Effects of stimulus duration on amplitude modulation processing with cochlear implants

Effects of pulse shape on pitch sensitivity of cochlear implant users

Contemporary cochlear implants (CIs) use cathodic-leading symmetric biphasic (C-BP) pulses for electrical stimulation. It remains unclear whether asymmetric pulses emphasizing the anodic or cathodic phase may improve spectral and temporal coding with CIs. This study tested place- and temporal-pitch sensitivity with C-BP, anodic-centered triphasic (A-TP), and cathodic-centered triphasic (C-TP) pulse trains on apical, middle, and basal electrodes in 10 implanted ears. Virtual channel ranking (VCR) thresholds (for place-pitch sensitivity) were measured at both a low and a high pulse rate of 99 (Experiment 1) and 1000 (Experiment 2) pulses per second (pps), and amplitude modulation frequency ranking (AMFR) thresholds (for temporal-pitch sensitivity) were measured at a 1000-pps pulse rate in Experiment 3. All stimuli were presented in monopolar mode. Results of all experiments showed that detection thresholds, most comfortable levels (MCLs), VCR thresholds, and AMFR thresholds were higher on more basal electrodes. C-BP pulses had longer active phase duration and thus lower detection thresholds and MCLs than A-TP and C-TP pulses. Compared to C-TP pulses, A-TP pulses had lower detection thresholds at the 99-pps but not the 1000-pps pulse rate, and had lower MCLs at both pulse rates. A-TP pulses led to lower VCR thresholds than C-BP pulses, and in turn than C-TP pulses, at the 1000-pps pulse rate. However, pulse shape did not affect VCR thresholds at the 99-pps pulse rate (possibly due to the fixed temporal pitch) or AMFR thresholds at the 1000-pps pulse rate (where the overall high performance may have reduced the changes with different pulse shapes). Notably, stronger polarity effect on VCR thresholds (or more improvement in VCR with A-TP than with C-TP pulses) at the 1000-pps pulse rate was associated with stronger polarity effect on detection thresholds at the 99-pps pulse rate (consistent with more degeneration of auditory nerve peripheral processes). The results suggest that A-TP pulses may improve place-pitch sensitivity or spectral coding for CI users, especially in situations with peripheral process degeneration.

Psychoacoustic and electroencephalographic responses to changes in amplitude modulation depth and frequency in relation to speech recognition in cochlear implantees

Temporal envelope modulations (TEMs) are one of the most important features that cochlear implant (CI) users rely on to understand speech. Electroencephalographic assessment of TEM encoding could help clinicians to predict speech recognition more objectively, even in patients unable to provide active feedback. The acoustic change complex (ACC) and the auditory steady-state response (ASSR) evoked by low-frequency amplitude-modulated pulse trains can be used to assess TEM encoding with electrical stimulation of individual CI electrodes. In this study, we focused on amplitude modulation detection (AMD) and amplitude modulation frequency discrimination (AMFD) with stimulation of a basal versus an apical electrode. In twelve adult CI users, we (a) assessed behavioral AMFD thresholds and (b) recorded cortical auditory evoked potentials (CAEPs), AMD-ACC, AMFD-ACC, and ASSR in a combined 3-stimulus paradigm. We found that the electrophysiological responses were significantly higher for apical than for basal stimulation. Peak amplitudes of AMFD-ACC were small and (therefore) did not correlate with speech-in-noise recognition. We found significant correlations between speech-in-noise recognition and (a) behavioral AMFD thresholds and (b) AMD-ACC peak amplitudes. AMD and AMFD hold potential to develop a clinically applicable tool for assessing TEM encoding to predict speech recognition in CI users.

Assessing the Relationship Between Pitch Perception and Neural Health in Cochlear Implant Users

Various neural health estimates have been shown to indicate the density of spiral ganglion neurons in animal and modeling studies of cochlear implants (CIs). However, when applied to human CI users, these neural health estimates based on psychophysical and electrophysiological measures are not consistently correlated with each other or with the speech recognition performance. This study investigated whether the neural health estimates have stronger correlations with the temporal and place pitch sensitivity than with the speech recognition performance. On five electrodes in 12 tested ears of eight adult CI users, polarity effect (PE), multipulse integration (MPI), and interphase gap (IPG) effect on the amplitude growth function (AGF) of electrically evoked compound action potential (ECAP) were measured to estimate neural health, while thresholds of amplitude modulation frequency ranking (AMFR) and virtual channel ranking (VCR) were measured to indicate temporal and place pitch sensitivity. AzBio sentence recognition in noise was measured using the clinical CI processor for each ear. The results showed significantly poorer AMFR and VCR thresholds on the basal electrodes than on the apical and middle electrodes. Across ears and electrodes, only the IPG offset effect on ECAP AGF had a nearly significant negative correlation with the VCR threshold after removing the outliers. No significant across-ear correlations were found between the mean neural health estimates, mean pitch-ranking thresholds, and AzBio sentence recognition score. This study suggests that the central axon demyelination reflected by the IPG offset effect may be important for the place pitch sensitivity of CI users and that the IPG offset effect may be used to predict the perceptual resolution of virtual channels for CI programming.

Comparing Sound-Field Speech-Auditory Brainstem Response Components between Cochlear Implant Users with Different Speech Recognition in Noise Scores

OBJECTIVES

Many studies have suggested that cochlear implant (CI) users vary in terms of speech recognition in noise. Studies in this field attribute this variety partly to subcortical auditory processing. Studying speech-Auditory Brainstem Response (speech-ABR) provides good information about speech processing; thus, this work was designed to compare speech-ABR components between two groups of CI users with good and poor speech recognition in noise scores.

MATERIALS & METHODS

The present study was conducted on two groups of CI users aged 8-10 years old. The first group (CI-good) consisted of 15 children with prelingual CI who had good speech recognition in noise performance. The second group (CI-poor) was matched with the first group, but they had poor speech recognition in noise performance. The speech-ABR test in a sound-field presentation was performed for all the participants.

RESULTS

The speech-ABR response showed more delay in C, D, E, F, O latencies in CI-poor than CI-good users (P <0.05), meanwhile no significant difference was observed in initial wave (V(t= -0.293, p= 0.771 and A (t= -1.051, p= 0.307). Analysis in spectral-domain showed a weaker representation of fundamental frequency as well as the first formant and high-frequency component of speech stimuli in the CI users with poor auditory performance.

CONCLUSIONS

Results revealed that CI users who showed poor auditory performance in noise performance had deficits in encoding the periodic portion of speech signals at the brainstem level. Also, this study could be as physiological evidence for poorer pitch processing in CI users with poor speech recognition in noise performance.

Mandarin tone recognition training with cochlear implant simulation: Amplitude envelope enhancement and cue weighting

With limited fundamental frequency (F0) cues, cochlear implant (CI) users recognize Mandarin tones using amplitude envelope. This study investigated whether tone recognition training with amplitude envelope enhancement may improve tone recognition and cue weighting with CIs. Three groups of CI-simulation listeners received training using vowels with amplitude envelope modified to resemble F0 contour (enhanced-amplitude-envelope training), training using natural vowels (natural-amplitude-envelope training), and exposure to natural vowels without training, respectively. Tone recognition with natural and enhanced amplitude envelope cues and cue weighting of amplitude envelope and F0 contour were measured in pre-, post-, and retention-tests. It was found that with similar pre-test performance, both training groups had better tone recognition than the no-training group after training. Only enhanced-amplitude-envelope training increased the benefits of amplitude envelope enhancement in the post- and retention-tests than in the pre-test. Neither training paradigm increased the cue weighting of amplitude envelope and F0 contour more than stimulus exposure. Listeners attending more to amplitude envelope in the pre-test tended to have better tone recognition with enhanced amplitude envelope cues before training and improve more in tone recognition after enhanced-amplitude-envelope training. The results suggest that auditory training and speech enhancement may bring maximum benefits to CI users when combined.

Sequential stream segregation with bilateral cochlear implants

Sequential stream segregation on the basis of binaural 'ear-of-entry', modulation rate and electrode place-of-stimulation cues was investigated in bilateral cochlear implant (CI) listeners using a rhythm anisochrony detection task. Sequences of alternating 'A' and 'B' bursts were presented via direct electrical stimulation and comprised either an isochronous timing structure or an anisochronous structure that was generated by delaying just the 'B' bursts. 'B' delay thresholds that enabled rhythm anisochrony detection were determined. Higher thresholds were assumed to indicate a greater likelihood of stream segregation, resulting specifically from stream integration breakdown. Results averaged across subjects showed that thresholds were significantly higher when monaural 'A' and 'B' bursts were presented contralaterally rather than ipsilaterally, and that diotic presentation of 'A', with a monaural 'B', yielded intermediate thresholds. When presented monaurally and ipsilaterally, higher thresholds were also found when successive bursts had mismatched rather than matched modulation rates. In agreement with previous studies, average delay thresholds also increased as electrode separation between bursts increased when presented ipsilaterally. No interactions were found between ear-of-entry, modulation rate and place-of-stimulation. However, combining moderate electrode difference cues with either diotic-'A' ear-of-entry cues or modulation-rate mismatch cues did yield greater threshold increases than observed with any of those cues alone. The results from the present study indicate that sequential stream segregation can be elicited in bilateral CI users by differences in the signal across ears (binaural cues), in modulation rate (monaural cues) and in place-of-stimulation (monaural cues), and that those differences can be combined to further increase segregation.

Saliency of Vowel Features in Neural Responses of Cochlear Implant Users

Cochlear implants restore hearing in deaf individuals, but speech perception remains challenging. Poor discrimination of spectral components is thought to account for limitations of speech recognition in cochlear implant users. We investigated how combined variations of spectral components along two orthogonal dimensions can maximize neural discrimination between two vowels, as measured by mismatch negativity. Adult cochlear implant users and matched normal-hearing listeners underwent electroencephalographic event-related potentials recordings in an optimum-1 oddball paradigm. A standard /a/ vowel was delivered in an acoustic free field along with stimuli having a deviant fundamental frequency (+3 and +6 semitones), a deviant first formant making it a /i/ vowel or combined deviant fundamental frequency and first formant (+3 and +6 semitones /i/ vowels). Speech recognition was assessed with a word repetition task. An analysis of variance between both amplitude and latency of mismatch negativity elicited by each deviant vowel was performed. The strength of correlations between these parameters of mismatch negativity and speech recognition as well as participants' age was assessed. Amplitude of mismatch negativity was weaker in cochlear implant users but was maximized by variations of vowels' first formant. Latency of mismatch negativity was later in cochlear implant users and was particularly extended by variations of the fundamental frequency. Speech recognition correlated with parameters of mismatch negativity elicited by the specific variation of the first formant. This nonlinear effect of acoustic parameters on neural discrimination of vowels has implications for implant processor programming and aural rehabilitation.

Cantonese Tone Perception for Children Who Use a Hearing Aid and a Cochlear Implant in Opposite Ears

OBJECTIVES

The ability to recognize tones is vital for speech perception in tonal languages. Cantonese has six tones, which are differentiated almost exclusively by pitch cues (tones 1 to 6). The differences in pitch contours among the tones are subtle, making Cantonese a challenging language for cochlear implant users. The addition of a hearing aid has been shown to improve speech perception in nontonal languages and in Mandarin Chinese. This study (1) investigates the Cantonese tone perception ability of children who use a cochlear implant and a hearing aid in opposite ears; (2) evaluates the effect of varying pitch height and pitch contour cues on Cantonese tone perception for these children; and (3) compares the Cantonese tone perception ability for using a hearing aid and a cochlear implant together versus an implant alone.

DESIGN

Eight native Cantonese speaking children using a cochlear implant and a hearing aid in opposite ears were assessed for tone perception and word identification. The tone perception test involved discriminating and ranking tone pairs from natural and artificially manipulated Cantonese tones with various pitch heights and/or pitch contours. The word identification test involved identifying Cantonese words in a four-alternative forced-choice task. All tests were performed in two device conditions: (1) cochlear implant and hearing aid together and (2) implant alone.

RESULTS

Seven of the 8 subjects performed significantly above chance in both tests using the cochlear implant alone. Results showed that both pitch height and/or pitch direction were important perceptual cues for implant users. Perception for some tones was improved by increasing the pitch height differences between the tones. The ability to discriminate and rank the tone 2/tone 5 contrast and the tone 4/tone 6 contrast was poor, as the tones in these contrasts are similar in pitch contours and onset frequencies. No significant improvement was observed after artificially increasing the pitch offset differences between the tones in the tone 2/tone 5 and the tone 4/tone 6 contrasts. Tone perception results were significantly better with the addition of the hearing aid in the nonimplanted ear compared with using the implant alone; however, word identification results were not significantly different between using the implant alone and using both the hearing aid and the implant together. None of the subjects performed worse in tone perception or in word identification when the hearing aid was added.

CONCLUSIONS

Reduced ability to perceive pitch contour cues, even when artificially exaggerated, may explain some of the difficulties in Cantonese word recognition for implant users. The addition of a contralateral hearing aid could be beneficial for Cantonese tone perception for some individuals with a unilateral implant. The results encouraged Cantonese speakers to trial a hearing aid in the nonimplanted ear when using a cochlear implant.

Mandarin Lexical Tone Acquisition in Cochlear Implant Users With Prelingual Deafness: A Review

PURPOSE

The purpose of this review article is to synthesize evidence from the fields of developmental linguistics and cochlear implant technology relevant to the production and perception of Mandarin lexical tone in cochlear implant users with prelingual deafness. The aim of this review was to identify potential factors that determine outcomes for tonal-language speaking cochlear implant users and possible directions for further research.

METHOD

A computerized database search of MEDLINE, CINAHL, Academic Search Premier, Web of Science, and Google Scholar was undertaken in June and July 2014. Search terms used were lexical tone AND tonal language, speech development AND/OR speech production AND/OR speech perception AND cochlear implants, and pitch perception AND cochlear implants, anywhere in the title or abstract.

CONCLUSION

Despite the demonstrated limitations of pitch perception in cochlear implant users, there is some evidence that typical production and perception of lexical tone is possible by cochlear implant users with prelingual deafness. Further studies are required to determine the factors that contribute to better outcomes to inform rehabilitation processes for cochlear implant users in tonal-language environments.

Results using the OPAL strategy in Mandarin speaking cochlear implant recipients

OBJECTIVE

To evaluate the effectiveness of an experimental pitch-coding strategy for improving recognition of Mandarin lexical tone in cochlear implant (CI) recipients.

DESIGN

Adult CI recipients were tested on recognition of Mandarin tones in quiet and speech-shaped noise at a signal-to-noise ratio of +10 dB; Mandarin sentence speech-reception threshold (SRT) in speech-shaped noise; and pitch discrimination of synthetic complex-harmonic tones in quiet. Two versions of the experimental strategy were examined: (OPAL) linear (1:1) mapping of fundamental frequency (F0) to the coded modulation rate; and (OPAL+) transposed mapping of high F0s to a lower coded rate. Outcomes were compared to results using the clinical ACE™ strategy.

STUDY SAMPLE

Five Mandarin speaking users of Nucleus® cochlear implants.

RESULTS

A small but significant benefit in recognition of lexical tones was observed using OPAL compared to ACE in noise, but not in quiet, and not for OPAL+ compared to ACE or OPAL in quiet or noise. Sentence SRTs were significantly better using OPAL+ and comparable using OPAL to those using ACE. No differences in pitch discrimination thresholds were observed across strategies.

CONCLUSIONS

OPAL can provide benefits to Mandarin lexical tone recognition in moderately noisy conditions and preserve perception of Mandarin sentences in challenging noise conditions.

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.

Mandarin Tone and Vowel Recognition in Cochlear Implant Users: Effects of Talker Variability and Bimodal Hearing

OBJECTIVES

For cochlear implant (CI) users with residual low-frequency acoustic hearing in the nonimplanted ear, bimodal hearing combining the use of a CI and a contralateral hearing aid (HA) may provide more salient talker voice cues than CI alone to handle the variability of talker identity across trials. This study tested the effects of talker variability, bimodal hearing, and their interaction on response accuracy and time of CI users' Mandarin tone, vowel, and syllable recognition (i.e., combined Mandarin tone and vowel recognition in this study).

DESIGN

Fifteen prelingually deafened native Mandarin-speaking CI users (at age 20 or lower) participated in this study. Four talkers each produced six Mandarin single-vowel syllables in four lexical tones. The stimuli were presented in quiet via a single loudspeaker. To study the effects of talker variability, Mandarin tone, vowel, and syllable recognition was tested in two presentation conditions: with stimuli blocked according to talker (blocked-talker condition) or mixed across talkers from trial to trial (mixed-talker condition). To explore the effects of bimodal hearing, two processor conditions were tested: CI alone or CI + HA. The cumulative response time was recorded as an indirect indicator of the cognitive load or listening effort in each condition. The correlations were computed between demographic/hearing factors (e.g., hearing thresholds in the nonimplanted ear) and bimodal performance/benefits (where bimodal benefits refer to the performance differences between CI alone and CI + HA).

RESULTS

Mandarin tone recognition with both CI alone and CI + HA was significantly poorer in the mixed-talker condition than in the blocked-talker condition, while vowel recognition was comparable in the two presentation conditions. Bimodal hearing significantly improved Mandarin tone recognition but not vowel recognition. Mandarin syllable recognition was significantly affected by both talker variability and bimodal hearing. The cumulative response time significantly reduced with CI + HA compared with CI alone, but remained invariant with respect to talker variability. There was no interaction between talker variability and bimodal hearing for any performance measure adopted in this study. Correlation analyses revealed that the bimodal performance and benefits in Mandarin tone, vowel, and syllable recognition could not be predicted by the hearing thresholds in the nonimplanted ear or by the demographic factors of the participants.

CONCLUSIONS

Talker variability from trial to trial significantly degraded Mandarin tone and syllable recognition performance in both the CI alone and CI + HA conditions. While bimodal hearing did not reduce the talker variability effects on Mandarin tone and syllable recognition, generally better Mandarin tone and syllable recognition performance with shorter response time (an indicator of less listening effort) was observed when a contralateral HA was used in conjunction with the CI. On the other hand, vowel recognition was not significantly affected by either talker variability or bimodal hearing, because ceiling effects could not be counted out of the vowel recognition results.

Deficits in the Sensitivity to Pitch Sweeps by School-Aged Children Wearing Cochlear Implants

Sensitivity to static changes in pitch has been shown to be poorer in school-aged children wearing cochlear implants (CIs) than children with normal hearing (NH), but it is unclear whether this is also the case for dynamic changes in pitch. Yet, dynamically changing pitch has considerable ecological relevance in terms of natural speech, particularly aspects such as intonation, emotion, or lexical tone information. Twenty one children with NH and 23 children wearing a CI participated in this study, along with 18 NH adults and 6 CI adults for comparison. Listeners with CIs used their clinically assigned settings with envelope-based coding strategies. Percent correct was measured in one- or three-interval two-alternative forced choice tasks, for the direction or discrimination of harmonic complexes based on a linearly rising or falling fundamental frequency. Sweep rates were adjusted per subject, in a logarithmic scale, so as to cover the full extent of the psychometric function. Data for up- and down-sweeps were fitted separately, using a maximum-likelihood technique. Fits were similar for up- and down-sweeps in the discrimination task, but diverged in the direction task because psychometric functions for down-sweeps were very shallow. Hits and false alarms were then converted into d′ and beta values, from which a threshold was extracted at a d′ of 0.77. Thresholds were very consistent between the two tasks and considerably higher (worse) for CI listeners than for their NH peers. Thresholds were also higher for children than adults. Factors such as age at implantation, age at profound hearing loss, and duration of CI experience did not play any major role in this sensitivity. Thresholds of dynamic pitch sensitivity (in either task) also correlated with thresholds for static pitch sensitivity and with performance in tasks related to speech prosody.

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.

Melodic interval perception by normal-hearing listeners and cochlear implant users

The perception of melodic intervals (sequential pitch differences) is essential to music perception. This study tested melodic interval perception in normal-hearing (NH) listeners and cochlear implant (CI) users. Melodic interval ranking was tested using an adaptive procedure. CI users had slightly higher interval ranking thresholds than NH listeners. Both groups' interval ranking thresholds, although not affected by root note, significantly increased with standard interval size and were higher for descending intervals than for ascending intervals. The pitch direction effect may be due to a procedural artifact or a difference in central processing. In another test, familiar melodies were played with all the intervals scaled by a single factor. Subjects rated how in tune the melodies were and adjusted the scaling factor until the melodies sounded the most in tune. CI users had lower final interval ratings and less change in interval rating as a function of scaling factor than NH listeners. For CI users, the root-mean-square error of the final scaling factors and the width of the interval rating function were significantly correlated with the average ranking threshold for ascending rather than descending intervals, suggesting that CI users may have focused on ascending intervals when rating and adjusting the melodies.

Enhancement of temporal cues to pitch in cochlear implants: effects on pitch ranking

The abilities to hear changes in pitch for sung vowels and understand speech using an experimental sound coding strategy (eTone) that enhanced coding of temporal fundamental frequency (F0) information were tested in six cochlear implant users, and compared with performance using their clinical (ACE) strategy. In addition, rate- and modulation rate-pitch difference limens (DLs) were measured using synthetic stimuli with F0s below 300 Hz to determine psychophysical abilities of each subject and to provide experience in attending to rate cues for the judgment of pitch. Sung-vowel pitch ranking tests for stimuli separated by three semitones presented across an F0 range of one octave (139-277 Hz) showed a significant benefit for the experimental strategy compared to ACE. Average d-prime (d') values for eTone (d' = 1.05) were approximately three time larger than for ACE (d' = 0.35). Similar scores for both strategies in the speech recognition tests showed that coding of segmental speech information by the experimental strategy was not degraded. Average F0 DLs were consistent with results from previous studies and for all subjects were less than or equal to approximately three semitones for F0s of 125 and 200 Hz.

Pitch contour identification with combined place and temporal cues using cochlear implants

This study investigated the integration of place- and temporal-pitch cues in pitch contour identification (PCI), in which cochlear implant (CI) users were asked to judge the overall pitch-change direction of stimuli. Falling and rising pitch contours were created either by continuously steering current between adjacent electrodes (place pitch), by continuously changing amplitude modulation (AM) frequency (temporal pitch), or both. The percentage of rising responses was recorded as a function of current steering or AM frequency change, with single or combined pitch cues. A significant correlation was found between subjects' sensitivity to current steering and AM frequency change. The integration of place- and temporal-pitch cues was most effective when the two cues were similarly discriminable in isolation. Adding the other (place or temporal) pitch cues shifted the temporal- or place-pitch psychometric functions horizontally without changing the slopes. PCI was significantly better with consistent place- and temporal-pitch cues than with inconsistent cues. PCI with single cues and integration of pitch cues were similar on different electrodes. The results suggest that CI users effectively integrate place- and temporal-pitch cues in relative pitch perception tasks. Current steering and AM frequency change should be coordinated to better transmit dynamic pitch information to CI users.

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.

Development of a temporal fundamental frequency coding strategy for cochlear implants

A sound-coding strategy for users of cochlear implants, named enhanced-envelope-encoded tone (eTone), was developed to improve coding of fundamental frequency (F0) in the temporal envelopes of the electrical stimulus signals. It is based on the advanced combinational encoder (ACE) strategy and includes additional processing that explicitly applies F0 modulation to channel envelope signals that contain harmonics of prominent complex tones. Channels that contain only inharmonic signals retain envelopes normally produced by ACE. The strategy incorporates an F0 estimator to determine the frequency of modulation and a harmonic probability estimator to control the amount of modulation enhancement applied to each channel. The F0 estimator was designed to provide an accurate estimate of F0 with minimal processing lag and robustness to the effects of competing noise. Error rates for the F0 estimator and accuracy of the harmonic probability estimator were compared with previous approaches and outcomes demonstrated that the strategy operates effectively across a range of signals and conditions that are relevant to cochlear implant users.

Encoding pitch contours using current steering

This study investigated cochlear implant (CI) users' ability to perceive pitch cues from time-varying virtual channels (VCs) to identify pitch contours. Seven CI users were tested on apical, medial, and basal electrode pairs with stimulus durations from 100 to 1000 ms. In one stimulus set, 9 pitch contours were created by steering current between the component electrodes and the VC halfway between the electrodes. Another stimulus set only contained 3 pitch contours (flat, falling, and rising). VC discrimination was also tested on the same electrodes. The total current level of dual-electrode stimuli was linearly interpolated between those of single-electrode stimuli to minimize loudness changes. The results showed that pitch contour identification (PCI) scores were similar across electrode locations, and significantly improved at longer durations. For durations longer than 300 ms, 2 subjects had nearly perfect 9-contour identification, and 5 subjects perfectly identified the 3 basic contours. Both PCI and VC discrimination varied greatly across subjects. Cumulative d(') values for VC discrimination were significantly correlated with 100-, 200-, and 500-ms PCI scores. These results verify the feasibility of encoding pitch contours using current steering, and suggest that identification of such pitch contours strongly relies on CI users' sensitivity to VCs.