Effects of lifetime noise exposure on the middle-age human auditory brainstem response, tinnitus and speech-in-noise intelligibility

Low-sound-level auditory processing in noise-exposed adults

Early signs of noise-induced hearing damage are difficult to identify, as they are often confounded by factors such as age, audiometric thresholds, or even music experience. Much previous research has focused on deficits observed at high intensity levels. In contrast, the present study was designed to test the hypothesis that noise exposure causes a degradation in low-sound-level auditory processing in humans, as a consequence of dysfunction of the inner hair cell pathway. Frequency difference limens (FDLs) and amplitude modulation depth discrimination (MDD) were measured for five center frequencies (0.75, 1, 3, 4 and 6 kHz) at 15 and 25 dB sensation level (SL), as a function of noise exposure, age, audiometric hearing loss, and music experience. Forty participants, aged 33-75 years, with normal hearing up to 1 kHz and mild-to-moderate hearing loss above 2 kHz, were tested. Participants had varying degrees of self-reported noise exposure, and varied in music experience. FDL worsened as a function of age. Participants with music experience outperformed the non-experienced in both the FDL and MDD tasks. MDD thresholds were significantly better for high-noise-exposed, than for low-noise-exposed, participants at 25 dB SL, particularly at 6 kHz. No effects of age or hearing loss were observed in the MDD. It is possible that the association between MDD thresholds and noise exposure was not causal, but instead was mediated by other factors that were not measured in the study. The association is consistent, qualitatively, with a hypothesized loss of compression due to outer hair cell dysfunction.

Association of occupational noise exposure, bilateral hearing loss with atherosclerotic cardiovascular disease risk in Chinese adults

We aimed to explore the association of occupational noise exposure with atherosclerotic cardiovascular disease (ASCVD) risk in Chinese adults. We included 21,412 participants from the Dongfeng-tongji Cohort Study, occupational noise exposure was evaluated through workplace noise level and/or the job titles, hearing loss was defined as a pure-tone mean of 25 dB or higher at 0.5, 1, 2, and 4 kHz in any ear. Compared with participants without occupational noise exposure, the 10-year ASCVD risk was significantly higher for noise exposure duration ≥20 years (OR = 1.20, 95%CI = 1.05-1.32) after adjusting for potential confounders. In the subgroup analysis, the association was only statistically significant in males (OR = 1.86, 95%CI = 1.12-3.14) and participants aged equal to or over 60 years old (OR = 1.20, 95%CI = 1.05-1.33), but not in females (OR = 1.15, 95%CI = 0.71-1.92) and aged below 60 (OR = 1.51, 95%CI = 0.75-2.85). In the subsample analyses (N = 10,165), bilateral hearing loss was associated with a higher risk of 10-year ASCVD (OR = 1.72, 95%CI = 1.30-2.30), especially for participants who were males (OR = 2.40, 95%CI = 1.61-3.42) and aged equal to or over 60 (OR = 1.85, 95%CI = 1.40-2.44). The present study suggests that occupational noise exposure may be a potential risk factor for ASCVD, especially for males and older participants.

Age-related central gain compensation for reduced auditory nerve output for people with normal audiograms, with and without tinnitus

Central gain compensation for reduced auditory nerve output has been hypothesized as a mechanism for tinnitus with a normal audiogram. Here, we investigate if gain compensation occurs with aging. For 94 people (aged 12-68 years, 64 women, 7 tinnitus) with normal or close-to-normal audiograms, the amplitude of wave I of the auditory brainstem response decreased with increasing age but was not correlated with wave V amplitude after accounting for age-related subclinical hearing loss and cochlear damage, a result indicative of age-related gain compensation. The correlations between age and wave I/III or III/V amplitude ratios suggested that compensation occurs at the wave III generator site. For each one of the seven participants with non-pulsatile tinnitus, the amplitude of wave I, wave V, and the wave I/V amplitude ratio were well within the confidence limits of the non-tinnitus participants. We conclude that increased central gain occurs with aging and is not specific to tinnitus.

The possibility of cochlear synaptopathy in young people using a personal listening device

OBJECTIVE

To evaluate the association of listening to music loudly through personal listening devices with cochlear synaptopathy in young adults.

METHODS

Fifty healthy young adults selected among 109 volunteers were included in the study. Participants of high risk (n=25) and low risk (n=25) groups estimated according to ETDNL (estimated total daily noise level) were evaluated using pure tone audiometry, tympanometry, matrix test, electrocochleography (EcochG) and auditory brainstem response (ABR) to evaluate the occurrence of cochlear synaptopathy.

RESULTS

Audiometric thresholds between the groups were not significantly different (p>0.05). High risk group participants showed poorer performance than the low-risk group on the TurMatrix test, in non-adaptive noise with -5 SNR and -7.5 SNR, and at the 50% understanding SNR level with headphones (p<0.01). There was no difference in the adaptive free field in noise test at which 50% understanding was achieved (p>0.05). The AP amplitudes on EcochG and wave V amplitudes on ABR were significantly smaller in the high-risk group (p<0.05). There was no association between ETDNL and I/V ratio on ABR.

CONCLUSION

Poorer performance in TurMatrix and other electrophysiologic tests revealed the negative effect of personal listening devices on the auditory system. Our findings support the hypothesis that personal listening devices could cause cochlear synaptopathy. Long-term studies are needed to determine the effects of binaural hearing and duration of noise exposure on the auditory system.

Contrasting mechanisms for hidden hearing loss: Synaptopathy vs myelin defects

Hidden hearing loss (HHL) is an auditory neuropathy characterized by normal hearing thresholds but reduced amplitudes of the sound-evoked auditory nerve compound action potential (CAP). In animal models, HHL can be caused by moderate noise exposure or aging, which induces loss of inner hair cell (IHC) synapses. In contrast, recent evidence has shown that transient loss of cochlear Schwann cells also causes permanent auditory deficits in mice with similarities to HHL. Histological analysis of the cochlea after auditory nerve remyelination showed a permanent disruption of the myelination patterns at the heminode of type I spiral ganglion neuron (SGN) peripheral terminals, suggesting that this defect could be contributing to HHL. To shed light on the mechanisms of different HHL scenarios observed in animals and to test their impact on type I SGN activity, we constructed a reduced biophysical model for a population of SGN peripheral axons whose activity is driven by a well-accepted model of cochlear sound processing. We found that the amplitudes of simulated sound-evoked SGN CAPs are lower and have greater latencies when heminodes are disorganized, i.e. they occur at different distances from the hair cell rather than at the same distance as in the normal cochlea. These results confirm that disruption of heminode positions causes desynchronization of SGN spikes leading to a loss of temporal resolution and reduction of the sound-evoked SGN CAP. Another mechanism resulting in HHL is loss of IHC synapses, i.e., synaptopathy. For comparison, we simulated synaptopathy by removing high threshold IHC-SGN synapses and found that the amplitude of simulated sound-evoked SGN CAPs decreases while latencies remain unchanged, as has been observed in noise exposed animals. Thus, model results illuminate diverse disruptions caused by synaptopathy and demyelination on neural activity in auditory processing that contribute to HHL as observed in animal models and that can contribute to perceptual deficits induced by nerve damage in humans.

Association Between Tinnitus Pitch and Consonant Recognition in Noise

Purpose Difficulties in speech-in-noise understanding are often reported in individuals with tinnitus. Building on our previous findings that speech-in-noise performance is correlated with subjective loudness of tinnitus, this study aimed to investigate the effect of tinnitus pitch on consonant recognition in noise. Method Pure-tone audiometry and the Quick Speech-in-Noise Test were conducted on 66 participants categorized into four groups by their hearing sensitivity and self-report of tinnitus. Consonant recognition scores at various frequency ranges were obtained at the 5 dB SNR condition of the Quick Speech-in-Noise Test. Participants with tinnitus also completed a tinnitus pitch-matching procedure. Correlation analyses were conducted between tinnitus pitch and the frequency of the worst consonant recognition, and the error rates based on word and sentence position were compared. Results Regardless of hearing sensitivity, tinnitus pitch did not correlate with the frequency of the worst consonant recognition. Sentence-initial word recognition was affected by hearing loss, whereas sentence-final word recognition was not affected by hearing loss or tinnitus. In contrast to individuals with normal hearing, participants with hearing loss varied in full-sentence recognition, with those reporting tinnitus exhibiting significantly higher error rates. Conclusions The findings suggest that the effect of tinnitus on consonant recognition in noise may involve higher level functions more than perceptual characteristics of tinnitus. Furthermore, for individuals with speech-in-noise concerns, clinical evaluation should address both hearing sensitivity and the presence of tinnitus. Future speech-in-noise studies should incorporate cognitive tests and, possibly, brain imaging to parse out the contribution of cognitive factors, such as cognitive control, in speech-in-noise in tinnitus.

Occupational Noise: Auditory and Non-Auditory Consequences

Occupational noise exposure accounts for approximately 16% of all disabling hearing losses, but the true value and societal costs may be grossly underestimated because current regulations only identify hearing impairments in the workplace if exposures result in audiometric threshold shifts within a limited frequency region. Research over the past several decades indicates that occupational noise exposures can cause other serious auditory deficits such as tinnitus, hyperacusis, extended high-frequency hearing loss, and poor speech perception in noise. Beyond the audiogram, there is growing awareness that hearing loss is a significant risk factor for other debilitating and potentially life-threatening disorders such as cardiovascular disease and dementia. This review discusses some of the shortcomings and limitations of current noise regulations in the United States and Europe.

Within-Subject Comparisons of the Auditory Brainstem Response and Uncomfortable Loudness Levels in Ears With and Without Tinnitus in Unilateral Tinnitus Subjects With Normal Audiograms

OBJECTIVE

To evaluate whether cochlear synaptopathy is a common pathophysiologic cause of tinnitus in individuals with normal audiograms.

STUDY DESIGN

Prospective study.

SETTING

Tertiary referral center.

METHODS

We enrolled 27 subjects with unilateral tinnitus and normal symmetric hearing thresholds, and 27 age- and sex-matched control subjects with normal symmetric hearing thresholds. We measured 1) the amplitudes of waves I and V with 90 dB nHL click stimuli in quiet conditions; 2) the latency shift of wave V with 80 dB nHL click stimuli in background noise, varying from 40 dB HL to 70 dB HL; and 3) uncomfortable loudness levels (UCLs) at 500 Hz and 3000 Hz pure tones.

RESULTS

There were no significant differences in the wave V/I amplitude ratio or the latency shift in wave V with increasing noise levels among the tinnitus ears (TEs), nontinnitus ears (NTEs), and control ears. There were no significant differences in UCLs at 500 Hz or 3000 Hz between TEs and NTEs, but the UCLs were lower in TEs (mean 111.3 dB or 104.1 dB) and NTEs (mean 109.4 dB or 100.6 dB) than in control ears (mean 117.9 dB or 114.1 dB, p < 0.017). No subject met our criteria for cochlear synaptopathy or increased central gain in terms of all three parameters.

CONCLUSION

Based on these results for UCL, increased central gain is a major mechanism of tinnitus in humans with normal audiograms. However, this compensatory mechanism for reduced auditory input may originate from other pathophysiologic factors rather than from cochlear synaptopathy.

Hearing Difficulties and Tinnitus in Construction, Agricultural, Music, and Finance Industries: Contributions of Demographic, Health, and Lifestyle Factors

High levels of occupational noise exposure increase the risk of hearing difficulties and tinnitus. However, differences in demographic, health, and lifestyle factors could also contribute to high levels of hearing difficulties and tinnitus in some industries. Data from a subsample (n = 22,936) of the U.K. Biobank were analyzed to determine to what extent differences in levels of hearing difficulties and tinnitus in high-risk industries (construction, agricultural, and music) compared with low-risk industries (finance) could be attributable to demographic, health, and lifestyle factors, rather than occupational noise exposure. Hearing difficulties were identified using a digits-in-noise speech recognition test. Tinnitus was identified based on self-report. Logistic regression analyses showed that occupational noise exposure partially accounted for higher levels of hearing difficulties in the agricultural industry compared with finance, and occupational noise exposure, older age, low socioeconomic status, and non-White ethnic background partially accounted for higher levels of hearing difficulties in the construction industry. However, the factors assessed in the model did not fully account for the increased likelihood of hearing difficulties in high-risk industries, suggesting that there are additional unknown factors which impact on hearing or that there was insufficient measurement of factors included in the model. The levels of tinnitus were greatest for music and construction industries compared with finance, and these differences were accounted for by occupational and music noise exposure, as well as older age. These findings emphasize the need to promote hearing conservation in occupational and music settings, with a particular focus on high-risk demographic subgroups.

Cochlear synaptopathy: new findings in animal and human research

In animal models, prolonged exposure (2 h) to high-level noise causes an irreparable damage to the synapses between the inner hair cells and auditory nerve fibers within the cochlea. Nevertheless, this injury does not necessarily alter the hearing threshold. Similar findings have been observed as part of typical aging in animals. This type of cochlear synaptopathy, popularly called "hidden hearing loss," has been a significant issue in neuroscience research and clinical audiology scientists. The results obtained in different investigations are inconclusive in their diagnosis and suggest new strategies for both prognosis and treatment of cochlear synaptopathy. Here we review the major physiological findings regarding cochlear synaptopathy in animals and humans and discuss mathematical models. We also analyze the potential impact of these results on clinical practice and therapeutic options.

Search for Electrophysiological Indices of Hidden Hearing Loss in Humans: Click Auditory Brainstem Response Across Sound Levels and in Background Noise

OBJECTIVES

Recent studies in animals indicate that even moderate levels of exposure to noise can damage synaptic ribbons between the inner hair cells and auditory nerve fibers without affecting audiometric thresholds, giving rise to the use of the term "hidden hearing loss" (HHL). Despite evidence across several animal species, there is little consistent evidence for HHL in humans. The aim of the study is to evaluate potential electrophysiological changes specific to individuals at risk for HHL.

DESIGN

Participants forming the high-risk experimental group consisted of 28 young normal-hearing adults who participated in marching band for at least 5 years. Twenty-eight age-matched normal-hearing adults who were not part of the marching band and had little or no history of recreational or occupational exposure to loud sounds formed the low-risk control group. Measurements included pure tone audiometry of conventional and high frequencies, distortion product otoacoustic emissions, and electrophysiological measures of auditory nerve and brainstem function as reflected in the click-evoked auditory brainstem response (ABR). In experiment 1, ABRs were recorded in a quiet background across stimulus levels (30-90 dB nHL) presented in 10 dB steps. In experiment 2, the ABR was elicited by a 70 dB nHL click stimulus presented in a quiet background, and in the presence of simultaneous ipsilateral continuous broadband noise presented at 50, 60, and 70 dB SPL using an insert earphone (Etymotic, ER2).

RESULTS

There were no differences between the low- and high-risk groups in audiometric thresholds or distortion product otoacoustic emission amplitude. Experiment 1 demonstrated smaller wave-I amplitudes at moderate and high sound levels for high-risk compared to low-risk group with similar wave III and wave V amplitude. Enhanced amplitude ratio V/I, particularly at moderate sound level (60 dB nHL), suggesting central compensation for reduced input from the periphery for high-risk group. The results of experiment 2 show that the decrease in wave I amplitude with increasing background noise level was relatively smaller for the high-risk compared to the low-risk group. However, wave V amplitude reduction was essentially similar for both groups. These results suggest that masking induced wave I amplitude reduction is smaller in individuals at high risk for cochlear synaptopathy. Unlike previous studies, we did not observe a difference in the noise-induced wave V latency shift between low- and high-risk groups.

CONCLUSIONS

Results of experiment 1 are consistent with findings in both animal studies (that suggest cochlear synaptopathy involving selective damage of low-spontaneous rate and medium-spontaneous rate fibers), and in several human studies that show changes in a range of ABR metrics that suggest the presence of cochlear synaptopathy. However, without postmortem examination by harvesting human temporal bone (the gold standard for identifying synaptopathy) with different noise exposure background, no direct inferences can be derived for the presence/extent of cochlear synaptopathy in high-risk group with high sound over-exposure history. Results of experiment 2 demonstrate that to the extent response amplitude reflects both the number of neural elements responding and the neural synchrony of the responding elements, the relatively smaller change in response amplitude for the high-risk group would suggest a reduced susceptibility to masking. One plausible mechanism would be that suppressive effects that kick in at moderate to high levels are different in these two groups, particularly at moderate levels of the masking noise. Altogether, a larger scale dataset with different noise exposure background, longitudinal measurements (changes due to recreational over-exposure by studying middle-school to high-school students enrolled in marching band) with an array of behavioral and electrophysiological tests are needed to understand the complex pathogenesis of sound over-exposure damage in normal-hearing individuals.

Electrophysiological markers of cochlear function correlate with hearing-in-noise performance among audiometrically normal subjects

Hearing loss caused by noise exposure, ototoxic drugs, or aging results from the loss of sensory cells, as reflected in audiometric threshold elevation. Animal studies show that loss of hair cells can be preceded by loss of auditory-nerve peripheral synapses, which likely degrades auditory processing. While this condition, known as cochlear synaptopathy, can be diagnosed in mice by a reduction of suprathreshold cochlear neural responses, its diagnosis in humans remains challenging. To look for evidence of cochlear nerve damage in normal hearing subjects, we measured their word recognition performance in difficult listening environments and compared it to cochlear function as assessed by otoacoustic emissions and click-evoked electrocochleography. Several electrocochleographic markers were correlated with word scores, whereas distortion product otoacoustic emissions were not. Specifically, the summating potential (SP) was larger and the cochlear nerve action potential (AP) was smaller in those with the worst word scores. Adding a forward masker or increasing stimulus rate reduced SP in the worst performers, suggesting that this potential includes postsynaptic components as well as hair cell receptor potentials. Results suggests that some of the variance in word scores among listeners with normal audiometric threshold arises from cochlear neural damage.NEW & NOTEWORTHY Recent animal studies suggest that millions of people may be at risk of permanent impairment from cochlear synaptopathy, the age-related and noise-induced degeneration of neural connections in the inner ear that "hides" behind a normal audiogram. This study examines electrophysiological responses to clicks in a large cohort of subjects with normal hearing sensitivity. The resultant correlations with word recognition performance are consistent with an important contribution cochlear neural damage to deficits in hearing in noise abilities.

Investigating the effects of noise exposure on self-report, behavioral and electrophysiological indices of hearing damage in musicians with normal audiometric thresholds

Musicians are at risk of hearing loss due to prolonged noise exposure, but they may also be at risk of early sub-clinical hearing damage, such as cochlear synaptopathy. In the current study, we investigated the effects of noise exposure on electrophysiological, behavioral and self-report correlates of hearing damage in young adult (age range = 18-27 years) musicians and non-musicians with normal audiometric thresholds. Early-career musicians (n = 76) and non-musicians (n = 47) completed a test battery including the Noise Exposure Structured Interview, pure-tone audiometry (PTA; 0.25-8 kHz), extended high-frequency (EHF; 12 and 16 kHz) thresholds, otoacoustic emissions (OAEs), auditory brainstem responses (ABRs), speech perception in noise (SPiN), and self-reported tinnitus, hyperacusis and hearing in noise difficulties. Total lifetime noise exposure was similar between musicians and non-musicians, the majority of which could be accounted for by recreational activities. Musicians showed significantly greater ABR wave I/V ratios than non-musicians and were also more likely to report experience of - and/or more severe - tinnitus, hyperacusis and hearing in noise difficulties, irrespective of noise exposure. A secondary analysis revealed that individuals with the highest levels of noise exposure had reduced outer hair cell function compared to individuals with the lowest levels of noise exposure, as measured by OAEs. OAE level was also related to PTA and EHF thresholds. High levels of noise exposure were also associated with a significant increase in ABR wave V latency, but only for males, and a higher prevalence and severity of hyperacusis. These findings suggest that there may be sub-clinical effects of noise exposure on various hearing metrics even at a relatively young age, but do not support a link between lifetime noise exposure and proxy measures of cochlear synaptopathy such as ABR wave amplitudes and SPiN. Closely monitoring OAEs, PTA and EHF thresholds when conventional PTA is within the clinically 'normal' range could provide a useful early metric of noise-induced hearing damage. This may be particularly relevant to early-career musicians as they progress through a period of intensive musical training, and thus interventions to protect hearing longevity may be vital.

Evoked Potentials Reveal Noise Exposure-Related Central Auditory Changes Despite Normal Audiograms

Purpose Complaints of auditory perceptual deficits, such as tinnitus and difficulty understanding speech in background noise, among individuals with clinically normal audiograms present a perplexing problem for audiologists. One potential explanation for these "hidden" auditory deficits is loss of the synaptic connections between the inner hair cells and their afferent auditory nerve fiber targets, a condition that has been termed cochlear synaptopathy. In animal models, cochlear synaptopathy can occur due to aging or exposure to noise or ototoxic drugs and is associated with reduced auditory brainstem response (ABR) wave I amplitudes. Decreased ABR wave I amplitudes have been demonstrated among young military Veterans and non-Veterans with a history of firearm use, suggesting that humans may also experience noise-induced synaptopathy. However, the downstream consequences of synaptopathy are unclear. Method To investigate how noise-induced reductions in wave I amplitude impact the central auditory system, the ABR, the middle latency response (MLR), and the late latency response (LLR) were measured in 65 young Veterans and non-Veterans with normal audiograms. Results In response to a click stimulus, the MLR was weaker for Veterans compared to non-Veterans, but the LLR was not reduced. In addition, low ABR wave I amplitudes were associated with a reduced MLR, but with an increased LLR. Notably, Veterans reporting tinnitus showed the largest mean LLRs. Conclusions These findings indicate that decreased peripheral auditory input leads to compensatory gain in the central auditory system, even among individuals with normal audiograms, and may impact auditory perception. This pattern of reduced MLR, but not LLR, was observed among Veterans even after statistical adjustment for sex and distortion product otoacoustic emission differences, suggesting that synaptic loss plays a role in the observed central gain. Supplemental Material https://doi.org/10.23641/asha.11977854.

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.

The ongoing search for cochlear synaptopathy in humans: Masked thresholds for brief tones in Threshold Equalizing Noise

This study aimed to advance towards a clinical diagnostic method for detection of cochlear synaptopathy with the hypothesis that synaptopathy should be manifested in elevated masked thresholds for brief tones. This hypothesis was tested in tinnitus sufferers, as they are thought to have some degree of synaptopathy. Near-normal-hearing tinnitus sufferers and their matched controls were asked to detect pure tones with durations of 5, 10, 100, and 200 ms presented in low- and high-level Threshold Equalizing Noise. In addition, lifetime noise exposure was estimated for all participants. Contrary to the hypothesis, there was no significant difference in masked thresholds for brief tones between tinnitus sufferers and their matched controls. Masked thresholds were also not related to lifetime noise exposure. There are two possible explanations of the results: 1) the participants in our study did not have cochlear synaptopathy, or 2) synaptopathy does not lead to elevated masked thresholds for brief tones. This study adds a new approach to the growing list of behavioral methods that attempted to detect potential signs of cochlear synaptopathy in humans.

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.

Auditory brainstem response demonstrates that reduced peripheral auditory input is associated with self-report of tinnitus

Tinnitus is one of the predicted perceptual consequences of cochlear synaptopathy, a type of age-, noise-, or drug-induced auditory damage that has been demonstrated in animal models to cause homeostatic changes in central auditory gain. Although synaptopathy has been observed in human temporal bones, assessment of this condition in living humans is limited to indirect non-invasive measures such as the auditory brainstem response (ABR). In animal models, synaptopathy is associated with a reduction in ABR wave I amplitude at suprathreshold stimulus levels. Several human studies have explored the relationship between wave I amplitude and tinnitus, with conflicting results. This study investigates the hypothesis that reduced peripheral auditory input due to synaptic/neuronal loss is associated with tinnitus. Wave I amplitude data from 193 individuals [43 with tinnitus (22%), 150 without tinnitus (78%)], who participated in up to 3 out of 4 different studies, were included in a logistic regression analysis to estimate the relationship between wave I amplitude and tinnitus at a variety of stimulus levels and frequencies. Statistical adjustment for sex and distortion product otoacoustic emissions (DPOAEs) was included. The results suggest that smaller wave I amplitudes and/or lower DPOAE levels are associated with an increased probability of tinnitus.

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.

Evaluation of dichotic listening performance in normal-hearing, noise-exposed young females

Recent animal studies have shown that intense noise exposures that produce robust temporary threshold shift (TTS) can inflict irreversible damage to the synaptic connections between the inner hair cells and auditory neurons. It was hypothesized that noise-induced cochlear synaptopathy may cause impaired acoustic encoding in the central auditory nervous system leading to impaired speech perception, particularly in challenging listening situations. The aim of the study was to evaluate the influence of high noise exposure background (NEB) on dichotic listening performance, speech-in-noise performance, and auditory brainstem responses (ABR) measured in young females with normal audiograms. The central hypothesis was that individuals with high NEB would exhibit reduced ABR wave I amplitude and subsequently would exhibit poorer performance on speech-in-noise and dichotic listening. In a sample of 32 females (14 with high NEB and 18 with low NEB) aged 18-35 years, the study compared behavioral hearing thresholds (from 250 to 16000 Hz), distortion-product otoacoustic emissions (DPOAEs, 1000-16000 Hz), click-evoked ABR, QuickSIN signal-to-noise ratio (SNR) loss and dichotic digit test (DDT). The results showed no clear association between NEB, and hearing thresholds, DPOAEs, click-evoked ABR measures, and QuickSIN SNR loss. Individuals with high NEB revealed significantly lower DDT scores and evidence of reduced right ear advantage compared to individuals with low NEB. The poorer performance in DDT and the ear asymmetry in DDT scores with normal ABR findings suggest that high NEB might alter the hemispheric organization of speech-sound processing and cognitive control. The clinical significance of the present findings is discussed.

Acquisition of Subcortical Auditory Potentials With Around-the-Ear cEEGrid Technology in Normal and Hearing Impaired Listeners

Even though the principles of recording brain electrical activity remain unchanged since their discovery, their acquisition has seen major improvements. The cEEGrid, a recently developed flex-printed multi-channel sensory array, can be placed around the ear and successfully record well-known cortical electrophysiological potentials such as late auditory evoked potentials (AEPs) or the P300. Due to its fast and easy application as well as its long-lasting signal recording window, the cEEGrid technology offers great potential as a flexible and ‘wearable’ solution for the acquisition of neural correlates of hearing. Early potentials of auditory processing such as the auditory brainstem response (ABR) are already used in clinical assessment of sensorineural hearing disorders and envelope following responses (EFR) have shown promising results in the diagnosis of suprathreshold hearing deficits. This study evaluates the suitability of the cEEGrid electrode configuration to capture these AEPs. cEEGrid potentials were recorded and compared to cap-EEG potentials for young normal-hearing listeners and older listeners with high-frequency sloping audiograms to assess whether the recordings are adequately sensitive for hearing diagnostics. ABRs were elicited by presenting clicks (70 and 100-dB peSPL) and stimulation for the EFRs consisted of 120 Hz amplitude-modulated white noise carriers presented at 70-dB SPL. Data from nine bipolar cEEGrid channels and one classical cap-EEG montage (earlobes to vertex) were analysed and outcome measures were compared. Results show that the cEEGrid is able to record ABRs and EFRs with comparable shape to those recorded using a conventional cap-EEG recording montage and the same amplifier. Signal strength is lower but can still produce responses above the individual neural electrophysiological noise floor. This study shows that the application of the cEEGrid can be extended to the acquisition of early auditory evoked potentials.

Envelope regularity discrimination

The ability to discriminate irregular from regular amplitude modulation was assessed using the "envelope regularity discrimination" test. The amount of irregularity was parametrically varied and quantified by an "irregularity index." Normative data were gathered for young subjects with normal audiometric thresholds. Parameters varied were the carrier and modulation frequencies, f_c and f_m, and the baseline modulation index, m. All tests were performed using a background threshold-equalizing noise. The main findings were (1) using f_c = 4000 Hz, f_m = 8 Hz, and m = 0.3, performance improved over the first two threshold runs and then remained roughly stable, and there was a high correlation between thresholds obtained at 80 dB sound pressure level (SPL) and at 20 dB sensation level; (2) using f_m = 8 Hz and m = 0.3 with a level of 80 dB SPL, thresholds did not vary significantly across f_c = 1000, 2000, and 4000 Hz; (3) using f_m = 8 Hz and f_c = 4000 Hz with a level of 80 dB SPL, thresholds did not vary significantly for m from 0.2 to 0.5; and (4) using m = 0.3 and f_c = 4000 Hz with a level of 80 dB SPL, thresholds improved with increasing f_m from 2 to 16 Hz. For all conditions, there was substantial individual variability, probably resulting from differences in "processing efficiency."

Rising-frequency chirp stimulus to effectively enhance wave-I amplitude of auditory brainstem response

This study aims to find an effective chirp signal that enhances the amplitude of wave-I of auditory brainstem response (ABR) to diagnose "cochlear synaptopathy." Although several chirp signals have been proposed to enhance the amplitude of wave-V, the effect on wave-I has not been clarified yet. Ten chirp signals, which have shorter group delays than the commonly used "CE-chirp," were produced to measure the amplitudes of wave-I and wave-V of the ABRs. The results show that one of the chirp signals significantly enhanced the amplitude of wave-I, where the group delay is approximately half of the CE-chirp.

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.

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.

The Role of Cognitive Control in Tinnitus and Its Relation to Speech-in-Noise Performance

Self-reported difficulties in speech-in-noise (SiN) recognition are common among tinnitus patients. Whereas hearing impairment that usually co-occurs with tinnitus can explain such difficulties, recent studies suggest that tinnitus patients with normal hearing sensitivity still show decreased SiN understanding, indicating that SiN difficulties cannot be solely attributed to changes in hearing sensitivity. In fact, cognitive control, which refers to a variety of top-down processes that human beings use to complete their daily tasks, has been shown to be critical for SiN recognition, as well as the key to understand cognitive inefficiencies caused by tinnitus. In this article, we review studies investigating the association between tinnitus and cognitive control using behavioral and brain imaging assessments, as well as those examining the effect of tinnitus on SiN recognition. In addition, three factors that can affect cognitive control in tinnitus patients, including hearing sensitivity, age, and severity of tinnitus, are discussed to elucidate the association among tinnitus, cognitive control, and SiN recognition. Although a possible central or cognitive involvement has always been postulated in the observed SiN impairments in tinnitus patients, there is as yet no direct evidence to underpin this assumption, as few studies have addressed both SiN performance and cognitive control in one tinnitus cohort. Future studies should aim at incorporating SiN tests with various subjective and objective methods that evaluate cognitive performance to better understand the relationship between SiN difficulties and cognitive control in tinnitus patients.