Auditory Learning and Adaptation after Cochlear Implantation: A Preliminary Study of Discrimination and Labeling of Vowel Sounds by Cochlear Implant Users

This study examined two possible reasons underlying longitudinal increases in vowel identification by cochlear implant users: improved labeling of vowel sounds and improved electrode discrimination. The Multidimensional Phoneme Identification (MPI) model was used to obtain ceiling estimates of vowel identification for each subject, given his/her electrode discrimination skills. Vowel identification scores were initially lower than the ceiling estimates, but they gradually approached them over the first few months post-implant. Taken together, the present results suggest that improved labeling is the main mechanism explaining post-implant increases in vowel identification.     

On the structure of phoneme categories in listeners with cochlear implants.

PURPOSE: To describe cochlear implant users' phoneme labeling, discrimination, and prototypes for a vowel and a sibilant contrast, and to assess the effects of 1 year's experience with prosthetic hearing. METHOD: Based on naturally produced clear examples of "boot," "beet," "said," and "shed" by 1 male and 1 female speaker, continua with 13 stimuli were synthesized for each contrast. Seven hearing controls labeled those stimuli and assigned them goodness ratings, as did 7 implant users at 1-month postimplant. One year later, these measures were repeated, and within category discrimination, d', was assessed. RESULTS: Compared with controls, implant users' vowel and sibilant labeling slopes were substantially shallower but improved over 1 year of prosthesis use. Their sensitivity to phonetic differences within phoneme categories was about half that of controls. The slopes of their goodness rating functions were shallower and did not improve. Their prototypes for the sibilant contrast (but not the vowels) were closer to one another and did not improve by moving apart. CONCLUSIONS: Implant users' phoneme labeling and within-category perceptual structure were anomalous at 1-month postimplant. After 1 year of prosthesis use, phoneme labeling categories had sharpened but within category discrimination was well below that of hearing controls.     

Effects of Stimulus Level on Speech Perception with Cochlear Prostheses

This study is one of a series that examines stimulus features important for cochlear implant function. Here, we examine effects of stimulus level. In subjects with cochlear implants, a number of psychophysical tests of temporal discrimination (pulse rate discrimination, gap detection, etc.) show marked improvement as a function of stimulus level through most or all of the dynamic range, while electrode-place discrimination can improve or degrade as a function of level. In this study, effects of these combined potential influences were studied by examining the effects of stimulus level on syllable identification. We tested two hypotheses: that syllable identification varies as a function of stimulus level and that level and electrode configuration interact in affecting syllable identification. We examined vowel and consonant identification as a function of stimulus level for bipolar and monopolar electrode configurations. We used experimental processor maps where upper and lower stimulation limits of each electrode pair were equated to eliminate confounding effects of dynamic range, which varies across subjects and electrodes. For each channel, stimulation amplitude was set to a fixed percentage of its dynamic range. Eight adult subjects with Nucleus CI24M implants were tested using the SPEAK processing strategy. With each electrode configuration, stimulus levels were tested from 0% to 90% of the dynamic range in nine steps. The effects on consonant and vowel identification were similar. Phoneme identification was usually better for monopolar than for bipolar stimulation. In the lower half of the dynamic range, syllable identification usually increased as a function of stimulus level. In the upper half of the dynamic range, syllable identification continued to increase as a function of level to 90% of the dynamic range for some subjects, while for others there was no appreciable change or a decrease as a function of level. Decreases in performance at high levels were more common with monopolar than bipolar stimulation. These results suggest that if speech processors are programmed to optimize level for each individual, speech perception performance could be improved.     

Phoneme recognition by deaf individuals using the multichannel nucleus cochlear implant.

Experiments have been carried out to determine which cues are used in phoneme identification by deaf individuals using a cochlear implant. Five deaf individuals with a Nucleus 22-channel cochlear implant were tested with open set speech audiometry in free field without lipreading. Speech material consisted of lists of Dutch words of the Consonant-Vowel-Consonant type (CVC-words). Word scores ranged from 0 to 22%, phoneme scores from 11 to 54%. For each subject the responses to the initial consonant, the vowel and the final consonant were entered into separate confusion matrices. Kruskal analysis, which provided a geometric representation of these confusions, showed that in the recognition of consonants the feature of voicing is all important. Vowels were identified on the basis of the frequencies of the first and second formants. In one subject the electrode array could only partially be inserted into the cochlea, leaving roughly half the second formant area of the electrode array outside the cochlea. For this subject vowel identification was based upon the first formant and vowel duration; there was no contribution of second formant information to vowel identification. Compressing the first and second formant frequency to the limited intracochlear array did not enhance transmission of second formant information and did not improve performance. The basic findings for consonant and vowel recognition could be explained by the speech coding strategy of the Nucleus speech processor in which voicing determines stimulus periodicity and formant frequencies determine channel selection. Kruskal analysis of phoneme confusions may aid in programming and evaluating the performance of the Nucleus cochlear implant.     

The effect of frequency allocation on phoneme recognition with the nucleus 22 cochlear implant

HYPOTHESIS: Phoneme recognition performance in patients implanted with the Nucleus 22 cochlear implant is affected by the frequency-to-electrode assignment. BACKGROUND: Multiple electrodes in modern cochlear implants are intended to deliver frequency-specific information to different tonotopic locations along the cochlea. However, the relation between the electrode locations, distribution of frequency information, and performance has not been explored thoroughly. METHODS: Ten listeners were tested on vowel and consonant identification tasks immediately after receiving each of the 15 speech processors. Experimental processors were created with 4, 7, and 20 activated electrodes. Five different frequency allocations were tested with all electrode conditions. RESULTS: For 7- and 20-electrode maps, best vowel recognition performance was obtained with frequency tables 7 and 9, with subjects showing best performance with the table with which they were most familiar. With 4-electrode maps, no change in vowel recognition performance was observed as a function of the frequency allocation. Consonant scores showed only a small effect of frequency allocation across all processors. Results were similar across listeners with different electrode insertion depths. CONCLUSION: The allocation of frequency ranges to electrodes in the Nucleus-22 cochlear implant can affect vowel recognition, when more than four electrodes are used, but is less important for consonant recognition. The allocation of frequency ranges to electrodes is an important factor in multichannel implants with more than four electrodes. The similarity of results across implant listeners with different electrode insertion depths implies that the optimal frequency allocation is one that best matches the allocation to which they've become accustomed, rather than one that matches the original tonotopic location of the electrodes.     

The effect of perimodiolar placement on speech perception and frequency discrimination by cochlear implant users

CONCLUSION: Neither speech understanding nor frequency discrimination ability was better in Nucleus Contour users than in Nucleus 24 straight electrode users. Furthermore, perimodiolar electrode placement does not result in better frequency discrimination. OBJECTIVES: We addressed three questions related to perimodiolar electrode placement. First, do patients implanted with the Contour electrode understand speech better than with an otherwise identical device that has a straight electrode? Second, do these groups have different frequency discrimination abilities? Third, is the distance of the electrode from the modiolus related to frequency discrimination ability? SUBJECTS AND METHODS: Contour and straight electrode users were matched on four important variables. We then tested these listeners on CNC word and HINT sentence identification tasks, and on a formant frequency discrimination task. We also examined X-rays and measured the distance of the electrodes from the modiolus to determine whether there is a relationship between this factor and frequency discrimination ability. RESULTS: Both speech understanding and frequency discrimination abilities were similar for listeners implanted with the Contour vs a straight electrode. Furthermore, there was no linear relationship between electrode-modiolus distance and frequency discrimination ability. However, we did note a second-order relationship between these variables, suggesting that frequency discrimination is worse when the electrodes are either too close or too far away from the modiolus.     

人工耳蜗植入后的声母识别率

目的：讨论人工耳蜗植入术后对汉语普通话声母识别的效果。方法：对４例全聋者人工耳蜗植入术后进行１．０￣１．５年的听觉语言训练。结果：４例人工耳蜗植入者平均声母识别率为７６％,韵母为９４．１％。结论：汉语普通话声母识别率低于韵母。带有相同共振峰持性后续音的声母之间发生错辨的机会较多。     

Effects of electrode location and spacing on phoneme recognition with the Nucleus-22 cochlear implant.

OBJECTIVE: The objective of this paper was to determine how phoneme identification was affected by the cochlear location and spacing of the electrodes in cochlear implant listeners. DESIGN: Subjects were initially programmed with the full complement of 20 active electrodes, in which each electrode was assigned to represent the output of one filter in the normal SPEAK processor. In the present study several four-electrode processors were constructed by assigning the output of more than one filter to a single electrode. In all conditions speech sounds were still analyzed into 20 frequency bands and processed according to the usual SPEAK processing strategy, but the location and spacing of the four stimulated electrode pairs were varied systematically. In Experiment I, the spacing between stimulated electrodes was fixed at 3.75 mm and the cochlear location of the four electrode pairs was shifted from the most-apical position up to 3.0 mm toward the base in 0.75 mm steps. In Experiment II, the spatial separation between the four electrode pairs (each bipolar-plus-one) was systematically changed from 1.5 mm to 4.5 mm while holding the most apical active electrode fixed. In Experiment III, the spacing of active electrodes was varied to represent equal tonotopic spacing to equal linear frequency intervals between pairs. Recognition of medial vowels and consonants was measured in three subjects with these custom four-electrode speech processors. RESULTS: In Experiment I, results showed that both vowel and consonant recognition were best when the electrodes were in the most apical locations. In Experiment II, best speech recognition occurred when electrode pairs were separated by 3 to 3.75 mm. In Experiment III, both vowel and consonant recognition scores decreased when the spacing of electrode pairs was changed from equal tonotopic spacing to equal linear frequency intervals. Overall, vowel and consonant recognition were best at the most apical electrode locations and when the spacing of electrodes matched the frequency intervals of the analysis filters. Consonant recognition was relatively robust to alterations in electrode location and spacing. The best vowel scores with four-electrode speech processors were about 10 percentage lower than scores obtained with the full 20-electrode speech processors. However, the best consonant scores with four-electrode speech processors were similar to those obtained with the full 20-electrode speech processors. Information transmission analysis revealed that temporal envelope cues (voicing and manner) were not strongly affected by changes in electrode location and spacing, whereas spectral cues, as represented by vowel recognition and consonantal place of articulation, were strongly affected. Both spectral and temporal phoneme cues were strongly affected by the degree of tonotopic warping, created by altering both the location and spacing of the activated electrodes. CONCLUSION: The cochlear location and spacing of the activated electrodes had a clear effect on phoneme recognition. Temporal cues were less affected by tonotopic shifts or linear tonotopic stretching or shrinking, but were susceptible to nonlinear tonotopic warping. Spectral cues were sensitive to all tonotopic manipulations: shifting, linear stretching, and nonlinear warping. However, the present experiments could not differentiate whether the optimal mapping between analysis frequency bands and stimulation electrodes was determined by the normal acoustic tonotopic pattern or by the pattern learned from experience with the 20-electrode implant.     

Digital speech processing for cochlear implants.

A rather general basic working hypothesis for cochlear implant research might be formulated as follows. Signal processing for cochlear implants should carefully select a subset of the total information contained in the sound signal and transform these elements into those physical stimulation parameters which can generate distinctive perceptions for the listener. Several new digital processing strategies have thus been implemented on a laboratory cochlear implant speech processor for the Nucleus 22-electrode system. One of the approaches (PES, pitch excited sampler) is based on the maximum peak channel vocoder concept whereby the spectral energy of a number of frequency bands is transformed into appropriate electrical stimulation parameters for up to 22 electrodes using a voice pitch synchronous pulse rate at any electrode. Another approach (CIS, continuous interleaved sampler) uses a maximally high pitch-independent stimulation pulse rate on a selected number of electrodes. As only one electrode can be stimulated at any instance of time, the rate of stimulation is limited by the required stimulus pulse widths (as determined individually for each subject) and some additional constraints and parameters which have to be optimized and fine tuned by psychophysical measurements. Evaluation experiments with 5 cochlear implant users resulted in significantly improved performance in consonant identification tests with the new processing strategies as compared with the subjects own wearable speech processors whereas improvements in vowel identification tasks were rarely observed. The pitch-synchronous coding (PES) resulted in worse performance compared to the coding without explicit pitch extraction (CIS).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of electrode configuration and frequency allocation on vowel recognition with the Nucleus-22 cochlear implant.

OBJECTIVE: This study was conducted to understand vowel recognition in cochlear implants as a function of the cochlear location and separation of the stimulated electrode pairs and as a function of the matching between speech spectral information and the location of the stimulated electrodes. DESIGN: Four-electrode speech processors with a continuous interleaved sampling speech processing strategy were implemented through a custom interface in five subjects implanted with the Nucleus-22 cochlear implant. The temporal envelopes from four broad frequency bands were used to modulate 500 pps, 100 microsec/phase interleaved pulse trains delivered to four electrode pairs. Ten different frequency allocations and five sets of four-electrode configurations were tested. Each frequency allocation represented the same cochlear extent but different cochlear locations based on Greenwood's frequency-to-place formula. Recognition of multi-talker medial vowels was measured for each combination of parameters with no period of practice or adjustment. RESULTS: Results showed that recognition of multi-talker vowels was highly dependent on frequency allocation for all electrode configurations. For a given electrode configuration maximum vowel recognition was observed with a specific frequency allocation. When the stimulated electrodes were shifted basally by 3 mm, the frequency allocation that produced the best performance also shifted basally by 3 mm. A similar pattern of vowel recognition was observed as a function of frequency allocation for electrode configurations that had the same apical-most electrode in each pair, regardless of location of the basal-most electrode in the pair. Subjects with different electrode insertion depths had similar trends in vowel recognition for each frequency allocation. CONCLUSIONS: For a given electrode configuration, the best performance was obtained with processors with a specific frequency allocation. In addition, the apical-most member of each electrode pair had a much stronger influence on vowel recognition in electric hearing. Finally, results from this study also suggest that over time, patients with implants can partially adapt to a basal shift in place of stimulation.     

Performance of subjects fit with the Advanced Bionics CII and Nucleus 3G cochlear implant devices

OBJECTIVE: To determine if subjects who used different cochlear implant devices and who were matched on consonant-vowel-consonant (CNC) identification in quiet would show differences in performance on speech-based tests of spectral and temporal resolution, speech understanding in noise, or speech understanding at low sound levels. DESIGN: The performance of 15 subjects fit with the CII Bionic Ear System (CII Bionic Ear behind-the-ear speech processor with the Hi-Resolution sound processing strategy; Advanced Bionics Corporation) was compared with the performance of 15 subjects fit with the Nucleus 24 electrode array and ESPrit 3G behind-the-ear speech processor with the advanced combination encoder speech coding strategy (cochlear corporation). SUBJECTS: Thirty adults with late-onset deafness and above-average speech perception abilities who used cochlear implants. MAIN OUTCOME MEASURES: Vowel recognition, consonant recognition, sentences in quiet (74, 64, and 54 dB SPL [sound pressure level]) and in noise (+10 and +5 dB SNR [signal-to-noise ratio]), voice discrimination, and melody recognition. RESULTS: Group differences in performance were significant in 4 conditions: vowel identification, difficult sentence material at +5 dB and +10 dB SNR, and a measure that quantified performance in noise and low input levels relative to performance in quiet. CONCLUSIONS: We have identified tasks on which there are between-group differences in performance for subjects matched on CNC word scores in quiet. We suspect that the differences in performance are due to differences in signal processing. Our next goal is to uncover the signal processing attributes of the speech processors that are responsible for the differences in performance.     

Covariation of cochlear implant users' perception and production of vowel contrasts and their identification by listeners with normal hearing.

This study investigates covariation of perception and production of vowel contrasts in speakers who use cochlear implants and identification of those contrasts by listeners with normal hearing. Formant measures were made of seven vowel pairs whose members are neighboring in acoustic space. The vowels were produced in carrier phrases by 8 postlingually deafened adults, before and after they received their cochlear implants (CI). Improvements in a speaker's production and perception of a given vowel contrast and normally hearing listeners' identification of that contrast in masking noise tended to occur together. Specifically, speakers who produced vowel pairs with reduced contrast in the pre-CI condition (measured by separation in the acoustic vowel space) and who showed improvement in their perception of these contrasts post-CI (measured with a phoneme identification test) were found to have enhanced production contrasts post-CI in many cases. These enhanced production contrasts were associated, in turn, with enhanced masked word recognition, as measured from responses of a group of 10 normally hearing listeners. The results support the view that restoring self-hearing allows a speaker to adjust articulatory routines to ensure sufficient perceptual contrast for listeners.     

Intensity operating range measures as predictors of word-recognition ability in cochlear implant subjects

The purposes of the experiment were to examine the appropriateness of pre- and post-implant intensity measures (thresholds, most comfortable listening level, and loudness discomfort level) as predictors of post-implant phoneme-recognition ability and to study the relationship between pre- and post-implant intensity measures. Pre-implant intensity measures were obtained on 16 subjects who were eventually implanted with either a Nucleus device (n = 8) or a Symbion device (n = 8). Phoneme scores on a NU-6 word list were obtained on these 16 subjects at 1 month post-implant. Post-implant intensity measures were also made on the 8 Symbion subjects at 1 month post-implant. The results showed that none of the pre-implant intensity measures correlated significantly with post-implant phoneme scores. In addition, pre-implant intensity measures did not correlate with the same post-implant intensity measures. However, post-implant MCLs and LDLs correlated significantly with phoneme scores as reflected by correlation coefficients that were larger than 0.8. These preliminary results suggest that although intensity measures may relate to phoneme-recognition ability, their use as predictive measures (as in pre-implant measures) for post-implant ability is questionable.     

Modification of speech encoding process of single channel cochlear implant utilizing digital delay circuit

Speech perception by single channel cochlear implant can be improved to some extent by modifying the encoding process of the speech signals. The principle of our encoding strategy is to present speech signals with appropriate delay time for lower frequency components of the speech sound, while without delay for higher frequency components. This is based on a cochlear physiology that higher frequency sounds are perceived earlier than lower frequency sounds, since the former is perceived at the basal turn and the latter at the apical turn. The following tests were performed without the aid of lip reading in the two patients implanted with a single channel cochlear implant of 3M-House design. Lower frequency components of the speech signals were digitally delayed with a Sony SDP-777 ES surround processor. The results indicated that the optimal delay time for identification of /i/, /e/ and /u/ needed longer delay time, while that of /a/ and /o/ need shorter delay time, corresponding to the difference between the first and second formant of each vowel. Comparison of vowel perception with and without delay coding showed that vowel identification with delay coding was superior to that without it; 30% vs. 16% in case 1, and 48% vs. 30% in case 2.     

Electrode discrimination and speech perception in young children using cochlear implants

OBJECTIVE: The aim was to determine the efficacy of a child-appropriate procedure to assess electrode discrimination ability in young children using cochlear implants and to investigate the relationship of electrode discrimination ability and speech perception performance in children implanted at a young age. DESIGN: An adaptation of the play audiometry procedure was used to assess electrode discrimination in seventeen 4- to 10-yr-old children. The children were required to respond with a game-like motor response when a repeating stimulation on a reference electrode "changed" to a different electrode. They were also assessed on a speech feature discrimination test, a closed-set word recognition test and a nonverbal intelligence task. RESULTS: Sixty-five percent of subjects demonstrated ability to discriminate adjacent electrodes in mid and apical regions of the cochlea, whilst the remaining subjects needed electrode separations of between two and nine electrodes for successful discrimination. In a forward stepwise regression analysis electrode discrimination ability was found to be the strongest factor in accounting for variance in the speech perception scores. Subject variables such as duration of deafness, nonverbal intelligence and implant experience did not significantly account for further variance in the speech perception scores for this group of children. CONCLUSIONS: Electrode discrimination ability was the strongest factor in predicting performance on speech perception measures in a group of children using cochlear implants.     

Incorporating ceiling effects during analysis of speech perception data from a paediatric cochlear implant cohort

Objective: To compare speech perception between children with a different age at cochlear implantation. Design: We evaluated speech perception by comparing consonant–vowel–consonant (auditory) (CVC(A)) scores at five-year follow-up of children implanted between 1997 and 2010. The proportion of children from each age-at-implantation group reaching the 95%CI of CVC(A) ceiling scores (>95%) was calculated to identify speech perception differences masked by ceiling effects. Study sample: 54 children implanted between 8 and 36 months. Results: Although ceiling effects occurred, a CVC(A) score difference between age-at-implantation groups was confirmed (H (4)?=?30.36; p?p?<0.001). A larger proportion of children implanted before 13 months compared to children implanted between 13 and 18 months reached ceiling scores. Logistic regression confirmed that age at implantation predicted whether a child reached a ceiling score. Conclusions: Ceiling effects can mask thorough delineation of speech perception. However, this study showed long-term speech perception outperformance of early implanted children (<18 months) either including or not accounting for ceiling effects during analysis. Development of long-term assessment tools not affected by ceiling effects is essential to maintain adequate assessment of young implanted infants.     

Psychophysics of a prototype peri-modiolar cochlear implant electrode array

Psychophysical measurements were performed in three hearing-impaired adult subjects implanted with a CI22 cochlear prosthesis (Cochlear Ltd.) fitted with a developmental peri-modiolar electrode array. The array was manufactured with a curvature approximating that of the inner wall of the scala tympani but, after straightening and insertion, lay on average about half way between the inner and outer walls of the scala. All subjects were tested with bipolar stimulation; two were also tested with monopolar, employing the most basal electrode as the return. Maximum comfortable level and threshold reduced with decreasing distance of electrode from the modiolus, whereas dynamic range increased. The linearity of the loudness growth function did not vary significantly with electrode position but the function was more non-linear for lower maximum comfortable levels. Current level discrimination, normalized with respect to dynamic range, improved with decreasing distance of electrode from the modiolus in two subjects. Pitch varied regularly with insertion depth of the stimulated electrode for bipolar stimulation in two subjects and also for monopolar stimulation in one subject. Electrode discrimination was enhanced by closeness to the modiolus. Whereas the forward masking patterns for bipolar stimulation of electrodes close to the modiolus had a sharp double-peaked structure, those for monopolar stimulation were flatter and had a single peak.     

Chronological changes of stimulation levels in prelingually deafened children with cochlear implant

This study investigated changes in long-term cochlear implant MAPs (values of the program to stimulate electrodes) in children, and examined whether there were significant differences between children and adults. Ten prelingually deafened children (aged from 2.7 to 7.8 years) who received the Nucleus 22 cochlear implant at Kyoto University between 1996 and 1998 participated in this study. Behavioral thresholds (T levels) and maximal comfortable levels (C levels) were evaluated at 3 months post-implant, and then every 6 months up to 4 years. Ten adult cochlear implant users were also evaluated as a control group. All subjects had used their implant for at least 4 years after device connection. All prelingually deafened children demonstrated slower but continuous improvements in speech perception and production abilities during the test period. T levels became stable at 30 months post-implant in children and at 24 months post-implant in adults. C levels became stable at 6 months post-implant in children and within 3 months post-implant in adults. In conclusion, T levels of prelingually deafened children became stabilized 30 months post-implant, while C levels stabilized 6 months post-implant. MAPs of prelingually deafened children were stabilized 30 months post-implant, which was longer than that in adult cochlear implant users.     

Patterns of phoneme identification error in cochlear and eighth-nerve disorders

The purpose of this study was to determine whether patterns of phoneme identification error differ among listeners with cochlear and retrocochlear auditory disorder. Speech intelligibility performance was analyzed in 15 patients with confirmed eighth-nerve disorder and in 15 patients with cochlear disorders, matched to the retrocochlear group for age and audiometric configuration, using confusion matrices derived from responses to a monosyllabic word list. Results indicated that: (1) vowel errors were more prevalent in the retrocochlear group and varied directly with increasing stimulus presentation level; and (2) consonant errors did not differ in type or relative frequency between the two groups, nor was there a level-dependent effect for consonant errors. These results are supported by the results of closed-set vowel identification tests and thus do not appear to be an artifact of open-set testing. Vowel errors may account for a major part of the speech 'rollover' phenomenon typical of retrocochlear auditory dysfunction.     

Frequency mapping in cochlear implants

OBJECTIVE: To understand the short-term ("acute") effects of parametric variations to the frequency-to-electrode mapping on phoneme identification by Nucleus-22 cochlear implant listeners. METHODS: Phoneme recognition was measured in five Nucleus-22 cochlear implant listeners using custom four-channel continuous interleaved sampler (CIS) processors. For the four-channel processors, speech signals were band-pass filtered into four broad frequency bands. The temporal envelope in each band was extracted by half-wave rectification and low-pass filtering at 160 Hz. The extracted envelope was then transformed to electric currents by a power function with an exponent of 0.2. The resulting electric currents were delivered to four electrode pairs (18,22), (13,17), (8,12), (3,7). The effect of frequency-to-electrode mapping was investigated by systematically varying the parameters of band-pass filters while fixing the electrode locations. Experiment 1 measured phoneme recognition as a function of the slope of band-pass filters. The slope of band-pass filters varied from 48 dB/octave to 6 dB/octave; the corner frequencies of band-pass filters were not varied. Experiment 2 measured phoneme recognition as a function of the distribution of band-pass filters across a fixed overall frequency range. The frequency divisions of a fixed overall frequency range were systematically varied from a logarithmic to a linear distribution. Experiment 3 measured phoneme recognition as a function of the bandwidth of the band-pass filters. The bandwidth of each filter varied from 0.2 to 2 octaves; the center frequencies for each band were not varied. No practice or feedback was provided for subjects in all experiments. RESULTS: The slope of the band-pass filters had little effect on both vowel and consonant recognition. A slight performance drop was observed for only the shallowest slope condition (6 dB/octave). In contrast, the distribution of the band-pass filters had a strong effect on vowel recognition but a weak effect on consonant recognition. Best performance was achieved when a logarithmic or near-logarithmic frequency distribution was used to divide the overall frequency range. The bandwidth of the band-pass filters had a moderate effect on both vowel and consonant recognition. Vowel scores dropped significantly when the bandwidth of filters was too broad, whereas consonant scores dropped significantly when a narrower bandwidth was used. CONCLUSION: Under "acute" testing conditions, phoneme recognition with a four-channel CIS strategy seems to be only mildly affected by the slope of the band-pass filters, but can be significantly affected by the distribution of filters as well as the bandwidth of the filters. Optimal or near-optimal performance can be achieved with a logarithmic frequency distribution. Vowels are more susceptible to broad bandwidths, whereas consonants are more susceptible to narrow bandwidths.