Evidence of noise-induced subclinical hearing loss using auditory brainstem responses and objective measures of noise exposure in humans

Effects of Noise Exposure on Peripheral Auditory Function, Binaural Envelope Coding, and Speech Perception in Student Musicians With Normal Hearing

OBJECTIVES

Musicians face an increased risk of hearing loss due to prolonged and repetitive exposure to high-noise levels. Detecting early signs of hearing loss, which are subtle and often elusive to traditional clinical tests like pure-tone audiometry, is essential. The objective of this study was to investigate the impact of noise exposure on the electrophysiological and perceptual aspects of subclinical hearing damage in young musicians with normal audiometric thresholds.

DESIGN

The study included 33 musicians and 33 nonmusicians, all aged between 21 and 35 years, with normal audiometric thresholds. Participants underwent a comprehensive test battery, which encompassed standard and extended high-frequency (EHF) pure-tone audiometry (0.25 to 16 kHz), a Noise Exposure Structured Interview, auditory brainstem responses (ABRs) to clicks at various presentation rates and levels, thresholds for detecting interaural envelope time difference, and a spatial release from masking (SRM) paradigm in which the target speech was presented in the presence of either colocated or spatially separated time-reversed two-talker babble.

RESULTS

The results indicated the musician group reported greater lifetime noise exposure than the nonmusician group, but the Noise Exposure Structured Interview scores were neither correlated with the ABR results nor with the speech perception outcomes. The ABR analyses indicated diminished level-dependent growth and increased rate-dependent decline in wave I amplitudes among musicians compared with nonmusicians. The student musicians exhibited better binaural envelope processing skills than nonmusicians, emphasizing their perceptual advantages in auditory processing associated with musicianship. Speech perception results indicated no significant differences in SRM between student musicians and nonmusicians. However, in both groups, individuals with poorer EHF hearing exhibited reduced SRM compared with those with normal EHF hearing, underscoring the importance of assessing and addressing EHF hearing.

CONCLUSIONS

Student musicians exhibit peripheral neural deficits; however, no clear relation was found between these neural deficits and their perceptual skills. Notably, reduced EHF thresholds were clearly related to reduced SRM, which poses a challenge for speech perception in complex multi-talker environments, affecting both musicians and nonmusicians alike.

Impact of tinnitus on chirp-evoked auditory brainstem response recorded using maximum length sequences

Tinnitus is considered a potential consequence of cochlear synaptopathy. While animal studies have shown that this neural deafferentation reduces wave I amplitudes of the auditory brainstem response (ABR) at suprathreshold levels, studies in humans with tinnitus have reported conflicting results. To enhance the sensitivity of ABRs in detecting neurophysiological conditions associated with tinnitus, this study aimed to examine chirp-evoked ABRs in normal-hearing individuals with tinnitus using the maximum length sequence (MLS) technique. Chirp stimuli improve synchronous neural discharge during ABR recording, while the MLS technique enables the extraction of evoked responses from overlapping waveforms. We hypothesized that this combined approach would more effectively reveal ABR morphological characteristics associated with tinnitus. The results indicated no significant difference in noise exposure between the tinnitus and control groups. However, the tinnitus group exhibited significantly larger wave I amplitude, prolonged wave V latency, and extended interpeak interval in MLS responses to chirps at the lowest stimulus rate of 13.8/s. These findings identify unique characteristics of MLS responses to chirps in individuals with tinnitus. We interpret our findings in relation to the ongoing discussion about the neurophysiological mechanisms of tinnitus. Further studies should be conducted to investigate possible etiologies of tinnitus.

Attitudes towards hearing, hearing loss, and hearing protection in university students

OBJECTIVE

Young adults are at risk for hearing loss caused by exposure to loud music. Intervention at this stage provides opportunities to support lifelong hearing protection use. This study explores attitudes related to hearing, hearing loss, and hearing protection among university students.

DESIGN

Qualitative interview design, supplemented by quantitative questionnaire data.

STUDY SAMPLE

18 university students, aged 18-24 years.

RESULTS

Students were uncertain about mechanisms of noise-induced hearing loss, did not feel vulnerable to permanent hearing damage from loud music, were unconvinced of hearing protection efficacy, and reported barriers to hearing protection use. Students emphasised the positive effects of loud music and reported an increased likelihood of using hearing protection were it used by peers. Music students appeared more aware of the negative effects of loud music exposure. Students reported conflicting attitudes regarding government regulation of hearing protection use.

CONCLUSION

Young adults require education about hearing protection from multiple, credible sources and need to understand the mechanisms behind noise-induced hearing loss in a way that makes it of high personal relevance.

Intrasubject variability in potential early markers of sensorineural hearing damage

The quest for noninvasive early markers for sensorineural hearing loss (SNHL) has yielded diverse measures of interest. However, comprehensive studies evaluating the test-retest reliability of multiple measures and stimuli within a single study are scarce, and a standardized clinical protocol for robust early markers of SNHL remains elusive. To address these gaps, this study explores the intra-subject variability of various potential electroencephalogram- (EEG-) biomarkers for cochlear synaptopathy (CS) and other SNHL-markers in the same individuals. Fifteen normal-hearing young adults underwent repeated measures of (extended high-frequency) pure-tone audiometry, speech-in-noise intelligibility, distortion-product otoacoustic emissions (DPOAEs), and auditory evoked potentials; comprising envelope following responses (EFR) and auditory brainstem responses (ABR). Results confirm high reliability in pure-tone audiometry, whereas the matrix sentence-test exhibited a significant learning effect. The reliability of DPOAEs varied across three evaluation methods, each employing distinct SNR-based criteria for DPOAE-datapoints. EFRs exhibited superior test-retest reliability compared to ABR-amplitudes. Our findings emphasize the need for careful interpretation of presumed noninvasive SNHL measures. While tonal-audiometry's robustness was corroborated, we observed a confounding learning effect in longitudinal speech audiometry. The variability in DPOAEs highlights the importance of consistent ear probe replacement and meticulous measurement techniques, indicating that DPOAE test-retest reliability is significantly compromised under less-than-ideal conditions. As potential EEG-biomarkers of CS, EFRs are preferred over ABR-amplitudes based on the current study results.

A Pilot Dashboard System to Track Cumulative Exposure to Sound Levels during Music Instruction: A Technical Report

BACKGROUND

 Music-induced hearing loss (MIHL) is a critical public health issue. During music instruction, students and teachers are at risk of developing hearing loss due to exposure to loud and unsafe sound levels that can exceed 100 dBA. Prevention of MIHL in music students must be a desired action of all music educators.

PURPOSE

 To promote deliberate changes in music instruction and encourage more moderate sound creation and exposure during music education, it is essential to equip the instructors with live dynamic tools to monitor the overall sound intensities during music instruction. Equally important data to convey to the instructors are information regarding the intensity and duration of sounds at specific frequency regions in the music they are generating. Unfortunately, there are no feasible techniques to track cumulative live music exposures at various frequencies nor are there any guidelines for safe music exposure.

RESEARCH DESIGN

 We created a visually appealing, user-friendly dashboard prototype system to display the accumulated time and intensity of sound exposure during live classes/rehearsals categorized into three frequency ranges. These visuals can be easily understood at a glance allowing musicians and instructors to make informed decisions about how to play music safely.

EXPERIMENTAL APPROACH AND RESULTS

 The dashboard included a collection of circular dial graphs that displayed in real time the accumulated sound exposure in the instructor's selected frequency range and showed the percentage of the maximum daily sound exposure based on the National Institute for Occupational Safety and Health Standards (NIOSH, 1998). Although NIOSH standards are not widely applied for music exposure, we propose that these standards can be used to provide initial guidelines to develop critical levels of music exposure. Additionally, the dashboard included a color-coded equalizer that displayed the instantaneous frequency distribution of sounds to indicate if sound levels at specific frequencies were too high even for short-term exposure.

CONCLUSION

 Less expensive than existing technology and more convenient to use, this dashboard will enable music instructors to make informed decisions on how to best adapt their teaching approaches to protect the hearing health of their students.

Electrophysiological Characteristics in Pediatric Cochlear Implantation

AIMS

Cochlear implantation is a potential intervention for individuals with severe to profound hearing loss, in particular in the pediatric population. This literature review aims to comprehensively evaluate the applications of electrophysiological tests in enhancing cochlear implant (CI) outcomes for children.

METHODS

A literature review searched Medline and PubMed databases for articles on electrophysiological tests in CI children, using the terms "electrophysiological tests," "children," and "cochlear implant." The systematic search leads to 72 eligible texts.

RESULTS

Electrophysiological tests can be used to test CI children without the need for their active participation. These tests can be helpful in identifying and improving the health of deaf children in various ways, such as determining the CI functional status, the semantic integration effects in CI children, the effect of central auditory structures in speech stimulus processing, the development of lexical-semantic in CI children, and tracking the maturation of the central auditory system. CI enhances central auditory nervous system (CANS) maturation and auditory/language skills.

CONCLUSION

The quality of electrophysiological tests can be improved to enhance hearing outcome prediction, postoperative physiology understanding, and hearing loss mechanisms. Electrophysiological tests study CANS maturation, identify lesions, aid CI programming, determine prognosis, and treatment outcomes.

A longitudinal study investigating the effects of noise exposure on behavioural, electrophysiological and self-report measures of hearing in musicians with normal audiometric thresholds

Musicians are at risk of hearing loss and tinnitus due to regular exposure to high levels of noise. This level of risk may have been underestimated previously since damage to the auditory system, such as cochlear synaptopathy, may not be easily detectable using standard clinical measures. Most previous research investigating hearing loss in musicians has involved cross-sectional study designs that may capture only a snapshot of hearing health in relation to noise exposure. The aim of this study was to investigate the effects of cumulative noise exposure on behavioural, electrophysiological, and self-report indices of hearing damage in early-career musicians and non-musicians with normal hearing over a 2-year period. Participants completed an annual test battery consisting of pure tone audiometry, extended high-frequency hearing thresholds, distortion product otoacoustic emissions (DPOAEs), speech perception in noise, auditory brainstem responses, and self-report measures of tinnitus, hyperacusis, and hearing in background noise. Participants also completed the Noise Exposure Structured Interview to estimate cumulative noise exposure across the study period. Linear mixed models assessed changes over time. The longitudinal analysis comprised 64 early-career musicians (female n = 34; age range at T0 = 18-26 years) and 30 non-musicians (female n = 20; age range at T0 = 18-27 years). There were few longitudinal changes as a result of musicianship. Small improvements over time in some measures may be attributable to a practice/test-retest effect. Some measures (e.g., DPOAE indices of outer hair cell function) were associated with noise exposure at each time point, but did not show a significant change over time. A small proportion of participants reported a worsening of their tinnitus symptoms, which participants attributed to noise exposure, or not using hearing protection. Future longitudinal studies should attempt to capture the effects of noise exposure over a longer period, taken at several time points, for a precise measure of how hearing changes over time. Hearing conservation programmes for "at risk" individuals should closely monitor DPOAEs to detect early signs of noise-induced hearing loss when audiometric thresholds are clinically normal.

Midlife Speech Perception Deficits: Impact of Extended High-Frequency Hearing, Peripheral Neural Function, and Cognitive Abilities

OBJECTIVES

The objectives of the present study were to investigate the effects of age-related changes in extended high-frequency (EHF) hearing, peripheral neural function, working memory, and executive function on speech perception deficits in middle-aged individuals with clinically normal hearing.

DESIGN

We administered a comprehensive assessment battery to 37 participants spanning the age range of 20 to 56 years. This battery encompassed various evaluations, including standard and EHF pure-tone audiometry, ranging from 0.25 to 16 kHz. In addition, we conducted auditory brainstem response assessments with varying stimulation rates and levels, a spatial release from masking (SRM) task, and cognitive evaluations that involved the Trail Making test (TMT) for assessing executive function and the Abbreviated Reading Span test (ARST) for measuring working memory.

RESULTS

The results indicated a decline in hearing sensitivities at EHFs and an increase in completion times for the TMT with age. In addition, as age increased, there was a corresponding decrease in the amount of SRM. The declines in SRM were associated with age-related declines in hearing sensitivity at EHFs and TMT performance. While we observed an age-related decline in wave I responses, this decline was primarily driven by age-related reductions in EHF thresholds. In addition, the results obtained using the ARST did not show an age-related decline. Neither the auditory brainstem response results nor ARST scores were correlated with the amount of SRM.

CONCLUSIONS

These findings suggest that speech perception deficits in middle age are primarily linked to declines in EHF hearing and executive function, rather than cochlear synaptopathy or working memory.

Hypoactivation of the central auditory system in listeners who are hypertolerant of background noise

Listeners exhibit varying levels of tolerance for background noise during speech communication. It has been proposed that low tolerance of background noise may be the consequence of abnormally amplified gain in the central auditory system (CAS). Here, using a dataset of young adults with normal hearing thresholds, we asked whether central gain mechanisms might also explain cases of hypertolerance of background noise, as well as cases of reduced, but not abnormal, tolerance. We used the auditory brainstem response to derive a measure of CAS gain (wave V/wave I ratio) to compare listeners' background noise tolerance while listening to speech, grouping them into three categories: hyper, high, and medium tolerance. We found that hypertolerant listeners had reduced CAS gain compared to those with high tolerance. This effect was driven by wave V not wave I. In addition, the medium tolerant listeners trended toward having reduced wave I and reduced wave V amplitudes and generally higher levels of exposure to loud sound, suggestive of the early stages of noise-compromised peripheral function without an apparent compensatory increase in central gain. Our results provide physiological evidence that 1) reduced CAS gain may account for hypertolerance of background noise but that 2) increased CAS gain is not a prerequisite for medium tolerance of background noise.NEW & NOTEWORTHY Our findings strengthen the proposed mechanistic connection between background noise tolerance and auditory physiology by suggesting a link between hypertolerance and reduced central auditory gain, measured by the auditory brainstem response.

Effects of cultural dynamics on everyday acoustic environmentsa)

Differences in acoustic environments have previously been linked to socioeconomic status (SES). However, it is crucial to acknowledge that cultural values can also play a significant role in shaping acoustic environments. The goal of this study was to investigate if social behaviors related to cultural heritage and SES could help us understand how Latinx and European college students in the U.S. have different acoustic environments. College students were given digital recorders to record their daily acoustic environments for two days. These recordings were used to (1) evaluate nearfield noise levels in their natural surroundings and (2) quantify the percentage of time participants spent on behavioral collectivistic activities such as socializing and interacting with others. Behavioral collectivism was examined as a mediator between cultural heritage, SES, and nearfield noise levels. Findings revealed that both SES and cultural heritage were associated with nearfield noise levels. However, behavioral collectivism mediated the relationship between culture and nearfield noise levels. These findings show that collectivist cultural norms significantly relate to Latinx' daily noise levels. The implications of these findings for public health and health inequities included promoting equitable auditory well-being and better knowledge of socio-cultural settings.

Effects of age and noise exposure history on auditory nerve response amplitudes: A systematic review, study, and meta-analysis

Auditory nerve (AN) function has been hypothesized to deteriorate with age and noise exposure. Here, we perform a systematic review of published studies and find that the evidence for age-related deficits in AN function is largely consistent across the literature, but there are inconsistent findings among studies of noise exposure history. Further, evidence from animal studies suggests that the greatest deficits in AN response amplitudes are found in noise-exposed aged mice, but a test of the interaction between effects of age and noise exposure on AN function has not been conducted in humans. We report a study of our own examining differences in the response amplitude of the compound action potential N1 (CAP N1) between younger and older adults with and without a self-reported history of noise exposure in a large sample of human participants (63 younger adults 18-30 years of age, 103 older adults 50-86 years of age). CAP N1 response amplitudes were smaller in older than younger adults. Noise exposure history did not appear to predict CAP N1 response amplitudes, nor did the effect of noise exposure history interact with age. We then incorporated our results into two meta-analyses of published studies of age and noise exposure history effects on AN response amplitudes in neurotypical human samples. The meta-analyses found that age effects across studies are robust (r = -0.407), but noise exposure effects are weak (r = -0.152). We conclude that noise exposure effects may be highly variable depending on sample characteristics, study design, and statistical approach, and researchers should be cautious when interpreting results. The underlying pathology of age-related and noise-induced changes in AN function are difficult to determine in living humans, creating a need for longitudinal studies of changes in AN function across the lifespan and histological examination of the AN from temporal bones collected post-mortem.

Effects of Age and Noise Exposure History on Auditory Nerve Response Amplitudes: A Systematic Review, Study, and Meta-Analysis

Auditory nerve (AN) function has been hypothesized to deteriorate with age and noise exposure. Here, we perform a systematic review of published studies and find that the evidence for age-related deficits in AN function is largely consistent across the literature, but there are inconsistent findings among studies of noise exposure history. Further, evidence from animal studies suggests that the greatest deficits in AN response amplitudes are found in noise-exposed aged mice, but a test of the interaction between effects of age and noise exposure on AN function has not been conducted in humans. We report a study of our own examining differences in the response amplitude of the compound action potential N1 (CAP N1) between younger and older adults with and without a self-reported history of noise exposure in a large sample of human participants (63 younger adults 18-30 years of age, 103 older adults 50-86 years of age). CAP N1 response amplitudes were smaller in older than younger adults. Noise exposure history did not appear to predict CAP N1 response amplitudes, nor did the effect of noise exposure history interact with age. We then incorporated our results into two meta-analyses of published studies of age and noise exposure history effects on AN response amplitudes in neurotypical human samples. The meta-analyses found that age effects across studies are robust (r=-0.407), but noise-exposure effects are weak (r=-0.152). We conclude that noise-exposure effects may be highly variable depending on sample characteristics, study design, and statistical approach, and researchers should be cautious when interpreting results. The underlying pathology of age-related and noise-induced changes in AN function are difficult to determine in living humans, creating a need for longitudinal studies of changes in AN function across the lifespan and histological examination of the AN from temporal bones collected post-mortem.

Heightened OAEs in young adult musicians: Influence of current noise exposure and training recency

Otoacoustic emissions (OAEs) are a non-invasive metric of cochlear function. Studies of OAEs in musicians have yielded mixed results, ranging from evidence of diminished OAEs in musicians-suggesting noise-induced hearing loss-to no difference when compared to non-musicians, or even a trend for stronger OAEs in musicians. The goal of this study was to use a large sample of college students with normal hearing (n = 160) to compare OAE SNRs in musicians and non-musicians and to explore potential effects of training recency and noise exposure on OAEs in these cohorts. The musician cohort included both active musicians (who at the time of enrollment practiced at least weekly) and past musicians (who had at least 6 years of training). All participants completed a questionnaire about recent noise exposure (previous 12 months), and a subset of participants (71 musicians and 15 non-musicians) wore a personal noise dosimeter for one week to obtain a more nuanced and objective measure of exposure to assess how different exposure levels may affect OAEs before the emergence of a clinically significant hearing loss. OAEs were tested using both transient-evoked OAEs (TEOAEs) and distortion-product OAEs (DPOAEs). As predicted from the literature, musicians experienced significantly higher noise levels than non-musicians based on both subjective (self-reported) and objective measures. Yet we found stronger TEOAEs and DPOAEs in musicians compared to non-musicians in the ∼1-5 kHz range. Comparisons between past and active musicians suggest that enhanced cochlear function in young adult musicians does not require active, ongoing musical practice. Although there were no significant relations between OAEs and noise exposure as measured by dosimetry or questionnaire, active musicians had weaker DPOAEs than past musicians when the entire DPOAE frequency range was considered (up to ∼16 kHz), consistent with a subclinical noise-induced hearing loss that only becomes apparent when active musicians are contrasted with a cohort of individuals with comparable training but without the ongoing risks of noise exposure. Our findings suggest, therefore, that separate norms should be developed for musicians for earlier detection of incipient hearing loss. Potential explanations for enhanced cochlear function in musicians include pre-existing (inborn or demographic) differences, training-related enhancements of cochlear function (e.g., upregulation of prestin, stronger efferent feedback mechanisms), or a combination thereof. Further studies are needed to determine if OAE enhancements offer musicians protection against damage caused by noise exposure.

Evaluation of Lifetime Noise Exposure History Reporting

PURPOSE

The purpose of this study is to critically evaluate lifetime noise exposure history (LNEH) reporting. First, two different approaches to evaluate the cumulative LNEH were compared. Second, individual LNEH was associated with the subjects' hearing status. Third, loudness estimates of exposure activities, by means of Jokitulppo- and Ferguson-based exposure levels, were compared with dosimeter sound-level measurements.

METHOD

One hundred one young adults completed the questionnaires, and a subgroup of 30 subjects underwent audiological assessment. Pure-tone audiometry, speech-in-noise intelligibility, distortion product otoacoustic emissions, auditory brainstem responses, and envelope following responses were included. Fifteen out of the 30 subjects took part in a noisy activity while wearing a dosimeter.

RESULTS

First, results demonstrate that the structured questionnaire yielded a greater amount of information pertaining to the diverse activities, surpassing the insights obtained from an open-ended questionnaire. Second, no significant correlations between audiological assessment and LNEH were found. Lastly, the results indicate that Ferguson-based exposure levels offer a more precise estimation of the actual exposure levels, in contrast to Jokitulppo-based estimates.

CONCLUSIONS

We propose several recommendations for determining the LNEH. First, it is vital to define accurate loudness categories and corresponding allocated levels, with a preference for the loudness levels proposed by Ferguson et al. (2019), as identified in this study. Second, a structured questionnaire regarding LNEH is recommended, discouraging open-ended questioning. Third, it is essential to include a separate category exclusively addressing work-related activities, encompassing various activities for more accurate surveying.

Subclinical hearing loss associated with aging

Objective

Contribute to clarifying the existence of subclinical hearing deficits associated with aging.

Design

In this work, we study and compare the auditory perceptual and electrophysiological performance of normal-hearing young and adult subjects (tonal audiometry, high-frequency tone threshold, a triplet of digits in noise, and click-evoked auditory brainstem response).

Study sample

45 normal hearing volunteers were evaluated and divided into two groups according to age. 27 subjects were included in the "young group" (mean 22.1 years), and 18 subjects (mean 42.22 years) were included in the "adult group."

Results

In the perceptual tests, the adult group presented significantly worse tonal thresholds in the high frequencies (12 and 16 kHz) and worse performance in the digit triplet tests in noise. In the electrophysiological test using the auditory brainstem response technique, the adult group presented significantly lower I and V wave amplitudes and higher V wave latencies at the supra-threshold level. At the threshold level, we observed a significantly higher latency in wave V in the adult group. In addition, in the partial correlation analysis, controlling for the hearing level, we observed a relationship (negative) between age and speech in noise performance and high-frequency thresholds. No significant association was observed between age and the auditory brainstem response.

Conclusion

The results are compatible with subclinical hearing loss associated with aging.

Auditory Electrophysiological and Perceptual Measures in Student Musicians with High Sound Exposure

This study aimed to determine (a) the influence of noise exposure background (NEB) on the peripheral and central auditory system functioning and (b) the influence of NEB on speech recognition in noise abilities in student musicians. Twenty non-musician students with self-reported low NEB and 18 student musicians with self-reported high NEB completed a battery of tests that consisted of physiological measures, including auditory brainstem responses (ABRs) at three different stimulus rates (11.3 Hz, 51.3 Hz, and 81.3 Hz), and P300, and behavioral measures including conventional and extended high-frequency audiometry, consonant-vowel nucleus-consonant (CNC) word test and AzBio sentence test for assessing speech perception in noise abilities at -9, -6, -3, 0, and +3 dB signal to noise ratios (SNRs). The NEB was negatively associated with performance on the CNC test at all five SNRs. A negative association was found between NEB and performance on the AzBio test at 0 dB SNR. No effect of NEB was found on the amplitude and latency of P300 and the ABR wave I amplitude. More investigations of larger datasets with different NEB and longitudinal measurements are needed to investigate the influence of NEB on word recognition in noise and to understand the specific cognitive processes contributing to the impact of NEB on word recognition in noise.

Differences in a Musician's Advantage for Speech-in-Speech Perception Based on Age and Task

PURPOSE

This study investigates the debate that musicians have an advantage in speech-in-noise perception from years of targeted auditory training. We also consider the effect of age on any such advantage, comparing musicians and nonmusicians (age range: 18-66 years), all of whom had normal hearing. We manipulate the degree of fundamental frequency (f _o) separation between the competing talkers, as well as use different tasks, to probe attentional differences that might shape a musician's advantage across ages.

METHOD

Participants (ranging in age from 18 to 66 years) included 29 musicians and 26 nonmusicians. They completed two tasks varying in attentional demands: (a) a selective attention task where listeners identify the target sentence presented with a one-talker interferer (Experiment 1), and (b) a divided attention task where listeners hear two vowels played simultaneously and identify both competing vowels (Experiment 2). In both paradigms, f _o separation was manipulated between the two voices (Δf _o = 0, 0.156, 0.306, 1, 2, 3 semitones).

RESULTS

Results show that increasing differences in f _o separation lead to higher accuracy on both tasks. Additionally, we find evidence for a musician's advantage across the two studies. In the sentence identification task, younger adult musicians show higher accuracy overall, as well as a stronger reliance on f _o separation. Yet, this advantage declines with musicians' age. In the double vowel identification task, musicians of all ages show an across-the-board advantage in detecting two vowels-and use f _o separation more to aid in stream separation-but show no consistent difference in double vowel identification.

CONCLUSIONS

Overall, we find support for a hybrid auditory encoding-attention account of music-to-speech transfer. The musician's advantage includes f _o, but the benefit also depends on the attentional demands in the task and listeners' age. Taken together, this study suggests a complex relationship between age, musical experience, and speech-in-speech paradigm on a musician's advantage.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.21956777.

Supra-threshold deficits in normal hearing military recruits exposed to impulse noise

The aim of this study was to determine the effect of impulse noise exposure on various proxy measures of cochlear synaptopathy in young military recruits. A total of 27 military recruits with exposure to firearm and artillery noise and 13 non exposed participants were recruited. All presented with normal hearing thresholds and the presence of distortion product otoacoustic emissions (DPOAEs). The Noise Exposure Structured Interview (NESI) was used to quantify noise exposure. Speech perception in noise (SPiN), equivalent rectangular bandwidth (ERB) of auditory filters, auditory brainstem response wave I amplitude, wave I amplitude growth function, wave I/V amplitude ratio, wave V latency, wave V latency shift with ipsilateral noise, and the summating potential/action potential ratio of the electrocochleography were measured. In military participants, SPiN was worse, ERB at 4 kHz was larger, wave I amplitude at 75 dBnHL was reduced, and wave V latency was delayed. However, no significant correlations were observed between NESI and auditory measures, once multiplicity of tests was controlled for. These results suggest that military recruits may exhibit supra-threshold deficits, despite presenting with normal hearing thresholds and presence of DPOAEs. Future studies should include a measure of auditory filters in their test battery.

Audiologic characterization using clinical physiological measures: Normative data from macaque monkeys

Clinical auditory physiological measures (e.g., auditory brainstem responses, ABRs, and distortion product otoacoustic emissions, DPOAEs) provide diagnostic specificity for differentially diagnosing overt hearing impairments, but they remain limited in their ability to detect specific sites of lesion and subtle levels of cochlear damage. Studies in animal models may hold the key to improve differential diagnosis due to the ability to induce tightly controlled and histologically verifiable subclinical cochlear pathologies. Here, we present a normative set of traditional and clinically novel physiological measures using ABRs and DPOAEs measured in a large cohort of male macaque monkeys. Given the high similarities between macaque and human auditory anatomy, physiology, and susceptibility to hearing damage, this normative data set will serve as a crucial baseline to investigate novel physiological measures to improve diagnostics. DPOAE amplitudes were robust at f2 = 1.22, L1/L2 = 65/55, increased with frequency up to 10 kHz, and exhibited high test re-test reliability. DPOAE thresholds were lowest from 2-10 kHz and highest < 2 kHz. ABRs with a standard clinical electrode montage (vertex-to-mastoid, VM) produced Waves I-IV with a less frequently observed Wave-I, and lower thresholds. ABRs with a vertex-to-tympanic membrane (VT) electrode montage produced a more robust Wave-I, but absent Waves II-IV and higher thresholds. Further study with the VM montage revealed amplitudes that increased with stimulus level and were largest in response to click stimuli, with Wave-II showing the largest ABR amplitude, followed by -IV and -I, with high inter- and intra-subject variability. ABR wave latencies decreased with stimulus level and frequency. When stimulus presentation rate increased or stimuli were presented in close temporal proximity, ABR amplitude decreased, and latency increased. These findings expand upon existing literature of normative clinical physiological data in nonhuman primates and lay the groundwork for future studies investigating the effects of noise-induced pathologies in macaques.

Abnormal neural adaptation consequent to combined exposure to jet fuel and noise

A fundamental property of first-order sensory neurons is the ability to alter their response properties as a function of change in the statistical parameters of an input signal. Such neural adaptation shapes the performance features of contiguous neural circuits that ultimately drive sensory discrimination. The current study focused on whether combined exposure to jet fuel and noise might alter the capacity of the auditory nerve to adapt to stimulus presentation speed. Young hooded Long-Evans 4-5 weeks old male rats were grouped and used in the current experiment. One group was exposed via inhalation to 1000 mg/m³ of jet propulsion fuel for 6 hr per day, 5 days per week for 4 weeks. Another group was exposed to a 5.5-11.3 kHz band-pass noise at 85 dB SPL for 6 hr per day, 5 days per week for 4 weeks. An additional group was simultaneously exposed to both jet fuel and noise. An age-matched group served as control and was not exposed to either jet fuel or noise. After experimental exposures, animals were given 4 weeks to recover and then assessed for neural adaptation. Both slow and fast rectangular voltage pulses were employed to elicit neuroelectric activity from the animals. Data demonstrated significant neural adaptation (1.46 μV shift) among controls, where neural activity decreased as the stimulus presentation speed rose from 10 to 100 per sec. This effect might also be observed in animals in the jet fuel treated and rats in the noise-exposed group. However, animals who were simultaneously exposed to both jet fuel and noise failed to exhibit neural adaptation. This abnormality appeared to be masked because independent slow and fast stimuli produced similar neural activity between controls and rats exposed to both jet fuel and noise. Therefore, neural adaptation assays may further be developed to unmask silent neurotoxicity consequent to physiochemical exposures.

A Home-Based Approach to Auditory Brainstem Response Measurement: Proof-of-Concept and Practical Guidelines

Broad-scale neuroscientific investigations of diverse human populations are difficult to implement. This is because the primary neuroimaging methods (magnetic resonance imaging, electroencephalography [EEG]) historically have not been portable, and participants may be unable or unwilling to travel to test sites. Miniaturization of EEG technologies has now opened the door to neuroscientific fieldwork, allowing for easier access to under-represented populations. Recent efforts to conduct auditory neuroscience outside a laboratory setting are reviewed and then an in-home technique for recording auditory brainstem responses (ABRs) and frequency-following responses (FFRs) in a home setting is introduced. As a proof of concept, we have conducted two in-home electrophysiological studies: one in 27 children aged 6 to 16 years (13 with autism spectrum disorder) and another in 12 young adults aged 18 to 27 years, using portable electrophysiological equipment to record ABRs and FFRs to click and speech stimuli, spanning rural and urban and multiple homes and testers. We validate our fieldwork approach by presenting waveforms and data on latencies and signal-to-noise ratio. Our findings demonstrate the feasibility and utility of home-based ABR/FFR techniques, paving the course for larger fieldwork investigations of populations that are difficult to test or recruit. We conclude this tutorial with practical tips and guidelines for recording ABRs and FFRs in the field and discuss possible clinical and research applications of this approach.

Examining the Profile of Noise-Induced Cochlear Synaptopathy Using iPhone Health App Data and Cochlear and Brainstem Electrophysiological Responses to Fast Clicks Rates

Little is known about objective classifying of noise exposure risk levels in personal listening device (PLD) users and electrophysiologic evidence of cochlear synaptopathy at very fast click rates. The aim of the study was to objectively classify noise exposure risk using iPhone Health app and identify signs of cochlear synaptopathy using behavioral and electrophysiologic measures. Thirty normal-hearing females (aged 18-26 years) were grouped based on their iPhone Health app's 6-month listening level and noise exposure data into low-risk and high-risk groups. They were assessed using a questionnaire, extended high-frequency (EHF) audiometry, QuickSIN test, distortion-product otoacoustic emission (DPOAE), and simultaneous recording of electrocochleography (ECochG) and auditory brainstem response (ABR) at three click rates (19.5/s, 97.7/s, 234.4/s). A series of ANOVAs and independent samples t -test were conducted for group comparison. Both groups had within-normal EHF hearing thresholds and DPOAEs. However, the high-risk participants were over twice as likely to suffer from tinnitus, had abnormally large summating potential to action potential amplitude and area ratios at fast rates, and had slightly smaller waves I and V amplitudes. The high-risk group demonstrated a profile of behavioral and objective signs of cochlear synaptopathy based on ECochG and ABR recordings at fast click rates. The findings in this study suggest that the iPhone Health app may be a useful tool for further investigation into cochlear synaptopathy in PLD users.

No effect of occupational noise exposure on auditory brainstem response and speech perception in noise

The primary aim of this study was to investigate whether auditory brainstem response (ABR) and speech perception in noise (SPiN) were associated with occupational noise exposure in normal hearing young factory workers. Forty young adults occupationally exposed to noise and 40 non-exposed young adults (control group) from Zhejiang province in China were selected. All participants presented with normal hearing thresholds and distortion product otoacoustic emissions. Participants were evaluated with the Mandarin Bamford-Kowal-Bench (BKB) test and ABR. The latter was obtained for click stimulus at 50, 60, 70, 80, and 90 dBnHL. Peak-to-trough amplitudes and latencies for waves I and V were obtained. The ABR wave I amplitude, the wave I/V amplitude ratio, the slope of the wave I amplitude growth as a function of stimulus intensity (AMP-I_Slope), and the wave V latency shift with ipsilateral noise (LAT-V_Slope) were used as ABR outcomes. Finally, equivalent continuous average sound pressure level normalized to 8 h (L_Aeq.8h) and cumulative noise exposure (CNE) were obtained for noise-exposed participants. No significant differences between groups were found for any ABR outcomes. Noise-exposed participants exhibited worse BKB scores than control group participants. A multivariate regression model showed that 23.3% of the variance in BKB scores was explained by group category (exposed vs. non-exposed) and hearing thresholds. However, since none of the ABR outcomes exploring cochlear synaptopathy were associated with noise exposure, we cannot conclude that cochlear synaptopathy was the contributing factor for the differences between groups for BKB scores. Factors that go beyond sensory processing may explain such results, especially given socio-economic differences between the noise-exposed and control groups. We conclude that in this sample of participants, occupational noise exposure was not associated with signs of cochlear synaptopathy as measured by ABR and BKB.

Threshold Equalizing Noise Test Reveals Suprathreshold Loss of Hearing Function, Even in the "Normal" Audiogram Range

Objectives:

The threshold equalizing noise (TEN(HL)) is a clinically administered test to detect cochlear “dead regions” (i.e., regions of loss of inner hair cell [IHC] connectivity), using a “pass/fail” criterion based on the degree of elevation of a masked threshold in a tone-detection task. With sensorineural hearing loss, some elevation of the masked threshold is commonly observed but usually insufficient to create a “fail” diagnosis. The experiment reported here investigated whether the gray area between pass and fail contained information that correlated with factors such as age or cumulative high-level noise exposure (>100 dBA sound pressure levels), possibly indicative of damage to cochlear structures other than the more commonly implicated outer hair cells.

Design:

One hundred and twelve participants (71 female) who underwent audiometric screening for a sensorineural hearing loss, classified as either normal or mild, were recruited. Their age range was 32 to 74 years. They were administered the TEN test at four frequencies, 0.75, 1, 3, and 4 kHz, and at two sensation levels, 12 and 24 dB above their pure-tone absolute threshold at each frequency. The test frequencies were chosen to lie either distinctly away from, or within, the 2 to 6 kHz region where noise-induced hearing loss is first clinically observed as a notch in the audiogram. Cumulative noise exposure was assessed by the Noise Exposure Structured Interview (NESI). Elements of the NESI also permitted participant stratification by music experience.

Results:

Across all frequencies and testing levels, a strong positive correlation was observed between elevation of TEN threshold and absolute threshold. These correlations were little-changed even after noise exposure and music experience were factored out. The correlations were observed even within the range of “normal” hearing (absolute thresholds ≤15 dB HL).

Conclusions:

Using a clinical test, sensorineural hearing deficits were observable even within the range of clinically “normal” hearing. Results from the TEN test residing between “pass” and “fail” are dominated by processes not related to IHCs. The TEN test for IHC-related function should therefore only be considered for its originally designed function, to generate a binary decision, either pass or fail.

The Relative and Combined Effects of Noise Exposure and Aging on Auditory Peripheral Neural Deafferentation: A Narrative Review

Animal studies have shown that noise exposure and aging cause a reduction in the number of synapses between low and medium spontaneous rate auditory nerve fibers and inner hair cells before outer hair cell deterioration. This noise-induced and age-related cochlear synaptopathy (CS) is hypothesized to compromise speech recognition at moderate-to-high suprathreshold levels in humans. This paper evaluates the evidence on the relative and combined effects of noise exposure and aging on CS, in both animals and humans, using histopathological and proxy measures. In animal studies, noise exposure seems to result in a higher proportion of CS (up to 70% synapse loss) compared to aging (up to 48% synapse loss). Following noise exposure, older animals, depending on their species, seem to either exhibit significant or little further synapse loss compared to their younger counterparts. In humans, temporal bone studies suggest a possible age- and noise-related auditory nerve fiber loss. Based on the animal data obtained from different species, we predict that noise exposure may accelerate age-related CS to at least some extent in humans. In animals, noise-induced and age-related CS in separation have been consistently associated with a decreased amplitude of wave 1 of the auditory brainstem response, reduced middle ear muscle reflex strength, and degraded temporal processing as demonstrated by lower amplitudes of the envelope following response. In humans, the individual effects of noise exposure and aging do not seem to translate clearly into deficits in electrophysiological, middle ear muscle reflex, and behavioral measures of CS. Moreover, the evidence on the combined effects of noise exposure and aging on peripheral neural deafferentation in humans using electrophysiological and behavioral measures is even more sparse and inconclusive. Further research is necessary to establish the individual and combined effects of CS in humans using temporal bone, objective, and behavioral measures.

Optimizing non-invasive functional markers for cochlear deafferentation based on electrocochleography and auditory brainstem responses

Accumulating evidence suggests that cochlear deafferentation may contribute to suprathreshold deficits observed with or without elevated hearing thresholds, and can lead to accelerated age-related hearing loss. Currently there are no clinical diagnostic tools to detect human cochlear deafferentation in vivo. Preclinical studies using a combination of electrophysiological and post-mortem histological methods clearly demonstrate cochlear deafferentation including myelination loss, mitochondrial damages in spiral ganglion neurons (SGNs), and synaptic loss between inner hair cells and SGNs. Since clinical diagnosis of human cochlear deafferentation cannot include post-mortem histological quantification, various attempts based on functional measurements have been made to detect cochlear deafferentation. So far, those efforts have led to inconclusive results. Two major obstacles to the development of in vivo clinical diagnostics include a lack of standardized methods to validate new approaches and characterize the normative range of repeated measurements. In this overview, we examine strategies from previous studies to detect cochlear deafferentation from electrocochleography and auditory brainstem responses. We then summarize possible approaches to improve these non-invasive functional methods for detecting cochlear deafferentation with a focus on cochlear synaptopathy. We identify conceptual approaches that should be tested to associate unique electrophysiological features with cochlear deafferentation.

Noise exposure levels predict blood levels of the inner ear protein prestin

Serological biomarkers of inner ear proteins are a promising new approach for studying human hearing. Here, we focus on the serological measurement of prestin, a protein integral to a human’s highly sensitive hearing, expressed in cochlear outer hair cells (OHCs). Building from recent nonhuman studies that associated noise-induced OHC trauma with reduced serum prestin levels, and studies suggesting subclinical hearing damage in humans regularly engaging in noisy activities, we investigated the relation between serum prestin levels and environmental noise levels in young adults with normal clinical audiograms. We measured prestin protein levels from circulating blood and collected noise level data multiple times over the course of the experiment using body-worn sound recorders. Results indicate that serum prestin levels have a negative relation with noise exposure: individuals with higher routine noise exposure levels tended to have lower prestin levels. Moreover, when grouping participants based on their risk for a clinically-significant noise-induced hearing loss, we found that prestin levels differed significantly between groups, even though behavioral hearing thresholds were similar. We discuss possible interpretations for our findings including whether lower serum levels may reflect subclinical levels of OHC damage, or possibly an adaptive, protective mechanism in which prestin expression is downregulated in response to loud environments.

Evidence for Loss of Activity in Low-Spontaneous-Rate Auditory Nerve Fibers of Older Adults

Auditory function declines with age, as evidenced by communication difficulties in challenging listening environments for older adults. Declining auditory function may arise, in part, from an age-related loss and/or inactivity of low-spontaneous-rate (SR) auditory nerve (AN) fibers, a subgroup of neurons important for suprathreshold processing. Compared to high-SR fibers, low-SR fibers take longer to recover from prior stimulation. Taking advantage of this difference, the forward-masked recovery function paradigm estimates the relative proportions of low- and high-SR fibers in the AN by quantifying the time needed for AN responses to recover from prior stimulation (ΔT_recovery). Due to the slower recovery of low-SR fibers, ANs that need more time to fully recover (longer ΔT_recovery) are estimated to have a larger proportion of low-SR fibers than ANs that need less time (shorter ΔT_recovery). To test the hypothesis that low-SR fiber activity is reduced in older humans, the current study assessed recovery functions in 32 older and 16 younger adults using the compound action potential. Results show that ΔT_recovery is shorter for older adults than for younger adults, consistent with a theorized age-related loss and/or inactivity of low-SR fibers. ΔT_recovery did not differ between individuals with and without a prior history of noise exposure as assessed by self-report. This study is the first to successfully assess forward-masked recovery functions in both younger and older adults and provides important insights into the structural and functional changes occurring in the AN with increasing age.

The Variability in Potential Biomarkers for Cochlear Synaptopathy After Recreational Noise Exposure

PURPOSE

Speech-in-noise tests and suprathreshold auditory evoked potentials are promising biomarkers to diagnose cochlear synaptopathy (CS) in humans. This study investigated whether these biomarkers changed after recreational noise exposure.

METHOD

The baseline auditory status of 19 normal-hearing young adults was analyzed using questionnaires, pure-tone audiometry, speech audiometry, and auditory evoked potentials. Nineteen subjects attended a music festival and completed the same tests again at Day 1, Day 3, and Day 5 after the music festival.

RESULTS

No significant relations were found between lifetime noise-exposure history and the hearing tests. Changes in biomarkers from the first session to the follow-up sessions were nonsignificant, except for speech audiometry, which showed a significant learning effect (performance improvement).

CONCLUSIONS

Despite the individual variability in prefestival biomarkers, we did not observe changes related to the noise-exposure dose caused by the attended event. This can indicate the absence of noise exposure-driven CS in the study cohort, or reflect that biomarkers were not sensitive enough to detect mild CS. Future research should include a more diverse study cohort, dosimetry, and results from test-retest reliability studies to provide more insight into the relationship between recreational noise exposure and CS. Supplemental Material https://doi.org/10.23641/asha.16821283.

Use of the auditory brainstem response for assessment of cochlear synaptopathy in humans

Although clinical use of the auditory brainstem response (ABR) to detect retrocochlear disorders has been largely replaced by imaging in recent years, the discovery of cochlear synaptopathy has thrown this foundational measure of auditory function back into the spotlight. Whereas modern imaging now allows for the noninvasive detection of vestibular schwannomas, imaging technology is not currently capable of detecting cochlear synaptopathy, the loss of the synaptic connections between the inner hair cells and afferent auditory nerve fibers. However, animal models indicate that the amplitude of the first wave of the ABR, a far-field evoked potential generated by the synchronous firing of auditory nerve fibers, is highly correlated with synaptic integrity. This has led to many studies investigating the use of the ABR as a metric of synaptopathy in humans. However, these studies have yielded mixed results, leading to a lack of consensus about the utility of the ABR as an indicator of synaptopathy. This review summarizes the animal and human studies that have investigated the ABR as a measure of cochlear synaptic function, discusses factors that may have contributed to the mixed findings and the lessons learned, and provides recommendations for future use of this metric in the research and clinical settings.

Sound level context modulates neural activity in the human brainstem

Optimal perception requires adaptation to sounds in the environment. Adaptation involves representing the acoustic stimulation history in neural response patterns, for example, by altering response magnitude or latency as sound-level context changes. Neurons in the auditory brainstem of rodents are sensitive to acoustic stimulation history and sound-level context (often referred to as sensitivity to stimulus statistics), but the degree to which the human brainstem exhibits such neural adaptation is unclear. In six electroencephalography experiments with over 125 participants, we demonstrate that the response latency of the human brainstem is sensitive to the history of acoustic stimulation over a few tens of milliseconds. We further show that human brainstem responses adapt to sound-level context in, at least, the last 44 ms, but that neural sensitivity to sound-level context decreases when the time window over which acoustic stimuli need to be integrated becomes wider. Our study thus provides evidence of adaptation to sound-level context in the human brainstem and of the timescale over which sound-level information affects neural responses to sound. The research delivers an important link to studies on neural adaptation in non-human animals.

Middle Ear Muscle Reflex and Word Recognition in "Normal-Hearing" Adults: Evidence for Cochlear Synaptopathy?

OBJECTIVES

Permanent threshold elevation after noise exposure, ototoxic drugs, or aging is caused by loss of sensory cells; however, animal studies show that hair cell loss is often preceded by degeneration of synapses between sensory cells and auditory nerve fibers. The silencing of these neurons, especially those with high thresholds and low spontaneous rates, degrades auditory processing and may contribute to difficulties in understanding speech in noise. Although cochlear synaptopathy can be diagnosed in animals by measuring suprathreshold auditory brainstem responses, its diagnosis in humans remains a challenge. In mice, cochlear synaptopathy is also correlated with measures of middle ear muscle (MEM) reflex strength, possibly because the missing high-threshold neurons are important drivers of this reflex. The authors hypothesized that measures of the MEM reflex might be better than other assays of peripheral function in predicting difficulties hearing in difficult listening environments in human subjects.

DESIGN

The authors recruited 165 normal-hearing healthy subjects, between 18 and 63 years of age, with no history of ear or hearing problems, no history of neurologic disorders, and unremarkable otoscopic examinations. Word recognition in quiet and in difficult listening situations was measured in four ways: using isolated words from the Northwestern University auditory test number six corpus with either (a) 0 dB signal to noise, (b) 45% time compression with reverberation, or (c) 65% time compression with reverberation, and (d) with a modified version of the QuickSIN. Audiometric thresholds were assessed at standard and extended high frequencies. Outer hair cell function was assessed by distortion product otoacoustic emissions (DPOAEs). Middle ear function and reflexes were assessed using three methods: the acoustic reflex threshold as measured clinically, wideband tympanometry as measured clinically, and a custom wideband method that uses a pair of click probes flanking an ipsilateral noise elicitor. Other aspects of peripheral auditory function were assessed by measuring click-evoked gross potentials, that is, summating potential (SP) and action potential (AP) from ear canal electrodes.

RESULTS

After adjusting for age and sex, word recognition scores were uncorrelated with audiometric or DPOAE thresholds, at either standard or extended high frequencies. MEM reflex thresholds were significantly correlated with scores on isolated word recognition, but not with the modified version of the QuickSIN. The highest pairwise correlations were seen using the custom assay. AP measures were correlated with some of the word scores, but not as highly as seen for the MEM custom assay, and only if amplitude was measured from SP peak to AP peak, rather than baseline to AP peak. The highest pairwise correlations with word scores, on all four tests, were seen with the SP/AP ratio, followed closely by SP itself. When all predictor variables were combined in a stepwise multivariate regression, SP/AP dominated models for all four word score outcomes. MEM measures only enhanced the adjusted r values for the 45% time compression test. The only other predictors that enhanced model performance (and only for two outcome measures) were measures of interaural threshold asymmetry.

CONCLUSIONS

Results suggest that, among normal-hearing subjects, there is a significant peripheral contribution to diminished hearing performance in difficult listening environments that is not captured by either threshold audiometry or DPOAEs. The significant univariate correlations between word scores and either SP/AP, SP, MEM reflex thresholds, or AP amplitudes (in that order) are consistent with a type of primary neural degeneration. However, interpretation is clouded by uncertainty as to the mix of pre- and postsynaptic contributions to the click-evoked SP. None of the assays presented here has the sensitivity to diagnose neural degeneration on a case-by-case basis; however, these tests may be useful in longitudinal studies to track accumulation of neural degeneration in individual subjects.

IGF-1 Haploinsufficiency Causes Age-Related Chronic Cochlear Inflammation and Increases Noise-Induced Hearing Loss

Insulin-like growth factor 1 (IGF-1) deficiency is an ultrarare syndromic human sensorineural deafness. Accordingly, IGF-1 is essential for the postnatal maturation of the cochlea and the correct wiring of hearing in mice. Less severe decreases in human IGF-1 levels have been associated with other hearing loss rare genetic syndromes, as well as with age-related hearing loss (ARHL). However, the underlying mechanisms linking IGF-1 haploinsufficiency with auditory pathology and ARHL have not been studied. Igf1-heterozygous mice express less Igf1 transcription and have 40% lower IGF-1 serum levels than wild-type mice. Along with ageing, IGF-1 levels decreased concomitantly with the increased expression of inflammatory cytokines, Tgfb1 and Il1b, but there was no associated hearing loss. However, noise exposure of these mice caused increased injury to sensory hair cells and irreversible hearing loss. Concomitantly, there was a significant alteration in the expression ratio of pro- and anti-inflammatory cytokines in Igf1^+/- mice. Unbalanced inflammation led to the activation of the stress kinase JNK and the failure to activate AKT. Our data show that IGF-1 haploinsufficiency causes a chronic subclinical proinflammatory age-associated state and, consequently, greater susceptibility to stressors. This work provides the molecular bases to further understand hearing disorders linked to IGF-1 deficiency.

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.

Is it necessary to perform occupational audiometric testing at 6-months of employment?

Introduction

Current Brazilian legislation requires that all workers exposed to noise are to be given an audiogram upon hiring, after 6 months of employment (first periodic test), and annually after the first periodic test. In other countries, the regulations of hearing conservation programs do not include the requirement for audiometric testing at 6 months of employment, but only annually. There is no evidence that the periodicity adopted by Brazilian legislation is the most appropriate.

Objective

The present study aimed to evaluate the first 3 occupational audiometric tests of workers exposed to noise.

Methods

Historical cohort study with cross-sectional analysis. Participants were all male metallurgy workers aged up to 40 years. The first 3 audiograms of each worker were analyzed: pre-employment audiometric test, periodic audiometric test 1, and periodic audiometric test 2. For each worker, mean frequency thresholds were calculated at 3, 4, and 6 kHz in the left and right ears for each test. Statistical analysis was performed using the nonparametric Wilcoxon test.

Results

A total of 988 workers were included. There was a significant difference in auditory thresholds between the pre-employment test and the 2 subsequent periodic tests for the right and left ears. There was no significant difference between Test1 and Test2 in either ear.

Conclusion

Given the lack of difference between the first 2 periodic tests, we believe that they could be merged into a single test, i.e., first periodic audiometric testing could be performed at 12 months of employment without compromising workers’ health.

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.

New Insights on the Effect of TNF Alpha Blockade by Gene Silencing in Noise-Induced Hearing Loss

Noise exposure represents the second most common cause of acquired sensorineural hearing loss and we observed that tumor necrosis factor α (TNFα) was involved in this context. The effect of Tnfα gene silencing on the expression profile related to the TNFα metabolic pathway in an experimental model of noise-induced hearing loss had not previously been studied. Methods: Single ears of Wistar rats were pretreated with Tnfα small interfering RNA (siRNA) by trans-tympanic administration 24 h before they were exposed to white noise (120 dBSPL for three hours). After 24 h of noise exposure, we analyzed the electrophysiological threshold and the amplitude of waves I, II, III, and IV in the auditory brain response click. In addition, qRT-PCR was performed to evaluate the TNFα metabolic pathway in the ears submitted or not to gene silencing. Results: Preservation of the electrophysiological threshold and the amplitude of waves was observed in the ears submitted to gene silencing compared to the ears not treated. Increased anti-apoptotic gene expression and decreased pro-apoptotic gene expression were found in the treated ears. Conclusion: Our results allow us to suggest that the blockade of TNFα by gene silencing was useful to prevent noise-induced hearing loss.

Noise Exposure May Diminish the Musician Advantage for Perceiving Speech in Noise

OBJECTIVE

Although numerous studies have shown that musicians have better speech perception in noise (SPIN) compared to nonmusicians, other studies have not replicated the "musician advantage for SPIN." One factor that has not been adequately addressed in previous studies is how musicians' SPIN is affected by routine exposure to high levels of sound. We hypothesized that such exposure diminishes the musician advantage for SPIN.

DESIGN

Environmental sound levels were measured continuously for 1 week via body-worn noise dosimeters in 56 college students with diverse musical backgrounds and clinically normal pure-tone audiometric averages. SPIN was measured using the Quick Speech in Noise Test (QuickSIN). Multiple linear regression modeling was used to examine how music practice (years of playing a musical instrument) and routine noise exposure predict QuickSIN scores.

RESULTS

Noise exposure and music practice were both significant predictors of QuickSIN, but they had opposing influences, with more years of music practice predicting better QuickSIN scores and greater routine noise exposure predicting worse QuickSIN scores. Moreover, mediation analysis suggests that noise exposure suppresses the relationship between music practice and QuickSIN scores.

CONCLUSIONS

Our findings suggest a beneficial relationship between music practice and SPIN that is suppressed by noise exposure.

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.

Recent Advancements in Gene and Stem Cell-Based Treatment Modalities: Potential Implications in Noise-Induced Hearing Loss

Noise-induced hearing loss (NIHL) poses a significant burden on not only the economics of health care but also the quality of life of an individual, as we approach an unprecedented age of longevity. In this article, we will delineate the current landscape of management of NIHL. We discuss the most recent results from in vitro and in vivo studies that determine the effectiveness of established pharmacotherapy such as corticosteroid and potential emerging therapies like N-acetyl cysteine and neurotrophins (NTs), as well as highlight ongoing clinical trials for these therapeutic agents. We present an overview of how the recent advancements in the field of gene-based and stem cell-based therapies can help in developing effective therapeutic strategies for NIHL. Gene-based therapies have shown exciting results demonstrating cochlear cellular regeneration using Atoh1, NRF2 as well as NT gene therapy employing viral vectors. In addition, we will discuss the recent advancements in genome-editing technologies, such as CRISPR/Cas9, and its potential role in NIHL therapy. We will further discuss the current state of stem cell therapy as it pertains to treating neurodegenerative conditions including NIHL. Embryonic stem cells, adult-derived stem cells, and induced pluripotent stem cells all represent an enticing reservoir of replacing damaged cells as a result of NIHL. Finally, we will discuss the barriers that need to be overcome to translate these promising treatment modalities to the clinical practice in pursuit of improving quality of life of patients having NIHL. Anat Rec, 303:516-526, 2020. © 2019 American Association for Anatomy.

Non-Invasive Assays of Cochlear Synaptopathy - Candidates and Considerations

Studies in multiple species, including in post-mortem human tissue, have shown that normal aging and/or acoustic overexposure can lead to a significant loss of afferent synapses innervating the cochlea. Hypothetically, this cochlear synaptopathy can lead to perceptual deficits in challenging environments and can contribute to central neural effects such as tinnitus. However, because cochlear synaptopathy can occur without any measurable changes in audiometric thresholds, synaptopathy can remain hidden from standard clinical diagnostics. To understand the perceptual sequelae of synaptopathy and to evaluate the efficacy of emerging therapies, sensitive and specific non-invasive measures at the individual patient level need to be established. Pioneering experiments in specific mice strains have helped identify many candidate assays. These include auditory brainstem responses, the middle-ear muscle reflex, envelope-following responses, and extended high-frequency audiograms. Unfortunately, because these non-invasive measures can be also affected by extraneous factors other than synaptopathy, their application and interpretation in humans is not straightforward. Here, we systematically examine six extraneous factors through a series of interrelated human experiments aimed at understanding their effects. Using strategies that may help mitigate the effects of such extraneous factors, we then show that these suprathreshold physiological assays exhibit across-individual correlations with each other indicative of contributions from a common physiological source consistent with cochlear synaptopathy. Finally, we discuss the application of these assays to two key outstanding questions, and discuss some barriers that still remain. This article is part of a Special Issue entitled: Hearing Loss, Tinnitus, Hyperacusis, Central Gain.

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.