Adaptation by a cochlear-implant patient to upward shifts in the frequency representation of speech

Gated Word Recognition by Postlingually Deafened Adults With Cochlear Implants: Influence of Semantic Context

PURPOSE

The main goal of this study was to investigate the minimum amount of sensory information required to recognize spoken words (isolation points [IPs]) in listeners with cochlear implants (CIs) and investigate facilitative effects of semantic contexts on the IPs.

METHOD

Listeners with CIs as well as those with normal hearing (NH) participated in the study. In Experiment 1, the CI users listened to unprocessed (full-spectrum) stimuli and individuals with NH listened to full-spectrum or vocoder processed speech. IPs were determined for both groups who listened to gated consonant-nucleus-consonant words that were selected based on lexical properties. In Experiment 2, the role of semantic context on IPs was evaluated. Target stimuli were chosen from the Revised Speech Perception in Noise corpus based on the lexical properties of the final words.

RESULTS

The results indicated that spectrotemporal degradations impacted IPs for gated words adversely, and CI users as well as participants with NH listening to vocoded speech had longer IPs than participants with NH who listened to full-spectrum speech. In addition, there was a clear disadvantage due to lack of semantic context in all groups regardless of the spectral composition of the target speech (full spectrum or vocoded). Finally, we showed that CI users (and users with NH with vocoded speech) can overcome such word processing difficulties with the help of semantic context and perform as well as listeners with NH.

CONCLUSION

Word recognition occurs even before the entire word is heard because listeners with NH associate an acoustic input with its mental representation to understand speech. The results of this study provide insight into the role of spectral degradation on the processing of spoken words in isolation and the potential benefits of semantic context. These results may also explain why CI users rely substantially on semantic context.

Design and evaluation of a cochlear implant strategy based on a "Phantom" channel

Unbalanced bipolar stimulation, delivered using charge balanced pulses, was used to produce "Phantom stimulation", stimulation beyond the most apical contact of a cochlear implant's electrode array. The Phantom channel was allocated audio frequencies below 300 Hz in a speech coding strategy, conveying energy some two octaves lower than the clinical strategy and hence delivering the fundamental frequency of speech and of many musical tones. A group of 12 Advanced Bionics cochlear implant recipients took part in a chronic study investigating the fitting of the Phantom strategy and speech and music perception when using Phantom. The evaluation of speech in noise was performed immediately after fitting Phantom for the first time (Session 1) and after one month of take-home experience (Session 2). A repeated measures of analysis of variance (ANOVA) within factors strategy (Clinical, Phantom) and interaction time (Session 1, Session 2) revealed a significant effect for the interaction time and strategy. Phantom obtained a significant improvement in speech intelligibility after one month of use. Furthermore, a trend towards a better performance with Phantom (48%) with respect to F120 (37%) after 1 month of use failed to reach significance after type 1 error correction. Questionnaire results show a preference for Phantom when listening to music, likely driven by an improved balance between high and low frequencies.

Learning auditory space: generalization and long-term effects

BACKGROUND

Previous findings have shown that humans can learn to localize with altered auditory space cues. Here we analyze such learning processes and their effects up to one month on both localization accuracy and sound externalization. Subjects were trained and retested, focusing on the effects of stimulus type in learning, stimulus type in localization, stimulus position, previous experience, externalization levels, and time.

METHOD

We trained listeners in azimuth and elevation discrimination in two experiments. Half participated in the azimuth experiment first and half in the elevation first. In each experiment, half were trained in speech sounds and half in white noise. Retests were performed at several time intervals: just after training and one hour, one day, one week and one month later. In a control condition, we tested the effect of systematic retesting over time with post-tests only after training and either one day, one week, or one month later.

RESULTS

With training all participants lowered their localization errors. This benefit was still present one month after training. Participants were more accurate in the second training phase, revealing an effect of previous experience on a different task. Training with white noise led to better results than training with speech sounds. Moreover, the training benefit generalized to untrained stimulus-position pairs. Throughout the post-tests externalization levels increased. In the control condition the long-term localization improvement was not lower without additional contact with the trained sounds, but externalization levels were lower.

CONCLUSION

Our findings suggest that humans adapt easily to altered auditory space cues and that such adaptation spreads to untrained positions and sound types. We propose that such learning depends on all available cues, but each cue type might be learned and retrieved differently. The process of localization learning is global, not limited to stimulus-position pairs, and it differs from externalization processes.

The process of spoken word recognition in the face of signal degradation

Though much is known about how words are recognized, little research has focused on how a degraded signal affects the fine-grained temporal aspects of real-time word recognition. The perception of degraded speech was examined in two populations with the goal of describing the time course of word recognition and lexical competition. Thirty-three postlingually deafened cochlear implant (CI) users and 57 normal hearing (NH) adults (16 in a CI-simulation condition) participated in a visual world paradigm eye-tracking task in which their fixations to a set of phonologically related items were monitored as they heard one item being named. Each degraded-speech group was compared with a set of age-matched NH participants listening to unfiltered speech. CI users and the simulation group showed a delay in activation relative to the NH listeners, and there is weak evidence that the CI users showed differences in the degree of peak and late competitor activation. In general, though, the degraded-speech groups behaved statistically similarly with respect to activation levels.

Comparing live to recorded speech in training the perception of spectrally shifted noise-vocoded speech

Two experimental groups were trained for 2 h with live or recorded speech that was noise-vocoded and spectrally shifted and was from the same text and talker. These two groups showed equivalent improvements in performance for vocoded and shifted sentences, and the group trained with recorded speech showed consistently greater improvements than untrained controls. Another group trained with unshifted noise-vocoded speech improved no more than untrained controls. Computer-based training thus appears at least as effective as labor-intensive live-voice training for improving the perception of spectrally shifted noise-vocoded speech, and by implication, for training of users of cochlear implants.

Frequency selectivity of contralateral residual acoustic hearing in bimodal cochlear implant users, and limitations on the ability to match the pitch of electric and acoustic stimuli

OBJECTIVE

To assess the reliability of across-ear, acoustic-electric pitch/timbre comparisons for determining effective characteristic frequencies of cochlear implant electrodes.

STUDY SAMPLE

Nine CI users with contralateral residual acoustic hearing.

DESIGN

Absolute acoustic thresholds in the unimplanted ear were measured and frequency selectivity was assessed via psychophysical tuning curves. An adjustment method was used to match the percepts elicited by pulse trains on individual electrodes with various acoustic signals (pure tones, narrow-band noises, and bandpass filtered pulse trains). The starting frequency of the acoustic signal was roved and matches were obtained at different loudness levels.

RESULTS

Acoustic frequency selectivity varied widely. Two subjects showed clear evidence of frequency selectivity extending above 500 Hz. Only these subjects produced consistent pitch matches over repeated measurements. For other subjects, the acoustic frequency eventually selected tended to correlate with the initially presented frequency. There was limited evidence of level effects and these were inconsistent across subjects and electrodes.

CONCLUSIONS

Across-modality pitch/timbre matching appears unlikely to provide a generally applicable method for determining the effective characteristic frequencies of cochlear implant electrodes. Frequency selectivity above 500 Hz may be necessary for consistent pitch/timbre matches.

Vowel confusion patterns in adults during initial 4 years of implant use

This study investigated adult cochlear implant users' (n = 39) vowel recognition and confusions by an open-set syllable test during 4 years of implant use, in a prospective repeated-measures design. Subjects' responses were coded for phoneme errors and estimated by the generalized mixed model. Improvement in overall vowel recognition was highest during the first 6 months, showing statistically significant change until 4 years, especially for the mediocre performers. The best performers improved statistically significantly until 18 months. The poorest performers improved until 12 months and exhibited more vowel confusions. No differences were found in overall vowel recognition between Nucleus24M/24R and Med-ElC40+ device users (matched comparison), but certain vowels showed statistically significant differences. Vowel confusions between adjacent vowels were evident, probably due to the implant users' inability to discriminate formant frequencies. Vowel confusions were also dominated by vowels whose average F1 and/or F2 frequencies were higher than the target vowel, indicating a basalward shift in the confusions.

Use of "phantom electrode" technique to extend the range of pitches available through a cochlear implant

OBJECTIVE

The range of pitch sensations available in cochlear implants (CIs) is conventionally thought to be limited by the location of the most apical and basal electrodes. However, partial bipolar stimulation, in which current is distributed to two intracochlear electrodes and one extracochlear electrode, can produce "phantom electrode" (PE) pitch percepts that extend beyond the pitch range available with physical electrodes. The goals of this study were (1) to determine the PE configuration that generated the lowest pitch relative to monopolar (MP) stimulation of the most apical electrode and (2) to determine the amount of pitch shift produced by different PE configurations.

DESIGN

Ten Advanced Bionics CI users (9 unilateral and 1 bilateral), implanted with the CII or HiRes 90k implant and the HiFocus 1, HiFocus 1j, or Helix electrode arrays participated in this study. PEs were created by simultaneously stimulating the primary and compensating electrodes in opposite phase. To test different PE configurations, the proportion of current delivered to the compensating electrode (sigma) and the electrode separation between the primary and compensatory electrode (D) were varied. To estimate the relative pitch of PEs, the lowest pitched PEs with primary electrodes 4 and 8 were compared with subsets of MP electrodes (1, 2, 3, 4, 5 and 5, 6, 7, 8, 9, respectively).

RESULTS

In all subjects, it was possible to identify sigma and D values that produced a PE that was lower in pitch than the MP stimulation of the primary electrode. In some subjects, increasing sigma and/or D produced progressively lower pitch percepts, whereas in others, PE pitch changed nonmonotonically with sigma and/or D. The amount of PE pitch shift could be estimated only for 14 cases; in seven cases, the pitch shift was <1 MP electrode, and in seven other cases, the pitch shift was between 1 and 2 MP electrodes.

CONCLUSIONS

PE stimulation can elicit pitch percepts lower than that of the most apical MP electrode; the PE pitch is lower by the equivalent of 0.5 to 2 MP electrodes.

The contribution of apical stimulation to Mandarin speech perception in users of the MED-EL COMBI 40+ cochlear implant

CONCLUSION

Not stimulating the apical cochlear region in tonal language speaking cochlear implantees significantly reduces discrimination of Mandarin vowels. The data presented here suggest that electrode arrays that allow complete cochlear coverage with stimulation pulses seem to be preferable over shorter arrays for use in cochlear implant (CI) indications.

OBJECTIVE

To assess the contribution of electrical stimulation beyond the first cochlear turn on tonal language speech perception.

METHODS

Twelve Mandarin-speaking users of the MED-EL COMBI 40+ cochlear implant with complete insertion of the standard COMBI 40+ electrode array participated in the study. Acute speech tests were performed in seven electrode configurations with stimulation either distributed over the whole length of the cochlea or restricted to the apical, middle or basal regions. The test battery comprised tone, consonant, and vowel identification in quiet as well as a sentence recognition task in quiet and noise.

RESULTS

While neither tone nor consonant identification depended crucially on the placement of the active electrodes, vowel identification and sentence recognition decreased significantly when the four apical electrodes were not stimulated.

Interactions between unsupervised learning and the degree of spectral mismatch on short-term perceptual adaptation to spectrally shifted speech

OBJECTIVES

Cochlear implant listeners are able to at least partially adapt to the spectral mismatch associated with the implant device and speech processor via daily exposure and/or explicit training. The overall goal of this study was to investigate interactions between short-term unsupervised learning (i.e., passive adaptation) and the degree of spectral mismatch in normal-hearing listeners' adaptation to spectrally shifted vowels.

DESIGN

Normal-hearing subjects were tested while listening to acoustic cochlear implant simulations. Unsupervised learning was measured by testing vowel recognition repeatedly over a 5 day period; no feedback or explicit training was provided. In experiment 1, subjects listened to 8-channel, sine-wave vocoded speech. The spectral envelope was compressed to simulate a 16 mm cochlear implant electrode array. The analysis bands were fixed and the compressed spectral envelope was linearly shifted toward the base by 3.6, 6, or 8.3 mm to simulate different insertion depths of the electrode array, resulting in a slight, moderate, or severe spectral shift. In experiment 2, half the subjects were exclusively exposed to a severe shift with 8 or 16 channels (exclusive groups), and half the subjects were exposed to 8-channel severely shifted speech, 16-channel severely shifted speech, and 8-channel moderately shifted speech, alternately presented within each test session (mixed group). The region of stimulation in the cochlea was fixed (16 mm in extent and 15 mm from the apex) and the analysis bands were manipulated to create the spectral shift conditions. To determine whether increased spectral resolution would improve adaptation, subjects were exposed to 8- or 16-channel severely shifted speech.

RESULTS

In experiment 1, at the end of the adaptation period, there was no significant difference between 8-channel speech that was spectrally matched and that shifted by 3.6 mm. There was a significant, but less-complete, adaptation to the 6 mm shift and no adaptation to the 8.3 mm shift. In experiment 2, for the mixed exposure group, there was significant adaptation to severely shifted speech with 8 channels and even greater adaptation with 16 channels. For the exclusive exposure group, there was no significant adaptation to severely shifted speech with either 8 or 16 channels.

CONCLUSIONS

These findings suggest that listeners are able to passively adapt to spectral shifts up to 6 mm. For spectral shifts beyond 6 mm, some passive adaptation was observed with mixed exposure to a smaller spectral shift, even at the expense of some low frequency information. Mixed exposure to the smaller shift may have enhanced listeners' access to spectral envelope details that were not accessible when listening exclusively to severely shifted speech. The results suggest that the range of spectral mismatch that can support passive adaptation may be larger than previously reported. Some amount of passive adaptation may be possible with severely shifted speech by exposing listeners to a relatively small mismatch in conjunction with the severe mismatch.

Spatial hearing and speech intelligibility in bilateral cochlear implant users

OBJECTIVE

The abilities to localize sounds and segregate speech from interfering sounds in a complex auditory environment were studied in a group of adults who use bilateral cochlear implants. The first aim of the study was to investigate the change in speech intelligibility under bilateral and unilateral listening modes as a function of bilateral experience during the first 6 mo of activation. The second aim was to look at whether localization and speech intelligibility in the presence of interfering speech are correlated and if the relationship is specific to the bilateral listening mode. The third aim was to examine whether sound lateralization (right versus left) emerges before sound localization within a hemifield.

DESIGN

Participants were 17 native English speaking adults with postlingual deafness. All subjects received the Nucleus 24 Contour implant in both ears, either during the same surgery or during two separate surgeries that were no more than 1 mo apart. Both devices for each subject were activated at the same time, regardless of surgical approach. Speech intelligibility was measured at 3 and 6 mo after activation. Target speech was presented at 0 degrees in front. Testing was conducted in quiet and in the presence of four-talker babble. The babble was located on the right, on the left, or in front (colocated with the target). Sound localization abilities were measured at the 3 mo interval. All testing was conducted under three listening modes: left ear alone, right ear alone, or bilateral.

RESULTS

On the speech-in-babble task, benefit of listening with two ears compared with one was greater when going from 3 to 6 mo of experience. This was evident when the target speech and interfering speech were spatially separated, but not when they were presented from the same location. At 3 mo postactivation of bilateral hearing, 82% of subjects demonstrated bilateral benefit when right/left discrimination was evaluated. In contrast, 47% of subjects showed a bilateral benefit when sound localization was evaluated, suggesting that directional hearing might emerge in a two-step process beginning with discrimination and converging on more fine-grained localization. The bilateral speech intelligibility scores were positively correlated with sound localization abilities, so that listeners who were better able to hear speech in babble were generally better able to identify source locations.

CONCLUSIONS

During the early stages of bilateral hearing through cochlear implants in postlingually deafened adults, there is an early emergence of spatial hearing skills. Although nearly all subjects can discriminate source locations to the right versus left, less than half are able to perform the more difficult task of identifying source locations in a multispeaker array. Benefits for speech intelligibility with one versus two implants improve with time, in particular when spatial cues are used to segregate speech and competing noise. Localization and speech-in-noise abilities in this group of patients are somewhat correlated.

Simulating the effects of spread of electric excitation on musical tuning and melody identification with a cochlear implant

PURPOSE

To determine why, in a pilot study, only 1 of 11 cochlear implant listeners was able to reliably identify a frequency-to-electrode map where the intervals of a familiar melody were played on the correct musical scale. The authors sought to validate their method and to assess the effect of pitch strength on musical scale recognition in normal-hearing listeners.

METHOD

Musical notes were generated as either sine waves or spectrally shaped noise bands, with a center frequency equal to that of a desired note and symmetrical (log-scale) reduction in amplitude away from the center frequency. The rate of amplitude reduction was manipulated to vary pitch strength of the notes and to simulate different degrees of current spread. The effect of the simulated degree of current spread was assessed on tasks of musical tuning/scaling, melody recognition, and frequency discrimination.

RESULTS

Normal-hearing listeners could accurately and reliably identify the appropriate musical scale when stimuli were sine waves or steeply sloping noise bands. Simulating greater current spread degraded performance on all tasks.

CONCLUSIONS

Cochlear implant listeners with an auditory memory of a familiar melody could likely identify an appropriate frequency-to-electrode map but only in cases where the pitch strength of the electrically produced notes is very high.

Horizontal-Plane Localization of Noise and Speech Signals by Postlingually Deafened Adults Fitted With Bilateral Cochlear Implants*

OBJECTIVES

The main purpose of the study was to assess the ability of adults with bilateral cochlear implants to localize noise and speech signals in the horizontal plane. A second objective was to measure the change in localization performance in these adults between approximately 5 and 15 mo after activation. A third objective was to evaluate the relative roles of interaural level difference (ILD) and interaural temporal difference (ITD) cues in localization by these subjects.

DESIGN

Twenty-two adults, all postlingually deafened and all bilaterally fitted with MED-EL COMBI 40+ cochlear implants, were tested in a modified source identification task. Subjects were tested individually in an anechoic chamber, which contained an array of 43 numbered loudspeakers extending from -90 degrees to +90 degrees azimuth. On each trial, a 200-msec signal (either a noise burst or a speech sample) was presented from one of 17 active loudspeakers (span: +/-80 degrees ), and the subject had to identify which source from the 43 loudspeakers in the array produced the signal. Subjects were tested in three conditions: left device only active, right device only active, and both devices active. Twelve of the 22 subjects were retested approximately 10 mo after their first test. In Experiment 2, the spectral content and rise-decay time of the noise stimulus were manipulated.

RESULTS

The relationship between source azimuth and response azimuth was characterized in terms of the adjusted constant error (ĉ). (1) With both devices active, ĉ for the noise stimulus varied from 8.1 degrees to 43.4 degrees (mean: 24.1 degrees ). By comparison, ĉ for a group of listeners with normal hearing ranged from 3.5 degrees to 7.8 degrees (mean: 5.6 degrees ). When subjects listened in unilateral mode (with one device turned off), ĉ was at or near chance (50.5 degrees ) in all cases. However, when considering unilateral performance on each subject's better side, average ĉ for the speech stimulus was 47.9 degrees , which was significantly (but only slightly) better than chance. (2) When listening bilaterally, error score was significantly lower for the speech stimulus (mean ĉ = 21.5 degrees ) than for the noise stimulus (mean ĉ = 24.1 degrees ). (3) As a group, the 12 subjects who were retested 10 mo after their first visit showed no significant improvement in localization performance during the intervening time. However, two subjects who performed very poorly during their first visit showed dramatic improvement (error scores were halved) over the intervening time. In Experiment 2, removing the high-frequency content of noise signals resulted in significantly poorer performance, but removing the low-frequency content or increasing the rise-decay time did not have an effect.

CONCLUSIONS

In agreement with previously reported data, subjects with bilateral cochlear implants localized sounds in the horizontal plane remarkably well when using both of their devices, but they generally could not localize sounds when either device was deactivated. They could localize the speech signal with slightly, but significantly better accuracy than the noise, possibly due to spectral differences in the signals, to the availability of envelope ITD cues with the speech but not the noise signal, or to more central factors related to the social salience of speech signals. For most subjects the remarkable ability to localize sounds has stabilized by 5 mo after activation. However, for some subjects who perform poorly initially, there can be substantial improvement past 5 mo. Results from Experiment 2 suggest that ILD cues underlie localization ability for noise signals, and that ITD cues do not contribute.

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

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

Speech recognition as a function of high-pass filter cutoff frequency for people with and without low-frequency cochlear dead regions

Regions in the cochlea with no (or very few) functioning inner hair cells and/or neurons are called "dead regions" (DRs). The recognition of high-pass filtered nonsense syllables was measured as a function of filter cutoff frequency for hearing-impaired people with and without low-frequency (apical) cochlear DRs. The diagnosis of any DR was made using the TEN(HL) test, and psychophysical tuning curves were used to define the edge frequency (f(e)) more precisely. Stimuli were amplified differently for each ear, using the "Cambridge formula." For subjects with low-frequency hearing loss but without DRs, scores were high (about 78%) for low cutoff frequencies, remained approximately constant for cutoff frequencies up to 862 Hz, and then worsened with increasing cutoff frequency. For subjects with low-frequency DRs, performance was typically poor for the lowest cutoff frequency (100 Hz), improved as the cutoff frequency was increased to about 0.57f(e), and worsened with further increases. These results indicate that people with low-frequency DRs are able to make effective use of frequency components that fall in the range 0.57f(e) to f(e), but that frequency components below 0.57f(e) have deleterious effects. The results have implications for the fitting of hearing aids to people with low-frequency DRs.

Continuous improvement in Mandarin lexical tone perception as the number of channels increased: a simulation study of cochlear implant

CONCLUSION

With reference to English phoneme recognition, where performance usually does not improve after six or eight channels in cochlear implants (CIs), increasing total channel numbers continuously improved perception of Mandarin tones.

OBJECTIVE

To test our hypothesis that current CI strategies might be modified to improve Mandarin lexical tonal perception.

MATERIALS AND METHODS

Lexical tonal perception tests using 48 monosyllables in Mandarin Chinese were conducted in 32 native Mandarin speakers with normal hearing. The performance of tonal perception was compared among the controlled factors, which were total channel number, number of channels allocated to the F0 spectrum, and whether there were spectral shifts in the electrode configuration. The experimental condition that preserves fine structure was used as a comparison.

RESULTS

The signal processing strategy using 16 channels--which is technically possible with current CI devices--produced better tonal perception than those using 12 or 8 channels. Increasing the number of fundamental channels did not improve tonal perception, and spectral shifts did not change tonal perception. An experimental condition (FiC12) that preserves the fine structure produced significantly better overall scores for tone perception than other experimental conditions with envelope strategies.

An electric frequency-to-place map for a cochlear implant patient with hearing in the nonimplanted ear

The aim of this study was to relate the pitch of high-rate electrical stimulation delivered to individual cochlear implant electrodes to electrode insertion depth and insertion angle. The patient (CH1) was able to provide pitch matches between electric and acoustic stimulation because he had auditory thresholds in his nonimplanted ear ranging between 30 and 60 dB HL over the range, 250 Hz to 8 kHz. Electrode depth and insertion angle were measured from high-resolution computed tomography (CT) scans of the patient's temporal bones. The scans were used to create a 3D image volume reconstruction of the cochlea, which allowed visualization of electrode position within the scala. The method of limits was used to establish pitch matches between acoustic pure tones and electric stimulation (a 1,652-pps, unmodulated, pulse train). The pitch matching data demonstrated that, for insertion angles of greater than 450 degrees or greater than approximately 20 mm insertion depth, pitch saturated at approximately 420 Hz. From 20 to 15 mm insertion depth pitch estimates were about one-half octave lower than the Greenwood function. From 13 to 3 mm insertion depth the pitch estimates were approximately one octave lower than the Greenwood function. The pitch match for an electrode only 3.4 mm into the cochlea was 3,447 Hz. These data are consistent with other reports, e.g., Boëx et al. (2006), of a frequency-to-place map for the electrically stimulated cochlea in which perceived pitches for stimulation on individual electrodes are significantly lower than those predicted by the Greenwood function for stimulation at the level of the hair cell.

Implications of deep electrode insertion on cochlear implant fitting

Using long Med-El Combi40+ electrode arrays, it is now possible to cover the whole range of the cochlea, up to about two turns. Such insertion depths have received little attention. To evaluate the contribution of deeply inserted electrodes, five Med-El cochlear implant users were tested on vowel and consonant identification tests with fittings with first one, two, and up to five apical electrodes being deactivated. In addition, subjects performed pitch-ranking experiments, using loudness-balanced stimuli, to identify electrodes creating pitch confusions. Radiographs were taken to measure each electrode insertion depth. All subjects used each modified fitting for two periods of about 3 weeks. During the experiment, the same stimulation rate and frequency range were maintained across all the fittings used for each individual subject. After each trial period the subject had to perform three consonant and three vowel identification tests. All subjects showed deep electrode insertions ranging from 605 degrees to 720 degrees. The two subjects with the deepest electrode insertions showed significantly increased vowel- and consonant-identification performances with fittings with the two or three most apical electrodes deactivated compared to their standard fitting with all available electrodes activated. The other three subjects did not show significant improvements in performance when one or two of their most apical electrodes were deactivated. Four out of five subjects preferred to continue use of a fitting with one or more apical electrodes deactivated. The two subjects with the deepest insertions also showed pitch confusions between their most apical electrodes. Two possible reasons for these results are discussed. One is to reduce neural interactions related to electrodes producing pitch confusions. Another is to improve the alignment of the frequency components of sounds coded by the electrical signals delivered to each electrode to the overall pitch of the auditory perception produced by the electrical stimulation of auditory nerve fibers.

Speech Recognition for Unilateral and Bilateral Cochlear Implant Modes in the Presence of Uncorrelated Noise Sources

OBJECTIVE

The purpose of the current investigation was to compare speech recognition in noise for bilateral and unilateral modes within postlingually deafened, adult bilateral cochlear implant recipients. In addition, it was of interest to evaluate the time course of the bilateral speech-recognition advantage and the effect of changing signal-to-noise ratio (SNR) on the magnitude of the bilateral advantage.

DESIGN

In the first experiment, 16 postlingually deafened adults who were bilaterally implanted with the MED-EL C40+ cochlear device were evaluated in unilateral left, unilateral right, and bilateral conditions 4 to 7 mo after activation. Speech recognition in the presence of five spatially separated, uncorrelated noise sources was evaluated using both a single fixed SNR of +10 dB and an adaptive-SNR method. In a follow-up study, a subset of 10 participants was re-evaluated using an identical fixed-SNR method 12 to 17 mo after activation to examine the time course of speech-recognition performance in both unilateral and bilateral modes at a single SNR. A third study was performed with a subset of six participants to examine performance over a range of SNRs. In this study, speech recognition was measured 12 to 17 mo after activation in quiet and at +5, +10, +15, and +20 dB SNRs using the same five uncorrelated noise sources.

RESULTS

The speech-recognition data revealed a significant bilateral advantage of 3.3 dB using the adaptive-SNR method. A significant bilateral advantage of 9% was also measured using a fixed +10 dB SNR. Results from the second study revealed that experience resulted in a significant (11 to 20%) increase in speech-recognition-in-noise performance for both unilateral and bilateral modes; however, the magnitude of the bilateral advantage was not affected by experience. Results from the third study revealed the largest bilateral advantage at the poorest SNR evaluated. In addition, performance in quiet was significantly better than that measured in the presence of noise, even at the +20 dB SNR.

CONCLUSIONS

The results of these experiments support a small but significant bilateral speech-recognition-in-noise advantage for cochlear implant recipients in an environment with multiple noise sources. This advantage is presumed to be attributable to the combined effects of binaural squelch and diotic summation. Although experience generally improved speech-recognition-in-noise performance in both unilateral and bilateral modes, a consistent bilateral advantage (approximately 10%) was measured at 4 to 7 mo and at 12 to 17 mo postactivation.

Perceptual adaptation to spectrally shifted vowels: training with nonlexical labels

Although normal-hearing (NH) and cochlear implant (CI) listeners are able to adapt to spectrally shifted speech to some degree, auditory training has been shown to provide more complete and/or accelerated adaptation. However, it is unclear whether listeners use auditory and visual feedback to improve discrimination of speech stimuli, or to learn the identity of speech stimuli. The present study investigated the effects of training with lexical and nonlexical labels on NH listeners' perceptual adaptation to spectrally degraded and spectrally shifted vowels. An eight-channel sine wave vocoder was used to simulate CI speech processing. Two degrees of spectral shift (moderate and severe shift) were studied with three training paradigms, including training with lexical labels (i.e., "hayed," "had," "who'd," etc.), training with nonlexical labels (i.e., randomly assigned letters "f," "b," "g," etc.), and repeated testing with lexical labels (i.e., "test-only" paradigm without feedback). All training and testing was conducted over 5 consecutive days, with two to four training exercises per day. Results showed that with the test-only paradigm, lexically labeled vowel recognition significantly improved for moderately shifted vowels; however, there was no significant improvement for severely shifted vowels. Training with nonlexical labels significantly improved the recognition of nonlexically labeled vowels for both shift conditions; however, this improvement failed to generalize to lexically labeled vowel recognition with severely shifted vowels. Training with lexical labels significantly improved lexically labeled vowel recognition with severely shifted vowels. These results suggest that storage and retrieval of speech patterns in the central nervous system is somewhat robust to tonotopic distortion and spectral degradation. Although training with nonlexical labels may improve discrimination of spectrally distorted peripheral patterns, lexically meaningful feedback is needed to identify these peripheral patterns. The results also suggest that training with lexically meaningful feedback may be beneficial to CI users, especially patients with shallow electrode insertion depths.

The right information may matter more than frequency-place alignment: simulations of frequency-aligned and upward shifting cochlear implant processors for a shallow electrode array insertion

OBJECTIVE

It has been claimed that speech recognition with a cochlear implant is dependent on the correct frequency alignment of analysis bands in the speech processor with characteristic frequencies (CFs) at electrode locations. However, the use of filters aligned in frequency to a relatively basal electrode array position leads to significant loss of lower frequency speech information. This study uses an acoustic simulation to compare two approaches to the matching of speech processor filters to an electrode array having a relatively shallow depth within the typical range, such that the most apical element is at a CF of 1851 Hz. Two noise-excited vocoder speech processors are compared, one with CF-matched filters, and one with filters matched to CFs at basilar membrane locations 6 mm more apical than electrode locations.

DESIGN

An extended crossover training design examined pre- and post-training performance in the identification of vowels and words in sentences for both processors. Subjects received about 3 hours of training with each processor in turn.

RESULTS

Training improved performance with both processors, but training effects were greater for the shifted processor. For a male talker, the shifted processor led to higher post-training scores than the frequency-aligned processor with both vowels and sentences. For a female talker, post-training vowel scores did not differ significantly between processors, whereas sentence scores were higher with the frequency-aligned processor.

CONCLUSIONS

Even for a shallow electrode insertion, we conclude that a speech processor should represent information from important frequency regions below 1 kHz and that the possible cost of frequency misalignment can be significantly reduced with listening experience.