Loud Music Exposure and Cochlear Synaptopathy in Young Adults: Isolated Auditory Brainstem Response Effects but No Perceptual Consequences

No evidence for a link between noise exposure and auditory temporal processing for young adults with normal audiograms

The link between lifetime noise exposure and temporal processing abilities was investigated for 45 normal-hearing participants, recruited from a population of undergraduate students, aged 18 to 23 years. A self-report instrument was employed to assess the amount of neuropathic noise (here defined as sounds with levels exceeding approximately 80 dBA) each participant had been exposed to and sensitivity to temporal-fine-structure and temporal-envelope information was determined using frequency discrimination and envelope irregularity detection tasks, respectively. Despite sizable individual variability in all measures, correlations between noise exposure and the ability to process temporal cues were small and non-significant.

Reliability of Measures Intended to Assess Threshold-Independent Hearing Disorders

OBJECTIVES

Recent animal studies have shown that noise exposure can cause cochlear synaptopathy without permanent threshold shift. Because the noise exposure preferentially damaged auditory nerve fibers that processed suprathreshold sounds (low-spontaneous rate fibers), it has been suggested that synaptopathy may underlie suprathreshold hearing deficits in humans. Recently, several researchers have suggested measures to identify the pathology or pathologies underlying suprathreshold hearing deficits in humans based on results from animal studies; however, the reliability of some of these measures have not been assessed. The purpose of this study was to assess the test-retest reliability of measures that may have the potential to relate suprathreshold hearing deficits to site(s)-of-lesion along the peripheral auditory system in humans.

DESIGN

Adults with audiometric normal hearing were tested on a battery of behavioral and physiologic measures that included (1) thresholds in quiet (TIQ), (2) thresholds in noise (TIN), (3) frequency-modulation detection threshold (FMDT), (4) word recognition in four listening conditions, (5) distortion-product otoacoustic emissions (DPOAE), (6) middle ear muscle reflex (MEMR), (7) tone burst-elicited auditory brainstem response (tbABR), and (8) speech-evoked ABR (sABR). Data collection for each measure was repeated over two visits separated by at least one week. The residuals of the correlation between the suprathreshold measures and TIQ serve as functional and quantitative proxies for threshold-independent hearing disorders because they represent the portion of the raw measures that is not dependent on TIQ. Reliability of the residual measures was assessed using intraclass correlation (ICC).

RESULTS

Reliability for the residual measures was good (ICC ≥ 0.75) for FMDT, DPOAEs, and MEMR. Residual measures showing moderate reliability (0.5 ≤ ICC < 0.75) were tbABR wave I amplitude, TIN, and word recognition in quiet, noise, and time-compressed speech with reverberation. Wave V of the tbABR, waves of the sABR, and recognition of time-compressed words had poor test-retest reliability (ICC < 0.5).

CONCLUSIONS

Reliability of residual measures was mixed, suggesting that care should be taken when selecting measures for diagnostic tests of threshold-independent hearing disorders. Quantifying hidden hearing loss as the variance in suprathreshold measures of auditory function that is not due to TIQ may provide a reliable estimate of threshold-independent hearing disorders in humans.

Sensitivity to Interaural Phase in Older Hearing-Impaired Listeners Correlates With Nonauditory Trail Making Scores and With a Spatial Auditory Task of Unrelated Peripheral Origin

Interaural phase difference (IPD) discrimination upper frequency limits and just-noticeable differences (JNDs), interaural level difference (ILD) JNDs, and diotic intensity JNDs were measured for 20 older hearing-impaired listeners with matched moderate sloping to severe sensorineural hearing losses. The JNDs were measured using tone stimuli at 500 Hz. In addition to these auditory tests, the participants completed a cognitive test (Trail Making Test). Significant performance improvements in IPD discrimination were observed across test sessions. Strong correlations were found between IPD and ILD discrimination performance. Very strong correlations were observed between IPD discrimination and Trail Making performance as well as strong correlations between ILD discrimination and Trail Making performance. These relationships indicate that interindividual variability in IPD discrimination performance did not exclusively reflect deficits specific to any auditory processing, including early auditory processing of temporal information. The observed relationships between spatial audition and cognition may instead be attributable to a modality-general spatial processing deficit and/or individual differences in global processing speed.

Hidden Hearing Loss: A Disorder with Multiple Etiologies and Mechanisms

Hidden hearing loss (HHL), a recently described auditory disorder, has been proposed to affect auditory neural processing and hearing acuity in subjects with normal audiometric thresholds, particularly in noisy environments. In contrast to central auditory processing disorders, HHL is caused by defects in the cochlea, the peripheral auditory organ. Noise exposure, aging, ototoxic drugs, and peripheral neuropathies are some of the known risk factors for HHL. Our knowledge of the causes and mechanisms of HHL are based primarily on animal models. However, recent clinical studies have also shed light on the etiology and prevalence of this cochlear disorder and how it may affect auditory perception in humans. Here, we review the current knowledge regarding the causes and cellular mechanisms of HHL, summarize information on available noninvasive tests for differential diagnosis, and discuss potential therapeutic approaches for treatment of HHL.

The association between subcortical and cortical fMRI and lifetime noise exposure in listeners with normal hearing thresholds

In animal models, exposure to high noise levels can cause permanent damage to hair-cell synapses (cochlear synaptopathy) for high-threshold auditory nerve fibers without affecting sensitivity to quiet sounds. This has been confirmed in several mammalian species, but the hypothesis that lifetime noise exposure affects auditory function in humans with normal audiometric thresholds remains unconfirmed and current evidence from human electrophysiology is contradictory. Here we report the auditory brainstem response (ABR), and both transient (stimulus onset and offset) and sustained functional magnetic resonance imaging (fMRI) responses throughout the human central auditory pathway across lifetime noise exposure. Healthy young individuals aged 25-40 years were recruited into high (n = 32) and low (n = 30) lifetime noise exposure groups, stratified for age, and balanced for audiometric threshold up to 16 kHz fMRI demonstrated robust broadband noise-related activity throughout the auditory pathway (cochlear nucleus, superior olivary complex, nucleus of the lateral lemniscus, inferior colliculus, medial geniculate body and auditory cortex). fMRI responses in the auditory pathway to broadband noise onset were significantly enhanced in the high noise exposure group relative to the low exposure group, differences in sustained fMRI responses did not reach significance, and no significant group differences were found in the click-evoked ABR. Exploratory analyses found no significant relationships between the neural responses and self-reported tinnitus or reduced sound-level tolerance (symptoms associated with synaptopathy). In summary, although a small effect, these fMRI results suggest that lifetime noise exposure may be associated with central hyperactivity in young adults with normal hearing thresholds.

The Role of Cognition in Common Measures of Peripheral Synaptopathy and Hidden Hearing Loss

Purpose The aim of this study was to quantify the portion of variance in several measures suggested to be indicative of peripheral noise-induced cochlear synaptopathy and hidden hearing disorder that can be attributed to individual cognitive capacity. Method Regression and relative importance analysis was used to model several behavioral and physiological measures of hearing in 32 adults ranging in age from 20 to 74 years. Predictors for the model were hearing sensitivity and performance on a number of cognitive tasks. Results There was a significant influence of cognitive capacity on several measures of cochlear synaptopathy and hidden hearing disorder. These measures include frequency modulation detection threshold, time-compressed word recognition in quiet and reverberation, and the strength of the frequency-following response of the speech-evoked auditory brainstem response. Conclusions Measures of hearing that involve temporal processing are significantly influenced by cognitive abilities, specifically, short-term and working memory capacity, executive function, and attention. Research using measures of temporal processing to diagnose peripheral disorders, such as noise-induced synaptopathy, need to consider cognitive influence even in a young, healthy population.

Noise-induced hearing loss and its prevention: Integration of data from animal models and human clinical trials

Animal models have been used to gain insight into the risk of noise-induced hearing loss (NIHL) and its potential prevention using investigational new drug agents. A number of compounds have yielded benefit in pre-clinical (animal) models. However, the acute traumatic injury models commonly used in pre-clinical testing are fundamentally different from the chronic and repeated exposures experienced by many human populations. Diverse populations that are potentially at risk and could be considered for enrollment in clinical studies include service members, workers exposed to occupational noise, musicians and other performing artists, and children and young adults exposed to non-occupational (including recreational) noise. Both animal models and clinical populations were discussed in this special issue, followed by discussion of individual variation in vulnerability to NIHL. In this final contribution, study design considerations for NIHL otoprotection in pre-clinical and clinical testing are integrated and broadly discussed with evidence-based guidance offered where possible, drawing on the contributions to this special issue as well as other existing literature. The overarching goals of this final paper are to (1) review and summarize key information across contributions and (2) synthesize information to facilitate successful translation of otoprotective drugs from animal models into human application.

Examining physiological and perceptual consequences of noise exposure

The consequences of noise exposure on the auditory system are not entirely understood. In animals, noise exposure causes selective synaptopathy-an uncoupling of auditory nerve fibers from sensory cells-mostly in fibers that respond to high sound levels. Synaptopathy can be measured physiologically in animals, but a direct relationship between noise exposure and synaptopathy in humans has yet to be proven. Sources of variability, such as age, indirect measures of noise exposure, and comorbid auditory disorders, obfuscate attempts to find concrete relationships between noise exposure, synaptopathy, and perceptual consequences. This study adds to the ongoing effort by examining relationships between noise exposure, auditory brainstem response (ABR) amplitudes, and speech perception in adults of various ages and audiometric thresholds and a subset of younger adults with clinically normal hearing. Regression models including noise exposure, age, hearing thresholds, and sex as covariates were compared to find a best-fitting model of toneburst ABR wave I amplitude at two frequencies and word recognition performance in three listening conditions: background noise, time compression, and time compression with reverberation. The data suggest the possibility of detecting synaptopathy in younger adults using physiological measures, but that age and comorbid hearing disorders may hinder attempts to assess noise-induced synaptopathy.

Investigating the Effect of Cochlear Synaptopathy on Envelope Following Responses Using a Model of the Auditory Nerve

The healthy auditory system enables communication in challenging situations with high levels of background noise. Yet, despite normal sensitivity to pure tones, many listeners complain about having difficulties in such situations. Recent animal studies demonstrated that noise overexposure that produces temporary threshold shifts can cause the loss of auditory nerve (AN) fiber synapses (i.e., cochlear synaptopathy, CS), which appears to predominantly affect medium- and low-spontaneous rate (SR) fibers. In the present study, envelope following response (EFR) magnitude-level functions were recorded in normal hearing (NH) threshold and mildly hearing-impaired (HI) listeners with thresholds elevated above 2 kHz. EFRs were elicited by sinusoidally amplitude modulated (SAM) tones presented in quiet with a carrier frequency of 2 kHz, modulated at 93 Hz, and modulation depths of 0.85 (deep) and 0.25 (shallow). While EFR magnitude-level functions for deeply modulated tones were similar for all listeners, EFR magnitudes for shallowly modulated tones were reduced at medium stimulation levels in some NH threshold listeners and saturated in all HI listeners for the whole level range. A phenomenological model of the AN was used to investigate the extent to which hair-cell dysfunction and/or CS could explain the trends observed in the EFR data. Hair-cell dysfunction alone, including postulated elevated hearing thresholds at extended high frequencies (EHF) beyond 8 kHz, could not account for the recorded EFR data. Postulated CS led to simulations generally consistent with the recorded data, but a loss of all types of AN fibers was required within the model framework. The effects of off-frequency contributions (i.e., away from the characteristic place of the stimulus) and the differential loss of different AN fiber types on EFR magnitude-level functions were analyzed. When using SAM tones in quiet as the stimulus, model simulations suggested that (1) EFRs are dominated by the activity of high-SR fibers at all stimulus intensities, and (2) EFRs at medium-to-high stimulus levels are dominated by off-frequency contributions.

Effects of Age, Cognition, and Neural Encoding on the Perception of Temporal Speech Cues

Older adults commonly report difficulty understanding speech, particularly in adverse listening environments. These communication difficulties may exist in the absence of peripheral hearing loss. Older adults, both with normal hearing and with hearing loss, demonstrate temporal processing deficits that affect speech perception. The purpose of the present study is to investigate aging, cognition, and neural processing factors that may lead to deficits on perceptual tasks that rely on phoneme identification based on a temporal cue - vowel duration. A better understanding of the neural and cognitive impairments underlying temporal processing deficits could lead to more focused aural rehabilitation for improved speech understanding for older adults. This investigation was conducted in younger (YNH) and older normal-hearing (ONH) participants who completed three measures of cognitive functioning known to decline with age: working memory, processing speed, and inhibitory control. To evaluate perceptual and neural processing of auditory temporal contrasts, identification functions for the contrasting word-pair WHEAT and WEED were obtained on a nine-step continuum of vowel duration, and frequency-following responses (FFRs) and cortical auditory-evoked potentials (CAEPs) were recorded to the two endpoints of the continuum. Multiple linear regression analyses were conducted to determine the cognitive, peripheral, and/or central mechanisms that may contribute to perceptual performance. YNH participants demonstrated higher cognitive functioning on all three measures compared to ONH participants. The slope of the identification function was steeper in YNH than in ONH participants, suggesting a clearer distinction between the contrasting words in the YNH participants. FFRs revealed better response waveform morphology and more robust phase-locking in YNH compared to ONH participants. ONH participants also exhibited earlier latencies for CAEP components compared to the YNH participants. Linear regression analyses revealed that cortical processing significantly contributed to the variance in perceptual performance in the WHEAT/WEED identification functions. These results suggest that reduced neural precision contributes to age-related speech perception difficulties that arise from temporal processing deficits.

Age Effects on Neural Representation and Perception of Silence Duration Cues in Speech

Purpose Degraded temporal processing associated with aging may be a contributing factor to older adults' hearing difficulties, especially in adverse listening environments. This degraded processing may affect the ability to distinguish between words based on temporal duration cues. The current study investigates the effects of aging and hearing loss on cortical and subcortical representation of temporal speech components and on the perception of silent interval duration cues in speech. Method Identification functions for the words DISH and DITCH were obtained on a 7-step continuum of silence duration (0-60 ms) prior to the final fricative in participants who are younger with normal hearing (YNH), older with normal hearing (ONH), and older with hearing impairment (OHI). Frequency-following responses and cortical auditory-evoked potentials were recorded to the 2 end points of the continuum. Auditory brainstem responses to clicks were obtained to verify neural integrity and to compare group differences in auditory nerve function. A multiple linear regression analysis was conducted to determine the peripheral or central factors that contributed to perceptual performance. Results ONH and OHI participants required longer silence durations to identify DITCH than did YNH participants. Frequency-following responses showed reduced phase locking and poorer morphology, and cortical auditory-evoked potentials showed prolonged latencies in ONH and OHI participants compared with YNH participants. No group differences were noted for auditory brainstem response Wave I amplitude or Wave V/I ratio. After accounting for the possible effects of hearing loss, linear regression analysis revealed that both midbrain and cortical processing contributed to the variance in the DISH-DITCH perceptual identification functions. Conclusions These results suggest that age-related deficits in the ability to encode silence duration cues may be a contributing factor in degraded speech perception. In particular, degraded response morphology relates to performance on perceptual tasks based on silence duration contrasts between words.

[Hidden hearing loss-damage to hearing processing even with low-threshold noise exposure?]

BACKGROUND

New research in animal models indicates that even at lower intensities, noise exposure can induce defects in the synapses of the auditory pathway. However, only very high levels of noise exposure lead to mechanical hair cell damage with lesions of the inner ear and measurable hearing loss (audiogram; distortion product otoacoustic emissions, DPOAE). This paper revises the literature, starting with a case study.

CASE HISTORY

A 41-year-old patient suffered from hearing loss and tinnitus in the right ear following a car accident with airbag deployment. Hearing loss recovered partially, tinnitus and difficulties in speech discrimination persisted. Audiometry showed typical high-frequency hearing loss (40 dB) and tonal tinnitus (8 kHz). Although DPOAE and ABR potentials (auditory brainstem response, wave III and V) were completely normal 6 months after the accident, there was no detectable cochlear action potential (CAP) in electrocochleography (ECochG).

DISCUSSION

These findings indicate recovery of initial hair cell damage, whereas synaptic transformation remains reduced and slight hearing loss and poor speech perception in complex listening situations persist. This phenomenon has been described as "hidden hearing loss" in newer literature. Although similar retrocochlear lesions in the auditory pathway could be detected in animal models, valid data in humans are currently lacking because no adequate diagnostic methods are available.

CONCLUSION

Noise trauma initially results in hair cell damage. After recovery, hearing loss may persist, which can be due to synaptic lesions in the first neuron. An adequate testbattery has to be developped.

Reliability and interrelations of seven proxy measures of cochlear synaptopathy

Investigations of cochlear synaptopathy in living humans rely on proxy measures of auditory nerve function. Numerous procedures have been developed, typically based on the auditory brainstem response (ABR), envelope-following response (EFR), or middle-ear-muscle reflex (MEMR). Validation is challenging, due to the absence of a gold-standard measure in humans. Some metrics correlate with synaptic survival in animal models, but translation between species is not straightforward; measurements in humans are likely to reflect greater error and greater variability from non-synaptopathic sources. The present study assessed the reliability of seven measures, as well as testing for correlations between them. Thirty-one young women with normal audiograms underwent repeated measurements of ABR wave I amplitude, ABR wave I growth, ABR wave V latency shift in noise, EFR amplitude, EFR growth with stimulus modulation depth, MEMR threshold, and an MEMR across-frequency difference measure. Intraclass correlation coefficients for ABR wave I amplitude, EFR amplitude, and MEMR threshold ranged from 0.85 to 0.93, suggesting that such tests can yield highly reliable results, given careful measurement techniques. The ABR and EFR difference measures exhibited only poor-to-moderate reliability. No significant correlations, nor any consistent trends, were observed between the various measures, providing no indication that these metrics reflect the same underlying physiological processes. Findings suggest that many proxy measures of cochlear synaptopathy should be regarded with caution, at least when employed in young adults with normal audiograms.

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Given careful measurement techniques, ABR and EFR amplitudes can be highly reliable.

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The same is true of MEMR thresholds and MEMR across-frequency threshold difference.

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Differential ABR and EFR measures exhibit only poor-to-moderate reliability.

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Correlations between measures are not evident in young people with normal audiograms.

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Proxy measures of synaptopathy in this population should be regarded with caution.

Age-Related Changes in the Auditory Brainstem Response and Suprathreshold Processing of Temporal and Spectral Modulation

The purpose of this study was to determine whether cochlear synaptopathy can be shown to be a viable basis for age-related hearing difficulties in humans and whether it manifests as deficient suprathreshold processing of temporal and spectral modulation. Three experiments were undertaken evaluating the effects of age on (a) the auditory brainstem response as a function of level, (b) temporal modulation detection as a function of level and background noise, and (c) spectral modulation as a function of level. Across the three experiments, a total of 21 older listeners with near-normal audiograms and 29 young listeners with audiometrically normal hearing participated. The auditory brainstem response experiment demonstrated reduced Wave I amplitudes and concomitant reductions in the amplitude ratios of Wave I to Wave V in the older listener group. These findings were interpreted as consistent with an electrophysiological profile of cochlear synaptopathy. The temporal and spectral modulation detection experiments, however, provided no support for the hypothesis of compromised suprathreshold processing in these domains. This pattern of results suggests that even if cochlear synaptopathy can be shown to be a viable basis for age-related hearing difficulties, then temporal and spectral modulation detection paradigms are not sensitive to its presence.

Relationship Between Subjective Reports of Temporary Threshold Shift and the Prevalence of Hearing Problems in Military Personnel

Traditional hearing conservation programs are based on the premise that noise dose, as measured by the time-weighted average noise level, is the primary risk factor associated with occupational hearing loss and that permanent threshold shifts are the most relevant outcome measures for determining when a noise-related hearing loss has occurred. However, recent studies in animal models have suggested that significant neurological damage to the hearing system can occur from noise exposures even when they are not severe enough to result in permanent threshold shifts. This has led to a number of studies attempting to relate subjective measures of noise exposure to subjective measures of hearing difficulty and suprathreshold measures of hearing performance (e.g., speech-in-noise tests). In this study, 3,330 U.S. service members volunteered to complete a survey on noise exposure, subjective hearing complaints, and tinnitus in conjunction with their annual hearing tests. Two questions were also included about the frequency and duration of temporary hearing losses that may have been experienced by the service member. The results show that subjective reports of temporary threshold shifts were substantially more predictive of tinnitus and other hearing complaints than more traditional questions based on the frequency of noise exposure.

Effects of Age and Noise Exposure on Proxy Measures of Cochlear Synaptopathy

Although there is strong histological evidence for age-related synaptopathy in humans, evidence for the existence of noise-induced cochlear synaptopathy in humans is inconclusive. Here, we sought to evaluate the relative contributions of age and noise exposure to cochlear synaptopathy using a series of electrophysiological and behavioral measures. We extended an existing cohort by including 33 adults in the age range 37 to 60, resulting in a total of 156 participants, with the additional older participants resulting in a weakening of the correlation between lifetime noise exposure and age. We used six independent regression models (corrected for multiple comparisons), in which age, lifetime noise exposure, and high-frequency audiometric thresholds were used to predict measures of synaptopathy, with a focus on differential measures. The models for auditory brainstem responses, envelope-following responses, interaural phase discrimination, and the co-ordinate response measure of speech perception were not statistically significant. However, both age and noise exposure were significant predictors of performance on the digit triplet test of speech perception in noise, with greater noise exposure (unexpectedly) predicting better performance in the 80 dB sound pressure level (SPL) condition and greater age predicting better performance in the 40 dB SPL condition. Amplitude modulation detection thresholds were also significantly predicted by age, with older listeners performing better than younger listeners at 80 dB SPL. Overall, the results are inconsistent with the predicted effects of synaptopathy.

Investigating peripheral sources of speech-in-noise variability in listeners with normal audiograms

A current initiative in auditory neuroscience research is to better understand why some listeners struggle to perceive speech-in-noise (SIN) despite having normal hearing sensitivity. Various hypotheses regarding the physiologic bases of this disorder have been proposed. Notably, recent work has suggested that the site of lesion underlying SIN deficits in normal hearing listeners may be either in "sub-clinical" outer hair cell damage or synaptopathic degeneration at the inner hair cell-auditory nerve fiber synapse. In this study, we present a retrospective investigation of these peripheral sources and their relationship with SIN performance variability in one of the largest datasets of young normal-hearing listeners presented to date. 194 participants completed detailed case history questionnaires assessing noise exposure, SIN complaints, tinnitus, and hyperacusis. Standard and extended high frequency audiograms, distortion product otoacoustic emissions, click-evoked auditory brainstem responses, and SIN performance measures were also collected. We found that: 1) the prevalence of SIN deficits in normal hearing listeners was 42% when based on subjective report and 8% when based on SIN performance, 2) hearing complaints and hyperacusis were more common in listeners with self-reported noise exposure histories than controls, 3) neither extended high frequency thresholds nor compound action potential amplitudes differed between noise-exposed and control groups, 4) extended high frequency hearing thresholds and compound action potential amplitudes were not predictive of SIN performance. These results suggest an association between noise exposure and hearing complaints in young, normal hearing listeners; however, SIN performance variability is not explained by peripheral auditory function to the extent that these measures capture subtle physiologic differences between participants.

Supra-threshold auditory brainstem response amplitudes in humans: Test-retest reliability, electrode montage and noise exposure

The auditory brainstem response (ABR) is a sub-cortical evoked potential in which a series of well-defined waves occur in the first 10 ms after the onset of an auditory stimulus. Wave V of the ABR, particularly wave V latency, has been shown to be remarkably stable over time in individual listeners. However, little attention has been paid to the reliability of wave I, which reflects auditory nerve activity. This ABR component has attracted interest recently, as wave I amplitude has been identified as a possible non-invasive measure of noise-induced cochlear synaptopathy. The current study aimed to determine whether ABR wave I amplitude has sufficient test-retest reliability to detect impaired auditory nerve function in an otherwise normal-hearing listener. Thirty normal-hearing females were tested, divided equally into low- and high-noise exposure groups. The stimulus was an 80 dB nHL click. ABR recordings were made from the ipsilateral mastoid and from the ear canal (using a tiptrode). Although there was some variability between listeners, wave I amplitude had high test-retest reliability, with an intraclass correlation coefficient (ICC) comparable to that for wave V amplitude. There were slight gains in reliability for wave I amplitude when recording from the ear canal (ICC of 0.88) compared to the mastoid (ICC of 0.85). The summating potential (SP) and ratio of SP to wave I were also quantified and found to be much less reliable than measures of wave I and V amplitude. Finally, we found no significant differences in the amplitude of any wave components between low- and high-noise exposure groups. We conclude that, if the other sources of between-subject variability can be controlled, wave I amplitude is sufficiently reliable to accurately characterize individual differences in auditory nerve function.

Impaired speech perception in noise with a normal audiogram: No evidence for cochlear synaptopathy and no relation to lifetime noise exposure

In rodents, noise exposure can destroy synapses between inner hair cells and auditory nerve fibers (“cochlear synaptopathy”) without causing hair cell loss. Noise-induced cochlear synaptopathy usually leaves cochlear thresholds unaltered, but is associated with long-term reductions in auditory brainstem response (ABR) amplitudes at medium-to-high sound levels. This pathophysiology has been suggested to degrade speech perception in noise (SPiN), perhaps explaining why SPiN ability varies so widely among audiometrically normal humans. The present study is the first to test for evidence of cochlear synaptopathy in humans with significant SPiN impairment. Individuals were recruited on the basis of self-reported SPiN difficulties and normal pure tone audiometric thresholds. Performance on a listening task identified a subset with “verified” SPiN impairment. This group was matched with controls on the basis of age, sex, and audiometric thresholds up to 14 kHz. ABRs and envelope-following responses (EFRs) were recorded at high stimulus levels, yielding both raw amplitude measures and within-subject difference measures. Past exposure to high sound levels was assessed by detailed structured interview. Impaired SPiN was not associated with greater lifetime noise exposure, nor with any electrophysiological measure. It is conceivable that retrospective self-report cannot reliably capture noise exposure, and that ABRs and EFRs offer limited sensitivity to synaptopathy in humans. Nevertheless, the results do not support the notion that noise-induced synaptopathy is a significant etiology of SPiN impairment with normal audiometric thresholds. It may be that synaptopathy alone does not have significant perceptual consequences, or is not widespread in humans with normal audiograms.

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Study of adults with impaired speech perception in noise (SPiN) and normal audiograms.

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A subset of those with reported SPiN impairment exhibited measurable SPiN deficits.

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SPiN-impaired participants were matched with controls for age, sex, and audiogram.

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Impaired SPiN was not associated with ABR or EFR measures of cochlear synaptopathy.

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Impaired SPiN was not associated with a detailed measure of lifetime noise exposure.