Agnès C. Léger

Effects of noise exposure on young adults with normal audiograms I: Electrophysiology

&NA; Noise‐induced cochlear synaptopathy has been demonstrated in numerous rodent studies. In these animal models, the disorder is characterized by a reduction in amplitude of wave I of the auditory brainstem response (ABR) to high‐level stimuli, whereas the response at threshold is unaffected. The aim of the present study was to determine if this disorder is prevalent in young adult humans with normal audiometric hearing. One hundred and twenty six participants (75 females) aged 18–36 were tested. Participants had a wide range of lifetime noise exposures as estimated by a structured interview. Audiometric thresholds did not differ across noise exposures up to 8 kHz, although 16‐kHz audiometric thresholds were elevated with increasing noise exposure for females but not for males. ABRs were measured in response to high‐pass (1.5 kHz) filtered clicks of 80 and 100 dB peSPL. Frequency‐following responses (FFRs) were measured to 80 dB SPL pure tones from 240 to 285 Hz, and to 80 dB SPL 4 kHz pure tones amplitude modulated at frequencies from 240 to 285 Hz (transposed tones). The bandwidth of the ABR stimuli and the carrier frequency of the transposed tones were chosen to target the 3–6 kHz characteristic frequency region which is usually associated with noise damage in humans. The results indicate no relation between noise exposure and the amplitude of the ABR. In particular, wave I of the ABR did not decrease with increasing noise exposure as predicted. ABR wave V latency increased with increasing noise exposure for the 80 dB peSPL click. High carrier‐frequency (envelope) FFR signal‐to‐noise ratios decreased as a function of noise exposure in males but not females. However, these correlations were not significant after the effects of age were controlled. The results suggest either that noise‐induced cochlear synaptopathy is not a significant problem in young, audiometrically normal adults, or that the ABR and FFR are relatively insensitive to this disorder in young humans, although it is possible that the effects become more pronounced with age. HighlightsLarge study on the effects of lifetime noise exposure in normal‐hearing young adults.No clear evidence for noise‐induced cochlear synaptopathy in ABR or FFR measures.Noise exposure associated with elevated 16‐kHz audiometric thresholds in females.

Toward a Diagnostic Test for Hidden Hearing Loss

Cochlear synaptopathy (or hidden hearing loss), due to noise exposure or aging, has been demonstrated in animal models using histological techniques. However, diagnosis of the condition in individual humans is problematic because of (a) test reliability and (b) lack of a gold standard validation measure. Wave I of the transient-evoked auditory brainstem response is a noninvasive electrophysiological measure of auditory nerve function and has been validated in the animal models. However, in humans, Wave I amplitude shows high variability both between and within individuals. The frequency-following response, a sustained evoked potential reflecting synchronous neural activity in the rostral brainstem, is potentially more robust than auditory brainstem response Wave I. However, the frequency-following response is a measure of central activity and may be dependent on individual differences in central processing. Psychophysical measures are also affected by intersubject variability in central processing. Differential measures may help to reduce intersubject variability due to unrelated factors. A measure can be compared, within an individual, between conditions that are affected differently by cochlear synaptopathy. Validation of the metrics is also an issue. Comparisons with animal models, computational modeling, auditory nerve imaging, and human temporal bone histology are all potential options for validation, but there are technical and practical hurdles and difficulties in interpretation. Despite the obstacles, a diagnostic test for hidden hearing loss is a worthwhile goal, with important implications for clinical practice and health surveillance.

Consonant identification in noise using Hilbert-transform temporal fine-structure speech and recovered-envelope speech for listeners with normal and impaired hearing

Consonant-identification ability was examined in normal-hearing (NH) and hearing-impaired (HI) listeners in the presence of steady-state and 10-Hz square-wave interrupted speech-shaped noise. The Hilbert transform was used to process speech stimuli (16 consonants in a-C-a syllables) to present envelope cues, temporal fine-structure (TFS) cues, or envelope cues recovered from TFS speech. The performance of the HI listeners was inferior to that of the NH listeners both in terms of lower levels of performance in the baseline condition and in the need for higher signal-to-noise ratio to yield a given level of performance. For NH listeners, scores were higher in interrupted noise than in steady-state noise for all speech types (indicating substantial masking release). For HI listeners, masking release was typically observed for TFS and recovered-envelope speech but not for unprocessed and envelope speech. For both groups of listeners, TFS and recovered-envelope speech yielded similar levels of performance and consonant confusion patterns. The masking release observed for TFS and recovered-envelope speech may be related to level effects associated with the manner in which the TFS processing interacts with the interrupted noise signal, rather than to the contributions of TFS cues per se.

Role of slow temporal modulations in speech identification for cochlear implant users

Abstract Objective: This study aimed to assess whether the capacity of cochlear implant (CI) users to identify speech is determined by their capacity to perceive slow (< 20 Hz) temporal modulations. Design: This was achieved by studying the correlation between (1) phoneme identification in quiet and in a steady-state or fluctuating (8 Hz) noises, and (2) amplitude-modulation detection thresholds (MDTs) at 8 Hz (i.e. slow temporal modulations). Study sample: Twenty-one CI users, unilaterally implanted with the same device, were tested in free field with their everyday clinical processor. Results: Extensive variability across subjects was observed for both phoneme identification and MDTs. Vowel and consonant identification scores in quiet were significantly correlated with MDTs at 8 Hz (r = − 0.47 for consonants, r = − 0.44 for vowels; p < 0.05). When the masker was a steady-state noise, only consonant identification scores tended to correlate with MDTs at 8 Hz (r = − 0.4; p = 0.07). When the masker was a fluctuating noise, consonant and vowel identification scores were not significantly correlated with MDTs at 8 Hz. Conclusions: Sensitivity to slow amplitude modulations is correlated with vowel and consonant perception in CI users. However, reduced sensitivity to slow modulations does not entirely explain the limited capacity of CI recipients to understand speech in noise.

Effects of spectral smearing on the identification of speech in noise filtered into low- and mid-frequency regions

Léger et al. [J. Acoust. Soc. Am. 131, 1502-1514 (2012)] reported deficits in the identification of consonants in noise by hearing-impaired listeners using stimuli filtered into low- or mid-frequency regions in which audiometric thresholds were normal or near-normal. The deficits could not be fully explained in terms of reduced audibility or temporal-envelope processing. However, previous studies indicate that the listeners may have had reduced frequency selectivity, with auditory filters broadened by a factor of about 1.3, despite having normal or near-normal audiometric thresholds in the tested regions. The present study aimed to determine whether the speech-perception deficits could be explained by such a small reduction of frequency selectivity. Consonant identification was measured for normal-hearing listeners in quiet and in unmodulated and modulated noises using the same method as Léger et al. The signal-to-noise ratio was set to -3 dB for the masked conditions. Various amounts of reduced frequency selectivity were simulated using a spectral-smearing algorithm. Performance was reduced only for spectral-smearing factors greater than 1.7. For all conditions, identification scores for hearing-impaired listeners could not be explained by a mild reduction of frequency selectivity.

The role of recovered envelope cues in the identification of temporal-fine-structure speech for hearing-impaired listeners.

Narrowband speech can be separated into fast temporal cues [temporal fine structure (TFS)], and slow amplitude modulations (envelope). Speech processed to contain only TFS leads to envelope recovery through cochlear filtering, which has been suggested to account for TFS-speech intelligibility for normal-hearing listeners. Hearing-impaired listeners have deficits with TFS-speech identification, but the contribution of recovered-envelope cues to these deficits is unknown. This was assessed for hearing-impaired listeners by measuring identification of disyllables processed to contain TFS or recovered-envelope cues. Hearing-impaired listeners performed worse than normal-hearing listeners, but TFS-speech intelligibility was accounted for by recovered-envelope cues for both groups.

Level variations in speech: Effect on masking release in hearing-impaired listeners

Acoustic speech is marked by time-varying changes in the amplitude envelope that may pose difficulties for hearing-impaired listeners. Removal of these variations (e.g., by the Hilbert transform) could improve speech reception for such listeners, particularly in fluctuating interference. Léger, Reed, Desloge, Swaminathan, and Braida [(2015b). J. Acoust. Soc. Am. 138, 389-403] observed that a normalized measure of masking release obtained for hearing-impaired listeners using speech processed to preserve temporal fine-structure (TFS) cues was larger than that for unprocessed or envelope-based speech. This study measured masking release for two other speech signals in which level variations were minimal: peak clipping and TFS processing of an envelope signal. Consonant identification was measured for hearing-impaired listeners in backgrounds of continuous and fluctuating speech-shaped noise. The normalized masking release obtained using speech with normal variations in overall level was substantially less than that observed using speech processed to achieve highly restricted level variations. These results suggest that the performance of hearing-impaired listeners in fluctuating noise may be improved by signal processing that leads to a decrease in stimulus level variations.

Effects of age on sensitivity to interaural time differences in envelope and fine structure, individually and in combination

Sensitivity to interaural time differences (ITDs) in envelope and temporal fine structure (TFS) of amplitude-modulated (AM) tones was assessed for young and older subjects, all with clinically normal hearing at the carrier frequencies of 250 and 500 Hz. Some subjects had hearing loss at higher frequencies. In experiment 1, thresholds for detecting changes in ITD were measured when the ITD was present in the TFS alone (ITDTFS), the envelope alone (ITDENV), or both (ITDTFS/ENV). Thresholds tended to be higher for the older than for the young subjects. ITDENV thresholds were much higher than ITDTFS thresholds, while ITDTFS/ENV thresholds were similar to ITDTFS thresholds. ITDTFS thresholds were lower than ITD thresholds obtained with an unmodulated pure tone, indicating that uninformative AM can improve ITDTFS discrimination. In experiment 2, equally detectable values of ITDTFS and ITDENV were combined so as to give consistent or inconsistent lateralization. There were large individual differences, but several subjects gave scores that were much higher than would be expected from the optimal combination of independent sources of information, even for the inconsistent condition. It is suggested that ITDTFS and ITDENV cues are processed partly independently, but that both cues influence lateralization judgments, even when one cue is uninformative.

Abnormal intelligibility of speech in competing speech and in noise in a frequency region where audiometric thresholds are near-normal for hearing-impaired listeners.

The ability to identify syllables in the presence of speech-shaped noise and a single-talker background was measured for 18 normal-hearing (NH) listeners, and for eight hearing-impaired (HI) listeners with near-normal audiometric thresholds for frequencies up to 1.5 kHz and a moderate to severe hearing loss above 2 kHz. The stimulus components were restricted to the low-frequency (≤1.5 kHz) region, where audiometric thresholds were classified clinically as normal or near normal for all listeners. Syllable identification in a speech background was measured as a function of the fundamental-frequency (F0) difference between competing voices (ranging from 1 semitone to ∼1 octave). HI listeners had poorer syllable intelligibility than NH listeners in all conditions. Intelligibility decreased by about the same amount for both groups when the F0 difference between competing voices was reduced. The results suggest that the ability to identify speech against noise or an interfering talker was disrupted in frequency regions of near-normal hearing for HI listeners, but that the ability to benefit from the tested F0 differences was not disrupted. This deficit was not predicted by the elevated absolute thresholds for speech in speech, but it was for speech in noise. It may result from supra-threshold auditory deficits associated with aging.

Evidence that hidden hearing loss does not vary systematically as a function of noise exposure in young adults with normal audiometric hearing

Cochlear synaptopathy, or “hidden hearing loss,” refers to a loss of synapses between inner hair cells and auditory nerve fibers, and is observed in rodent models as a consequence of noise exposure and/or aging. In humans, cochlear synaptopathy is not thought to be detectable by pure tone audiometry, as thresholds to soft sounds in the rodent models are not permanently elevated. One hundred and forty audiometrically normal participants below the age of 35 and with a range of lifetime noise exposures performed an extensive battery of tests, including electrophysiological measures, psychophysical tests, and speech-in-noise tests. Inter-aural phase discrimination, amplitude modulation detection, and spatial release from masking on a speech task were found to be sensitive to noise exposure; however, these trends are weak and only the phase discrimination task followed the predicted direction (i.e., high noise exposed individuals showing elevated thresholds). None of the electrophysiological measures, includin...