

Temporal modulation transfer functions in cochlear implantees using a method that limits overall loudness cues

Temporal modulation transfer functions (TMTFs) were measured for six users of cochlear implants, using different carrier rates and levels. Unlike most previous studies investigating modulation detection, the experimental design limited potential effects of overall loudness cues. Psychometric functions (percent correct discrimination of modulated from unmodulated stimuli versus modulation depth) were obtained. For each modulation depth, each modulated stimulus was loudness balanced to the unmodulated reference stimulus, and level jitter was applied in the discrimination task. The loudness-balance data showed that the modulated stimuli were louder than the unmodulated reference stimuli with the same average current, thus confirming the need to limit loudness cues when measuring modulation detection. TMTFs measured in this way had a low-pass characteristic, with a cut-off frequency (at comfortably loud levels) similar to that for normal-hearing listeners. A reduction in level caused degradation in modulation detection efficiency and a lower-cut-off frequency (i.e. poorer temporal resolution). An increase in carrier rate also led to a degradation in modulation detection efficiency, but only at lower levels or higher modulation frequencies. When detection thresholds were expressed as a proportion of dynamic range, there was no effect of carrier rate for the lowest modulation frequency (50 Hz) at either level.

Detection of pulse trains in the electrically stimulated cochlea: effects of cochlear health

Perception of electrical stimuli varies widely across users of cochlear implants and across stimulation sites in individual users. It is commonly assumed that the ability of subjects to detect and discriminate electrical signals is dependent, in part, on conditions in the implanted cochlea, but evidence supporting that hypothesis is sparse. The objective of this study was to define specific relationships between the survival of tissues near the implanted electrodes and the functional responses to electrical stimulation of those electrodes. Psychophysical and neurophysiological procedures were used to assess stimulus detection as a function of pulse rate under the various degrees of cochlear pathology. Cochlear morphology, assessed post-mortem, ranged from near-normal numbers of hair cells, peripheral processes and spiral ganglion cells, to complete absence of hair cells and peripheral processes and small numbers of surviving spiral ganglion cells. The psychophysical and neurophysiological studies indicated that slopes and levels of the threshold versus pulse rate functions reflected multipulse integration throughout the 200 ms pulse train with an additional contribution of interactions between adjacent pulses at high pulse rates. The amount of multipulse integration was correlated with the health of the implanted cochlea with implications for perception of more complex prosthetic stimuli.

Cochlear infrastructure for electrical hearing

Although the cochlear implant is already the world's most successful neural prosthesis, opportunities for further improvement abound. Promising areas of current research include work on improving the biological infrastructure in the implanted cochlea to optimize reception of cochlear implant stimulation and on designing the pattern of electrical stimulation to take maximal advantage of conditions in the implanted cochlea. In this review we summarize what is currently known about conditions in the cochlea of deaf, implanted humans and then review recent work from our animal laboratory investigating the effects of preserving or reinnervating tissues on psychophysical and electrophysiological measures of implant function. Additionally we review work from our human laboratory on optimizing the pattern of electrical stimulation to better utilize strengths in the cochlear infrastructure. Histological studies of human temporal bones from implant users and from people who would have been candidates for implants show a range of pathologic conditions including spiral ganglion cell counts ranging from approximately 2% to 92% of normal and partial hair cell survival in some cases. To duplicate these conditions in a guinea pig model, we use a variety of deafening and implantation procedures as well as post-deafening therapies designed to protect neurons and/or regenerate neurites. Across populations of human patients, relationships between nerve survival and functional measures such as speech have been difficult to demonstrate, possibly due to the numerous subject variables that can affect implant function and the elapsed time between functional measures and postmortem histology. However, psychophysical studies across stimulation sites within individual human subjects suggest that biological conditions near the implanted electrodes contribute significantly to implant function, and this is supported by studies in animal models comparing histological findings to psychophysical and electrophysiological data. Results of these studies support the efforts to improve the biological infrastructure in the implanted ear and guide strategies which optimize stimulation patterns to match patient-specific conditions in the cochlea.

Detection and rate discrimination of amplitude modulation in electrical hearing

Three experiments were designed to examine temporal envelope processing by cochlear implant (CI) listeners. In experiment 1, the hypothesis that listeners' modulation sensitivity would in part determine their ability to discriminate between temporal modulation rates was examined. Temporal modulation transfer functions (TMTFs) obtained in an amplitude modulation detection (AMD) task were compared to threshold functions obtained in an amplitude modulation rate discrimination (AMRD) task. Statistically significant nonlinear correlations were observed between the two measures. In experiment 2, results of loudness-balancing showed small increases in the loudness of modulated over unmodulated stimuli beyond a modulation depth of 16%. Results of experiment 3 indicated small but statistically significant effects of level-roving on the overall gain of the TMTF, but no impact of level-roving on the average shape of the TMTF across subjects. This suggested that level-roving simply increased the task difficulty for most listeners, but did not indicate increased use of intensity cues under more challenging conditions. Data obtained with one subject, however, suggested that the most sensitive listeners may derive some benefit from intensity cues in these tasks. Overall, results indicated that intensity cues did not play an important role in temporal envelope processing by the average CI listener.

Acoustic temporal modulation detection and speech perception in cochlear implant listeners

The goals of the present study were to measure acoustic temporal modulation transfer functions (TMTFs) in cochlear implant listeners and examine the relationship between modulation detection and speech recognition abilities. The effects of automatic gain control, presentation level and number of channels on modulation detection thresholds (MDTs) were examined using the listeners' clinical sound processor. The general form of the TMTF was low-pass, consistent with previous studies. The operation of automatic gain control had no effect on MDTs when the stimuli were presented at 65 dBA. MDTs were not dependent on the presentation levels (ranging from 50 to 75 dBA) nor on the number of channels. Significant correlations were found between MDTs and speech recognition scores. The rates of decay of the TMTFs were predictive of speech recognition abilities. Spectral-ripple discrimination was evaluated to examine the relationship between temporal and spectral envelope sensitivities. No correlations were found between the two measures, and 56% of the variance in speech recognition was predicted jointly by the two tasks. The present study suggests that temporal modulation detection measured with the sound processor can serve as a useful measure of the ability of clinical sound processing strategies to deliver clinically pertinent temporal information.

Effects of electrode configuration on cochlear implant modulation detection thresholds

Cochlear implant function, as assessed by psychophysical measures, varies from one stimulation site to another within a patient's cochlea. This suggests that patient performance might be improved by selection of the best-functioning sites for the processor map. In evaluating stimulation sites for such a strategy, electrode configuration is an important variable. Variation across stimulation sites in loudness-related measures (detection thresholds and maximum comfortable loudness levels), is much larger for stimulation with bipolar electrode configurations than with monopolar configurations. The current study found that, in contrast to the loudness-related measures, magnitudes of across-site means and the across-site variances of modulation detection thresholds were not dependent on electrode configuration, suggesting that the mechanisms underlying variation in these various psychophysical measures are not all the same. The data presented here suggest that bipolar and monopolar electrode configurations are equally effective in identifying good and poor stimulation sites for modulation detection but that the across-site patterns of modulation detection thresholds are not the same for the two configurations. Therefore, it is recommended to test all stimulation sites using the patient's clinically assigned electrode configuration when performing psychophysical evaluation of a patient's modulation detection acuity to select sites for the processor map.

Relationship between gap detection thresholds and loudness in cochlear-implant users

Gap detection threshold (GDT) is a commonly used measure of temporal acuity in cochlear-implant (CI) recipients. This measure, like other measures of temporal acuity, shows considerable variation across subjects and also varies across stimulation sites within subjects. The aims of this study were (1) to determine whether across-site variation in GDTs would be reduced or maintained with increased stimulation levels; (2) to determine whether across-site variation in GDTs at low stimulation levels was related to differences in loudness percepts at those same levels; and (3) to determine whether matching loudness levels could reduce across-site differences in GDTs. Thresholds and maximum comfortable loudness levels were measured in postlingually deaf adults using all available sites in their electrode arrays. All sites were then surveyed at 30% of the dynamic range (DR) to examine across-site variation. Two sites with the largest difference in GDTs were then selected and for those two sites GDTs were measured at multiple levels of the DR (10%, 30%, 50%, 70%, and 90%). Stimuli consisted of 500 ms trains of symmetric-biphasic pulses, 40 μs/phase, presented at a rate of 1000 pps using a monopolar (MP1+2) electrode configuration. To examine perceptual differences in loudness, the selected sites were loudness-matched at the same levels of the DR. Variations in GDTs and loudness patterns were observed across stimulation sites and across subjects. Variations in GDTs across sites tended to decrease with increasing stimulation levels. For the majority of the subjects, stimuli at a given level in %DR were perceived louder at sites with better GDTs than those presented at the same level in %DR at sites with poorer GDTs. These results suggest that loudness is a contributing factor to across-site variation in GDTs and that CI fittings based on more detailed loudness matching could reduce across-site variation and improve perceptual acuity.

Probing the electrode-neuron interface with focused cochlear implant stimulation

Cochlear implants are highly successful neural prostheses for persons with severe or profound hearing loss who gain little benefit from hearing aid amplification. Although implants are capable of providing important spectral and temporal cues for speech perception, performance on speech tests is variable across listeners. Psychophysical measures obtained from individual implant subjects can also be highly variable across implant channels. This review discusses evidence that such variability reflects deviations in the electrode-neuron interface, which refers to an implant channel's ability to effectively stimulate the auditory nerve. It is proposed that focused electrical stimulation is ideally suited to assess channel-to-channel irregularities in the electrode-neuron interface. In implant listeners, it is demonstrated that channels with relatively high thresholds, as measured with the tripolar configuration, exhibit broader psychophysical tuning curves and smaller dynamic ranges than channels with relatively low thresholds. Broader tuning implies that frequency-specific information intended for one population of neurons in the cochlea may activate more distant neurons, and a compressed dynamic range could make it more difficult to resolve intensity-based information, particularly in the presence of competing noise. Degradation of both types of cues would negatively affect speech perception.

Effects of stimulation rate on modulation detection and speech recognition by cochlear implant users

OBJECTIVE

This study investigated the effect of low to moderate electrical stimulation rates (275, 350, 500 and 900 pps/ch) on modulation detection ability of cochlear implant subjects, and the relationship between modulation detection and speech perception as a function of rate.

DESIGN

A repeated ABCD experimental design for the four rate conditions was employed. A sinusoidally amplitude modulated acoustic signal was presented to the audio input of a research processor. Stimuli were presented at an acoustic level that produced electrical levels close to the subjects' most comfortable level (MCL) of stimulation and at an acoustic level 20 dB below this.

STUDY SAMPLE

Ten postlingually deaf adult users of the Nucleus CI24 cochlear implant participated.

RESULTS

Acoustic modulation detection thresholds (MDTs), averaged across the subject group, were significantly better for rates of 500 pps/ch compared to the other rates examined for stimuli presented at MCL. In addition, there was a significant relation between speech perception in noise and acoustic MDTs at MCL.

CONCLUSIONS

The benefits obtained in speech perception and modulation detection as a function of rate were attributed to an increased electrical dynamic range as a function of stimulation rate, at least for rates up to 500 pps/ch.

Encoding and decoding amplitude-modulated cochlear implant stimuli--a point process analysis

Cochlear implant speech processors stimulate the auditory nerve by delivering amplitude-modulated electrical pulse trains to intracochlear electrodes. Studying how auditory nerve cells encode modulation information is of fundamental importance, therefore, to understanding cochlear implant function and improving speech perception in cochlear implant users. In this paper, we analyze simulated responses of the auditory nerve to amplitude-modulated cochlear implant stimuli using a point process model. First, we quantify the information encoded in the spike trains by testing an ideal observer's ability to detect amplitude modulation in a two-alternative forced-choice task. We vary the amount of information available to the observer to probe how spike timing and averaged firing rate encode modulation. Second, we construct a neural decoding method that predicts several qualitative trends observed in psychophysical tests of amplitude modulation detection in cochlear implant listeners. We find that modulation information is primarily available in the sequence of spike times. The performance of an ideal observer, however, is inconsistent with observed trends in psychophysical data. Using a neural decoding method that jitters spike times to degrade its temporal resolution and then computes a common measure of phase locking from spike trains of a heterogeneous population of model nerve cells, we predict the correct qualitative dependence of modulation detection thresholds on modulation frequency and stimulus level. The decoder does not predict the observed loss of modulation sensitivity at high carrier pulse rates, but this framework can be applied to future models that better represent auditory nerve responses to high carrier pulse rate stimuli. The supplemental material of this article contains the article's data in an active, re-usable format.

Effects of stimulus duration on amplitude modulation processing with cochlear implants

This study investigated temporal integration processes underlying cochlear implant (CI) users' amplitude modulation processing. Thresholds for modulation detection (AMDTs) and modulation frequency discrimination (AMFDTs) were measured for 50-, 100-, and 200-Hz modulation frequencies with stimulus durations from 50 to 400 ms in eight adult CI users. The results showed significant interactions between modulation frequency and stimulus duration for AMDTs and AMFDTs. The data suggest that temporal integration limits CI users' sensitivity to low temporal pitch over short durations, and that temporal integration over longer durations may not enhance CI users' sensitivity to high temporal pitch.

Speech recognition and temporal amplitude modulation processing by Mandarin-speaking cochlear implant users

OBJECTIVES

Fundamental frequency (F0) information is important to Chinese tone and speech recognition. Cochlear implant (CI) speech processors typically provide limited F0 information via temporal envelopes delivered to stimulating electrodes. Previous studies have shown that English-speaking CI users' speech performance is correlated with amplitude modulation detection thresholds (AMDTs). The present study investigated whether Chinese-speaking CI users' speech performance (especially tone recognition) is correlated with temporal processing capabilities.

DESIGN

Chinese tone, vowel, consonant, and sentence recognition were measured in 10 native Mandarin-speaking CI users via clinically assigned speech processors. AMDTs were measured in the same subjects for 20- and 100-Hz amplitude modulated (AM) stimuli presented to a middle electrode at five stimulation levels that spanned the dynamic range. To further investigate the CI users' sensitivity to temporal envelope cues, AM frequency discrimination thresholds (AMFDTs) were measured for two standard AM frequencies (50 and 100 Hz), presented to the same middle electrode at 30% and 70% dynamic range with a fixed modulation depth (50%).

RESULTS

Results showed that AMDTs significantly improved with increasing stimulation level and that individual subjects exhibited markedly different AMDT functions. AMFDTs also improved with increasing stimulation level and were better with the 100-Hz standard AM frequency than with the 50-Hz standard AM frequency. Statistical analyses revealed that both mean AMDTs (averaged for 20- or 100-Hz AM across all stimulation levels) and mean AMFDTs (averaged for the 50-Hz standard AM frequency across both stimulation levels) were significantly correlated with tone, consonant, and sentence recognition scores, but not with vowel recognition scores. Mean AMDTs were also significantly correlated with mean AMFDTs.

CONCLUSIONS

These preliminary results, obtained from a limited number of subjects, demonstrate the importance of temporal processing to CI speech recognition. The results further suggest that CI users' Chinese tone and speech recognition may be improved by enhancing temporal envelope cues delivered by speech processing algorithms.