Auditory training with spectrally shifted speech: implications for cochlear implant patient auditory rehabilitation

Categories of auditory performance and speech intelligibility ratings of early-implanted children without speech training

OBJECTIVE

To assess whether speech therapy can lead to better results for early cochlear implantation (CI) children.

PATIENTS

A cohort of thirty-four congenitally profoundly deaf children who underwent CI before the age of 18 months at the Sixth Hospital affiliated with Shanghai Jiaotong University from January 2005 to July 2008 were included. Nineteen children received speech therapy in rehabilitation centers (ST), whereas the remaining fifteen cases did not (NST), but were exposed to the real world, as are normal hearing children.

METHODS

All children were assessed before surgery and at 6, 12, and 24 months after surgery with the Categories of Auditory Performance test (CAP) and the Speech Intelligibility Rating (SIR). Each assessment was given by the same therapist who was blind to the situation of the child at each observation interval. CAP and SIR scores of the groups were compared at each time point.

RESULTS

Our study showed that the auditory performance and speech intelligibility of trained children were almost the same as to those of untrained children with early implantation. The CAP and SIR scores of both groups increased with increased time of implant use during the follow-up period, and at each time point, the median scores of the two groups were about equal.

CONCLUSIONS

These results indicate that great communication benefits are achieved by early implantation (<18 months) without routine speech therapy. The results exemplify the importance of enhanced social environments provided by everyday life experience for human brain development and reassure parents considering cochlear implants where speech training is unavailable.

Consonant recognition as a function of the number of stimulation channels in the Hybrid short-electrode cochlear implant

Consonant recognition was measured as a function of the number of stimulation channels for Hybrid short-electrode cochlear implant (CI) users, long-electrode CI users, and normal-hearing (NH) listeners in quiet and background noise. Short-electrode CI subjects were tested with 1-6 channels allocated to a frequency range of 1063-7938 Hz. Long-electrode CI subjects were tested with 1-6, 8, or 22 channels allocated to 188-7938 Hz, or 1-6 or 15 channels from the basal 15 electrodes allocated to 1063-7938 Hz. NH listeners were tested with simulations of each CI group/condition. Despite differences in intracochlear electrode spacing for equivalent channel conditions, all CI subject groups performed similarly at each channel condition and improved up to at least four channels in quiet and noise. All CI subject groups underperformed relative to NH subjects. These preliminary findings suggest that the limited channel benefit seen for CI users may not be due solely to increases in channel interactions as a function of electrode density. Other factors such as pre-operative patient history, location of stimulation in the base versus apex, or a limit on the number of electric channels that can be processed cognitively, may also interact with the effects of electrode contact spacing along the cochlea.

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.

Effects of lower frequency-to-electrode allocations on speech and pitch perception with the hybrid short-electrode cochlear implant

Because some users of a Hybrid short-electrode cochlear implant (CI) lose their low-frequency residual hearing after receiving the CI, we tested whether increasing the CI speech processor frequency allocation range to include lower frequencies improves speech perception in these individuals. A secondary goal was to see if pitch perception changed after experience with the new CI frequency allocation. Three subjects who had lost all residual hearing in the implanted ear were recruited to use an experimental CI frequency allocation with a lower frequency cutoff than their current clinical frequency allocation. Speech and pitch perception results were collected at multiple time points throughout the study. In general, subjects showed little or no improvement for speech recognition with the experimental allocation when the CI was worn with a hearing aid in the contralateral ear. However, all 3 subjects showed changes in pitch perception that followed the changes in frequency allocations over time, consistent with previous studies showing that pitch perception changes upon provision of a CI.

New frontiers in cochlear implantation: acoustic plus electric hearing, hearing preservation, and more

As cochlear implant technology has changed, so have implantation criteria. In addition to profoundly deaf individuals, candidacy has expanded to include those with significant remaining acoustic hearing. This article describes the devices that are now in clinical trial, discusses the rationale as to why residual hearing preservation is important, details the surgical techniques for hearing preservation, and shares the clinical results of electric plus acoustic processing. That a video is available online.

Audiovisual cues and perceptual learning of spectrally distorted speech

Two experiments investigate the effectiveness of audiovisual (AV) speech cues (cues derived from both seeing and hearing a talker speak) in facilitating perceptual learning of spectrally distorted speech. Speech was distorted through an eight channel noise-vocoder which shifted the spectral envelope of the speech signal to simulate the properties of a cochlear implant with a 6 mm place mismatch: Experiment I found that participants showed significantly greater improvement in perceiving noise-vocoded speech when training gave AV cues than when it gave auditory cues alone. Experiment 2 compared training with AV cues with training which gave written feedback. These two methods did not significantly differ in the pattern of training they produced. Suggestions are made about the types of circumstances in which the two training methods might be found to differ in facilitating auditory perceptual learning of speech.

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 acoustic cues by children with cochlear implants

PURPOSE

This study examined the use of different acoustic cues in auditory perception of consonant and vowel contrasts by profoundly deaf children with a cochlear implant (CI) in comparison to age-matched children and young adults with normal hearing.

METHOD

A speech sound categorization task in an XAB format was administered to 15 children ages 5-6 with a CI (mean age at implant: 1;8 [years;months]), 20 normal-hearing age-matched children, and 21 normal-hearing adults. Four contrasts were examined: //-/a/, /i/-/i/, /bu/-/pu/, and /fu/-/su/. Measures included phoneme endpoint identification, individual cue reliance, cue weighting, and classification slope.

RESULTS

The children with a CI used the spectral cues in the /fu/-/su/ contrast less effectively than the children with normal hearing, resulting in poorer phoneme endpoint identification and a shallower classification slope. Performance on the other 3 contrasts did not differ significantly. Adults consistently showed steeper classification slopes than the children, but similar cue-weighting patterns were observed in all 3 groups.

CONCLUSIONS

Despite their different auditory input, children with a CI appear to be able to use many acoustic cues effectively in speech perception. Most importantly, children with a CI and normal-hearing children were observed to use similar cue-weighting patterns.

Mismatch negativity and adaptation measures of the late auditory evoked potential in cochlear implant users

A better understanding of the neural correlates of large variability in cochlear implant (CI) patients' speech performance may allow us to find solutions to further improve CI benefits. The present study examined the mismatch negativity (MMN) and the adaptation of the late auditory evoked potential (LAEP) in 10 CI users. The speech syllable /da/ and 1-kHz tone burst were used to examine the LAEP adaptation. The amount of LAEP adaptation was calculated according to the averaged N1-P2 amplitude for the LAEPs evoked by the last 3 stimuli and the amplitude evoked by the first stimulus. For the MMN recordings, the standard stimulus (1-kHz tone) and the deviant stimulus (2-kHz tone) were presented in an oddball condition. Additionally, the deviants alone were presented in a control condition. The MMN was derived by subtracting the response to the deviants in the control condition from the oddball condition. Results showed that good CI performers displayed a more prominent LAEP adaptation than moderate-to-poor performers. Speech performance was significantly correlated to the amount of LAEP adaptation for the 1-kHz tone bursts. Good performers displayed large MMNs and moderate-to-poor performers had small or absent MMNs. The abnormal electrophysiological findings in moderate-to-poor performers suggest that long-term deafness may cause damage not only at the auditory cortical level, but also at the cognitive level.

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.

Effects of semantic context and feedback on perceptual learning of speech processed through an acoustic simulation of a cochlear implant

The effect of feedback and materials on perceptual learning was examined in listeners with normal hearing who were exposed to cochlear implant simulations. Generalization was most robust when feedback paired the spectrally degraded sentences with their written transcriptions, promoting mapping between the degraded signal and its acoustic-phonetic representation. Transfer-appropriate processing theory suggests that such feedback was most successful because the original learning conditions were reinstated at testing: Performance was facilitated when both training and testing contained degraded stimuli. In addition, the effect of semantic context on generalization was assessed by training listeners on meaningful or anomalous sentences. Training with anomalous sentences was as effective as that with meaningful sentences, suggesting that listeners were encouraged to use acoustic-phonetic information to identify speech than to make predictions from semantic context.

Resistance to learning binaurally mismatched frequency-to-place maps: implications for bilateral stimulation with cochlear implants

Simulations of monaural cochlear implants in normal hearing listeners have shown that the deleterious effects of upward spectral shifting on speech perception can be overcome with training. This study simulates bilateral stimulation with a unilateral spectral shift to investigate whether listeners can adapt to upward-shifted speech information presented together with contralateral unshifted information. A six-channel, dichotic, interleaved sine-carrier vocoder simulated a binaurally mismatched frequency-to-place map. Odd channels were presented to one ear with an upward frequency shift equivalent to 6 mm on the basilar membrane, while even channels were presented to the contralateral ear unshifted. In Experiment 1, listeners were trained for 5.3 h with either the binaurally mismatched processor or with just the shifted monaural bands. In Experiment 2, the duration of training was 10 h, and the trained condition alternated between those of Experiment 1. While listeners showed learning in both experiments, intelligibility with the binaurally mismatched processor never exceeded, intelligibility with just the three unshifted bands, suggesting that listeners did not benefit from combining the mismatched maps, even though there was clear scope to do so. Frequency-place map alignment may thus be of importance when optimizing bilateral devices of the type studied here.

Effects of training on recognition of musical instruments presented through cochlear implant simulations

BACKGROUND

The simulation of the CI (cochlear implant) signal presents a degraded representation of each musical instrument, which makes recognition difficult.

PURPOSE

To examine the efficiency and effectiveness of three types of training on recognition of musical instruments as presented through simulations of the sounds transmitted through a CI.

RESEARCH DESIGN

Participants were randomly assigned to one of three training conditions: repeated exposure, feedback, and direct instruction.

STUDY SAMPLE

Sixty-six adults with normal hearing.

INTERVENTION

Each participant completed three training sessions per week, over a five-week time period, in which they listened to the CI simulations of eight different musical instruments.

DATA COLLECTION AND ANALYSIS

Analyses on percent of instruments identified correctly showed statistically significant differences between recognition accuracy of the three training conditions (p < .01).

RESULTS

those assigned to the direct instruction group showed the greatest improvement over the five-week training period as well as sustained improvement after training. The feedback group achieved the next highest level of recognition accuracy. The repeated exposure group showed modest improvement during the first three-week time period, but no subsequent improvements.

CONCLUSIONS

These results indicate that different types of training are differentially effective with regard to improving recognition of musical instruments presented through a degraded signal, which has practical implications for the auditory rehabilitation of persons who use cochlear implants.

The Hybrid cochlear implant: a review

The Hybrid S or 'short-electrode' cochlear implant was developed to treat patients with a severe to profound hearing loss limited to the high frequencies. The short electrode is implanted into just the base or high-frequency region of the cochlea, with the goal of preserving residual low-frequency hearing. As a result, electric stimulation can be combined with acoustic stimulation in the same ear (and the opposite ear); this is one instance of 'acoustic plus electric' (A + E) stimulation. In this paper, we will review the latest findings from the first two stages of the clinical trial for the Hybrid concept in the United States. Generally, we will review surgical techniques, clinical trial criteria, residual hearing preservation, improvements in speech perception in quiet, and predictive factors for patient benefit. We will also discuss the significant benefit of A + E stimulation for speech perception in noise and musical measures of melody and instrument recognition, as well as valuable insights into central auditory nervous system plasticity gained from the use of a very short electrode array.

Development and plasticity of intra- and intersensory information processing

The functional architecture of sensory brain regions reflects an ingenious biological solution to the competing demands of a continually changing sensory environment. While they are malleable, they have the constancy necessary to support a stable sensory percept. How does the functional organization of sensory brain regions contend with these antithetical demands? Here we describe the functional organization of auditory and multisensory (i.e., auditory-visual) information processing in three sensory brain structures: (1) a low-level unisensory cortical region, the primary auditory cortex (A1); (2) a higher-order multisensory cortical region, the anterior ectosylvian sulcus (AES); and (3) a multisensory subcortical structure, the superior colliculus (SC). We then present a body of work that characterizes the ontogenic expression of experience-dependent influences on the operations performed by the functional circuits contained within these regions. We will present data to support the hypothesis that the competing demands for plasticity and stability are addressed through a developmental transition in operational properties of functional circuits from an initially labile mode in the early stages of postnatal development to a more stable mode in the mature brain that retains the capacity for plasticity under specific experiential conditions. Finally, we discuss parallels between the central tenets of functional organization and plasticity of sensory brain structures drawn from animal studies and a growing literature on human brain plasticity and the potential applicability of these principles to the audiology clinic.

Progress in restoration of hearing with the auditory brainstem implant

Fifty years ago auditory scientists were very skeptical about the potential of new prosthetic approaches that electrically stimulated the auditory nerve, the cochlear nuclei (CN), and the inferior colliculus (IC). In those decades, the basilar membrane was considered to play a fundamental and irreplaceable role as a fine spectrum analyzer in hearing physiology, and therefore it was thought that electrical stimulation of the auditory system would have never produced functionally useful hearing. Over the last 30 years, cochlear implants (CIs) have improved steadily to the point where the average sentence recognition with modern multichannel devices is better than 90% correct. More recently, similar performance has been observed with electric stimulation of the brainstem with auditory brainstem implants (ABIs). However, it is clear that to fully understand hearing and to design the next generation of prosthetic devices we must better understand the ear-brain relationship. Indeed some aspects of hearing do not require the intricate complexities of cochlear physiological responses, while other auditory tasks rely critically on specialized details of cochlear processing. The progress in electrical stimulation of the central auditory system requires us to reconsider the patient selection criteria for different implant devices, in particular to evaluate the possibility of ABIs for etiologies with poor outcomes with CIs. In the present review, the latest outcomes in restoration of hearing with ABI are presented. New guidelines are proposed for device selection for different etiologies and future research is suggested to further refine the process of matching an individual patient to the most appropriate implant device.

Recognition of spectrally degraded phonemes by younger, middle-aged, and older normal-hearing listeners

The effects of spectral degradation on vowel and consonant recognition abilities were measured in young, middle-aged, and older normal-hearing (NH) listeners. Noise-band vocoding techniques were used to manipulate the number of spectral channels and frequency-to-place alignment, thereby simulating cochlear implant (CI) processing. A brief cognitive test battery was also administered. The performance of younger NH listeners exceeded that of the middle-aged and older listeners, when stimuli were severely distorted (spectrally shifted); the older listeners performed only slightly worse than the middle-aged listeners. Significant intragroup variability was present in the middle-aged and older groups. A hierarchical multiple-regression analysis including data from all three age groups suggested that age was the primary factor related to shifted vowel recognition performance, but verbal memory abilities also contributed significantly to performance. A second regression analysis (within the middle-aged and older groups alone) revealed that verbal memory and speed of processing abilities were better predictors of performance than age alone. The overall results from the current investigation suggested that both chronological age and cognitive capacities contributed to the ability to recognize spectrally degraded phonemes. Such findings have important implications for the counseling and rehabilitation of adult CI recipients.

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.

Comparison of word-, sentence-, and phoneme-based training strategies in improving the perception of spectrally distorted speech

PURPOSE

To compare the effectiveness of 3 self-administered strategies for auditory training that might improve speech perception by adult users of cochlear implants. The strategies are based, respectively, on discriminating isolated words, words in sentences, and phonemes in nonsense syllables.

METHOD

Participants were 18 normal-hearing adults who listened to speech processed by a noise-excited vocoder to simulate the information provided by a cochlear implant. They were assigned randomly to word-, sentence-, or phoneme-based training and underwent 9 training sessions (20 min each) on separate days over a 2- to 3-week period. The effectiveness of training was assessed as the improvement in accuracy of discriminating vowels and consonants, as well as identifying words in sentences, relative to participants' best performance in repeated tests prior to training.

RESULTS

Word- and sentence-based training led to significant improvements in the ability to identify words in sentences that were significantly larger than the improvements produced by phoneme-based training. There were no significant differences between the effectiveness of word- and sentence-based training. No significant improvements in consonant or vowel discrimination were found for the sentence- or phoneme-based training groups, but some improvements were found for the word-based training group.

CONCLUSION

The word- and sentence-based training strategies were more effective than the phoneme-based strategy at improving the perception of spectrally distorted speech.

Effects of upper-frequency boundary and spectral warping on speech intelligibility in electrical stimulation

Speech understanding was tested for seven listeners using 12-electrode Med-El cochlear implants (CIs) and six normal-hearing listeners using a CI simulation. Eighteen different types of processing were evaluated, which varied the frequency-to-tonotopic place mapping and the upper boundary of the frequency and stimulation range. Spectrally unwarped and warped conditions were included. Unlike previous studies on this topic, the lower boundary of the frequency and stimulation range was fixed while the upper boundary was varied. For the unwarped conditions, only eight to ten channels were needed in both quiet and noise to achieve no significant degradation in speech understanding compared to the normal 12-electrode speech processing. The unwarped conditions were often the best conditions for understanding speech; however, small changes in frequency-to-place mapping (<0.77 octaves for the most basal electrode) yielded no significant degradation in performance from the nearest unwarped condition. A second experiment measured the effect of feedback training for both the unwarped and warped conditions. Improvements were found for the unwarped and frequency-expanded conditions, but not for the compressed condition. These results have implications for new CI processing strategies, such as the inclusion of spectral localization cues.

Cochlear implant speech processor frequency allocations may influence pitch perception

OBJECTIVE

To investigate the effects of assigning cochlear implant speech processor frequencies normally associated with more apical cochlear locations to the shallow insertion depths of the Iowa/Nucleus Hybrid electrode.

STUDY DESIGN

Subjects using the Hybrid implant for more than 1 year were tested on speech recognition with Consonant-Nucleus-Consonant words and consonant stimuli. Pitch sensations of individual electrodes were also measured electrically through the implant and acoustically in the contralateral ear.

SETTING

Tertiary care center.

RESULTS

Most subjects showed large improvements in speech recognition within 12 months after implantation. Furthermore, after longer periods of 24-plus months, some individuals were able to achieve high levels of consonant discrimination with electric-only processing comparable to long-electrode patients with deeper electrode insertions. Pitch perceptions obtained from individual electrodes in these subjects were closer to the frequency map assigned an electrode than the place-frequency predicted from cochlear location.

CONCLUSION

These results suggest that over time, pitch sensations may be determined more by the implant map than by cochlear location. In other words, the brain may adapt to spectral mismatches by remapping pitch. Furthermore, patients can perform well with shifted frequency allocations for speech recognition. The successful application of shifted frequency allocations also supports the idea of shallower insertions and greater preservation of residual hearing for all cochlear implants, regardless of the patient's frequency range of usable residual hearing.

Maximizing cochlear implant patients' performance with advanced speech training procedures

Advances in implant technology and speech processing have provided great benefit to many cochlear implant patients. However, some patients receive little benefit from the latest technology, even after many years' experience with the device. Moreover, even the best cochlear implant performers have great difficulty understanding speech in background noise, and music perception and appreciation remain major challenges. Recent studies have shown that targeted auditory training can significantly improve cochlear implant patients' speech recognition performance. Such benefits are not only observed in poorly performing patients, but also in good performers under difficult listening conditions (e.g., speech noise, telephone speech, music, etc.). Targeted auditory training has also been shown to enhance performance gains provided by new implant devices and/or speech processing strategies. These studies suggest that cochlear implantation alone may not fully meet the needs of many patients, and that additional auditory rehabilitation may be needed to maximize the benefits of the implant device. Continuing research will aid in the development of efficient and effective training protocols and materials, thereby minimizing the costs (in terms of time, effort and resources) associated with auditory rehabilitation while maximizing the benefits of cochlear implantation for all recipients.

Perceptual learning and auditory training in cochlear implant recipients

Learning electrically stimulated speech patterns can be a new and difficult experience for cochlear implant (CI) recipients. Recent studies have shown that most implant recipients at least partially adapt to these new patterns via passive, daily-listening experiences. Gradually introducing a speech processor parameter (eg, the degree of spectral mismatch) may provide for more complete and less stressful adaptation. Although the implant device restores hearing sensation and the continued use of the implant provides some degree of adaptation, active auditory rehabilitation may be necessary to maximize the benefit of implantation for CI recipients. Currently, there are scant resources for auditory rehabilitation for adult, postlingually deafened CI recipients. We recently developed a computer-assisted speech-training program to provide the means to conduct auditory rehabilitation at home. The training software targets important acoustic contrasts among speech stimuli, provides auditory and visual feedback, and incorporates progressive training techniques, thereby maintaining recipients' interest during the auditory training exercises. Our recent studies demonstrate the effectiveness of targeted auditory training in improving CI recipients' speech and music perception. Provided with an inexpensive and effective auditory training program, CI recipients may find the motivation and momentum to get the most from the implant device.

Variables predicting outcomes on listening and communication enhancement (LACE) training

Results from a large study of adults who completed a randomized crossover study of listening and communication enhancement (LACE) training were analysed to observe trends. The objective of this study was to determine predictors for greatest improvement following this four-week adaptive auditory training and aural rehabilitation program. Subjects with the poorest scores on the baseline tests, particularly those with the greatest degree of hearing loss, poorest scores on measures of degraded and competing speech, and those with the highest hearing handicap scores, were more likely to have greater improvement overall. However, there was considerable variability among the subjects, and some subjects' positive subjective reports belie smaller overall measured gains. Information collected from both the testing and the counseling of the patient should be taken into consideration when determining whether to proceed with LACE training.

Effect of training rate on recognition of spectrally shifted speech

OBJECTIVE

Previous studies have shown that the protocol used for auditory training may significantly affect the outcome of training. However, it is unclear how often training should be performed to maximize its benefit. The present study investigated how the frequency of training contributed to normal-hearing listeners' adaptation to spectrally shifted speech.

METHODS

Eighteen normal-hearing listeners were trained with spectrally shifted and compressed speech via an 8-channel acoustic simulation of cochlear implant speech processing. Five short training sessions (1 hr per session) were completed by each subject; subjects were trained at one of three training rates: five sessions per week, three sessions per week, or one session per week. Subjects were trained to identify medial vowels presented in a cVc format; depending on the level of difficulty, the number of response choices was increased and/or the acoustic differences between vowels were reduced. Vowel and consonant recognition was measured before and after training as well as at regular intervals during the training period. Sentence recognition was measured before and after training only.

RESULTS

Results showed that pretraining vowel recognition scores were poor (14.0% correct, on average) for all subjects, due to the severe spectral shift. After five sessions of targeted vowel contrast training, there was a significant improvement of shifted vowel recognition for most subjects. The mean improvement was comparable (approximately 15 percentage points) across the three training rate conditions, despite significant intersubject variability in pre- and pretraining baseline performance. There was no significant difference in training outcomes among the three training rates. Spectrally shifted consonant and sentence recognition also improved by approximately 20 percentage points after training, even though consonants and sentences were not explicitly trained. Similar to vowel recognition, there was no significant difference in training outcomes among the three training rates for shifted consonant and sentence recognition.

CONCLUSIONS

The results demonstrated that the training rate had little effect on normal-hearing listeners' adaptation to spectrally shifted speech, at least for the training periods (ranging from 1 to 5 wk) used in the present study. The outcome of auditory training may depend more strongly on the amount of training (i.e., total number of training sessions) rather than the frequency of training (i.e., daily or once per week). Although more frequent training may accelerate listeners' adaptation to spectrally shifted speech, there may be significant benefits from training as little as one session per week. The results of the present study suggest that appropriate training schedules can be developed to optimize the effectiveness, efficiency, and effort associated with hearing-impaired patients' auditory rehabilitation.

Combined Effects of Frequency Compression-Expansion and Shift on Speech Recognition

OBJECTIVE

To explore combined acute effects of frequency shift and compression-expansion on speech recognition, using noiseband vocoder processing.

DESIGN

Recognition of vowels and consonants, processed with a noiseband vocoder, was measured with five normal-hearing subjects, between the ages of 27 and 35 yr. The speech signal was filtered into 8 or 16 analysis bands and the envelopes were extracted from each band. The carrier noise bands were modulated by the envelopes and resynthesized to produce the processed speech. In the baseline matched condition, the frequency ranges of the corresponding analysis and carrier bands were the same. In the shift only condition, the frequency ranges of the carrier bands were shifted up or down relative to the analysis bands. In the compression and expansion only conditions, the analysis band range was made larger or smaller, respectively, than the carrier band range. By applying the shift to carrier bands and compression or expansion to analysis bands simultaneously, the combined effects of the two spectral distortions on speech recognition were explored.

RESULTS

When the spectral distortions of compression-expansion or shift were applied separately, the performance was reduced from the baseline matched condition. However, when the two spectral degradations were applied simultaneously, a compensatory effect was observed; the reduction in performance was smaller for some combinations compared to the reduction observed for each distortion individually.

CONCLUSIONS

The results of the present study are consistent with previous vocoder studies with normal-hearing subjects that showed a negative effect of spectral mismatch between analysis and carrier bands on speech recognition. The present results further show that matching the frequency ranges of 1 to 2 kHz, which contain important speech information, can be more beneficial for speech recognition than matching the overall frequency ranges, in certain conditions.

Changes in pitch with a cochlear implant over time

In the normal auditory system, the perceived pitch of a tone is closely linked to the cochlear place of vibration. It has generally been assumed that high-rate electrical stimulation by a cochlear implant electrode also evokes a pitch sensation corresponding to the electrode's cochlear place ("place" code) and stimulation rate ("temporal" code). However, other factors may affect electric pitch sensation, such as a substantial loss of nearby nerve fibers or even higher-level perceptual changes due to experience. The goals of this study were to measure electric pitch sensations in hybrid (short-electrode) cochlear implant patients and to examine which factors might contribute to the perceived pitch. To look at effects of experience, electric pitch sensations were compared with acoustic tone references presented to the non-implanted ear at various stages of implant use, ranging from hookup to 5 years. Here, we show that electric pitch perception often shifts in frequency, sometimes by as much as two octaves, during the first few years of implant use. Additional pitch measurements in more recently implanted patients at shorter time intervals up to 1 year of implant use suggest two likely contributions to these observed pitch shifts: intersession variability (up to one octave) and slow, systematic changes over time. We also found that the early pitch sensations for a constant electrode location can vary greatly across subjects and that these variations are strongly correlated with speech reception performance. Specifically, patients with an early low-pitch sensation tend to perform poorly with the implant compared to those with an early high-pitch sensation, which may be linked to less nerve survival in the basal end of the cochlea in the low-pitch patients. In contrast, late pitch sensations show no correlation with speech perception. These results together suggest that early pitch sensations may more closely reflect peripheral innervation patterns, while later pitch sensations may reflect higher-level, experience-dependent changes. These pitch shifts over time not only raise questions for strict place-based theories of pitch perception, but also imply that experience may have a greater influence on cochlear implant perception than previously thought.

Effects of computer-assisted speech training on Mandarin-speaking hearing-impaired children

The present study investigated whether moderate amounts of computer-assisted speech training can improve the speech recognition performance of hearing-impaired children. Ten Mandarin-speaking children (3 hearing aid users and 7 cochlear implant users) participated in the study. Training was conducted at home using a personal computer for half an hour per day, 5 days per week, for a period of 10 weeks. Results showed significant improvements in subjects' vowel, consonant, and tone recognition performance after training. The improved performance was largely retained for 2 months after training was completed. These results suggest that moderate amounts of auditory training, using a computer-based auditory rehabilitation tool with minimal supervision, can be effective in improving the speech performance of hearing-impaired children.

Effectiveness of computer-based auditory training in improving the perception of noise-vocoded speech

Five experiments were designed to evaluate the effectiveness of "high-variability" lexical training in improving the ability of normal-hearing subjects to perceive noise-vocoded speech that had been spectrally shifted to simulate tonotopic misalignment. Two approaches to training were implemented. One training approach required subjects to recognize isolated words, while the other training approach required subjects to recognize words in sentences. Both approaches to training improved the ability to identify words in sentences. Improvements following a single session (lasting 1-2 h) of auditory training ranged between 7 and 12 %pts and were significantly larger than improvements following a visual control task that was matched with the auditory training task in terms of the response demands. An additional three sessions of word- and sentence-based training led to further improvements, with the average overall improvement ranging from 13 to 18% pts. When a tonotopic misalignment of 3 mm rather than 6 mm was simulated, training with several talkers led to greater generalization to new talkers than training with a single talker. The results confirm that computer-based lexical training can help overcome the effects of spectral distortions in speech, and they suggest that training materials are most effective when several talkers are included.

Computer-Assisted Speech Training for Cochlear Implant Patients: Feasibility, Outcomes, and Future Directions

Learning electrically stimulated speech patterns can be a new and difficult experience for cochlear implant patients. Cochlear implantation alone may not fully meet the needs of many patients, and additional auditory rehabilitation may be necessary to maximize the benefits of the implant device. A recently developed computer-assisted speech-training program provides cochlear implant patients with the means to conduct auditory rehabilitation at home. The training software targets important acoustic contrasts between speech stimuli and provides auditory and visual feedback as well as progressive training, thereby maintaining patients' interest in the auditory training exercises. Recent scientific studies have demonstrated the effectiveness of such specialized auditory training programs in improving cochlear implant patients' speech recognition performance. Provided with an inexpensive and accessible auditory training program, cochlear implant patients may find the motivation and momentum to get the most from the implant device.

Frequency map for the human cochlear spiral ganglion: implications for cochlear implants

The goals of this study were to derive a frequency-position function for the human cochlear spiral ganglion (SG) to correlate represented frequency along the organ of Corti (OC) to location along the SG, to determine the range of individual variability, and to calculate an "average" frequency map (based on the trajectories of the dendrites of the SG cells). For both OC and SG frequency maps, a potentially important limitation is that accurate estimates of cochlear place frequency based upon the Greenwood function require knowledge of the total OC or SG length, which cannot be determined in most temporal bone and imaging studies. Therefore, an additional goal of this study was to evaluate a simple metric, basal coil diameter that might be utilized to estimate OC and SG length. Cadaver cochleae (n = 9) were fixed <24 h postmortem, stained with osmium tetroxide, microdissected, decalcified briefly, embedded in epoxy resin, and examined in surface preparations. In digital images, the OC and SG were measured, and the radial nerve fiber trajectories were traced to define a series of frequency-matched coordinates along the two structures. Images of the cochlear turns were reconstructed and measurements of basal turn diameter were made and correlated with OC and SG measurements. The data obtained provide a mathematical function for relating represented frequency along the OC to that of the SG. Results showed that whereas the distance along the OC that corresponds to a critical bandwidth is assumed to be constant throughout the cochlea, estimated critical band distance in the SG varies significantly along the spiral. Additional findings suggest that measurements of basal coil diameter in preoperative images may allow prediction of OC/SG length and estimation of the insertion depth required to reach specific angles of rotation and frequencies. Results also indicate that OC and SG percentage length expressed as a function of rotation angle from the round window is fairly constant across subjects. The implications of these findings for the design and surgical insertion of cochlear implants are discussed.

Estimation of Vowel Recognition With Cochlear Implant Simulations

Because there are many parameters in the cochlear implant (CI) device that can be optimized for individual patients, it is important to estimate a parameter's effect before patient evaluation. In this paper, Mel-frequency cepstrum coefficients (MFCCs) were used to estimate the acoustic vowel space for vowel stimuli processed by the CI simulations. The acoustic space was then compared to vowel recognition performance by normal-hearing subjects listening to the same processed speech. Five CI speech processor parameters were simulated to produce different degree of spectral resolution, spectral smearing, spectral warping, spectral shifting, and amplitude distortion. The acoustic vowel space was highly correlated with normal hearing subjects' vowel recognition performance for parameters that affected the spectral channels and spectral smearing. However, the acoustic vowel space was not significantly correlated with perceptual performance for parameters that affected the degree of spectral warping, spectral shifting, and amplitude distortion. In particular, while spectral warping and shifting did not significantly reshape the acoustic space, vowel recognition performance was significantly affected by these parameters. The results from the acoustic analysis suggest that the CI device can preserve phonetic distinctions under conditions of spectral warping and shifting. Auditory training may help CI patients better perceive these speech cues transmitted by their speech processors.

Vowel recognition via cochlear implants and noise vocoders: effects of formant movement and duration

Previous work has demonstrated that normal-hearing individuals use fine-grained phonetic variation, such as formant movement and duration, when recognizing English vowels. The present study investigated whether these cues are used by adult postlingually deafened cochlear implant users, and normal-hearing individuals listening to noise-vocoder simulations of cochlear implant processing. In Experiment 1, subjects gave forced-choice identification judgments for recordings of vowels that were signal processed to remove formant movement and/or equate vowel duration. In Experiment 2, a goodness-optimization procedure was used to create perceptual vowel space maps (i.e., best exemplars within a vowel quadrilateral) that included F1, F2, formant movement, and duration. The results demonstrated that both cochlear implant users and normal-hearing individuals use formant movement and duration cues when recognizing English vowels. Moreover, both listener groups used these cues to the same extent, suggesting that postlingually deafened cochlear implant users have category representations for vowels that are similar to those of normal-hearing individuals.

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.

Frequency transposition around dead regions simulated with a noiseband vocoder

In sensorineural hearing loss, damage to inner hair cells or the auditory nerve may result in dead regions in the cochlea, where the information transmission is disrupted. In cochlear implants, similar dead regions might appear if the spiral ganglia do not function. Shannon et al. [J. Assoc. Res. Otolaryngol. 3, 185-199 (2002)] simulated dead regions of varying size and location using a noiseband vocoder. Phoneme recognition by normal-hearing subjects was measured under two frequency-place mapping conditions: the frequency range corresponding to the dead region was (1) removed or (2) reassigned to bands adjacent to the dead region to simulate the off-frequency stimulation of neurons at the edge of a dead region. The present study extends the results of Shannon et al. by including a frequency transposition mapping condition, where the overall acoustic input frequency range was distributed over the entire remaining nondead region. The frequency transposed map provided more acoustic information when compared to the map with the frequency range corresponding to the dead region removed. However, speech perception did not improve for many simulated dead region conditions, possibly due to the spectral distortions in the frequency-place mapping.