The perception of temporal modulations by cochlear implant patients

A computational modelling framework for assessing information transmission with cochlear implants

Computational models are useful tools to investigate scientific questions that would be complicated to address using an experimental approach. In the context of cochlear-implants (CIs), being able to simulate the neural activity evoked by these devices could help in understanding their limitations to provide natural hearing. Here, we present a computational modelling framework to quantify the transmission of information from sound to spikes in the auditory nerve of a CI user. The framework includes a model to simulate the electrical current waveform sensed by each auditory nerve fiber (electrode-neuron interface), followed by a model to simulate the timing at which a nerve fiber spikes in response to a current waveform (auditory nerve fiber model). Information theory is then applied to determine the amount of information transmitted from a suitable reference signal (e.g., the acoustic stimulus) to a simulated population of auditory nerve fibers. As a use case example, the framework is applied to simulate published data on modulation detection by CI users obtained using direct stimulation via a single electrode. Current spread as well as the number of fibers were varied independently to illustrate the framework capabilities. Simulations reasonably matched experimental data and suggested that the encoded modulation information is proportional to the total neural response. They also suggested that amplitude modulation is well encoded in the auditory nerve for modulation rates up to 1000 Hz and that the variability in modulation sensitivity across CI users is partly because different CI users use different references for detecting modulation.

Listening Effort in Prelingual Cochlear Implant Recipients: Effects of Spectral and Temporal Auditory Processing and Contralateral Acoustic Hearing

OBJECTIVE

Considering the impact of listening effort (LE) on auditory perception, attention, and memory, it is a significant aspect in the daily hearing experiences of cochlear implant (CI) recipients. Reduced spectral and temporal information on an acoustic signal can make listening more difficult; as a result, it is important to understand the relationship between LE and spectral and temporal auditory processing capacities in CI receivers.

STUDY DESIGN, SETTING, AND PATIENTS

This study used spectral ripple discrimination and temporal modulation transfer function to evaluate 20 prelingually deafened and early implanted CI recipients. The speech perception in noise test (primary) and the digit recall task (DRT-secondary) were used to assess LE using the dual-task paradigm. To assess the effects of acoustic hearing, contralateral acoustic hearing thresholds between 125 Hz and 8 kHz with a hearing aid were also acquired. To examine the relationship between the research variables, correlation coefficients were generated. Furthermore, the Mann-Whitney U test was used to compare unilateral and bimodal users.

RESULTS

There was a statistically significant correlation between LE and spectral ripple discrimination (r = 0.56; p = 0.011), 125 Hz (r = 0.51; p = 0.020), 250 Hz (r = 0.48; p = 0.030), 500 Hz (r = 0.45; p = 0.045), 1,000 Hz (r = 0.51; p = 0.023), 2000 Hz (r = 0.48; p = 0.031), and 4,000 Hz (r = 0.48; p = 0.031), whereas no statistically significant correlations were observed between temporal modulation transfer function in four frequencies and LE. There was no statistically significant difference between unilateral and bimodal CI recipients ( p > 0.05).

CONCLUSION

As a result of the improved signal-to-noise ratio in the auditory environment, CI users with better spectral resolutions and acoustic hearing have a reduced LE. On the other hand, temporal auditory processing, as measured by temporal modulation detection, does not contribute to the LE.

Asymmetric temporal envelope sensitivity: Within- and across-ear envelope comparisons in listeners with bilateral cochlear implants

For listeners with bilateral cochlear implants (BiCIs), patient-specific differences in the interface between cochlear implant (CI) electrodes and the auditory nerve can lead to degraded temporal envelope information, compromising the ability to distinguish between targets of interest and background noise. It is unclear how comparisons of degraded temporal envelope information across spectral channels (i.e., electrodes) affect the ability to detect differences in the temporal envelope, specifically amplitude modulation (AM) rate. In this study, two pulse trains were presented simultaneously via pairs of electrodes in different places of stimulation, within and/or across ears, with identical or differing AM rates. Results from 11 adults with BiCIs indicated that sensitivity to differences in AM rate was greatest when stimuli were paired between different places of stimulation in the same ear. Sensitivity from pairs of electrodes was predicted by the poorer electrode in the pair or the difference in fidelity between both electrodes in the pair. These findings suggest that electrodes yielding poorer temporal fidelity act as a bottleneck to comparisons of temporal information across frequency and ears, limiting access to the cues used to segregate sounds, which has important implications for device programming and optimizing patient outcomes with CIs.

Modulation Depth Discrimination by Cochlear Implant Users

Cochlear implants (CIs) convey the amplitude envelope of speech by modulating high-rate pulse trains. However, not all of the envelope may be necessary to perceive amplitude modulations (AMs); the effective envelope depth may be limited by forward and backward masking from the envelope peaks. Three experiments used modulated pulse trains to measure which portions of the envelope can be effectively processed by CI users as a function of AM frequency. Experiment 1 used a three-interval forced-choice task to test the ability of CI users to discriminate less-modulated pulse trains from a fully modulated standard, without controlling for loudness. The stimuli in experiment 2 were identical, but a two-interval task was used in which participants were required to choose the less-modulated interval, ignoring loudness. Catch trials, in which judgements based on level or modulation depth would give opposing answers, were included. Experiment 3 employed novel stimuli whose modulation envelope could be modified below a variable point in the dynamic range, without changing the loudness of the stimulus. Overall, results showed that substantial portions of the envelope are not accurately encoded by CI users. In experiment 1, where loudness cues were available, participants on average were insensitive to changes in the bottom 30% of their dynamic range. In experiment 2, where loudness was controlled, participants appeared insensitive to changes in the bottom 50% of the dynamic range. In experiment 3, participants were insensitive to changes in the bottom 80% of the dynamic range. We discuss potential reasons for this insensitivity and implications for CI speech-processing strategies.

Deficient Recurrent Cortical Processing in Congenital Deafness

The influence of sensory experience on cortical feedforward and feedback interactions has rarely been studied in the auditory cortex. Previous work has documented a dystrophic effect of deafness in deep cortical layers, and a reduction of interareal couplings between primary and secondary auditory areas in congenital deafness which was particularly pronounced in the top-down direction (from the secondary to the primary area). In the present study, we directly quantified the functional interaction between superficial (supragranular, I to III) and deep (infragranular, V and VI) layers of feline’s primary auditory cortex A1, and also between superficial/deep layers of A1 and a secondary auditory cortex, namely the posterior auditory field (PAF). We compared adult hearing cats under acoustic stimulation and cochlear implant (CI) stimulation to adult congenitally deaf cats (CDC) under CI stimulation. Neuronal activity was recorded from auditory fields A1 and PAF simultaneously with two NeuroNexus electrode arrays. We quantified the spike field coherence (i.e., the statistical dependence of spike trains at one electrode with local field potentials on another electrode) using pairwise phase consistency (PPC). Both the magnitude as well as the preferred phase of synchronization was analyzed. The magnitude of PPC was significantly smaller in CDCs than in controls. Furthermore, controls showed no significant difference between the preferred phase of synchronization between supragranular and infragranular layers, both in acoustic and electric stimulation. In CDCs, however, there was a large difference in the preferred phase between supragranular and infragranular layers. These results demonstrate a loss of synchrony and for the first time directly document a functional decoupling of the interaction between supragranular and infragranular layers of the primary auditory cortex in congenital deafness. Since these are key for the influence of top-down to bottom-up computations, the results suggest a loss of recurrent cortical processing in congenital deafness and explain the outcomes of previous studies by deficits in intracolumnar microcircuitry.

Relationship Between Peripheral and Psychophysical Measures of Amplitude Modulation Detection in Cochlear Implant Users

OBJECTIVE

This study investigates the relationship between electrophysiological and psychophysical measures of amplitude modulation (AM) detection. Prior studies have reported both measures of AM detection recorded separately from cochlear implant (CI) users and acutely deafened animals, but no study has made both measures in the same CI users. Animal studies suggest a progressive loss of high-frequency encoding as one ascends the auditory pathway from the auditory nerve to the cortex. Because the CI speech processor uses the envelope of an ongoing acoustic signal to modulate pulse trains that are subsequently delivered to the intracochlear electrodes, it is of interest to explore auditory nerve responses to modulated stimuli. In addition, psychophysical AM detection abilities have been correlated with speech perception outcomes. Thus, the goal was to explore how the auditory nerve responds to AM stimuli and to relate those physiologic measures to perception.

DESIGN

Eight patients using Cochlear Ltd. Implants participated in this study. Electrically evoked compound action potentials (ECAPs) were recorded using a 4000 pps pulse train that was sinusoidally amplitude modulated at 125, 250, 500, and 1000 Hz rates. Responses were measured for each pulse over at least one modulation cycle for an apical, medial, and basal electrode. Psychophysical modulation detection thresholds (MDTs) were also measured via a three-alternative forced choice, two-down, one-up adaptive procedure using the same modulation frequencies and electrodes.

RESULTS

ECAPs were recorded from individual pulses in the AM pulse train. ECAP amplitudes varied sinusoidally, reflecting the sinusoidal variation in the stimulus. A modulated response amplitude (MRA) metric was calculated as the difference in the maximal and minimum ECAP amplitudes over the modulation cycles. MRA increased as modulation frequency increased, with no apparent cutoff (up to 1000 Hz). In contrast, MDTs increased as the modulation frequency increased. This trend is inconsistent with the physiologic measures. For a fixed modulation frequency, correlations were observed between MDTs and MRAs; this trend was evident at all frequencies except 1000 Hz (although only statistically significant for 250 and 500 Hz AM rates), possibly an indication of central limitations in processing of high modulation frequencies. Finally, peripheral responses were larger and psychophysical thresholds were lower in the apical electrodes relative to basal and medial electrodes, which may reflect better cochlear health and neural survival evidenced by lower preoperative low-frequency audiometric thresholds and steeper growth of neural responses in ECAP amplitude growth functions for apical electrodes.

CONCLUSIONS

Robust ECAPs were recorded for all modulation frequencies tested. ECAP amplitudes varied sinusoidally, reflecting the periodicity of the modulated stimuli. MRAs increased as the modulation frequency increased, a trend we attribute to neural adaptation. For low modulation frequencies, there are multiple current steps between the peak and valley of the modulation cycle, which means successive stimuli are more similar to one another and neural responses are more likely to adapt. Higher MRAs were correlated with lower psychophysical thresholds at low modulation frequencies but not at 1000 Hz, implying a central limitation to processing of modulated stimuli.

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.

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.

An exploratory study of visual sequential processing in children with cochlear implants

OBJECTIVE

The objective of the study was to compare visual sequential processing in school-age children with cochlear implants (CIs) and their normal-hearing (NH) peers. Visual sequential processing was examined using both behavioral and an event-related potential (ERP) measures.

METHODS

Eighteen children with CIs and nineteen children who had hearing within normal limits (NH) participated in the behavioral study. Subtests from the Test of Visual Perceptual Skills and the Sensory Integration and Praxis Test were administered to all children. ERP measures were collected from five children with CI and five age-matched peers. Peak latencies (N200 and P300) and reaction times for visual sequential processing were compared in these two groups.

RESULTS

The findings of the study revealed significant group differences in visual sequential memory and visuo-motor sequencing tasks suggesting that children with severe-profound hearing loss may have difficulties in visual sequential tasks. The study also revealed longer P300 latencies and longer reaction times for a visual sequential matching task in children with CI when compared to their NH peers suggesting slower or delayed processing of visual sequential stimuli.

CONCLUSIONS

This exploratory study involving behavioral and ERP measures showed that as a group, children with prelingual, severe-profound hearing loss who use CIs have difficulties with visual sequential processing. These findings may have implications for rehabilitation for children with hearing loss in the light of recent evidence that accurate and efficient processing of sequentially presented visual stimuli is important for language and reading outcomes.

Simulating electrical modulation detection thresholds using a biophysical model of the auditory nerve

Modulation detection thresholds (MDTs) assess listeners' sensitivity to changes in the temporal envelope of a signal and have been shown to strongly correlate with speech perception in cochlear implant users. MDTs are simulated with a stochastic model of a population of auditory nerve fibers that has been verified to accurately simulate a number of physiologically important temporal response properties. The procedure to estimate detection thresholds has previously been applied to stimulus discrimination tasks. The population model simulates the MDT-stimulus intensity relationship measured in cochlear implant users. The model also recreates the shape of the modulation transfer function and the relationship between MDTs and carrier rate. Discrimination based on fluctuations in synchronous firing activity predicts better performance at low carrier rates, but quantitative measures of modulation coding predict better neural representation of high carrier rate stimuli. Manipulating the number of fibers and a temporal integration parameter, the width of a sliding temporal integration window, varies properties of the MDTs, such as cutoff frequency and peak threshold. These results demonstrate the importance of using a multi-diameter fiber population in modeling the MDTs and demonstrate a wider applicability of this model to simulating behavioral performance in cochlear implant listeners.

Modulation frequency discrimination with single and multiple channels in cochlear implant users

Temporal envelope cues convey important speech information for cochlear implant (CI) users. Many studies have explored CI users' single-channel temporal envelope processing. However, in clinical CI speech processors, temporal envelope information is processed by multiple channels. Previous studies have shown that amplitude modulation frequency discrimination (AMFD) thresholds are better when temporal envelopes are delivered to multiple rather than single channels. In clinical fitting, current levels on single channels must often be reduced to accommodate multi-channel loudness summation. As such, it is unclear whether the multi-channel advantage in AMFD observed in previous studies was due to coherent envelope information distributed across the cochlea or to greater loudness associated with multi-channel stimulation. In this study, single- and multi-channel AMFD thresholds were measured in CI users. Multi-channel component electrodes were either widely or narrowly spaced to vary the degree of overlap between neural populations. The reference amplitude modulation (AM) frequency was 100 Hz, and coherent modulation was applied to all channels. In Experiment 1, single- and multi-channel AMFD thresholds were measured at similar loudness. In this case, current levels on component channels were higher for single-than for multi-channel AM stimuli, and the modulation depth was approximately 100% of the perceptual dynamic range (i.e., between threshold and maximum acceptable loudness). Results showed no significant difference in AMFD thresholds between similarly loud single- and multi-channel modulated stimuli. In Experiment 2, single- and multi-channel AMFD thresholds were compared at substantially different loudness. In this case, current levels on component channels were the same for single- and multi-channel stimuli ("summation-adjusted" current levels) and the same range of modulation (in dB) was applied to the component channels for both single- and multi-channel testing. With the summation-adjusted current levels, loudness was lower with single than with multiple channels and the AM depth resulted in substantial stimulation below single-channel audibility, thereby reducing the perceptual range of AM. Results showed that AMFD thresholds were significantly better with multiple channels than with any of the single component channels. There was no significant effect of the distribution of electrodes on multi-channel AMFD thresholds. The results suggest that increased loudness due to multi-channel summation may contribute to the multi-channel advantage in AMFD, and that overall loudness may matter more than the distribution of envelope information in the cochlea.

Getting a decent (but sparse) signal to the brain for users of cochlear implants

The challenge in getting a decent signal to the brain for users of cochlear implants (CIs) is described. A breakthrough occurred in 1989 that later enabled most users to understand conversational speech with their restored hearing alone. Subsequent developments included stimulation in addition to that provided with a unilateral CI, either with electrical stimulation on both sides or with acoustic stimulation in combination with a unilateral CI, the latter for persons with residual hearing at low frequencies in either or both ears. Both types of adjunctive stimulation produced further improvements in performance for substantial fractions of patients. Today, the CI and related hearing prostheses are the standard of care for profoundly deaf persons and ever-increasing indications are now allowing persons with less severe losses to benefit from these marvelous technologies. The steps in achieving the present levels of performance are traced, and some possibilities for further improvements are mentioned. This article is part of a Special Issue entitled <Lasker Award>.

Identification and multiplicity of double vowels in cochlear implant users

PURPOSE

The present study examined cochlear implant (CI) users' perception of vowels presented concurrently (i.e., double vowels) to further our understanding of auditory grouping in electric hearing.

METHOD

Identification of double vowels and single vowels was measured with 10 CI subjects. Fundamental frequencies (F0s) of vowels were either 100 + 100 Hz or 100 + 300 Hz. Vowels were presented either synchronously or with a time delay. In "Double" sessions, subjects were given only double vowels. In "Double + Single" sessions, while double and single vowels were presented, subjects reported the number and identity of the vowel(s). In addition to clinical settings, stimuli were delivered via an experimental method that interleaved pulse streams of two vowels.

RESULTS

Although the time delay between vowels had a large effect on identification, the effect of change in fundamental frequency (ΔF0) was modest. Enumeration was poor in general, and identification of synchronous vowels was above chance in only the Double sessions with a priori knowledge about presentation. Interleaved presentation of vowel streams provided no benefit for identification and a marginal benefit for enumeration.

CONCLUSIONS

The results demonstrate the importance of episodic context for CI users. Unreliable perception of multiplicity observed in the present results suggests that auditory grouping in CIs may be driven by a schema-based process.

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.

Single- and multi-channel modulation detection in cochlear implant users

Single-channel modulation detection thresholds (MDTs) have been shown to predict cochlear implant (CI) users' speech performance. However, little is known about multi-channel modulation sensitivity. Two factors likely contribute to multichannel modulation sensitivity: multichannel loudness summation and the across-site variance in single-channel MDTs. In this study, single- and multi-channel MDTs were measured in 9 CI users at relatively low and high presentation levels and modulation frequencies. Single-channel MDTs were measured at widely spaced electrode locations, and these same channels were used for the multichannel stimuli. Multichannel MDTs were measured twice, with and without adjustment for multichannel loudness summation (i.e., at the same loudness as for the single-channel MDTs or louder). Results showed that the effect of presentation level and modulation frequency were similar for single- and multi-channel MDTs. Multichannel MDTs were significantly poorer than single-channel MDTs when the current levels of the multichannel stimuli were reduced to match the loudness of the single-channel stimuli. This suggests that, at equal loudness, single-channel measures may over-estimate CI users' multichannel modulation sensitivity. At equal loudness, there was no significant correlation between the amount of multichannel loudness summation and the deficit in multichannel MDTs, relative to the average single-channel MDT. With no loudness compensation, multichannel MDTs were significantly better than the best single-channel MDT. The across-site variance in single-channel MDTs varied substantially across subjects. However, the across-site variance was not correlated with the multichannel advantage over the best single channel. This suggests that CI listeners combined envelope information across channels instead of attending to the best channel.

A sensitive period for the impact of hearing loss on auditory perception

Manipulations of the sensory environment typically induce greater changes to the developing nervous system than they do in adulthood. The relevance of these neural changes can be evaluated by examining the age-dependent effects of sensory experience on quantitative measures of perception. Here, we measured frequency modulation (FM) detection thresholds in adult gerbils and investigated whether diminished auditory experience during development or in adulthood influenced perceptual performance. Bilateral conductive hearing loss (CHL) of ≈30 dB was induced either at postnatal day 10 or after sexual maturation. All animals were then trained as adults to detect a 5 Hz FM embedded in a continuous 4 kHz tone. FM detection thresholds were defined as the minimum deviation from the carrier frequency that the animal could reliably detect. Normal-hearing animals displayed FM thresholds of 25 Hz. Inducing CHL, either in juvenile or adult animals, led to a deficit in FM detection. However, this deficit was greater for juvenile onset hearing loss (89 Hz) relative to adult onset hearing loss (64 Hz). The effects could not be attributed to sensation level, nor were they correlated with proxies for attention. The thresholds displayed by CHL animals were correlated with shallower psychometric function slopes, suggesting that hearing loss was associated with greater variance of the decision variable, consistent with increased internal noise. The results show that decreased auditory experience has a greater impact on perceptual skills when initiated at an early age and raises the possibility that altered development of CNS synapses may play a causative role.

Variations in carrier pulse rate and the perception of amplitude modulation in cochlear implant users

OBJECTIVES

A major focus of recent attempts to enhance cochlear implant (CI) systems has been to increase the rate at which pulses are delivered to the electrode array. One basis for these attempts has been the expectation that faster stimulation rates would lead to an enhanced representation of temporal modulation information. However, there is recent physiological and behavioral evidence to suggest that the reverse may be the case. Here, the effects of stimulation rate on the perception of amplitude modulation were assessed using both modulation detection and modulation frequency discrimination tasks for a range of pulse rates extending considerably higher than the highest rate tested in previous studies and for different speech-relevant modulation frequencies.

DESIGN

Detection of sinusoidal amplitude modulation was assessed in five CI users using monopolar pulse trains presented to a single electrode at rates of 482, 723, 1447, 2894, and 5787 pulses per second (pps). Adaptive procedures were used to find the minimal detectable modulation depth at modulation frequencies of 10 and 100 Hz and at carrier levels of 25%, 50%, and 75% of the electrode's dynamic range. Discrimination of modulation frequency was examined for the same range of pulse rates for the highest carrier level. Similar adaptive procedures determined the minimum increase in modulation frequency that could be detected relative to reference modulation frequencies of 10, 100, and 200 Hz. In both tasks, level roving was implemented to minimize possible loudness cues.

RESULTS

Consistent with previous evidence, modulation detection thresholds were better for higher carrier levels and lower modulation frequencies. When modulation depth at threshold was expressed in terms of the ratio of the depth of the modulation and the carrier level in dB (i.e., 20 log m), performance was significantly better at lower pulse rates. However, when modulation depth was expressed relative to dynamic range, the effect of pulse rate was no longer significant, reflecting the fact that dynamic range increases with pulse rate. Modulation frequency discrimination clearly worsened with increasing modulation frequency, but there was no significant effect of pulse rate.

CONCLUSIONS

In contrast to some recent evidence, no clearly harmful effect of higher pulse rates on modulation perception was found. However, even with very fast stimulation rates, tested over a wide range of modulation frequencies and with two different tasks, there is no evidence of benefit from faster stimulation rates in the perception of amplitude modulation.

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.

Developmental neuroplasticity after cochlear implantation

Cortical development is dependent on stimulus-driven learning. The absence of sensory input from birth, as occurs in congenital deafness, affects normal growth and connectivity needed to form a functional sensory system, resulting in deficits in oral language learning. Cochlear implants bypass cochlear damage by directly stimulating the auditory nerve and brain, making it possible to avoid many of the deleterious effects of sensory deprivation. Congenitally deaf animals and children who receive implants provide a platform to examine the characteristics of cortical plasticity in the auditory system. In this review, we discuss the existence of time limits for, and mechanistic constraints on, sensitive periods for cochlear implantation and describe the effects of multimodal and cognitive reorganization that result from long-term auditory deprivation.

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 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.

Temporal codes for amplitude contrast in auditory cortex

The encoding of sound level is fundamental to auditory signal processing, and the temporal information present in amplitude modulation is crucial to the complex signals used for communication sounds, including human speech. The modulation transfer function, which measures the minimum detectable modulation depth across modulation frequency, has been shown to predict speech intelligibility performance in a range of adverse listening conditions and hearing impairments, and even for users of cochlear implants. We presented sinusoidal amplitude modulation (SAM) tones of varying modulation depths to awake macaque monkeys while measuring the responses of neurons in the auditory core. Using spike train classification methods, we found that thresholds for modulation depth detection and discrimination in the most sensitive units are comparable to psychophysical thresholds when precise temporal discharge patterns rather than average firing rates are considered. Moreover, spike timing information was also superior to average rate information when discriminating static pure tones varying in level but with similar envelopes. The limited utility of average firing rate information in many units also limited the utility of standard measures of sound level tuning, such as the rate level function (RLF), in predicting cortical responses to dynamic signals like SAM. Response modulation typically exceeded that predicted by the slope of the RLF by large factors. The decoupling of the cortical encoding of SAM and static tones indicates that enhancing the representation of acoustic contrast is a cardinal feature of the ascending auditory pathway.

Amplitude modulation and loudness in cochlear implantees

The effect of amplitude modulation of pulse trains on the loudness perceived by cochlear implantees was investigated for different overall levels of the signal, modulation depth and the carrier rate of the pulse train. Equally loud and threshold levels were determined for a variety of signal levels, modulation depths and carrier rates in six cochlear implantees. The pattern of results was consistent with the predictions of a previously published loudness model of McKay et al. (J Acoust Soc Am 113:2054-2063, 2003). The degree to which the loudness of modulated stimuli differed from the loudness elicited by an unmodulated pulse train with equivalent average current depended on the modulation depth and the absolute current level of the unmodulated stimulus. The effect of carrier rate on this measure was predictable solely from the effect of rate on absolute current level for equal loudness. The results have important implications for the interpretation of experiments measuring modulation detection that do not control loudness cues. We show that several previously published results regarding the effect of carrier rate and added noise on modulation detection could be reinterpreted in the light of these findings.

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.

Cochlear-implant high pulse rate and narrow electrode configuration impair transmission of temporal information to the auditory cortex

In the most commonly used cochlear prosthesis systems, temporal features of sound are signaled by amplitude modulation of constant-rate pulse trains. Several convincing arguments predict that speech reception should be optimized by use of pulse rates > or approximately 2,000 pulses per second (pps) and by use of intracochlear electrode configurations that produce restricted current spread (e.g., bipolar rather than monopolar configurations). Neither of those predictions has been borne out in consistent improvements in speech reception. Neurons in the auditory cortex of anesthetized guinea pigs phase lock to the envelope of sine-modulated electric pulse trains presented through a cochlear implant. The present study used that animal model to quantify the effects of carrier pulse rate, electrode configuration, current level, and modulator wave shape on transmission of temporal information from a cochlear implant to the auditory cortex. Modulation sensitivity was computed using a signal-detection analysis of cortical phase-locking vector strengths. Increasing carrier pulse rate in 1-octave steps from 254 to 4,069 pps resulted in systematic decreases in sensitivity. Comparison of sine- versus square-wave modulator waveforms demonstrated that some, but not all, of the loss of modulation sensitivity at high pulse rates was a result of the decreasing size of pulse-to-pulse current steps at the higher rates. Use of a narrow bipolar electrode configuration, compared with the monopolar configuration, produced a marked decrease in modulation sensitivity. Results from this animal model suggest explanations for the failure of high pulse rates and/or bipolar electrode configurations to produce hoped-for improvements in speech reception.

Cochlear implants: a remarkable past and a brilliant future

The aims of this paper are to (i) provide a brief history of cochlear implants; (ii) present a status report on the current state of implant engineering and the levels of speech understanding enabled by that engineering; (iii) describe limitations of current signal processing strategies; and (iv) suggest new directions for research. With current technology the "average" implant patient, when listening to predictable conversations in quiet, is able to communicate with relative ease. However, in an environment typical of a workplace the average patient has a great deal of difficulty. Patients who are "above average" in terms of speech understanding, can achieve 100% correct scores on the most difficult tests of speech understanding in quiet but also have significant difficulty when signals are presented in noise. The major factors in these outcomes appear to be (i) a loss of low-frequency, fine structure information possibly due to the envelope extraction algorithms common to cochlear implant signal processing; (ii) a limitation in the number of effective channels of stimulation due to overlap in electric fields from electrodes; and (iii) central processing deficits, especially for patients with poor speech understanding. Two recent developments, bilateral implants and combined electric and acoustic stimulation, have promise to remediate some of the difficulties experienced by patients in noise and to reinstate low-frequency fine structure information. If other possibilities are realized, e.g., electrodes that emit drugs to inhibit cell death following trauma and to induce the growth of neurites toward electrodes, then the future is very bright indeed.

Auditory cortex phase locking to amplitude-modulated cochlear implant pulse trains

Cochlear implant speech processors transmit temporal features of sound as amplitude modulation of constant-rate electrical pulse trains. This study evaluated the central representation of amplitude modulation in the form of phase-locked firing of neurons in the auditory cortex. Anesthetized pigmented guinea pigs were implanted with cochlear electrode arrays. Stimuli were 254 pulse/s (pps) trains of biphasic electrical pulses, sinusoidally modulated with frequencies of 10-64 Hz and modulation depths of -40 to -5 dB re 100% (i.e., 1-56.2% modulation). Single- and multiunit activity was recorded from multi-site silicon-substrate probes. The maximum frequency for significant phase locking (limiting modulation frequency) was >or=60 Hz for 42% of recording sites, whereas phase locking to pulses of unmodulated pulse trains rarely exceeded 30 pps. The strength of phase locking to frequencies >or=40 Hz often varied nonmonotonically with modulation depth, commonly peaking at modulation depths around -15 to -10 dB. Cortical phase locking coded modulation frequency reliably, whereas a putative rate code for frequency was confounded by rate changes with modulation depth. Group delay computed from the slope of mean phase versus modulation frequency tended to increase with decreasing limiting modulation frequency. Neurons in cortical extragranular layers had lower limiting modulation frequencies than did neurons in thalamic afferent layers. Those observations suggest that the low-pass characteristic of cortical phase locking results from intracortical filtering mechanisms. The results show that cortical neurons can phase lock to modulated electrical pulse trains across the range of modulation frequencies and depths presented by cochlear implant speech processors.

Use of a single channel dedicated to conveying enhanced temporal periodicity cues in cochlear implants: effects on prosodic perception and vowel identification

The continuous interleaved sampling (CIS) strategy for cochlear implants has well-established limitations for the perception of pitch changes in speech. This study investigated a modification of CIS in which one channel was dedicated to the transmission of a temporal encoding of fundamental frequency (F0). Normal hearing subjects listening to noise-excited vocoders, and implantees were tested on labelling the pitch movement of diphthongal glides, on using intonation information to identify sentences as question or statement, and on vowel recognition. There were no significant differences between modified processing and CIS in vowel recognition. However, while there was limited evidence of improved pitch perception relative to CIS with simplified F0 modulation applied to the most basal channel, in general it appears that for most implant users, restricting F0-related modulation to one channel does not provide significantly enhanced pitch information.

Vowel recognition via cochlear implants and noise vocoders: effects of formant movement and duration

Previous work has demonstrated that normal-hearing individuals use fine-grained phonetic variation, such as formant movement and duration, when recognizing English vowels. The present study investigated whether these cues are used by adult postlingually deafened cochlear implant users, and normal-hearing individuals listening to noise-vocoder simulations of cochlear implant processing. In Experiment 1, subjects gave forced-choice identification judgments for recordings of vowels that were signal processed to remove formant movement and/or equate vowel duration. In Experiment 2, a goodness-optimization procedure was used to create perceptual vowel space maps (i.e., best exemplars within a vowel quadrilateral) that included F1, F2, formant movement, and duration. The results demonstrated that both cochlear implant users and normal-hearing individuals use formant movement and duration cues when recognizing English vowels. Moreover, both listener groups used these cues to the same extent, suggesting that postlingually deafened cochlear implant users have category representations for vowels that are similar to those of normal-hearing individuals.

Effects of envelope-vocoder processing on F0 discrimination and concurrent-vowel identification

OBJECTIVE

The aim of this study was to examine the effects of envelope-vocoder sound processing on listeners' ability to discriminate changes in fundamental frequency (F0) in anechoic and reverberant conditions and on their ability to identify concurrent vowels based on differences in F0.

DESIGN

In the first experiment, F0 difference limens (F0DLs) were measured as a function of number of envelope-vocoder frequency channels (1, 4, 8, 24, and 40 channels, and unprocessed) in four normal-hearing listeners, with degree of simulated reverberation (no, mild, and severe reverberation) as a parameter. In the second experiment, vowel identification was measured as a function of the F0 difference between two simultaneous vowels in six normal-hearing listeners, with the number of vocoder channels (8 and 24 channels, and unprocessed) as a parameter.

RESULTS

Reverberation was detrimental to F0 discrimination in conditions with fewer numbers of vocoder channels. Despite the reasonable F0DLs (<1 semitone) with 24- and 8-channel vocoder processing, listeners were unable to benefit from F0 differences between the competing vowels in the concurrent-vowel paradigm.

CONCLUSIONS

The overall detrimental effects of vocoder processing are probably are due to the poor spectral representation of the lower-order harmonics. The F0 information carried in the temporal envelope is weak, susceptible to reverberation, and may not suffice for source segregation. To the extent that vocoder processing simulates cochlear implant processing, users of current implant processing schemes are unlikely to benefit from F0 differences between competing talkers when listening to speech in complex environments. The results provide further incentive for finding a way to make the information from low-order, resolved harmonics available to cochlear implant users.

Sensory temporal processing in adults with early hearing loss

This study examined tactile and visual temporal processing in adults with early loss of hearing. The tactile task consisted of punctate stimulations that were delivered to one or both hands by a mechanical tactile stimulator. Pairs of light emitting diodes were presented on a display for visual stimulation. Responses consisted of YES or NO judgments as to whether the onset of the pairs of stimuli was perceived simultaneously or non-simultaneously. Tactile and visual temporal thresholds were significantly higher for the deaf group when compared to controls. In contrast to controls, tactile and visual temporal thresholds for the deaf group did not differ when presentation locations were examined. Overall findings of this study support the notion that temporal processing is compromised following early deafness regardless of the spatial location in which the stimuli are presented.

Effect of stimulation rate on speech perception in adult users of the Med-El CIS speech processing strategy

The primary aim of the study was to determine the effect of altering channel stimulation rate on the performance of adult cochlear implant users. Six adult users of the Med-El CIS processing strategy underwent tests of categorical identification of synthetic speech, tests of sentence recognition and tests of consonant recognition in three listening conditions: high channel stimulation rate (ranging from 1500 to 2020 pps/ch), a medium rate (800 pps/ch) and a low rate (400 pps/ch). Number of channels was held constant across rate conditions. With the categorical identification task, performance varied by acoustic cue type but did not vary with rate. With the consonant recognition task performance varied by phonological feature, but there was also no significant effect of rate. However, two subjects showed markedly reduced sentence scores at lower rates. Results suggests that higher stimulation rates with the CIS strategy may be beneficial to speech perception in some cases.

Enhancement of temporal periodicity cues in cochlear implants: effects on prosodic perception and vowel identification

Standard continuous interleaved sampling processing, and a modified processing strategy designed to enhance temporal cues to voice pitch, were compared on tests of intonation perception, and vowel perception, both in implant users and in acoustic simulations. In standard processing, 400 Hz low-pass envelopes modulated either pulse trains (implant users) or noise carriers (simulations). In the modified strategy, slow-rate envelope modulations, which convey dynamic spectral variation crucial for speech understanding, were extracted by low-pass filtering (32 Hz). In addition, during voiced speech, higher-rate temporal modulation in each channel was provided by 100% amplitude-modulation by a sawtooth-like wave form whose periodicity followed the fundamental frequency (F0) of the input. Channel levels were determined by the product of the lower- and higher-rate modulation components. Both in acoustic simulations and in implant users, the ability to use intonation information to identify sentences as question or statement was significantly better with modified processing. However, while there was no difference in vowel recognition in the acoustic simulation, implant users performed worse with modified processing both in vowel recognition and in formant frequency discrimination. It appears that, while enhancing pitch perception, modified processing harmed the transmission of spectral information.

Effects of amplitude compression on first- and second-order modulation detection thresholds in cochlear implant listeners

The aim of this study was to examine the effects of instantaneous non-linear amplitude mapping on the detection of single-component and multicomponent temporal envelopes. To address this issue, first- and second-order amplitude modulation detection thresholds were measured in four cochlear implant users with the intervention of the compression device of the implant processor. The compression device is set to produce either a strongly or a weakly logarithmic mapping of stimulus amplitude to electrical amplitude. 'First-order' modulation detection thresholds indicate the ability of listeners to detect sinusoidal amplitude modulation (SAM) applied to a white noise carrier; they are measured as a function of the rate of that modulation, fm. 'Second-order' modulation detection thresholds indicate the ability to detect sinusoidal modulation applied to the depth of a sinusoidally amplitude-modulated signal (here, a 16-Hz sinusoidally amplitude-modulated white noise); they are measured as a function of the rate of the modulation applied to the modulation depth (referred to as fm'). In each task, stimuli are transformed by the implant processor and are presented through one electrode at approximately the same level. The results show that, in cochlear implant listeners, both first- and second-order modulation detection thresholds measured at the lower rates (< or =7 Hz) decrease slightly by about 3-6dB when the stronger compression is used. No effect of compression is observed at higher rates. These results suggest that instantaneous logarithmic amplitude mapping has beneficial- but limited-effects on the detection of single-component and multicomponent temporal envelopes. These results are discussed in light of current models of temporal envelope processing.

Enhancing temporal cues to voice pitch in continuous interleaved sampling cochlear implants

The limited spectral resolution of cochlear implant systems means that voice pitch perception depends on weak temporal envelope cues. Enhancement of such cues was investigated in implant users and in acoustic simulations. Subjects labeled the pitch movement of processed synthetic diphthongal glides. In standard processing, noise carriers (simulations) or pulse trains (implant users) were modulated by 400 Hz low-pass envelopes. In modified processing, carriers were modulated by two components: (1) Slow-rate (<32 Hz) envelope modulations, conveying dynamic spectral shape changes crucial for speech; (2) a simplified waveform (e.g., a sawtooth) matching the periodicity of the input diphthong. In both normal listeners and implant users performance was better with modified processing, though temporal envelope cues were less effective with higher F0. Factors contributing to the advantage for modified processing may include increased modulation depth and use of a modulation waveform featuring a rapid onset in each period, resulting in a clearer representation of F0 in the neural firing pattern. Eliminating slow-rate spectral dynamics, so that within-channel amplitude changes solely reflected F0, showed that dynamic spectral variation obscured temporal pitch cues. Though significant, advantages for modified processing were small, suggesting that the potential for developing strategies delivering enhanced pitch perception is limited.

Evaluating the function of phonetic perceptual phenomena within speech recognition: an examination of the perception of /d/-/t/ by adult cochlear implant users

This study examined whether cochlear implant users must perceive differences along phonetic continua in the same way as do normal hearing listeners (i.e., sharp identification functions, poor within-category sensitivity, high between-category sensitivity) in order to recognize speech accurately. Adult postlingually deafened cochlear implant users, who were heterogeneous in terms of their implants and processing strategies, were tested on two phonetic perception tasks using a synthetic /da/-/ta/ continuum (phoneme identification and discrimination) and two speech recognition tasks using natural recordings from ten talkers (open-set word recognition and forced-choice /d/-/t/ recognition). Cochlear implant users tended to have identification boundaries and sensitivity peaks at voice onset times (VOT) that were longer than found for normal-hearing individuals. Sensitivity peak locations were significantly correlated with individual differences in cochlear implant performance; individuals who had a /d/-/t/ sensitivity peak near normal-hearing peak locations were most accurate at recognizing natural recordings of words and syllables. However, speech recognition was not strongly related to identification boundary locations or to overall levels of discrimination performance. The results suggest that perceptual sensitivity affects speech recognition accuracy, but that many cochlear implant users are able to accurately recognize speech without having typical normal-hearing patterns of phonetic perception.

Spectral and temporal cues to pitch in noise-excited vocoder simulations of continuous-interleaved-sampling cochlear implants

Four-band and single-band noise-excited vocoders were used in acoustic simulations to investigate spectral and temporal cues to melodic pitch in the output of a cochlear implant speech processor. Noise carriers were modulated by amplitude envelopes extracted by half-wave rectification and low-pass filtering at 32 or 400 Hz. The four-band, but not the single-band processors, may preserve spectral correlates of fundamental frequency (F0). Envelope smoothing at 400 Hz preserves temporal correlates of F0, which are eliminated with 32-Hz smoothing. Inputs to the processors were sawtooth frequency glides, in which spectral variation is completely determined by F0, or synthetic diphthongal vowel glides, whose spectral shape is dominated by varying formant resonances. Normal listeners labeled the direction of pitch movement of the processed stimuli. For processed sawtooth waves, purely temporal cues led to decreasing performance with increasing F0. With purely spectral cues, performance was above chance despite the limited spectral resolution of the processors. For processed diphthongs, performance with purely spectral cues was at chance, showing that spectral envelope changes due to formant movement obscured spectral cues to F0. Performance with temporal cues was poorer for diphthongs than for sawtooths, with very limited discrimination at higher F0. These data suggest that, for speech signals through a typical cochlear implant processor, spectral cues to pitch are likely to have limited utility, while temporal envelope cues may be useful only at low F0.

Temporal processing and speech recognition in cochlear implant users

Although the speech recognition performance of cochlear implant patients has steadily improved in recent years, considerable variability remains in implant patient outcomes. Here, we report on a correlation between cochlear implant patients' auditory temporal processing and speech recognition abilities. Modulation detection thresholds were measured as a function of stimulation level; phoneme recognition was also assessed. Results showed marked individual differences in the shape and absolute sensitivity of subjects' modulation threshold functions. Statistical analysis revealed a strong correlation between phoneme recognition scores and subjects' mean modulation thresholds, demonstrating the contribution of temporal processing to implant speech perception. In correlating a basic psychophysical measure to speech performance, these results should contribute toward improved speech processor design and rehabilitation tools for cochlear implant patients.

Coding of the fundamental frequency in continuous interleaved sampling processors for cochlear implants

In this study the perception of the fundamental frequency (F0) of periodic stimuli by cochlear implant users is investigated. A widely used speech processor is the Continuous Interleaved Sampling (CIS) processor, for which the fundamental frequency appears as temporal fluctuations in the envelopes at the output. Three experiments with four users of the LAURA (Registered trade mark of Philips Hearing Implants, now Cochlear Technology Centre Europe) cochlear implant were carried out to examine the influence of the modulation depth of these envelope fluctuations on pitch discrimination. In the first experiment, the subjects were asked to discriminate between two SAM (sinusoidally amplitude modulated) pulse trains on a single electrode channel differing in modulation frequency ( deltaf = 20%). As expected, the results showed a decrease in the performance for smaller modulation depths. Optimal performance was reached for modulation depths between 20% and 99%, depending on subject, electrode channel, and modulation frequency. In the second experiment, the smallest noticeable difference in F0 of synthetic vowels was measured for three algorithms that differed in the obtained modulation depth at the output: the default CIS strategy, the CIS strategy in which the F0 fluctuations in the envelope were removed (FLAT CIS), and a third CIS strategy, which was especially designed to control and increase the depth of these fluctuations (F0 CIS). In general, performance was poorest for the FLAT CIS strategy, where changes in F0 are only apparent as changes of the average amplitude in the channel outputs. This emphasizes the importance of temporal coding of F0 in the speech envelope for pitch perception. No significantly better results were obtained for the F0 CIS strategy compared to the default CIS strategy, although the latter results in envelope modulation depths at which sub-optimal scores were obtained in some cases of the first experiment. This indicates that less modulation is needed if all channels are stimulated with synchronous F0 fluctuations. This hypothesis is confirmed in a third experiment where subjects performed significantly better in a pitch discrimination task with SAM pulse trains, if three channels were stimulated concurrently, as opposed to only one.

Variation in speech perception scores among children with cochlear implants

OBJECTIVE

The objective of this study was to identify common factors affecting speech perception scores in children with cochlear implants.

DESIGN

Speech perception data for 167 implanted children were collected at two cochlear implant centres in Melbourne and Sydney. The data comprised audition-alone scores on open-set word and sentence tests. Children were selected on the basis that they had a Nucleus 22-electrode cochlear implant. The average age of the children was 5 yr. Information was also collected about 12 factors that may have influenced speech perception scores for each child. Analysis of covariance was used to identify factors that significantly affected speech perception scores. Pearson pairwise correlation coefficients were also calculated for all factors analyzed.

RESULTS

The analyses in this study identified factors that accounted for 51%, 34%, and 45% of the variance in phoneme, word and sentence perception scores. Scores decreased by 1.4 to 2.4% per year of profound deafness prior to implantation. Children who normally use oral communication scored significantly higher than children normally using sign or simultaneous oral and sign communication. Children implanted in Sydney scored higher on average than children implanted in Melbourne.

CONCLUSIONS

The results show that a significant part of the variation in speech perception scores is systematically related to audiological and environmental factors for each child. The reasons for significant differences between children using different communication modes or from different clinics were not identified.

Intensity discrimination and detection of amplitude modulation in electric hearing

Wojtczak and Viemeister [J. Acoust. Soc. Am. 106, 1917-1924 (1999)] demonstrated a close relationship between intensity difference limens (DLs) and 4-Hz amplitude modulation (AM) detection thresholds in normal-hearing acoustic listeners. The present study demonstrates a similar relationship between intensity DLs and AM detection thresholds in cochlear-implant listeners, for gated stimuli. This suggests that acoustic and cochlear-implant listeners make use of a similar decision variable to perform intensity discrimination and modulation detection tasks. It can be shown that the absence of compression in electric hearing does not preclude this possibility.

Responses of inferior colliculus neurons to amplitude-modulated intracochlear electrical pulses in deaf cats

Current cochlear prostheses use amplitude-modulated pulse trains to encode acoustic signals. In this study we examined the responses of inferior colliculus (IC) neurons to sinusoidal amplitude-modulated pulses and compared the maximum unmodulated pulse rate (Fmax) to which they responded with the maximum modulation frequency (maxFm) that they followed. Consistent with previous results, responses to unmodulated pulses were all low-pass functions of pulse rate. Mean Fmax to unmodulated pulses was 104 pulses per second (pps) and modal Fmax was 60 pps. Above Fmax IC neurons ceased responding except for an onset burst at the beginning of the stimulus. However, IC neurons responded to much higher pulse rates when these pulses were amplitude modulated; 74% were relatively insensitive to carrier rate and responded to all modulated carriers including those exceeding 600 pps. In contrast, the responses of these neurons (70%) were low-pass functions of modulation frequency, and the remaining (30%) had band-pass functions with a maxFm of 42 and 34 Hz, respectively. Thus temporal resolution of IC neurons for modulated frequencies is significantly lower than that for unmodulated pulses. These two measures of temporal resolution (Fmax and maxFm) were uncorrelated (r(2) = 0.101). Several parameters influenced the amplitude and temporal structure of modulation responses including modulation depth, overall intensity and modulation-to-carrier rate ratio. We observed distortions in unit responses to amplitude-modulated signals when this ratio was 1/4 to 1/6. Since most current cochlear implant speech processors permit ratios that are significantly greater than this, severe distortion and signal degradation may occur frequently in these devices.

Modulation detection interference in cochlear implant subjects

The aim of this study was to determine whether detection thresholds for amplitude modulated signals on a single electrode were influenced by a masking modulation on a second electrode in cochlear implant users. Data were collected from four post-linguistically deafened subjects using the Cochlear Limited prosthesis. Investigated were the effects of the spatial separation between test and masker electrodes, 0 to 5 electrodes (0 to 3.75 mm), and the amount of masking modulation: 24%, 48%, 72%, and 96% above detection thresholds. Initially, modulation detection thresholds for stimulation on a single electrode without masking modulation were obtained for a set of six electrodes in the middle of the array. Modulation detection thresholds on a fixed test electrode were then obtained with unmodulated and modulated masking on a second electrode, which was one of the six electrodes in the initial study. In both studies, thresholds were measured for modulated pulse duration at the modulation frequencies of 10-200 Hz. In the first study, the shape of the detection thresholds as a function of modulation frequency, the temporal modulation transfer function, generally resembled a low-pass filter for two subjects. For the other two subjects, the functions were relatively flat across modulation frequencies. In the second study, unmodulated masking resulted in a small elevation in detection thresholds across electrodes. Modulation detection interference (MDI), the difference between thresholds for the modulated maskers and the unmodulated masker, was greater for larger amounts of masking modulation than for smaller amounts of masking modulation. For three of the four subjects, MDI was higher for smaller spatial separations between the two electrodes than for larger spatial separations suggesting that a portion of MDI may be due to overlap of neural excitation distributions produced by stimulation on two electrodes in close proximity on the array.

Design for an inexpensive but effective cochlear implant

Widespread application of cochlear implants is limited by cost, especially in developing countries. In this article we present a design for a low-cost but effective cochlear implant system. The system includes a speech processor, four pairs of transmitting and receiving coils, and an electrode array with four monopolar electrodes. All implanted components are passive, reducing to a minimum the complexity of manufacture and allowing high reliability. A four-channel continuous interleaved sampling strategy is used for the speech processor. The processor and transmission link have been evaluated in tests with a subject previously implanted with the Ineraid electrode array and percutaneous connector. A prototype of the link, consisting of four pairs of transmitting and external receiving coils, was used, with the outputs of the receiving coils directed to four intracochlear electrodes through the percutaneous connector. The subject achieved speech reception scores with the prototype system that were equivalent to those achieved with a standard laboratory implementation of a continuous interleaved sampling processor with current-controlled stimuli.

Reduction in excitability of the auditory nerve following electrical stimulation at high stimulus rates. II. Comparison of fixed amplitude with amplitude modulated stimuli

We have previously shown that acute electrical stimulation of the auditory nerve using charge-balanced biphasic current pulses presented continuously can lead to a prolonged decrement in auditory nerve excitability (Tykocinski et al., Hear. Res. 88 (1995), 124-142). This work also demonstrated a reduction in electrically evoked auditory brainstem response (EABR) amplitude decrement when using an otherwise equivalent pulse train with a 50% duty cycle. In the present study we have extended this work in order to compare the effects of electrical stimulation using both fixed amplitude electrical pulse trains and amplitude modulated (AM) pulse trains that more accurately model the dynamic stimulus paradigms used in cochlear implants. EABRs were recorded from guinea pigs following acute stimulation using AM trains of charge-balanced biphasic current pulses. The extent of stimulus-induced reductions in the EABR were compared with our previous results using either fixed amplitude continuous, or 50% duty cycle pulse trains operating at 0.34 microC/phase (2 mA, 170 micros/phase) at 400 or 1000 pulses/s (Tykocinski et al., Hear. Res. 88 (1995) 124-142). The AM pulse train, operating at the same rates, was based on a 1-s sequence of the most extensively activated electrode of a Nucleus Mini-22 cochlear implant using the SPEAK speech processing strategy exposed to 4-talker babble, and delivered the same total charge as the fixed amplitude 50% duty cycle pulse train. Two hours of continuous stimulation induced a significant, rate-dependent reduction in auditory nerve excitability, and showed only a slight post-stimulus recovery for monitoring periods of up to 6 hours. Following 2 or 4 h of stimulation using an otherwise equivalent pulse train with a 50% duty cycle or the AM pulse train, significantly less reduction in the EABR was observed, and recovery to pre-stimulus levels was generally rapid and complete. These differences in the extent of the recovery between the continuous waveform and both the 50% duty cycle and AM waveforms were statistically significant for both 400 and 1000 pulses/s stimuli. Consistent with our previous results, the stimulus changes observed using AM pulse trains were rate dependent, with higher rate stimuli evoking more extensive stimulus-induced changes. The present findings show that while stimulus-induced reductions in neural excitability are dependent on the extent of stimulus-induced neuronal activity, the use of an AM stimulus paradigm further reduces post-stimulus neural fatigue.

The future of cochlear implants

Remarkable progress has been made in recent years in the design and application of processing strategies for cochlear implants. Most notably, use of the new spectral peak (SPEAK) and continuous interleaved sampling (CIS) strategies have provided large improvements in speech reception performance compared with prior strategies (NIH Consensus Statement, 1995; Skinner et al., 1994a; Wilson et al., 1991). All major manufacturers of multichannel implant systems, including Advanced Bionics Corp., Bionic Systems, Cochlear Pty. Ltd., and Med El, now offer CIS or CIS-like strategies in their speech processors. The SPEAK strategy was developed by Cochlear Pty. Ltd and continues to be one of the options available in that company's devices. The principal purpose of this editorial is to present some of the many possibilities for further improvements in performance. To the extent that such possibilities are realized, implant systems of the future may be quite different from present systems, with different processing strategies, electrode designs, telemetry features, and fitting procedures.