Indication of a relation between speech perception and temporal resolution for cochlear implantees

Impact of Aging and the Electrode-to-Neural Interface on Temporal Processing Ability in Cochlear-Implant Users: Gap Detection Thresholds

Accurate processing of temporal information is critical to understanding speech through a cochlear implant (CI). This has potential implications for the growing population of CI users who are ≥65 years of age because of age-related auditory temporal processing deficits. The goal of this study was to measure temporal processing ability in a gap detection task in younger, middle-aged, and older CI users and to determine the relative contributions of chronological age and peripheral neural survival to performance. Single-electrode gap detection thresholds (GDTs) were measured using direct stimulation at five electrode locations and three electrical stimulation rates. The relationship between peripheral status (e.g., electrode-to-neural interface) and GDTs was assessed by the slope of the electrically evoked compound action potential (ECAP) amplitude growth function. Results showed that ECAP slope was the strongest subject-level predictor of GDTs. Steeper ECAP slopes, which are partially indicative of better peripheral function, were associated with better GDTs in younger participants. However, ECAP slope significantly interacted with stimulation rate and age, suggesting that ECAP slopes were not predictive of GDTs in middle-aged and older participants at some stimulation rates. ECAP slope was also related to age, with middle-aged and older participants exhibiting relatively shallow slopes and smaller ranges of slopes compared with younger participants. This pattern of ECAP results limited the evaluation of the independent effects of aging per se and peripheral status on temporal processing ability.

Behavioral Measures of Temporal Processing and Speech Perception in Cochlear Implant Users

BACKGROUND

Although most cochlear implant (CI) users achieve improvements in speech perception, there is still a wide variability in speech perception outcomes. There is a growing body of literature that supports the relationship between individual differences in temporal processing and speech perception performance in CI users. Previous psychophysical studies have emphasized the importance of temporal acuity for overall speech perception performance. Measurement of gap detection thresholds (GDTs) is the most common measure currently used to assess temporal resolution. However, most GDT studies completed with CI participants used direct electrical stimulation not acoustic stimulation and they used psychoacoustic research paradigms that are not easy to administer clinically. Therefore, it is necessary to determine if the variance in GDTs assessed with clinical measures of temporal processing such as the Randomized Gap Detection Test (RGDT) can be used to explain the variability in speech perception performance.

PURPOSE

The primary goal of this study was to investigate the relationship between temporal processing and speech perception performance in CI users.

RESEARCH DESIGN

A correlational study investigating the relationship between behavioral GDTs (assessed with the RGDT or the Expanded Randomized Gap Detection Test) and commonly used speech perception measures (assessed with the Speech Recognition Test [SRT], Central Institute for the Deaf W-22 Word Recognition Test [W-22], Consonant-Nucleus-Consonant Test [CNC], Arizona Biomedical Sentence Recognition Test [AzBio], Bamford-Kowal-Bench Speech-in-Noise Test [BKB-SIN]).

STUDY SAMPLE

Twelve postlingually deafened adult CI users (24-83 yr) and ten normal-hearing (NH; 22-30 yr) adults participated in the study.

DATA COLLECTION AND ANALYSIS

The data were collected in a sound-attenuated test booth. After measuring pure-tone thresholds, GDTs and speech perception performance were measured. The difference in performance between-participant groups on the aforementioned tests, as well as the correlation between GDTs and speech perception performance was examined. The correlations between participants' biologic factors, performance on the RGDT and speech perception measures were also explored.

RESULTS

Although some CI participants performed as well as the NH listeners, the majority of the CI participants displayed temporal processing impairments (GDTs > 20 msec) and poorer speech perception performance than NH participants. A statistically significant difference was found between the NH and CI test groups in GDTs and some speech tests (SRT, W-22, and BKB-SIN). For the CI group, there were significant correlations between GDTs and some measures of speech perception (CNC Phoneme, AzBio, BKB-SIN); however, no significant correlations were found between biographic factors and GDTs or speech perception performance.

CONCLUSIONS

Results support the theory that the variability in temporal acuity in CI users contributes to the variability in speech performance. Results also indicate that it is reasonable to use the clinically available RGDT to identify CI users with temporal processing impairments for further appropriate rehabilitation.

The effects of noise vocoding on gap detection thresholds

Gap detection threshold (GDT), the shortest silent interval a person can perceive, is a commonly used measure of temporal processing resolution. The purposes of this study were: (1) to examine the effects of noise vocoding, which has been used to simulate what signals sound like through a cochlear implant, on GDTs in normal-hearing subjects, and (2) to further the understanding of neural mechanisms underlying gap detection using the Auditory Late Response (ALR). Thirteen normal listeners participated. In behavioral tests, the GDTs were determined for the original and vocoded stimuli. In ALR recordings, the subjects were presented with auditory stimuli with and without containing gaps and stimuli with and without being vocoded. Results showed that GDTs were significantly elevated for vocoded stimuli with spectral resolutions of 4 and 20 channels compared to those for the original stimuli. A gap effect was observed in the post-gap ALR. Current densities for N1 peaks evoked by stimuli with zero- vs. non-zero ms gaps, pre- vs. post-gap markers, and original vs. vocoded stimuli were obtained using the standardized low-resolution brain electromagnetic tomography (sLORETA) method. Paired comparisons of pre- and post-gap current density values were made. Results showed a statistical difference between the N1s evoked by pre- vs. post-gap markers, with the activation in the middle frontal gyrus and precentral gyrus. The results suggest that: (1) noise vocoding does affect temporal processing resolution assessed with GDTs, (2) gap detection may involve the recruitment of cognitive neural resources, and (3) the ALR has a potential value of objectively estimating temporal processing resolution.

Neural adaptation and behavioral measures of temporal processing and speech perception in cochlear implant recipients

The objective was to determine if one of the neural temporal features, neural adaptation, can account for the across-subject variability in behavioral measures of temporal processing and speech perception performance in cochlear implant (CI) recipients. Neural adaptation is the phenomenon in which neural responses are the strongest at the beginning of the stimulus and decline following stimulus repetition (e.g., stimulus trains). It is unclear how this temporal property of neural responses relates to psychophysical measures of temporal processing (e.g., gap detection) or speech perception. The adaptation of the electrical compound action potential (ECAP) was obtained using 1000 pulses per second (pps) biphasic pulse trains presented directly to the electrode. The adaptation of the late auditory evoked potential (LAEP) was obtained using a sequence of 1-kHz tone bursts presented acoustically, through the cochlear implant. Behavioral temporal processing was measured using the Random Gap Detection Test at the most comfortable listening level. Consonant nucleus consonant (CNC) word and AzBio sentences were also tested. The results showed that both ECAP and LAEP display adaptive patterns, with a substantial across-subject variability in the amount of adaptation. No correlations between the amount of neural adaptation and gap detection thresholds (GDTs) or speech perception scores were found. The correlations between the degree of neural adaptation and demographic factors showed that CI users having more LAEP adaptation were likely to be those implanted at a younger age than CI users with less LAEP adaptation. The results suggested that neural adaptation, at least this feature alone, cannot account for the across-subject variability in temporal processing ability in the CI users. However, the finding that the LAEP adaptive pattern was less prominent in the CI group compared to the normal hearing group may suggest the important role of normal adaptation pattern at the cortical level in speech perception.

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.

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.

Sensitivity to isolated and concurrent intensity and fundamental frequency increments by cochlear implant users under natural listening conditions

Sensitivity to acoustic cues in cochlear implant (CI) listening under natural conditions is a potentially complex interaction between a number of simultaneous factors, and may be difficult to predict. In the present study, sensitivity was measured under conditions that approximate those of natural listening. Synthesized words having increases in intensity or fundamental frequency (F0) in a middle stressed syllable were presented in soundfield to normal-hearing listeners and to CI listeners using their everyday speech processors and programming. In contrast to the extremely fine sensitivity to electrical current observed when direct stimulation of single electrodes is employed, difference limens (DLs) for intensity were larger for the CI listeners by a factor of 2.4. In accord with previous work, F0 DLs were larger by almost one order of magnitude. In a second experiment, it was found that the presence of concurrent intensity and F0 increments reduced the mean DL to half that of either cue alone for both groups of subjects, indicating that both groups combine concurrent cues with equal success. Although sensitivity to either cue in isolation was not related to word recognition in CI users, the listeners having lower combined-cue thresholds produced better word recognition scores.

Effects of stimulation rate, mode and level on modulation detection by cochlear implant users

In cochlear implant (CI) patients, temporal processing is often poorest at low listening levels, making perception difficult for low-amplitude temporal cues that are important for consonant recognition and/or speech perception in noise. It remains unclear how speech processor parameters such as stimulation rate and stimulation mode may affect temporal processing, especially at low listening levels. The present study investigated the effects of these parameters on modulation detection by six CI users. Modulation detection thresholds (MDTs) were measured as functions of stimulation rate, mode, and level. Results show that for all stimulation rate and mode conditions, modulation sensitivity was poorest at quiet listening levels, consistent with results from previous studies. MDTs were better with the lower stimulation rate, especially for quiet-to-medium listening levels. Stimulation mode had no significant effect on MDTs. These results suggest that, although high stimulation rates may better encode temporal information and widen the electrode dynamic range, CI patients may not be able to access these enhanced temporal cues, especially at the lower portions of the dynamic range. Lower stimulation rates may provide better recognition of weak acoustic envelope information.

Cortical responses to promontorial stimulation in postlingual deafness

Electrical stimulation with a transtympanic electrode on the promontory of the middle ear allows the tasks of gap detection and temporal difference limen (TDL) to be carried out by both normally hearing and deaf subjects. Previous neuroimaging of normally hearing subjects revealed a region in the right posterior temporal lobe that is crucial to duration discrimination. The present study tested the hypothesis that postlingually deaf subjects recruit this area when they make subtle temporal discriminations. Fourteen postlingually deaf adult cochlear implant candidates were stimulated in the ear chosen for implantation. Altered cerebral activity was recorded with positron emission tomography as incremental 15-O-labelled water uptake. On stimulation with tone bursts, we found bilateral activity close to the primary auditory cortex in all subjects. However, subjects performing well on the TDL task demonstrated right-lateralized fronto-temporal and left-lateralized temporal activity in the respective TDL and gap-detection tasks, while subjects who failed to detect duration differences of less than 200 ms in the TDL discrimination task only had frontal and occipital rather than temporal lobe activation. We conclude that the ability to involve the right posterior temporal region is important to duration discrimination. This ability can be evaluated pre-operatively.

Gap detection by early-deafened cochlear-implant subjects

Two studies investigating gap-detection thresholds were conducted with cochlear-implant subjects whose onset of profound hearing loss was very early in life. The Cochlear Limited multiple-electrode prosthesis was used. The first study investigated the effects of pulse rate (200, 500, and 1000 pulses/s) and stimulus duration (500 and 1000 ms) on gap thresholds in 15 subjects. Average gap thresholds were 1.8 to 32.1 ms. There was essentially no effect of pulse rate and for almost all subjects, no effect of stimulus duration. For two subjects, performance was poorer for the 1000-ms stimulus duration. The second study investigated the relationships between gap thresholds, subject variables, and speech-perception scores. Data from the first study were combined with those from previous studies [Busby et al., Audiology 31, 95-111 (1992); Tong et al., J. Acoust. Soc. Am. 84, 951-962 (1988)], providing data from 27 subjects. A significant negative correlation was found between age at onset of deafness and gap thresholds and most variability in gap thresholds was for the congenitally deaf subjects. Significant negative correlations were found between gap thresholds and word scores for open-set Bamford-Kowal-Bench (BKB) sentences in the auditory-visual condition and lipreading enhancement scores for the same test.

Auditory temporal resolution and open speech recognition in cochlear implant recipients

The recognition of speech events depends, among other factors, on the resolution function. The present study was conducted to evaluate the auditory temporal resolution function in cochlear implant (CI) subjects by a gap detection threshold (GDT) test. Fourteen postlingual deaf subjects who used the Nucleus 22 Channel CI system participated in the study. They were divided into two subgroups: (1) six patients who did not achieve a significant open speech recognition score; and (2) eight patients who obtained different levels of open speech recognition ability (OSRA). Gap detection thresholds were found for six noise burst durations: 85, 65, 52, 36, 25 and 10 msec. Absolute values of GDT in the CI group were longer compared with those obtained from normal-hearing subjects. The CI values varied in consistence with burst duration; namely, as burst duration decreased, GD increased. This trend was found in CI recipients with OSRA only. Examination of the relationship between GDT and OSRA indicated that the majority of CI recipients who had OSRA showed lower GDT.