Cochlear synaptopathy in acquired sensorineural hearing loss: Manifestations and mechanisms

Common causes of hearing loss in humans - exposure to loud noise or ototoxic drugs and aging - often damage sensory hair cells, reflected as elevated thresholds on the clinical audiogram. Recent studies in animal models suggest, however, that well before this overt hearing loss can be seen, a more insidious, but likely more common, process is taking place that permanently interrupts synaptic communication between sensory inner hair cells and subsets of cochlear nerve fibers. The silencing of affected neurons alters auditory information processing, whether accompanied by threshold elevations or not, and is a likely contributor to a variety of perceptual abnormalities, including speech-in-noise difficulties, tinnitus and hyperacusis. Work described here will review structural and functional manifestations of this cochlear synaptopathy and will consider possible mechanisms underlying its appearance and progression in ears with and without traditional 'hearing loss' arising from several common causes in humans.

Tinnitus with a normal audiogram: Relation to noise exposure but no evidence for cochlear synaptopathy

In rodents, exposure to high-level noise can destroy synapses between inner hair cells and auditory nerve fibers, without causing hair cell loss or permanent threshold elevation. Such "cochlear synaptopathy" is associated with amplitude reductions in wave I of the auditory brainstem response (ABR) at moderate-to-high sound levels. Similar ABR results have been reported in humans with tinnitus and normal audiometric thresholds, leading to the suggestion that tinnitus in these cases might be a consequence of synaptopathy. However, the ABR is an indirect measure of synaptopathy and it is unclear whether the results in humans reflect the same mechanisms demonstrated in rodents. Measures of noise exposure were not obtained in the human studies, and high frequency audiometric loss may have impacted ABR amplitudes. To clarify the role of cochlear synaptopathy in tinnitus with a normal audiogram, we recorded ABRs, envelope following responses (EFRs), and noise exposure histories in young adults with tinnitus and matched controls. Tinnitus was associated with significantly greater lifetime noise exposure, despite close matching for age, sex, and audiometric thresholds up to 14 kHz. However, tinnitus was not associated with reduced ABR wave I amplitude, nor with significant effects on EFR measures of synaptopathy. These electrophysiological measures were also uncorrelated with lifetime noise exposure, providing no evidence of noise-induced synaptopathy in this cohort, despite a wide range of exposures. In young adults with normal audiograms, tinnitus may be related not to cochlear synaptopathy but to other effects of noise exposure.

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

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

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

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

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

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

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

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

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

Translational issues in cochlear synaptopathy

Understanding the biology of the previously underappreciated sensitivity of cochlear synapses to noise insult, and its clinical consequences, is becoming a mission for a growing number of auditory researchers. In addition, several research groups have become interested in developing therapeutic approaches that can reverse synaptopathy and restore hearing function. One of the major challenges to realizing the potential of synaptopathy rodent models is that current clinical audiometric approaches cannot yet reveal the presence of this subtle cochlear pathology in humans. This has catalyzed efforts, both from basic and clinical perspectives, to investigate novel means for diagnosing synaptopathy and to determine the main functional consequences for auditory perception and hearing abilities. Such means, and a strong concordance between findings in pre-clinical animal models and clinical studies in humans, are important for developing and realizing therapeutics. This paper frames the key outstanding translational questions that need to be addressed to realize this ambitious goal.

Effects of noise exposure on young adults with normal audiograms II: Behavioral measures

An estimate of lifetime noise exposure was used as the primary predictor of performance on a range of behavioral tasks: frequency and intensity difference limens, amplitude modulation detection, interaural phase discrimination, the digit triplet speech test, the co-ordinate response speech measure, an auditory localization task, a musical consonance task and a subjective report of hearing ability. One hundred and thirty-eight participants (81 females) aged 18-36 years were tested, with a wide range of self-reported noise exposure. All had normal pure-tone audiograms up to 8 kHz. It was predicted that increased lifetime noise exposure, which we assume to be concordant with noise-induced cochlear synaptopathy, would elevate behavioral thresholds, in particular for stimuli with high levels in a high spectral region. However, the results showed little effect of noise exposure on performance. There were a number of weak relations with noise exposure across the test battery, although many of these were in the opposite direction to the predictions, and none were statistically significant after correction for multiple comparisons. There were also no strong correlations between electrophysiological measures of synaptopathy published previously and the behavioral measures reported here. Consistent with our previous electrophysiological results, the present results provide no evidence that noise exposure is related to significant perceptual deficits in young listeners with normal audiometric hearing. It is possible that the effects of noise-induced cochlear synaptopathy are only measurable in humans with extreme noise exposures, and that these effects always co-occur with a loss of audiometric sensitivity.

Translating animal models to human therapeutics in noise-induced and age-related hearing loss

Acquired sensorineural hearing loss is one of the most prevalent chronic diseases, and aging and acoustic overexposure are common contributors. Decades of study in animals and humans have clarified the cellular targets and perceptual consequences of these forms of hearing loss, and preclinical studies have led to the development of therapeutics designed to slow, prevent or reverse them. Here, we review the histopathological changes underlying age-related and noise-induced hearing loss and the functional consequences of these pathologies. Based on these relations, we consider the ambiguities that arise in diagnosing underlying pathology from minimally invasive tests of auditory function, and how those ambiguities present challenges in the design and interpretation of clinical trials.

Effects of cochlear synaptopathy on middle-ear muscle reflexes in unanesthetized mice

Cochlear synaptopathy, i.e. the loss of auditory-nerve connections with cochlear hair cells, is seen in aging, noise damage, and other types of acquired sensorineural hearing loss. Because the subset of auditory-nerve fibers with high thresholds and low spontaneous rates (SRs) is disproportionately affected, audiometric thresholds are relatively insensitive to this primary neural degeneration. Although suprathreshold amplitudes of wave I of the auditory brainstem response (ABR) are attenuated in synaptopathic mice, there is not yet a robust diagnostic in humans. The middle-ear muscle reflex (MEMR) might be a sensitive metric (Valero et al., 2016), because low-SR fibers may be important drivers of the MEMR (Liberman and Kiang, 1984; Kobler et al., 1992). Here, to test the hypothesis that narrowband reflex elicitors can identify synaptopathic cochlear regions, we measured reflex growth functions in unanesthetized mice with varying degrees of noise-induced synaptopathy and in unexposed controls. To separate effects of the MEMR from those of the medial olivocochlear reflex, the other sound-evoked cochlear feedback loop, we used a mutant mouse strain with deletion of the acetylcholine receptor required for olivocochlear function. We demonstrate that the MEMR is normal when activated from non-synaptopathic cochlear regions, is greatly weakened in synaptopathic regions, and is a more sensitive indicator of moderate synaptopathy than the suprathreshold amplitude of ABR wave I.

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

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

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.

Reliability and interrelations of seven proxy measures of cochlear synaptopathy

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

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

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

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

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

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

Effects of age on electrophysiological measures of cochlear synaptopathy in humans

Age-related cochlear synaptopathy (CS) has been shown to occur in rodents with minimal noise exposure, and has been hypothesized to play a crucial role in age-related hearing declines in humans. Because CS affects mainly low-spontaneous rate auditory nerve fibers, differential electrophysiological measures such as the ratio of the amplitude of wave I of the auditory brainstem response (ABR) at high to low click levels (WIH/WIL), and the difference between frequency following response (FFR) levels to shallow and deep amplitude modulated tones (FFRS-FFRD), have been proposed as CS markers. However, age-related audiometric threshold shifts, particularly prominent at high frequencies, may confound the interpretation of these measures in cross-sectional studies of age-related CS. To address this issue, we measured WIH/WIL and FFRS-FFRD using highpass masking (HP) noise to eliminate the contribution of high-frequency cochlear regions to the responses in a cross-sectional sample of 102 subjects (34 young, 34 middle-aged, 34 older). WIH/WIL in the presence of the HP noise did not decrease as a function of age. However, in the absence of HP noise, WIH/WIL showed credible age-related decreases even after partialing out the effects of audiometric threshold shifts. No credible age-related decreases of FFRS-FFRD were found. Overall, the results do not provide evidence of age-related CS in the low-frequency region where the responses were restricted by the HP noise, but are consistent with the presence of age-related CS in higher frequency regions.

Envelope following response measurements in young veterans are consistent with noise-induced cochlear synaptopathy

Animal studies have demonstrated that noise exposure can lead to the loss of the synapses between the inner hair cells and their afferent auditory nerve fiber targets without impacting auditory thresholds. Although several non-invasive physiological measures appear to be sensitive to cochlear synaptopathy in animal models, including auditory brainstem response (ABR) wave I amplitude, the envelope following response (EFR), and the middle ear muscle reflex (MEMR), human studies of these measures in samples that are expected to vary in terms of the degree of noise-induced synaptopathy have resulted in mixed findings. One possible explanation for the differing results is that synaptopathy risk is lower for recreational noise exposure than for occupational or military noise exposure. The goal of this analysis was to determine if EFR magnitude and ABR wave I amplitude are reduced among young Veterans with a history of military noise exposure compared with non-Veteran controls with minimal noise exposure. EFRs and ABRs were obtained in a sample of young (19-35 years) Veterans and non-Veterans with normal audiograms and robust distortion product otoacoustic emissions (DPOAEs). The statistical analysis is consistent with a reduction in mean EFR magnitude and ABR wave I amplitude (at 90 dB peSPL) for Veterans with a significant history of noise exposure compared with non-Veteran controls. These findings are in agreement with previous ABR wave I amplitude findings in young Veterans and are consistent with animal models of noise-induced cochlear synaptopathy.

Auditory brainstem response latency in forward masking, a marker of sensory deficits in listeners with normal hearing thresholds

In rodent models, acoustic exposure too modest to elevate hearing thresholds can nonetheless cause auditory nerve fiber deafferentation, interfering with the coding of supra-threshold sound. Low-spontaneous rate nerve fibers, important for encoding acoustic information at supra-threshold levels and in noise, are more susceptible to degeneration than high-spontaneous rate fibers. The change in auditory brainstem response (ABR) wave-V latency with noise level has been shown to be associated with auditory nerve deafferentation. Here, we measured ABR in a forward masking paradigm and evaluated wave-V latency changes with increasing masker-to-probe intervals. In the same listeners, behavioral forward masking detection thresholds were measured. We hypothesized that 1) auditory nerve fiber deafferentation increases forward masking thresholds and increases wave-V latency and 2) a preferential loss of low-spontaneous rate fibers results in a faster recovery of wave-V latency as the slow contribution of these fibers is reduced. Results showed that in young audiometrically normal listeners, a larger change in wave-V latency with increasing masker-to-probe interval was related to a greater effect of a preceding masker behaviorally. Further, the amount of wave-V latency change with masker-to-probe interval was positively correlated with the rate of change in forward masking detection thresholds. Although we cannot rule out central contributions, these findings are consistent with the hypothesis that auditory nerve fiber deafferentation occurs in humans and may predict how well individuals can hear in noisy environments.

Animal models of hidden hearing loss: Does auditory-nerve-fiber loss cause real-world listening difficulties?

Afferent innervation of the cochlea by the auditory nerve declines during aging and potentially after sound overexposure, producing the common pathology known as cochlear synaptopathy. Auditory-nerve-fiber loss is difficult to detect with the clinical audiogram and has been proposed to cause 'hidden hearing loss' including impaired speech-in-noise perception. While evidence that auditory-nerve-fiber loss causes hidden hearing loss in humans is controversial, behavioral animal models hold promise to rigorously test this hypothesis because neural lesions can be induced and histologically validated. Here, we review recent animal behavioral studies on the impact of auditory-nerve-fiber loss on perception in a range of species. We first consider studies of tinnitus and hyperacusis inferred from acoustic startle reflexes, followed by a review of operant-conditioning studies of the audiogram, temporal integration for tones of varying duration, temporal resolution of gaps in noise, and tone-in-noise detection. Studies quantifying the audiogram show that tone-in-quiet sensitivity is unaffected by auditory-nerve-fiber loss unless neural lesions exceed 80%, at which point large deficits are possible. Changes in other aspects of perception, which were typically investigated for moderate-to-severe auditory-nerve-fiber loss of 50-70%, appear heterogeneous across studies and might be small compared to impairment caused by hair-cell pathologies. Future studies should pursue recent findings that behavioral sensitivity to brief tones and silent gaps in noise may be particularly vulnerable to auditory-nerve-fiber loss. Furthermore, aspects of auditory perception linked to central inhibition and fine neural response timing, such as modulation masking release and spatial hearing, may be productive directions for further animal behavioral research.

Identifying three otopathologies in humans

OBJECTIVES

Hearing-in-noise (HIN) is a primary complaint of both the hearing impaired and the hearing aid user. Both auditory nerve (AN) function and outer hair cell (OHC) function are thought to contribute to HIN, but their relative contributions are still being elucidated. OHCs play a critical role in HIN by fine tuning the motion of the basilar membrane. Further, animal studies suggest that cochlear (auditory) synaptopathy, which is the loss of synaptic contact between hair cells and the AN, may be another cause of HIN difficulty. While there is evidence that cochlear synaptopathy occurs in animal models, there is debate as to whether cochlear synaptopathy is clinically significant in humans, which may be due to disparate methods of measuring noise exposure in humans and our high variability in susceptibility to noise damage. Rather than use self-reported noise exposure to define synaptopathic groups, this paper assumes that the general population exhibits a range of noise exposures and resulting otopathologies and defines cochlear synaptopathy "operationally" as low CAP amplitude accompanied by normal DPOAE levels in persons with low pure tone averages. The first question is whether the standard audiogram detects AN dysfunction and OHC dysfunction? The second question is whether HIN performance is primarily dependent on AN function, OHC function, or both functions?

DESIGN

Adult subjects have been recruited to participate in an ongoing study and variables such as age, self-reported gender, pure tone audiometry (0.25-20 kHz), subjective perception of HIN difficulty, Quick Speech-in Noise (QuickSIN) test, 45% time compressed word recognition (WR) in 10% reverberation and WR in the presence of ipsilateral speech-weighted noise have been collected. These variables were correlated with OHC function measured by distortion-product otoacoustic emission (DPOAE) signal to-noise-ratio (SNR), and AN function measured by compound action potential (CAP) peak amplitude and ratio to summating potential measured using electrocochleography.

RESULTS

Synaptopathy, by this operational definition, may be present in as many as 30% of individuals with normal hearing. Persons hearing within normal limits may exhibit HIN difficulties, and persons with hearing within normal limits may exhibit two distinct types of otopathologies undetected by the standard audiogram (a.k.a. hidden hearing loss) namely operational cochlear synaptopathy and OHC dysfunction. AN untuning secondary to OHC dysfunction is a third otopathology that occurs in subjects with a Mild-Moderate sensorineural hearing loss (SNHL). Clinical norms for each of these otopathologies are presented. Finally, the data show that operational cochlear synaptopathy does not correlate with HIN dysfunction. Rather, HIN performance is primarily governed by OHC function, while AN untuning also plays a lesser but statistically significant role.

CONCLUSIONS

The results of this study suggest the following: (1) persons hearing within normal limits may exhibit HIN difficulties; (2) persons hearing within normal limits may exhibit undetected otopathologies, namely AN dysfunction and OHC dysfunction; (3) AN untuning secondary to OHC dysfunction occurs in subjects with Mild-Moderate SNHL; (4) HIN performance is primarily governed by OHC function rather than AN function.

Use of non-invasive measures to predict cochlear synapse counts

Cochlear synaptopathy, the loss of synaptic connections between inner hair cells (IHCs) and auditory nerve fibers, has been documented in animal models of aging, noise, and ototoxic drug exposure, three common causes of acquired sensorineural hearing loss in humans. In each of these models, synaptopathy begins prior to changes in threshold sensitivity or loss of hair cells; thus, this underlying injury can be hidden behind a normal threshold audiogram. Since cochlear synaptic loss cannot be directly confirmed in living humans, non-invasive assays will be required for diagnosis. In animals with normal auditory thresholds, the amplitude of wave 1 of the auditory brainstem response (ABR) is highly correlated with synapse counts. However, synaptopathy can also co-occur with threshold elevation, complicating the use of the ABR alone as a diagnostic measure. Using an age-graded series of mice and a partial least squares regression approach to model structure-function relationships, this study shows that the combination of a small number of ABR and distortion product otoacoustic emission (DPOAE) measurements can predict synaptic ribbon counts at various cochlear frequencies to within 1-2 synapses per IHC of their true value. In contrast, the model, trained using the age-graded series of mice, overpredicted synapse counts in a small sample of young noise-exposed mice, perhaps due to differences in the underlying pattern of damage between aging and noise-exposed mice. These results provide partial validation of a noninvasive approach to identify synaptic/neuronal loss in humans using ABRs and DPOAEs.

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.

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.

Neural Contributions to the Cochlear Summating Potential: Spiking and Dendritic Components

Using electrocochleography, the summating potential (SP) is a deflection from baseline to tones and an early rise in the response to clicks. Here, we use normal hearing gerbils and gerbils with outer hair cells removed with a combination of furosemide and kanamycin to investigate cellular origins of the SP. Round window electrocochleography to tones and clicks was performed before and after application of tetrodotoxin to prevent action potentials, and then again after kainic acid to prevent generation of an EPSP. With appropriate subtractions of the response curves from the different conditions, the contributions to the SP from outer hair cells, inner hair cell, and neural "spiking" and "dendritic" responses were isolated. Like hair cells, the spiking and dendritic components had opposite polarities to tones - the dendritic component had negative polarity and the spiking component had positive polarity. The magnitude of the spiking component was larger than the dendritic across frequencies and intensities. The onset to tones and to clicks followed a similar sequence; the outer hair cells responded first, then inner hair cells, then the dendritic component, and then the compound action potential of the spiking response. These results show the sources of the SP include at least the four components studied, and that these have a mixture of polarities and magnitudes that vary across frequency and intensity. Thus, multiple possible interactions must be considered when interpreting the SP for clinical uses.

Effects of age on psychophysical measures of auditory temporal processing and speech reception at low and high levels

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We found little evidence of greater age-related hearing declines at high sound levels.

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There are age-related temporal-processing declines independent of hearing loss.

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No evidence of age-related speech-reception deficits independent of hearing loss.

Age-related cochlear synaptopathy (CS) has been shown to occur in rodents with minimal noise exposure, and has been hypothesized to play a crucial role in age-related hearing declines in humans. It is not known to what extent age-related CS occurs in humans, and how it affects the coding of supra-threshold sounds and speech in noise. Because in rodents CS affects mainly low- and medium-spontaneous rate (L/M-SR) auditory-nerve fibers with rate-level functions covering medium-high levels, it should lead to greater deficits in the processing of sounds at high than at low stimulus levels. In this cross-sectional study the performance of 102 listeners across the age range (34 young, 34 middle-aged, 34 older) was assessed in a set of psychophysical temporal processing and speech reception in noise tests at both low, and high stimulus levels. Mixed-effect multiple regression models were used to estimate the effects of age while partialing out effects of audiometric thresholds, lifetime noise exposure, cognitive abilities (assessed with additional tests), and musical experience. Age was independently associated with performance deficits on several tests. However, only for one out of 13 tests were age effects credibly larger at the high compared to the low stimulus level. Overall these results do not provide much evidence that age-related CS, to the extent to which it may occur in humans according to the rodent model of greater L/M-SR synaptic loss, has substantial effects on psychophysical measures of auditory temporal processing or on speech reception in noise.

Review of blast noise and the auditory system

The auditory system is particularly vulnerable to blast injury due to the ear's role as a highly sensitive pressure transducer. Over the past several decades, studies have used a variety of animal models and experimental procedures to recreate blast-induced acoustic trauma. Given the developing nature of this field and our incomplete understanding of molecular mechanisms underlying blast-related auditory disturbances, an updated discussion about these studies is warranted. Here, we comprehensively review well-established blast-related auditory pathology including tympanic membrane perforation and hair cell loss. In addition, we discuss important mechanistic studies that aim to bridge gaps in our current understanding of the molecular and microstructural events underlying blast-induced cochlear, auditory nerve, brainstem, and central auditory system damage. Key findings from the recent literature include the association between endolymphatic hydrops and cochlear synaptic loss, blast-induced neuroinflammatory markers in the peripheral and central auditory system, and therapeutic approaches targeting biochemical markers of blast injury. We conclude that blast is an extreme form of noise exposure. Blast waves produce cochlear damage that appears similar to, but more extreme than, the standard noise exposure protocols used in auditory research. However, experimental variations in studies of blast-induced acoustic trauma make it challenging to compare and interpret data across studies.

Automated extraction of auditory brainstem response latencies and amplitudes by means of non-linear curve registration

BACKGROUND AND OBJECTIVES

Animal results have suggested that auditory brainstem responses (ABRs) to transient sounds presented at supra-threshold levels may be useful for measuring hearing damage that is hidden to current audiometric tests. Evaluating such ABRs requires extracting the latencies and amplitudes of relevant deflections, or "waves". Currently, this is mostly done by human observers manually picking the waves' peaks and troughs in each individual response - a process that is both time-consuming and requiring of expert experience. Here, we propose a highly automated procedure for extracting individual ABR wave latencies and amplitudes based on the well-established methodology of non-linear curve registration.

METHODS

First, the to-be-analysed individual ABRs are temporally aligned - either with one another or, if available, with a pre-existing template - by locally compressing or stretching their time axes with smooth and invertible time warping functions. Then, the individual latencies and amplitudes of relevant ABR waves are obtained by picking the latencies of the waves' peaks and troughs on the common (aligned) time axis and combining these with the individual aligned responses and inverse time warping functions.

RESULTS

Using an example ABR data set with a wide range of response latencies and signal-to-noise ratios (SNRs), we test different choices for fitting the time warping functions. We cross-validate the warping results using independent response replicates and compare automatically and manually extracted latencies and amplitudes for ABR waves I and V. Using a Bayesian approach, we show that, for the best registration condition, automatic and manual data were statistically similar.

CONCLUSIONS

Non-linear curve registration can be used to temporally align individual ABRs and extract their wave latencies and amplitudes in a way that closely matches results from manual picking.

The search for correlates of age-related cochlear synaptopathy: Measures of temporal envelope processing and spatial release from speech-on-speech masking

Older adults often experience difficulties understanding speech in adverse listening conditions. It has been suggested that for listeners with normal and near-normal audiograms, these difficulties may, at least in part, arise from age-related cochlear synaptopathy. The aim of this study was to assess if performance on auditory tasks relying on temporal envelope processing reveal age-related deficits consistent with those expected from cochlear synaptopathy. Listeners aged 20 to 66 years were tested using a series of psychophysical, electrophysiological, and speech-perception measures using stimulus configurations that promote coding by medium- and low-spontaneous-rate auditory-nerve fibers. Cognitive measures of executive function were obtained to control for age-related cognitive decline. Results from the different tests were not significantly correlated with each other despite a presumed reliance on common mechanisms involved in temporal envelope processing. Only gap-detection thresholds for a tone in noise and spatial release from speech-on-speech masking were significantly correlated with age. Increasing age was related to impaired cognitive executive function. Multivariate regression analyses showed that individual differences in hearing sensitivity, envelope-based measures, and scores from nonauditory cognitive tests did not significantly contribute to the variability in spatial release from speech-on-speech masking for small target/masker spatial separation, while age was a significant contributor.

Loss of synaptic ribbons is an early cause in ROS-induced acquired sensorineural hearing loss

Considerable evidence of reactive oxygen species (ROS) involvement in cochlear hair cell (HC) loss, leading to acquired sensorineural hearing loss (SNHL), were reported. Cochlear synaptopathy between HCs and spiral ganglion neurons has been gathering attention as a cochlear HC loss precursor not detectable by normal auditory evaluation. However, the molecular mechanisms linking ROS with HC loss, as well as the relationship between ROS and cochlear synaptopathy have not been elucidated. Here, we examined these linkages using NOX4-TG mice, which constitutively produce ROS without stimulation. mRNA levels of Piccolo 1, a major component of the synaptic ribbon (a specialized structure surrounded by synaptic vesicles in HCs), were decreased in postnatal day 6 NOX4-TG mice cochleae compared to those in WT mice; they were also decreased by noise exposure in 2-week-old WT cochleae. As noise exposure induces ROS production, this suggests that the synaptic ribbon is a target of ROS. The level of CtBP2, another synaptic ribbon component, was significantly lower in NOX4-TG cochleae of 1-month-old and 4-month-old mice compared to that in WT mice, although no significant differences were noted at 1.5- and 2-months. The decrease in CtBP2 plateaued in 4-month-old NOX4-TG, while it gradually decreased from 1 to 6 months in WT mice. Furthermore, CtBP2 level in 2-month-old NOX4-TG mice decreased significantly after exposure to cisplatin and noise compared to that in WT mice. These findings suggest that ROS lead to developmental delays and early degeneration of synaptic ribbons, which could be potential targets for novel therapeutics for ROS-induced SNHL.