Discriminability and Perceptual Saliency of Temporal and Spectral Cues for Final Fricative Consonant Voicing in Simulated Cochlear-Implant and Bimodal Hearing.

Multiple redundant acoustic cues can contribute to the perception of a single phonemic contrast. This study investigated the effect of spectral degradation on the discriminability and perceptual saliency of acoustic cues for identification of word-final fricative voicing in "loss" versus "laws", and possible changes that occurred when low-frequency acoustic cues were restored. Three acoustic cues that contribute to the word-final /s/-/z/ contrast (first formant frequency [F1] offset, vowel-consonant duration ratio, and consonant voicing duration) were systematically varied in synthesized words. A discrimination task measured listeners' ability to discriminate differences among stimuli within a single cue dimension. A categorization task examined the extent to which listeners make use of a given cue to label a syllable as "loss" versus "laws" when multiple cues are available. Normal-hearing listeners were presented with stimuli that were either unprocessed, processed with an eight-channel noise-band vocoder to approximate spectral degradation in cochlear implants, or low-pass filtered. Listeners were tested in four listening conditions: unprocessed, vocoder, low-pass, and a combined vocoder + low-pass condition that simulated bimodal hearing. Results showed a negative impact of spectral degradation on F1 cue discrimination and a trading relation between spectral and temporal cues in which listeners relied more heavily on the temporal cues for "loss-laws" identification when spectral cues were degraded. Furthermore, the addition of low-frequency fine-structure cues in simulated bimodal hearing increased the perceptual saliency of the F1 cue for "loss-laws" identification compared with vocoded speech. Findings suggest an interplay between the quality of sensory input and cue importance.

The role of continuous low-frequency harmonicity cues for interrupted speech perception in bimodal hearing.

Low-frequency acoustic cues have been shown to enhance speech perception by cochlear-implant users, particularly when target speech occurs in a competing background. The present study examined the extent to which a continuous representation of low-frequency harmonicity cues contributes to bimodal benefit in simulated bimodal listeners. Experiment 1 examined the benefit of restoring a continuous temporal envelope to the low-frequency ear while the vocoder ear received a temporally interrupted stimulus. Experiment 2 examined the effect of providing continuous harmonicity cues in the low-frequency ear as compared to restoring a continuous temporal envelope in the vocoder ear. Findings indicate that bimodal benefit for temporally interrupted speech increases when continuity is restored to either or both ears. The primary benefit appears to stem from the continuous temporal envelope in the low-frequency region providing additional phonetic cues related to manner and F1 frequency; a secondary contribution is provided by low-frequency harmonicity cues when a continuous representation of the temporal envelope is present in the low-frequency, or both ears. The continuous temporal envelope and harmonicity cues of low-frequency speech are thought to support bimodal benefit by facilitating identification of word and syllable boundaries, and by restoring partial phonetic cues that occur during gaps in the temporally interrupted stimulus.

Top-Down Processes in Simulated Electric-Acoustic Hearing: The Effect of Linguistic Context on Bimodal Benefit for Temporally Interrupted Speech.

OBJECTIVES: Previous studies have documented the benefits of bimodal hearing as compared with a cochlear implant alone, but most have focused on the importance of bottom-up, low-frequency cues. The purpose of the present study was to evaluate the role of top-down processing in bimodal hearing by measuring the effect of sentence context on bimodal benefit for temporally interrupted sentences. It was hypothesized that low-frequency acoustic cues would facilitate the use of contextual information in the interrupted sentences, resulting in greater bimodal benefit for the higher context (CUNY) sentences than for the lower context (IEEE) sentences. DESIGN: Young normal-hearing listeners were tested in simulated bimodal listening conditions in which noise band vocoded sentences were presented to one ear with or without low-pass (LP) filtered speech or LP harmonic complexes (LPHCs) presented to the contralateral ear. Speech recognition scores were measured in three listening conditions: vocoder-alone, vocoder combined with LP speech, and vocoder combined with LPHCs. Temporally interrupted versions of the CUNY and IEEE sentences were used to assess listeners' ability to fill in missing segments of speech by using top-down linguistic processing. Sentences were square-wave gated at a rate of 5 Hz with a 50% duty cycle. Three vocoder channel conditions were tested for each type of sentence (8, 12, and 16 channels for CUNY; 12, 16, and 32 channels for IEEE) and bimodal benefit was compared for similar amounts of spectral degradation (matched-channel comparisons) and similar ranges of baseline performance. Two gain measures, percentage-point gain and normalized gain, were examined. RESULTS: Significant effects of context on bimodal benefit were observed when LP speech was presented to the residual-hearing ear. For the matched-channel comparisons, CUNY sentences showed significantly higher normalized gains than IEEE sentences for both the 12-channel (20 points higher) and 16-channel (18 points higher) conditions. For the individual gain comparisons that used a similar range of baseline performance, CUNY sentences showed bimodal benefits that were significantly higher (7% points, or 15 points normalized gain) than those for IEEE sentences. The bimodal benefits observed here for temporally interrupted speech were considerably smaller than those observed in an earlier study that used continuous speech. Furthermore, unlike previous findings for continuous speech, no bimodal benefit was observed when LPHCs were presented to the LP ear. CONCLUSIONS: Findings indicate that linguistic context has a significant influence on bimodal benefit for temporally interrupted speech and support the hypothesis that low-frequency acoustic information presented to the residual-hearing ear facilitates the use of top-down linguistic processing in bimodal hearing. However, bimodal benefit is reduced for temporally interrupted speech as compared with continuous speech, suggesting that listeners' ability to restore missing speech information depends not only on top-down linguistic knowledge but also on the quality of the bottom-up sensory input.

Vowel identification by cochlear implant users: Contributions of duration cues and dynamic spectral cues.

A recent study from our laboratory assessed vowel identification in cochlear implant (CI) users, using full /dVd/ syllables and partial (center- and edges-only) syllables with duration cues neutralized [Donaldson, Rogers, Cardenas, Russell, and Hanna (2013). J. Acoust. Soc. Am. 134, 3021-3028]. CI users' poorer performance for partial syllables as compared to full syllables, and for edges-only syllables as compared to center-only syllables, led to the hypotheses (1) that CI users may rely strongly on vowel duration cues; and (2) that CI users have more limited access to dynamic spectral cues than steady-state spectral cues. The present study tested those hypotheses. Ten CI users and ten young normal hearing (YNH) listeners heard full /dVd/ syllables and modified (center- and edges-only) syllables in which vowel duration cues were either preserved or eliminated. The presence of duration cues significantly improved vowel identification scores in four CI users, suggesting a strong reliance on duration cues. Duration effects were absent for the other CI users and the YNH listeners. On average, CI users and YNH listeners demonstrated similar performance for center-only stimuli and edges-only stimuli having the same total duration of vowel information. However, three CI users demonstrated significantly poorer performance for the edges-only stimuli, indicating apparent deficits of dynamic spectral processing.

Vowel identification by cochlear implant users: contributions of static and dynamic spectral cues.

Previous research has shown that normal hearing listeners can identify vowels in syllables on the basis of either quasi-static or dynamic spectral cues; however, it is not known how well cochlear implant (CI) users with current-generation devices can make use of these cues. The present study assessed vowel identification in adult CI users and a comparison group of young normal hearing (YNH) listeners. Stimuli were naturally spoken /dVd/ syllables and modified syllables that retained only quasi-static spectral cues from an 80-ms segment of the vowel center ("C80" stimuli) or dynamic spectral cues from two 20-ms segments of the vowel edges ("E20" stimuli). YNH listeners exhibited near-perfect performance for the unmodified (99.8%) and C80 (92.9%) stimuli and maintained good performance for the E20 stimuli (70.2%). CI users exhibited poorer average performance than YNH listeners for the Full stimuli (72.3%) and proportionally larger reductions in performance for the C80 stimuli (41.8%) and E20 stimuli (29.0%). Findings suggest that CI users have difficulty identifying vowels on the basis of spectral cues in the absence of duration cues, and have limited access to brief dynamic spectral cues. Error analyses suggest that CI users may rely strongly on vowel duration cues when those cues are available.

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

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

Within-subjects comparison of the HiRes and Fidelity120 speech processing strategies: speech perception and its relation to place-pitch sensitivity.

OBJECTIVES: Previous studies have confirmed that current steering can increase the number of discriminable pitches available to many cochlear implant (CI) users; however, the ability to perceive additional pitches has not been linked to improved speech perception. The primary goals of this study were to determine (1) whether adult CI users can achieve higher levels of spectral cue transmission with a speech processing strategy that implements current steering (Fidelity120) than with a predecessor strategy (HiRes) and, if so, (2) whether the magnitude of improvement can be predicted from individual differences in place-pitch sensitivity. A secondary goal was to determine whether Fidelity120 supports higher levels of speech recognition in noise than HiRes. DESIGN: A within-subjects repeated measures design evaluated speech perception performance with Fidelity120 relative to HiRes in 10 adult CI users. Subjects used the novel strategy (either HiRes or Fidelity120) for 8 wks during the main study; a subset of five subjects used Fidelity120 for three additional months after the main study. Speech perception was assessed for the spectral cues related to vowel F1 frequency, vowel F2 frequency, and consonant place of articulation; overall transmitted information for vowels and consonants; and sentence recognition in noise. Place-pitch sensitivity was measured for electrode pairs in the apical, middle, and basal regions of the implanted array using a psychophysical pitch-ranking task. RESULTS: With one exception, there was no effect of strategy (HiRes versus Fidelity120) on the speech measures tested, either during the main study (N = 10) or after extended use of Fidelity120 (N = 5). The exception was a small but significant advantage for HiRes over Fidelity120 for consonant perception during the main study. Examination of individual subjects' data revealed that 3 of 10 subjects demonstrated improved perception of one or more spectral cues with Fidelity120 relative to HiRes after 8 wks or longer experience with Fidelity120. Another three subjects exhibited initial decrements in spectral cue perception with Fidelity120 at the 8-wk time point; however, evidence from one subject suggested that such decrements may resolve with additional experience. Place-pitch thresholds were inversely related to improvements in vowel F2 frequency perception with Fidelity120 relative to HiRes. However, no relationship was observed between place-pitch thresholds and the other spectral measures (vowel F1 frequency or consonant place of articulation). CONCLUSIONS: Findings suggest that Fidelity120 supports small improvements in the perception of spectral speech cues in some Advanced Bionics CI users; however, many users show no clear benefit. Benefits are more likely to occur for vowel spectral cues (related to F1 and F2 frequency) than for consonant spectral cues (related to place of articulation). There was an inconsistent relationship between place-pitch sensitivity and improvements in spectral cue perception with Fidelity120 relative to HiRes. This may partly reflect the small number of sites at which place-pitch thresholds were measured. Contrary to some previous reports, there was no clear evidence that Fidelity120 supports improved sentence recognition in noise.

Vowel identification by younger and older listeners: relative effectiveness of vowel edges and vowel centers.

Young normal-hearing (YNH) and older normal-hearing (ONH) listeners identified vowels in naturally produced /bVb/ syllables and in modified syllables that consisted of variable portions of the vowel edges (silent-center [SC] stimuli) or vowel center (center-only [CO] stimuli). Listeners achieved high levels of performance for all but the shortest stimuli, indicating that they were able to access vowel cues throughout the syllable. ONH listeners performed similarly to YNH listeners for most stimuli, but performed more poorly for the shortest CO stimuli. SC and CO stimuli were equally effective in supporting vowel identification except when acoustic information was limited to 20 ms.

BKB-SIN and ANL predict perceived communication ability in cochlear implant users.

OBJECTIVE: Cochlear implant (CI) users typically report that speech recognition becomes substantially more difficult in the presence of background noise. This perception is consistent with objective measures of speech recognition showing that CI users require more favorable signal-to-noise ratios than normal-hearing (NH) listeners to achieve equivalent speech recognition. However, recent research in hearing aid users suggests that noise tolerance or the "willingness to listen in noise" may also influence perceived communication ability. Therefore, the purpose of this study was to evaluate the extent to which speech recognition in noise and noise tolerance predict perceived communication ability among adult CI users. DESIGN: Twenty adult CI users were evaluated on an objective test of speech recognition in noise (Bamford-Kowal-Bench Sentences in Noise [BKB-SIN] test) and a measure of noise tolerance (Acceptable Noise Level [ANL] test) and completed a self-report measure of communication difficulty (Abbreviated Profile of Hearing Aid Benefit [APHAB]). Relationships between BKB-SIN scores, ANL scores, and aided APHAB scores for the ease of communication, background noise, reverberation, and Global scales were assessed. In addition, BKB-SIN scores and ANL scores for the CI users were compared with scores for a control group of 23 NH listeners. RESULTS: CI users demonstrated substantially poorer BKB-SIN scores than NH listeners, as expected; however, their ANL scores were similar to those for NH listeners. BKB-SIN scores and ANL scores were not systematically related to one another. Each measure accounted for more than one third of the variance in CI users' aided APHAB Global scores; together, the two measures accounted for 72% of that variance. CONCLUSIONS: Both speech recognition in noise and noise tolerance are strongly associated with CI users' self-perceived communication ability. The two measures seem to reflect different factors that influence an individual's communication experience; thus, both may provide useful clinical information. The establishment of formal criteria for BKB-SIN scores and ANL scores that are predictive of excessive communication difficulty may help to identify CI users who could benefit from additional audiologic rehabilitation.

Forward-masked spatial tuning curves in cochlear implant users.

Forward-masked psychophysical spatial tuning curves (fmSTCs) were measured in twelve cochlear-implant subjects, six using bipolar stimulation (Nucleus devices) and six using monopolar stimulation (Clarion devices). fmSTCs were measured at several probe levels on a middle electrode using a fixed-level probe stimulus and variable-level maskers. The average fmSTC slopes obtained in subjects using bipolar stimulation (3.7 dBmm) were approximately three times steeper than average slopes obtained in subjects using monopolar stimulation (1.2 dBmm). Average spatial bandwidths were about half as wide for subjects with bipolar stimulation (2.6 mm) than for subjects with monopolar stimulation (4.6 mm). None of the tuning curve characteristics changed significantly with probe level. fmSTCs replotted in terms of acoustic frequency, using Greenwood's [J. Acoust. Soc. Am. 33, 1344-1356 (1961)] frequency-to-place equation, were compared with forward-masked psychophysical tuning curves obtained previously from normal-hearing and hearing-impaired acoustic listeners. The average tuning characteristics of fmSTCs in electric hearing were similar to the broad tuning observed in normal-hearing and hearing-impaired acoustic listeners at high stimulus levels. This suggests that spatial tuning is not the primary factor limiting speech perception in many cochlear implant users.

Effects of vowel context on the recognition of initial and medial consonants by cochlear implant users.

OBJECTIVE: Scores on consonant-recognition tests are widely used as an index of speech-perception ability in cochlear implant (CI) users. The consonant stimuli in these tests are typically presented in the /alpha/ vowel context, even though consonants in conversational speech occur in many other contexts. For this reason, it would be useful to know whether vowel context has any systematic effect on consonant recognition in this population. The purpose of the present study was to compare consonant recognition for the /alpha, i/, and /u/ vowel contexts for consonants presented in both initial (Cv) and medial (vCv) positions. DESIGN: Twenty adult CI users with one of three different implanted devices underwent consonant-confusion testing. Twelve stimulus conditions that differed according to vowel context (/alpha, i, u/), consonant position (Cv, vCv), and talker gender (male, female) were assessed in each subject. RESULTS: Mean percent-correct consonant-recognition scores were slightly (5 to 8%) higher for the /alpha/ and /u/ vowel contexts than for the /i/ vowel context for both initial and medial consonants. This general pattern was observed for both male and female talkers, for subjects with better and poorer average consonant-recognition performance, and for subjects using low, medium, and high stimulation rates in their speech processors. In contrast to the mean data, many individual subjects demonstrated large effects of vowel context. For 10 of 20 subjects, consonant-recognition scores varied by 15% or more across vowel contexts in one or more stimulus conditions. Similar to the mean data, these differences generally reflected better performance for the /alpha/ and /u/ vowel contexts than for the /i/ vowel context. An analysis of consonant features showed that overall performance was best for the voicing feature, followed by the manner and place features, and that the place feature showed the strongest effect of vowel context. Vowel-context effects were strongest for the six consonants /d, j, n, k, m/, and /l/. For three of these consonants (/j, n, k/), the back vowels /alpha/ and /u/ produced substantially (30 to 35%) higher mean scores than the front vowel /i/. For each of the remaining three consonants, a unique pattern was observed in which a different single vowel produced substantially higher scores than the others. Several additional consonants (/s, g, w, b/, and /d/) showed strong context effects in either the initial consonant or medial consonant position. Overall, voiceless stop, nasal, and glide-liquid consonants showed the strongest effects of vowel context, whereas the voiceless fricative and voiceless affricate consonants were least affected. Consistent with the feature analysis, a qualitative assessment of phoneme errors for the six key consonants indicated that vowel-context effects stem primarily from changes in the number of place-of-articulation errors made in each context. CONCLUSIONS: Vowel context has small but significant effects on consonant-recognition scores for the "average" CI listener, with the back vowels /alpha/ and /u/ producing better performance than the front vowel /i/. In contrast to the average results, however, the effects of vowel context are sizable in some individual subjects. This suggests that it may be beneficial to assess consonant recognition using two vowels, such as /alpha/ and /i/, which produce better and poorer performance, respectively. The present results underscore previous findings that poor transmission of spectral speech cues limits consonant-recognition performance in CI users. Spectral cue transmission may be hindered not only by poor spectral resolution in these listeners but also by the brief duration and dynamic nature of consonant place-of-articulation cues.

Place-pitch discrimination of single- versus dual-electrode stimuli by cochlear implant users (L).

Simultaneous or near-simultaneous activation of adjacent cochlear implant electrodes can produce pitch percepts intermediate to those produced by each electrode separately, thereby increasing the number of place-pitch steps available to cochlear implant listeners. To estimate how many distinct pitches could be generated with simultaneous dual-electrode stimulation, the present study measured place-pitch discrimination thresholds for single- versus dual-electrode stimuli in users of the Clarion CII device. Discrimination thresholds were expressed as the proportion of current directed to the secondary electrode of the dual-electrode pair. For 16 of 17 electrode pairs tested in six subjects, thresholds ranged from 0.11 to 0.64, suggesting that dual-electrode stimuli can produce 2-9 discriminable pitches between the pitches of single electrodes. Some subjects demonstrated a level effect, with better place-pitch discrimination at higher stimulus levels. Equal loudness was achieved with dual-electrode stimuli at net current levels that were similar to or slightly higher than those for single-electrode stimuli.

Effects of pulse rate and electrode array design on intensity discrimination in cochlear implant users.

The effects of pulse rate on intensity discrimination were evaluated in 14 subjects with Clarion C-I cochlear implants. Subjects had a standard [Clarion spiral electrode array (SPRL group)] or perimodiolar electrode array [Clarion HiFocus electrode array with electrode positioning system (HF+EPS group)]. Weber fractions for intensity discrimination [ Wf(dB)= 10 log deltaI/I] were evaluated at five levels over dynamic range at each of three pulse rates (200, 1625 and 6500 pps) using monopolar stimulation. Weber fractions were smaller for 200 pps stimuli than for 1625 or 6500 pps stimuli in both groups. Weber fractions were significantly smaller for SPRL subjects (mean Wf(dB) = -9.1 dB) than for HF+EPS subjects (mean Wf(dB) = -6.7 dB). Intensity difference limens (DLs) expressed as a percentage of dynamic range (DR) (deltaI%DR= deltaI/DRdB* 100) did not vary systematically with pulse rate in either group. Larger intensity DLs combined with smaller dynamic ranges led to fewer intensity steps over the dynamic range for HF+EPS subjects (average 9 steps) compared to SPRL subjects (average 23 steps). The observed effects of pulse rate and electrode array design may stem primarily from an inverse relationship between absolute current amplitude and the size of intensity DLs. The combination of smaller dynamic ranges and larger Weber fractions in HF+EPS subjects could be the result of increased variability of neural outputs in these subjects.

Effects of presentation level on phoneme and sentence recognition in quiet by cochlear implant listeners.

OBJECTIVE: The objectives of this study were to characterize the effects of presentation level on speech recognition in quiet by cochlear implant users with the Nucleus 22 SPEAK and Clarion v1.2 CIS speech-processing strategies, and to relate speech recognition at low presentation levels to stimulus audibility as measured by sound field thresholds. It was hypothesized that speech recognition performance in both Nucleus SPEAK and Clarion CIS participants would decrease as presentation level was decreased below 50 to 60 dBA, due to audibility limitations. However, it was expected that such level effects would be less severe in CIS participants than in SPEAK participants because the Clarion v1.2 device encodes a wider acoustic dynamic range (up to 60 dB) than the Nucleus 22 device (30 dB). DESIGN: Performance-intensity (P-I) functions for vowels, consonants and sentences in quiet were obtained from each participant. P-I functions incorporated speech levels of 70, 60, 50, 40 and 30 dBA. Subjects used their clinical speech processor maps and adjusted the loudness (volume/sensitivity) controls on their processors so that speech presented at 60 dBA was comfortably loud. Maps were created using default clinical procedures and were not adjusted to optimize sound field thresholds. Sound field thresholds and dynamic ranges were measured for warbled pure tones with frequencies of 250 to 6000 Hz. RESULTS: Consonant and sentence recognition showed strong level effects for both SPEAK and CIS participants, with performance decreasing substantially at levels below 50 dBA in most individuals. Vowel recognition showed weaker level effects. For all three speech materials, SPEAK and CIS participants demonstrated similar mean performance at 70 dBA; however, SPEAK participants showed larger reductions in performance than CIS participants with decreasing level. Sound field thresholds were more sensitive for CIS participants than for SPEAK participants, supporting the hypothesis that performance differences were related to audibility. CONCLUSIONS: Cochlear implant listeners are unable to maintain good speech recognition at low presentation levels due to reduced stimulus audibility, and this may significantly limit their ability to communicate in daily life. It is likely that audibility differences between SPEAK and CIS participants in the present study can be attributed at least partly to differences in the acoustic dynamic range used by the respective processors. However, several additional factors may have contributed to differences in audibility and perception of soft speech among individual listeners with both devices. These include the minimum and maximum electrical stimulation levels specified in participants' maps and the speech processor sensitivity setting used for testing.

Intensity discrimination and increment detection in cochlear-implant users.

Intensity difference limens (DLs) were measured in users of the Nucleus 22 and Clarion v1.2 cochlear implants and in normal-hearing listeners to better understand mechanisms of intensity discrimination in electric and acoustic hearing and to evaluate the possible role of neural adaptation. Intensity DLs were measured for three modes of presentation: gated (intensity increments gated synchronously with the pedestal), fringe (intensity increments delayed 250 or 650 ms relative to the onset of the pedestal), and continuous (intensity increments occur in the presence of a pedestal that is played throughout the experimental run). Stimuli for cochlear-implant listeners were trains of biphasic pulses; stimuli for normal-hearing listeners were a 1-kHz tone and a wideband noise. Clarion cochlear-implant listeners showed level-dependent effects of presentation mode. At low pedestal levels, gated thresholds were generally similar to thresholds obtained in the fringe and continuous conditions. At higher pedestal levels, however, the fringe and continuous conditions produced smaller intensity DLs than the gated condition, similar to the gated-continuous difference in intensity DLs observed in acoustic hearing. Nucleus cochlear-implant listeners did not show consistent threshold differences for the gated and fringe conditions, and were not tested in the continuous condition. It is not clear why a difference between gated and fringe thresholds occurred for the Clarion but not the Nucleus subjects. Normal-hearing listeners showed improved thresholds for the continuous condition relative to the gated condition, but the effect was larger for the 1-kHz tonal carrier than for the noise carrier. Findings suggest that adaptation occurring central to the inner hair cell synapse mediates the gated-continuous difference observed in Clarion cochlear-implant listeners and may also contribute to the gated-continuous difference in acoustic hearing.

Patterns of phoneme perception errors by listeners with cochlear implants as a function of overall speech perception ability.

Many studies have noted great variability in speech perception ability among postlingually deafened adults with cochlear implants. This study examined phoneme misperceptions for 30 cochlear implant listeners using either the Nucleus-22 or Clarion version 1.2 device to examine whether listeners with better overall speech perception differed qualitatively from poorer listeners in their perception of vowel and consonant features. In the first analysis, simple regressions were used to predict the mean percent-correct scores for consonants and vowels for the better group of listeners from those of the poorer group. A strong relationship between the two groups was found for consonant identification, and a weak, nonsignificant relationship was found for vowel identification. In the second analysis, it was found that less information was transmitted for consonant and vowel features to the poorer listeners than to the better listeners; however, the pattern of information transmission was similar across groups. Taken together, results suggest that the performance difference between the two groups is primarily quantitative. The results underscore the importance of examining individuals' perception of individual phoneme features when attempting to relate speech perception to other predictor variables.

Psychophysical recovery from pulse-train forward masking in electric hearing.

Psychophysical pulse-train forward-masking (PTFM) recovery functions were measured in fifteen subjects with the Nucleus mini-22 cochlear implant and six subjects with the Clarion cochlear implant. Masker and probe stimuli were 500-Hz trains of 200- or 77-micros/phase biphasic current pulses. Electrode configurations were bipolar for Nucleus subjects and monopolar for Clarion subjects. Masker duration was 320 ms. Probe duration was either 10 ms or 30 ms. Recovery functions were measured for a high-level masker on a middle electrode in all 21 subjects, on apical and basal electrodes in 7 of the Nucleus and 3 of the Clarion subjects, and for multiple masker levels on the middle electrode in 8 Nucleus subjects and 6 Clarion subjects. Recovery functions were described by an exponential process in which threshold shift (in microA) decreased exponentially with increasing time delay between the offset of the masker pulse train and the offset of the probe pulse train. All but 3 of the 21 subjects demonstrated recovery time constants on a middle electrode that were less than 95 ms. The mean time constant for these 18 subjects was 54 ms (s.d. 17 ms). Three other subjects tested on three electrodes exhibited time constants larger than 95 ms from an apical electrode only. Growth-of-masking slopes depended upon time delay, as expected from an exponential recovery process, i.e., progressively shallower slopes were observed at time delays of 10 ms and 50 ms. Recovery of threshold shift (in microA) for PTFM in electrical hearing behaves inthe same way as recovery of threshold shift (in dB) for pure-tone forward masking in acoustic hearing. This supports the concept that linear microamps are the electrical equivalent of acoustic decibels. Recovery from PTFM was not related to speech recognition in a simple manner. Three subjects with prolonged PTFM recovery demonstrated poor speech scores. The remaining subjects with apparently normal PTFM recovery demonstrated speech scores ranging from poor to excellent. Findings suggest that normal PTFM recovery is only one of several factors associated with good speech recognition in cochlear-implant listeners. Comparisons of recovery curves for 10- and 30-ms probe durations in two subjects showed little or no temporal integration at time delays less than 95 ms where recovery functions have steep slopes. The same subjects exhibited large amounts of temporal integration at longer time delays where recovery slopes are more gradual. This suggests that probe detection depends primarily on detection of the final pulses in the probe stimulus and supports the use of offset-to-offset time delays for characterizing PTFM recovery in electric hearing.