Identifying Subclinical Hearing Loss: Extended Audiometry and Word Recognition in Noise.

INTRODUCTION: Normal-hearing people often have complaints about the ability to recognize speech in noise. Such disabilities are not typically assessed with conventional audiometry. Suprathreshold temporal deficits might contribute to reduced word recognition in noise as well as reduced temporally based binaural release of masking for speech. Extended high-frequency audibility (>8 kHz) has also been shown to contribute to speech perception in noise. The primary aim of this study was to compare conventional audiometric measures with measures that could reveal subclinical deficits. METHODS: Conventional and extended high-frequency audiometry was done with 119 normal-hearing people ranging in age from 18 to 72. The ability to recognize words in noise was evaluated with and without differences in temporally based spatial cues. A low-uncertainty, closed-set word recognition task was used to limit cognitive influences. RESULTS: In normal-hearing listeners, word recognition in noise ability decreases significantly with increasing pure-tone average (PTA). On average, signal-to-noise ratios worsened by 5.7 and 6.0 dB over the normal range, for the diotic and dichotic conditions, respectively. When controlling for age, a significant relationship remained in the diotic condition. Measurement error was estimated at 1.4 and 1.6 dB for the diotic and dichotic conditions, respectively. Controlling for both PTA and age, EHF-PTAs showed significant partial correlations with SNR50 in both conditions (ρ = 0.30 and 0.23). Temporally based binaural release of masking worsened with age by 1.94 dB from 18 to 72 years old but showed no significant relationship with either PTA. CONCLUSIONS: All three assessments in this study demonstrated hearing problems independently of those observed in conventional audiometry. Considerable degradations in word recognition in noise abilities were observed as PTAs increased within the normal range. The use of an efficient words-in-noise measure might help identify functional hearing problems for individuals that are traditionally normal hearing. Extended audiometry provided additional predictive power for word recognition in noise independent of both the PTA and age. Temporally based binaural release of masking for word recognition decreased with age independent of PTAs within the normal range, indicating multiple mechanisms of age-related decline with potential clinical impact.

Mandarin tone recognition in English speakers with normal hearing and with cochlear implants.

Objective: Mandarin-speaking cochlear implant users have difficulty perceiving tonal changes in speech with current signal processing strategies. The purpose of this study was to evaluate whether English-speaking cochlear implant and normal hearing listeners can be trained to recognise closed-set Mandarin tones. The validity of using native-English speakers to evaluate Mandarin tone perception in cochlear implants was tested.Design: Two groups of native-English speaking participants were evaluated. All listeners were given training rounds and evaluation rounds in which their tonal identification was tested. The normal-hearing group was also tested with acoustic simulations of the traditional Continuous Interleaved Sampling (CIS) strategy.Study sample: Ten normal-hearing English speakers and seven cochlear implant listeners participated.Results: The normal-hearing group correctly identified unprocessed tones at 87% and CIS-processed tones at 58% on average. The cochlear implant listeners achieved 56% correct identification on average.Conclusions: This level of performance for native English speaking CI users was comparable to previous studies using native Mandarin-speaking CI listeners, which showed a mean of 59% in 19 CI users.

Effects of age and hearing mechanism on spectral resolution in normal hearing and cochlear-implanted listeners.

Spectral resolution limits speech perception with a cochlear implant (CI) in post-lingually deaf adults. However, the development of spectral resolution in pre-lingually deaf implanted children is not well understood. Acoustic spectral resolution was measured as a function of age (school-age versus adult) in CI and normal-hearing (NH) participants using spectral ripple discrimination (SRD). A 3-alternative forced-choice task was used to obtain SRD thresholds at five ripple depths. Effects of age and hearing method on SRD and spectral modulation transfer function (SMTF) slope (reflecting frequency resolution) and x-intercept (reflecting across-channel intensity resolution) were examined. Correlations between SRD, SMTF parameters, age, and speech perception in noise were studied. Better SRD in NH than CI participants was observed at all depths. SRD thresholds and SMTF slope correlated with speech perception in CI users. When adjusted for floor performance, x-intercept did not correlate with SMTF slope or speech perception. Age and x-intercept correlations were positive and significant in NH but not CI children suggesting that across-channel intensity resolution matures during school-age in NH children. No evidence for maturation of spectral resolution beyond early school-age in pre-lingually deaf implanted CI users was found in the present study.

Nonlinguistic Outcome Measures in Adult Cochlear Implant Users Over the First Year of Implantation.

OBJECTIVES: Postlingually deaf cochlear implant users' speech perception improves over several months after implantation due to a learning process which involves integration of the new acoustic information presented by the device. Basic tests of hearing acuity might evaluate sensitivity to the new acoustic information and be less sensitive to learning effects. It was hypothesized that, unlike speech perception, basic spectral and temporal discrimination abilities will not change over the first year of implant use. If there were limited change over time and the test scores were correlated with clinical outcome, the tests might be useful for acute diagnostic assessments of hearing ability and also useful for testing speakers of any language, many of which do not have validated speech tests. DESIGN: Ten newly implanted cochlear implant users were tested for speech understanding in quiet and in noise at 1 and 12 months postactivation. Spectral-ripple discrimination, temporal-modulation detection, and Schroeder-phase discrimination abilities were evaluated at 1, 3, 6, 9, and 12 months postactivation. RESULTS: Speech understanding in quiet improved between 1 and 12 months postactivation (mean 8% improvement). Speech in noise performance showed no statistically significant improvement. Mean spectral-ripple discrimination thresholds and temporal-modulation detection thresholds for modulation frequencies of 100 Hz and above also showed no significant improvement. Spectral-ripple discrimination thresholds were significantly correlated with speech understanding. Low FM detection and Schroeder-phase discrimination abilities improved over the period. Individual learning trends varied, but the majority of listeners followed the same stable pattern as group data. CONCLUSIONS: Spectral-ripple discrimination ability and temporal-modulation detection at 100-Hz modulation and above might serve as a useful diagnostic tool for early acute assessment of cochlear implant outcome for listeners speaking any native language.

Clinical evaluation of music perception, appraisal and experience in cochlear implant users.

OBJECTIVES: The objectives were to evaluate the relationships among music perception, appraisal, and experience in cochlear implant users in multiple clinical settings and to examine the viability of two assessments designed for clinical use. DESIGN: Background questionnaires (IMBQ) were administered by audiologists in 14 clinics in the United States and Canada. The CAMP included tests of pitch-direction discrimination, and melody and timbre recognition. The IMBQ queried users on prior musical involvement, music listening habits pre and post implant, and music appraisals. STUDY SAMPLE: One-hundred forty-five users of Advanced Bionics and Cochlear Ltd cochlear implants. RESULTS: Performance on pitch direction discrimination, melody recognition, and timbre recognition tests were consistent with previous studies with smaller cohorts, as well as with more extensive protocols conducted in other centers. Relationships between perceptual accuracy and music enjoyment were weak, suggesting that perception and appraisal are relatively independent for CI users. CONCLUSIONS: Perceptual abilities as measured by the CAMP had little to no relationship with music appraisals and little relationship with musical experience. The CAMP and IMBQ are feasible for routine clinical use, providing results consistent with previous thorough laboratory-based investigations.

Validation of a clinical assessment of spectral-ripple resolution for cochlear implant users.

OBJECTIVES: Nonspeech psychophysical tests of spectral resolution, such as the spectral-ripple discrimination task, have been shown to correlate with speech-recognition performance in cochlear implant (CI) users. However, these tests are best suited for use in the research laboratory setting and are impractical for clinical use. A test of spectral resolution that is quicker and could more easily be implemented in the clinical setting has been developed. The objectives of this study were (1) To determine whether this new clinical ripple test would yield individual results equivalent to the longer, adaptive version of the ripple-discrimination test; (2) To evaluate test-retest reliability for the clinical ripple measure; and (3) To examine the relationship between clinical ripple performance and monosyllabic word recognition in quiet for a group of CI listeners. DESIGN: Twenty-eight CI recipients participated in the study. Each subject was tested on both the adaptive and the clinical versions of spectral ripple discrimination, as well as consonant-nucleus-consonant word recognition in quiet. The adaptive version of spectral ripple used a two-up, one-down procedure for determining spectral ripple discrimination threshold. The clinical ripple test used a method of constant stimuli, with trials for each of 12 fixed ripple densities occurring six times in random order. Results from the clinical ripple test (proportion correct) were then compared with ripple-discrimination thresholds (in ripples per octave) from the adaptive test. RESULTS: The clinical ripple test showed strong concurrent validity, evidenced by a good correlation between clinical ripple and adaptive ripple results (r = 0.79), as well as a correlation with word recognition (r = 0.7). Excellent test-retest reliability was also demonstrated with a high test-retest correlation (r = 0.9). CONCLUSIONS: The clinical ripple test is a reliable nonlinguistic measure of spectral resolution, optimized for use with CI users in a clinical setting. The test might be useful as a diagnostic tool or as a possible surrogate outcome measure for evaluating treatment effects in hearing.

Relationship between channel interaction and spectral-ripple discrimination in cochlear implant users.

Cochlear implant (CI) users can achieve remarkable speech understanding, but there is great variability in outcomes that is only partially accounted for by age, residual hearing, and duration of deafness. Results might be improved with the use of psychophysical tests to predict which sound processing strategies offer the best potential outcomes. In particular, the spectral-ripple discrimination test offers a time-efficient, nonlinguistic measure that is correlated with perception of both speech and music by CI users. Features that make this "one-point" test time-efficient, and thus potentially clinically useful, are also connected to controversy within the CI field about what the test measures. The current work examined the relationship between thresholds in the one-point spectral-ripple test, in which stimuli are presented acoustically, and interaction indices measured under the controlled conditions afforded by direct stimulation with a research processor. Results of these studies include the following: (1) within individual subjects there were large variations in the interaction index along the electrode array, (2) interaction indices generally decreased with increasing electrode separation, and (3) spectral-ripple discrimination improved with decreasing mean interaction index at electrode separations of one, three, and five electrodes. These results indicate that spectral-ripple discrimination thresholds can provide a useful metric of the spectral resolution of CI users.

Psychoacoustic performance and music and speech perception in prelingually deafened children with cochlear implants.

The number of pediatric cochlear implant (CI) recipients has increased substantially over the past 10 years, and it has become more important to understand the underlying mechanisms of the variable outcomes in this population. In this study, psychoacoustic measures of spectral-ripple and Schroeder-phase discrimination, the Clinical Assessment of Music Perception, and consonant-nucleus-consonant (CNC) word recognition in quiet and spondee reception threshold (SRT) in noise tests have been presented to 11 prelingually deafened CI users, aged 8-16 years with at least 5 years of CI experience. The children's performance was compared to the previously reported results of postlingually deafened adult CI users. The average spectral-ripple threshold (n = 10) was 2.08 ripples/octave. The average Schroeder-phase discrimination was 67.3% for 50 Hz and 56.5% for 200 Hz (n = 9). The Clinical Assessment of Music Perception test showed that the average complex pitch direction discrimination was 2.98 semitones. The mean melody score was at a chance level, and the mean timbre score was 34.1% correct. The mean CNC word recognition score was 68.6%, and the mean SRT in steady noise was -8.5 dB SNR. The children's spectral-ripple resolution, CNC word recognition, and SRT in noise performances were, within statistical bounds, the same as in a population of postlingually deafened adult CI users. However, Schroeder-phase discrimination and music perception were generally poorer than in the adults. It is possible then that this poorer performance seen in the children might be partly accounted for by the delayed maturation in their temporal processing ability, and because of this, the children's performance may have been driven more by their spectral sensitivity.

Improved perception of speech in noise and Mandarin tones with acoustic simulations of harmonic coding for cochlear implants.

Harmonic and temporal fine structure (TFS) information are important cues for speech perception in noise and music perception. However, due to the inherently coarse spectral and temporal resolution in electric hearing, the question of how to deliver harmonic and TFS information to cochlear implant (CI) users remains unresolved. A harmonic-single-sideband-encoder [(HSSE); Nie et al. (2008). Proceedings of IEEE International Conference on Acoustics, Speech, and Signal Processing; Lie et al., (2010). Proceedings of IEEE International Conference on Acoustics, Speech, and Signal Processing] strategy has been proposed that explicitly tracks the harmonics in speech and transforms them into modulators conveying both amplitude modulation and fundamental frequency information. For unvoiced speech, HSSE transforms the TFS into a slowly varying yet still noise-like signal. To investigate its potential, four- and eight-channel vocoder simulations of HSSE and the continuous-interleaved-sampling (CIS) strategy were implemented, respectively. Using these vocoders, five normal-hearing subjects' speech recognition performance was evaluated under different masking conditions; another five normal-hearing subjects' Mandarin tone identification performance was also evaluated. Additionally, the neural discharge patterns evoked by HSSE- and CIS-encoded Mandarin tone stimuli were simulated using an auditory nerve model. All subjects scored significantly higher with HSSE than with CIS vocoders. The modeling analysis demonstrated that HSSE can convey temporal pitch cues better than CIS. Overall, the results suggest that HSSE is a promising strategy to enhance speech perception with CIs.

The ability of cochlear implant users to use temporal envelope cues recovered from speech frequency modulation.

Previous studies have demonstrated that normal-hearing listeners can understand speech using the recovered "temporal envelopes," i.e., amplitude modulation (AM) cues from frequency modulation (FM). This study evaluated this mechanism in cochlear implant (CI) users for consonant identification. Stimuli containing only FM cues were created using 1, 2, 4, and 8-band FM-vocoders to determine if consonant identification performance would improve as the recovered AM cues become more available. A consistent improvement was observed as the band number decreased from 8 to 1, supporting the hypothesis that (1) the CI sound processor generates recovered AM cues from broadband FM, and (2) CI users can use the recovered AM cues to recognize speech. The correlation between the intact and the recovered AM components at the output of the sound processor was also generally higher when the band number was low, supporting the consonant identification results. Moreover, CI subjects who were better at using recovered AM cues from broadband FM cues showed better identification performance with intact (unprocessed) speech stimuli. This suggests that speech perception performance variability in CI users may be partly caused by differences in their ability to use AM cues recovered from FM speech cues.

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.

Evidence of across-channel processing for spectral-ripple discrimination in cochlear implant listeners.

Spectral-ripple discrimination has been used widely for psychoacoustical studies in normal-hearing, hearing-impaired, and cochlear implant listeners. The present study investigated the perceptual mechanism for spectral-ripple discrimination in cochlear implant listeners. The main goal of this study was to determine whether cochlear implant listeners use a local intensity cue or global spectral shape for spectral-ripple discrimination. The effect of electrode separation on spectral-ripple discrimination was also evaluated. Results showed that it is highly unlikely that cochlear implant listeners depend on a local intensity cue for spectral-ripple discrimination. A phenomenological model of spectral-ripple discrimination, as an "ideal observer," showed that a perceptual mechanism based on discrimination of a single intensity difference cannot account for performance of cochlear implant listeners. Spectral modulation depth and electrode separation were found to significantly affect spectral-ripple discrimination. The evidence supports the hypothesis that spectral-ripple discrimination involves integrating information from multiple channels.

Psychoacoustic abilities associated with music perception in cochlear implant users.

OBJECTIVES: This study was designed to determine what acoustic elements are associated with musical perception ability in cochlear implant (CI) users and to understand how acoustic elements, which are important to good speech perception, contribute to music perception in CI users. It was hypothesized that the variability in the performance of music and speech perception may be related to differences in the sensitivity to specific acoustic features such as spectral changes or temporal modulations, or both. DESIGN: A battery of hearing tasks was administered to 42 CI listeners. The Clinical Assessment of Music Perception was used, which evaluates complex-tone pitch-direction discrimination, melody recognition, and timbre recognition. To investigate spectral and temporal processing, spectral-ripple discrimination and Schroeder-phase discrimination abilities were evaluated. Speech perception ability in quiet and noise was also evaluated. Relationships between Clinical Assessment of Music Perception subtest scores, spectral-ripple discrimination thresholds, Schroeder-phase discrimination scores, and speech recognition scores were assessed. RESULTS: Spectral-ripple discrimination was shown to correlate with all three aspects of music perception studied. Schroeder-phase discrimination was generally not predictive of music perception outcomes. Music perception ability was significantly correlated with speech perception ability. Nearly half of the variance in melody and timbre recognition was predicted jointly by spectral-ripple and pitch-direction discrimination thresholds. Similar results were observed on speech recognition as well. CONCLUSIONS: This study suggests that spectral-ripple discrimination is significantly associated with music perception in CI users. A previous report showed that spectral-ripple discrimination is significantly correlated with speech recognition in quiet and in noise. This study also showed that speech recognition and music perception are also related to one another. Spectral-ripple discrimination ability seems to reflect a wide range of hearing abilities in CI users. The results suggest that materially improving spectral resolution could provide significant benefits in music and speech perception outcomes in CI users.

Hybrid Music Perception Outcomes: Implications for Melody and Timbre Recognition in Cochlear Implant Recipients.

OBJECTIVE: To examine whether or not electric-acoustic music perception outcomes, observed in a recent Hybrid L24 clinical trial, were related to the availability of low-frequency acoustic cues not present in the electric domain. STUDY DESIGN: Prospective, repeated-measures, within-subject design. SETTING: Academic research hospital. SUBJECTS: Nine normally hearing individuals. INTERVENTION: Simulated electric-acoustic hearing in normally hearing individuals. MAIN OUTCOMES MEASURES: Acutely measured melody and timbre recognition scores from the University of Washington Clinical Assessment of Music Perception (CAMP) test. RESULTS: Melody recognition scores were consistently better for listening conditions that included low-frequency acoustic information. Mean scores for both acoustic (73.5%, S.D. = 15.5%) and electric-acoustic (67.9%, S.D. = 21.2%) conditions were significantly better (p < 0.001) than electric alone (39.2%, S.D. = 18.1%). This was not the case for timbre recognition for which scores were more variable across simulated listening modes with no significant differences found in mean scores across electric (36.1%, S.D. = 17.7%), acoustic (38.0%, S.D. = 20.4%), and electric-acoustic (40.7%, S.D. = 19.7%) conditions (p > 0.05). CONCLUSION: Recipients of hybrid cochlear implants demonstrate music perception abilities superior to those observed in traditional cochlear implant recipients. Results from the present study support the notion that electric-acoustic stimulation confers advantages related to the availability of low-frequency acoustic hearing, most particularly for melody recognition. However, timbre recognition remains more limited for both hybrid and traditional cochlear implant users. Opportunities remain for new coding strategies to improve timbre perception.

Improving performance in noise for hearing aids and cochlear implants using coherent modulation filtering.

This study evaluated the maximal attainable performance of speech enhancement strategies based on coherent modulation filtering. An optimal adaptive coherent modulation filtering algorithm was designed to enhance known signals from a target talker in two-talker babble noise. The algorithm was evaluated in a closed-set, speech-recognition-in-noise task. The speech reception threshold (SRT) was measured using a one-down, one-up adaptive procedure. Five hearing-impaired subjects and five cochlear implant users were tested in three processing conditions: (1) original sounds; (2) fixed coherent modulation filtered sounds; and (3) optimal coherent modulation filtered sounds. Six normal-hearing subjects were tested with a 6-channel cochlear implant simulation of sounds processed in the same three conditions. Significant improvements in SRTs were observed when the signal was processed with the optimal coherent modulation filtering algorithm. There was no benefit when the signal was processed with the fixed modulation filter. The current study suggested that coherent modulation filtering might be a promising method for front-end processing in hearing aids and cochlear implants. An approach such as hidden Markov models could be used to generalize the optimal coherent modulation filtering algorithm to unknown utterances and to extend it to open-set speech.

Clinical assessment of music perception in cochlear implant listeners.

OBJECTIVE: Cochlear implants (CI) have provided tremendous benefit for speech recognition in quiet for patients with severe and profound hearing impairment, but implant users still have great difficulty perceiving music. The purpose of this study was to develop a test to quantify music perception by CI listeners in a clinically practical manner that could be standardized for administration at any implant center. STUDY DESIGN: Prospective convenience sample. SETTING: Hearing research center at an academic hospital. PATIENTS: Eight CI listeners, including 5 men and 3 women with implant experience ranging from 0.5 to 6 years, participated in this study. They represented a variety of implant devices and strategies. INTERVENTION: Administration of the Clinical Assessment of Music Perception test in a standardized sound field. MAIN OUTCOME MEASURES: Music perception was assessed using a computerized test comprising pitch direction discrimination, melody identification, and timbre identification. The pitch subtest used a 2-alternative forced-choice adaptive procedure to determine a threshold interval for discrimination of complex pitch direction change. The melody and timbre subtests assessed recognition of 12 isochronous melodies and 8 musical instruments, respectively. RESULTS: Testing demonstrated a broad range of perceptual accuracy on all 3 subtests. Test duration averaged less than 45 minutes. CONCLUSION: Clinical Assessment of Music Perception is an efficient computerized test that may be used to measure 3 different aspects of music perception in CI users in a standardized and clinically practical manner.

Spectral-ripple resolution correlates with speech reception in noise in cochlear implant users.

Speech perception ability in noise is one of the most practical measures of success with a cochlear implant; however, with experience, this ability can change dramatically over time, making it a less than ideal tool for comparing performance among different processing strategies. This study examined performance on a spectral discrimination task and compared it to speech perception in noise. An adaptive procedure was used to determine the spectral-ripple density that subjects could discriminate. A closed-set, forced-choice adaptive procedure was used to determine speech reception thresholds for words in two-talker babble and in speech-shaped, steady-state noise. Spectral-ripple thresholds (ripples/octave) were significantly correlated with speech reception thresholds (dB SNR) in noise for 29 cochlear implant users (r = -0.55, p = 0.002 in two-talker babble; r = -0.62, p = 0.0004 in steady-state noise), demonstrating that better spectral resolution was associated with better speech perception in noise. A significant correlation was also found between the spectral-ripple discrimination ability and word recognition in quiet (r = 0.50, p = 0.009). In addition, test-retest reliability for spectral-ripple discrimination was good, and no learning was observed. The present study demonstrates that the spectral-ripple discrimination test, which is time efficient and nonlinguistic, would be a useful tool to evaluate cochlear implant performance with different signal processing strategies.

Effects of temporal fine structure on the lateralization of speech and on speech understanding in noise.

This study evaluated the role of temporal fine structure in the lateralization and understanding of speech in six normal-hearing listeners. Interaural time differences (ITDs) were introduced to invoke lateralization. Speech reception thresholds (SRTs) were evaluated in backgrounds of two-talker babble and speech-shaped noise. Two-syllable words with ITDs of 0 and 700 micros were used as targets. A vocoder technique, which systematically randomized fine structure, was used to evaluate the effects of fine structure on these tasks. Randomization of temporal fine structure was found to significantly reduce the ability of normal-hearing listeners to lateralize words, although for many listeners, good lateralization performance was achieved with as much as 80% fine-structure randomization. Most listeners demonstrated some rudimentary ability to lateralize with 100% fine-structure randomization. When ITDs were 0 micros, randomization of fine structure had a much greater effect on SRT in two-talker babble than in speech-shaped noise. Binaural advantages were also observed. In steady noise, the difference in SRT between words with 0- vs 700-micros ITDs was, on average, 6 dB with no fine-structure randomization and 2 dB with 100% fine-structure randomization. In two-talker babble this difference was 1.9 dB and, for most listeners, showed little effect of the degree of fine-structure randomization. These results suggest that (1) improved delivery of temporal fine structure would improve speech understanding in noise for implant recipients, (2) bilateral implant recipients might benefit from temporal envelope ITDs, and (3) improved delivery of temporal information could improve binaural benefits.

Current-level discrimination in the context of interleaved, multichannel stimulation in cochlear implants: effects of number of stimulated electrodes, pulse rate, and electrode separation.

The ability of cochlear implantees to detect an increment in current level at one of many stimulated electrodes was investigated. Such changes in the electric profile provide information for cochlear implantees to discriminate numerous sounds, especially vowels. In Experiment 1, sensitivity to increases in current level at one stimulation site in the electric profile decreased as the number of stimulated electrodes increased. This outcome was most likely a result of decreased stimulus levels at individual electrodes that were required to retain a comfortable loudness when the number of active electrodes was increased. Experiment 2 investigated the effects of pulse rate and separation between stimulation sites when the levels in percent of dynamic range and number of stimulated electrodes were held constant. The effect of pulse rate and electrode separation varied among listeners. The sensitivity of 6 of 9 listeners was best at the pulse rate that they used clinically. This might have been the result of adaptation to the clinical pulse rate, or listeners might have chosen their inherently best pulse rate during the clinical fitting.