Noise-induced cochlear neuropathy is selective for fibers with low spontaneous rates

C57BL/6-derived mice and the Cdh23ahl allele - Background matters

C57BL/6-derived mice are the most utilised mice in biomedical research, and yet actually there is no such thing as a generic C57BL/6 mouse. Instead, there are more than 150 C57BL/6-derived sub-strains recognised by the Mouse Genome Informatics (MGI) database, each of which carry sub-strain-specific fixed genetic differences that can potentially lead to phenotypic differences affecting a single, or multiple biological systems. One of the most widely known strain-specific alleles is the Cdh23ahl allele, a single nucleotide change that predisposes C57BL/6-derived mice to a progressive hearing loss that starts in the high-frequency region. As such, this allele is of particular relevance to auditory researchers. However, a recent study, comparing C57BL/6NTac mice with a co-isogenic strain in which the Cdh23ahl allele has been 'repaired' using genome editing, suggests that the Cdh23ahl allele may have a broader effect on phenotype expressivity of mouse mutants impacting not just the auditory system, but other organ systems as well. Here, using the Cdh23ahl allele as an exemplar, we discuss the importance of knowing, understanding and reporting the genetic background of mouse mutants.

Cochlear neural contributions to triple network changes in tinnitus, hyperacusis & misophonia? A perspective review

What do tinnitus, the perception of sounds without an internal or external source of noise, hyperacusis, the pathological hypersensitivity to noise, or misophonia, an intolerance to certain everyday noises, have in common, and what differentiates them? A large number of excellent studies focused in the last few decades on identifying the neural correlates of tinnitus, hyperacusis, or misophonia on the basis of central triple-network changes. In this perspective review we explicitly examine, possible differential and causal involvement of peripheral components as a presumptive trigger that may drive observed triple-network changes. Based on our results, we venture to hypothesize that: (i) tinnitus, hyperacusis, and misophonia can occur despite clinically normal hearing thresholds, and are likely causally independent of sex and age, (ii) tinnitus and hyperacusis, but possibly also misophonia are related to altered auditory processing that through desynchronized (tinnitus) or hyperactive (hyperacusis, misophonia) bottom-up ascending processing potentially explains the activity changes in, e.g., default or salient brain networks, as suggested in various studies of these different diseases. (iii) In misophonia a stress-induced top-down influence, as deep as the auditory nerve fibers, may be discussed as a contributor to generating misophonia-trigger sounds, a hypothesis that can be tested in future studies. We hope that the selective consideration of a possible interaction between peripheral and central components will help to minimize the greatest handicap of these pathologies to date towards successful therapy: the lack of clarification of the underlying causative mechanism of the diseases.

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.

Deciphering Auditory Hyperexcitability in Otogl Mutant Mice Unravels an Auditory Neuropathy Mechanism

Auditory neuropathies affect the spiral ganglion neurons of the auditory nerve or their synapses with the sensory hair cells, distorting the sound information transmitted from the ear to the brain. Deciphering the underlying pathophysiological mechanisms remains challenging owing to the diversity of spiral ganglion neuron subtypes and associated central auditory circuits. An auditory neuropathy mechanism is unraveled by investigating the origin of auditory hyperexcitability in a mouse model for hereditary congenital deafness. Otogl encodes the large Otogelin-like protein, which is related to secreted epithelial mucins and is implicated in the mechanical stimulation of cochlear outer hair cells. Heterozygous Otogl+/- mutant mice display auditory hyperexcitability, highlighted by their susceptibility to audiogenic seizures induced by loud sounds. It is shown that Otogl is transiently expressed in a subpopulation of spiral ganglion neurons during cochlear development. Despite their apparently normal hearing, Otogl+/- mice display poor activation of the spiral ganglion neurons processing loud sounds and an elevation of the activation threshold of the middle the ear muscle reflex that attenuates loud sounds. The findings reveal how a neuropathy affecting spiral ganglion neurons specialized in loud sound processing and associated with the middle the ear muscle reflex can manifest itself as auditory hyperexcitability.

Phenotypic changes of auditory nerve fibers after excitotoxicity

There is a substantial body of evidence elucidating the pathophysiological aspects of excitotoxicity in the mammalian cochlea. However, the question of whether the resultant damage is reversible remains unresolved. To replicate an excitotoxic event, we investigated the long-term effects of kainate application in gerbil cochleae. Surprisingly, despite persistent synapse loss, the compound action potential of the auditory nerve fully recovered. This functional retrieval was associated with a phenotypic change in auditory nerve fibers. Thresholds were improved along the tonotopic axis. High-spontaneous rate (SR) fibers largely populated the apical region, while low-SR fibers from the basal region exhibited sound-driven activity indistinguishable from control high-SR fibers. This functional phenotype change may support the full recovery of neural response thresholds and amplitudes after excitotoxicity. Furthermore, hyperresponsiveness of the auditory nerve fibers could be a crucial factor in the development of hyperactivity in the central auditory pathways, a common occurrence following acoustic overstimulation.

Silk Devices for Tympanic Membrane Repair Show No Ototoxicity in a Rat Model

HYPOTHESIS

Implantation of ClearDrum ® , a biodegradable silk fibroin device, into the middle ear (ME) cavity in a rat model will not affect hearing thresholds as measured by auditory brainstem response.

BACKGROUND

Chronic otitis media (COM) is a recurrent middle ear infection often accompanied by tympanic membrane perforation. Our laboratory has developed a biodegradable silk fibroin device (ClearDrum®) designed to treat tympanic membrane perforations in COM. ClearDrum ® acts as a prosthetic eardrum, providing a substrate on which tympanic membrane cells can grow and acts as a long-lasting implant.

METHODS

Two formulations were tested based on silk/glycerol and silk/polyurethane blends. Animals were anesthetized, and either a ClearDrum ® formulation or an autologous cartilage graft was surgically implanted into the middle ear. Thresholds of the auditory brainstem response (ABR) were measured at the time of implantation and at 4 and 12 weeks post-implantation to assess hearing after the implantation. After the final measurements at 12 weeks, middle ear and cochlea were harvested for histological assessment.

RESULTS

No significant differences in ABR thresholds between male and female animals were found at any timepoint. Results showed that there were no detrimental effects of either Cleardrum ® formulation on ABR thresholds as compared with implantation of autologous cartilage and no presence of inflammation within the middle ear or cochlea.

CONCLUSION

Our ABR data suggest no substantial ototoxic effects on outer or inner hair cells and provides some support toward clinical translation of ClearDrum devices.

The Auditory Brainstem Response Diagnoses Alzheimer-Like Disease in the 5xFAD Mouse Model

Early and accurate diagnosis of Alzheimer's disease (AD) will be key for effective personalized treatment plans ( Cummings, 2023). Significant difficulties in auditory processing have been frequently reported in many patients with mild cognitive impairment, the prodromal form of AD ( Tarawneh et al., 2022), making it an outstanding candidate as AD diagnostic biomarker. However, the efficiency of diagnosis with this parameter has not been explored. Here we show that when male mice with amyloidosis begin to show memory decline, changes in the auditory brainstem response (ABR) to clicks enable the reliable diagnosis of disease using a machine learning algorithm. Interpretation of the machine learning diagnosis revealed that the upper levels of the auditory pathway, including the inferior colliculus, were the probable sources of the defects. Histological analyses show that in these locations, neuroinflammation and plaque deposition temporally correlate with behavioral changes consistent with memory loss. While these findings are tempered by the caveat that they derive from amyloidosis mice, we propose that ABR measurements be evaluated as an additional rapid, low-cost, noninvasive biomarker to assist the diagnostic testing of early-stage AD.

Impact of Low-Level Blast Exposure From Military Training and Career Cumulation on Hearing Outcomes

INTRODUCTION

Occupational trades expose military service members to unique sound exposure profiles, distinct in frequency and amplitude to those commonly studied in industrial settings. Characteristics of the energy exposure incurred through blast exposure exercises a distinct mechanism of injury in the auditory system that yield acute and cumulative health impacts. The following paper considers physiological and self-reported data attributed to repeated low-level blast exposure. We focused our analysis to examine objective audiological metrics of blast overpressure damage to quantify the impact of discrete and accumulated blast exposure for military service members.

MATERIALS AND METHODS

This study was reviewed and approved by the Human Research Ethics Committee of Defense Research and Development Canada. Data were collected in a cross-sectional, repeated measures observational design. Participants comprised service members who took part in a training course using controlled explosives (n = 46) and an age- and sex-matched control group (n = 42) with noise exposure but no history of blast exposure. Extensive demographic and subjective health data were captured additional to a battery of quantitative health data. Nonlinear mixed effect models with bivariate generalized additive architecture, Friedman's test, and repeated measures ANOVA models were applied.

RESULTS

Subjective tinnitus and hearing scores were sensitive to life-time blast overpressure exposure (P = .034) and a delta of low- (2-4 kHz) and high-frequency (4-8 kHz) audiometric threshold averages significantly differentiated by both cumulative (P = .0016) and training course exposure (P = .0142). Furthermore, audiometric values showed progressive increasing variance in high frequencies (>4  kHz) in blast-exposed participants with worsening audiometric outcomes. Variation in high-frequency hearing thresholds may be attributed to differences in energy exposure because of structural differences between individuals or differences in exposure profiles (produced either by diversity in PPE compliance or exposure sources).

CONCLUSION

Military members exposed to blasts in a training environment and accumulated through a career experience adverse hearing and audiometric impacts. However, better measures of hearing injury should be pursued to improve understanding of injury mechanism and injury progression which may inform risk and medical interventions. Blast-induced hearing injuries are dynamic and change over time between acute and chronic phases which are further exacerbated with multiple blast exposures presented consecutively.

Extended high-frequency hearing and suprathreshold neural synchrony in the auditory brainstem

Elevated hearing thresholds in the extended high frequencies (EHFs) (>8 kHz) are often associated with poorer speech-in-noise recognition despite a clinically normal audiogram. However, whether EHF hearing loss is associated with disruptions in neural processing within the auditory brainstem remains uncertain. The objective of the present study was to investigate whether elevated EHF thresholds influence neural processing at lower frequencies in individuals with normal audiograms. Auditory brainstem responses (ABRs) were recorded at a suprathreshold level (80 dB normal hearing level) from 45 participants with clinically normal hearing. The recording protocol was optimized to obtain robust wave I of the ABR. Results revealed no significant relationship between the pure tone average for EHFs and any ABR metrics at either rate, while adjusting for the effects of age, sex, and hearing thresholds at standard frequencies (0.25-8 kHz). Rate-dependent significant sex effects for wave I and V amplitude, I-V amplitude ratio, and III and V latency were observed. Elevated EHF hearing thresholds do not significantly affect the brainstem processing in the lower frequencies (<8 kHz).

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.

Degradation in Binaural and Spatial Hearing, and Auditory Temporal Processing Abilities, as a Function of Aging

Objectives

Sensorineural hearing loss is common with advancing age, but even with normal or near-normal hearing in older persons, performance deficits are often seen for suprathreshold listening tasks such as understanding speech in background noise or localizing sound direction. This suggests there is also a more central source of the problem. Objectives of this study were to examine as a function of age (young adult to septuagenarian) performance on: 1) a spatial acuity task examining localization ability, and a spatial speech-in-noise (SSIN) recognition task, both measured in a hemi-anechoic sound field using a circular horizontal-plane loudspeaker array, and 2) a suprathreshold auditory temporal processing task and a spectro-temporal processing task, both measured under headphones. Further, we examined any correlations between age, hearing thresholds including extended high frequency (EHF: >8000 Hz), and these measures.

Design

Subjects were 48 adults, aged 21 to 78, with either normal hearing or only a mild sensorineural hearing loss through 4000 Hz. The localization task measured minimum audible angle (MAA) for 500 and 4000 Hz 1/3 rd octave narrowband noise (NBN) in diffuse background noise for both an on-axis (reference source 0°) and off-axis (reference source 45°) listening condition at signal-to-noise ratios (SNRs) of -3, -6, -9, and -12 dB. SSIN testing was also completed for key word recognition in sentences in multi-talker babble noise; specifically, the separation between speech and noise loudspeakers was adaptively varied to determine the difference needed for 40% and 80% correct performance levels. Finally, auditory temporal processing ability was examined using the Temporal Fine Structure (TFS) test, and the Spectro-Temporal Modulation (STM) test.

Results

Spatial acuity was poorer (larger MAAs) in older compared to younger subjects, particularly in the more adverse listening conditions (off-axis, and poorer SNRs). The SSIN data also showed declining mean performance with age at both criterion levels, emerging in the middle age group (> 40 years), but was not correlated with standard audiometric hearing thresholds. Decreased performance on the TFS and STM tasks was dependent on age, emerging only in the older (> 60 years) and middle (>40 years) age groups, respectively; neither was dependent on hearing thresholds. Results of multiple regression analyses suggest that SSIN recognition scales with the ability of the subjects to use both low-frequency binaural temporal fine structure as well as higher-frequency binaural envelope cues, both of which are impacted by aging but not necessarily audiometric hearing thresholds. Finally, EHF range hearing thresholds significantly decreased with age, but performance on tasks remained significantly correlated with age when controlled for EHF hearing.

Conclusions

Particularly for more adverse listening conditions, age-related deficits, but not hearing-threshold-related deficits, were found on both of the spatial hearing tasks and in temporal and spectro-temporal processing abilities. It may be that deficits in temporal processing ability contribute to poorer spatial hearing performance in older subjects due to inaccurate coding of binaural/interaural timing information sent from the periphery to the binaural brainstem. In addition, EHF hearing loss may be a coexisting factor in the reduced performance seen in older subjects.

Diffusion-Weighted Magnetic Resonance Imaging: A Diagnostic Tool for Auditory (Axonal) Neuropathy

BACKGROUND

Axonal neuropathies are disorders that impair neural transmission, leading to substantial sensory deficits. In the auditory system, axonal degeneration can disrupt auditory processing, causing significant hearing difficulties. Understanding the extent of axonal degeneration and its impact on auditory function is crucial for improving diagnosis and management. This study aims to quantify axonal degeneration in the VIIIth nerve using diffusion-weighted MRI and to correlate these findings with auditory function.

METHODS

Fifty-two children and adults participated. A total of, 27 with normal hearing, 7 with cochlear hearing loss and 18 with auditory neuropathy (AN). Hearing thresholds and dMRI data was collected for all participants and the VIIIth nerve was evaluated using the fixel-based analysis metric of Apparent Fibre Density (AFD).

RESULTS

AFD was significantly lower in participants with AN compared to participants with normal hearing and cochlear hearing loss (p < 0.05). 9/18 participants with AN exhibited AFD values ≥ 2 standard deviations below the normal range. Additionally, AFD was strongly correlated with hearing thresholds in participants with no evidence of cochlear dysfunction (r = -0.776, p < 0.001), suggesting reduced auditory nerve fibre density is associated with impaired sound detection.

CONCLUSIONS

dMRI-derived AFD is a sensitive marker for axonal degeneration in the VIIIth nerve. This study provides the first in vivo evidence linking VIIIth nerve microstructure with hearing thresholds, highlighting the potential of dMRI in diagnosing and monitoring AN. The findings suggest that dMRI could be a valuable tool in clinical settings for assessing auditory nerve health and guiding treatment strategies for individuals with AN.

Deciphering Compromised Speech-in-Noise Intelligibility in Older Listeners: The Role of Cochlear Synaptopathy

Speech intelligibility declines with age and sensorineural hearing damage (SNHL). However, it remains unclear whether cochlear synaptopathy (CS), a recently discovered form of SNHL, significantly contributes to this issue. CS refers to damaged auditory-nerve synapses that innervate the inner hair cells and there is currently no go-to diagnostic test available. Furthermore, age-related hearing damage can comprise various aspects (e.g., hair cell damage, CS) that each can play a role in impaired sound perception. To explore the link between cochlear damage and speech intelligibility deficits, this study examines the role of CS for word recognition among older listeners. We first validated an envelope-following response (EFR) marker for CS using a Budgerigar model. We then applied this marker in human experiments, while restricting the speech material's frequency content to ensure that both the EFR and the behavioral tasks engaged similar cochlear frequency regions. Following this approach, we identified the relative contribution of hearing sensitivity and CS to speech intelligibility in two age-matched (65-year-old) groups with clinically normal (n = 15, 8 females) or impaired audiograms (n = 13, 8 females). Compared to a young normal-hearing control group (n = 13, 7 females), the older groups demonstrated lower EFR responses and impaired speech reception thresholds. We conclude that age-related CS reduces supra-threshold temporal envelope coding with subsequent speech coding deficits in noise that cannot be explained based on hearing sensitivity alone.

Neural and behavioral binaural hearing impairment and its recovery following moderate noise exposure

Noise-induced cochlear synaptopathy has been studied for over 25 years with no known diagnosis for this disorder in humans. This type of "hidden hearing loss" induces a loss of synapses in the inner ear but no change in audiometric thresholds. Recent studies have shown that by two months post synaptopathy-inducing noise exposure, synapses in some animal species can regenerate. Animal studies to date have focused primarily on peripheral hearing measures to diagnose ribbon synapse loss, while suggesting binaural listening deficits such as speech-reception-in-noise result from this disorder, but haven't accounted for the possible regeneration of synapses. To address this, we measured binaural physiological and behavioral function, the latter utilizing the pre-pulse inhibition of acoustic startle method, in both male and female adult guinea pigs following exposure to noise that has been shown to induce cochlear synaptopathy. Physiological measurements extended to 2 months post noise exposure to characterize any deficit and subsequent recovery. While common audiological assessments showed temporary threshold shift, reduced evoked potential amplitudes indicative of synaptopathy and measurable binaural electrophysiological hearing deficits post exposure, all measures recovered by 2 months. Suspected regeneration of synaptic ribbons occurred by 2 months post exposure and cochlear histology revealed no synaptic loss 4 months post exposure. Our results show that the same noise exposure protocol demonstrated to cause synaptic loss in prior studies causes physiological binaural processing deficits in the brainstem and that the recovery of neural binaural processing coincides with the regeneration of synapses shown in previous studies and normal binaural hearing behavior.

Efficacy of Hearing Aids in Patients with Hearing Difficulties in Noise: Focus on Hidden Hearing Loss

Objectives: Hearing aids (HAs) have been used for standard high-frequency hearing loss and tinnitus, but their effects on speech intelligibility in noise (SIN) in people with normal hearing, including hidden hearing loss (HHL), have been little explored. Methods: We included in a prospective cohort study patients who experience poor SIN and have normal pure tone average in quiet conditions or slight HL. We used open-fit HAs. The FraMatrix test was used to explore SIN. The benefit of HAs in daily situations and for tinnitus was evaluated. Results: Forty patients were explored, including 12 with HHL. FraMatrix and quality of life improved significantly (p < 0.001) with HAs, especially in those with HHL (2.0 dB SNR of speech recognition threshold improvement). HAs also effectively suppress tinnitus. Participants used HAs 9.9 h per day. Conclusions: This study shows an improvement of SIN with HAs in patients with slight HL but more importantly in patients with HHL.

Noise-induced ribbon synapse loss in the mouse basal cochlear region does not reduce inner hair cell exocytosis

Noise-induced hearing loss is one of the most common forms of hearing loss in adults and also one of the most common occupational diseases. Extensive previous work has shown that the highly sensitive synapses of the inner hair cells (IHCs) may be the first target for irreparable damage and permanent loss in the noise-exposed cochlea, more precisely in the cochlear base. However, how such synaptic loss affects the synaptic physiology of the IHCs in this particularly vulnerable part of the cochlea has not yet been investigated. To address this question, we exposed 3-4-week-old C57BL/6J mice to 8-16 kHz noise for 2 h under isoflurane anesthesia. We then employed hearing measurements, immunohistochemistry and patch-clamp to assess IHC synaptic function. Two noise sound pressure levels (SPLs) were used to evoke acute hearing threshold elevations with different levels of recovery 2 weeks post-exposure. Regardless of noise intensity, the exposure resulted in a loss of approximately 25-36% of ribbon synapses in the basal portions of the cochlea that persisted 2 weeks after exposure. Perforated patch-clamp recordings were made in the IHCs of the basal regions of the cochlea where the greatest synaptic losses were observed. Depolarization-evoked calcium currents in IHCs 2 weeks after exposure were slightly but not significantly smaller as compared to controls from age-matched non-exposed animals. Exocytic changes monitored as changes in membrane capacitance did not follow that trend and remained similar to controls despite significant loss of ribbons, likely reflecting increased exocytosis at the remaining synapses. Additionally, we report for the first time that acute application of isoflurane reduces IHC calcium currents, which may have implications for noise-induced IHC synaptic loss.

The relationship and interdependence of auditory thresholds, proposed behavioural measures of hidden hearing loss, and physiological measures of auditory function

OBJECTIVES

Standard diagnostic measures focus on threshold elevation but hearing concerns may occur independently of threshold elevation - referred to as "hidden hearing loss" (HHL). A deeper understanding of HHL requires measurements that locate dysfunction along the auditory pathway. This study aimed to describe the relationship and interdependence between certain behavioural and physiological measures of auditory function that are thought to be indicative of HHL.

DESIGN

Data were collected on a battery of behavioural and physiological measures of hearing. Threshold-dependent variance was removed from each measure prior to generating a multiple regression model of the behavioural measures using the physiological measures.

STUDY SAMPLE

224 adults in the United States with audiometric thresholds ≤65 dB HL.

RESULTS

Thresholds accounted for between 21 and 60% of the variance in our behavioural measures and 5-51% in our physiological measures of hearing. There was no evidence that the behavioural measures of hearing could be predicted by the selected physiological measures.

CONCLUSIONS

Several proposed behavioural measures for HHL: thresholds-in-noise, frequency-modulation detection, and speech recognition in difficult listening conditions, are influenced by hearing sensitivity and are not predicted by outer hair cell or auditory nerve physiology. Therefore, these measures may not be able to assess threshold-independent hearing disorders.

In vivo AAV9-Myo7a gene rescue restores hearing and cholinergic efferent innervation in inner hair cells

In the mammalian cochlea, sensory hair cells are crucial for the transduction of acoustic stimuli into electrical signals, which are then relayed to the central auditory pathway via spiral ganglion neuron (SGN) afferent dendrites. The SGN output is directly modulated by inhibitory cholinergic axodendritic synapses from the efferent fibers originating in the superior olivary complex. When the adult cochlea is subjected to noxious stimuli or aging, the efferent system undergoes major rewiring, such that it reestablishes direct axosomatic contacts with the inner hair cells (IHCs), which occur only transiently during prehearing stages of development. The trigger, origin, and degree of efferent plasticity in the cochlea remains largely unknown. Using functional and morphological approaches, we demonstrate that efferent plasticity in the adult cochlea occurs as a direct consequence of mechanoelectrical transducer current dysfunction. We also show that, different from prehearing stages of development, the lateral olivocochlear - but not the medial olivocochlear - efferent fibers are those that form the axosomatic synapses with the IHCs. The study also demonstrates that in vivo restoration of IHC function using AAV-Myo7a rescue reestablishes the synaptic profile of adult IHCs and improves hearing, highlighting the potential of using gene-replacement therapy for progressive hearing loss.

Models optimized for real-world tasks reveal the task-dependent necessity of precise temporal coding in hearing

Neurons encode information in the timing of their spikes in addition to their firing rates. Spike timing is particularly precise in the auditory nerve, where action potentials phase lock to sound with sub-millisecond precision, but its behavioral relevance remains uncertain. We optimized machine learning models to perform real-world hearing tasks with simulated cochlear input, assessing the precision of auditory nerve spike timing needed to reproduce human behavior. Models with high-fidelity phase locking exhibited more human-like sound localization and speech perception than models without, consistent with an essential role in human hearing. However, the temporal precision needed to reproduce human-like behavior varied across tasks, as did the precision that benefited real-world task performance. These effects suggest that perceptual domains incorporate phase locking to different extents depending on the demands of real-world hearing. The results illustrate how optimizing models for realistic tasks can clarify the role of candidate neural codes in perception.

Age-related myelin deficits in the auditory brain stem contribute to cocktail-party deficits

Age-related hearing loss consists of both peripheral and central components and is an increasing global health concern. While peripheral hearing loss is well understood, central hearing loss- age-related changes in the central auditory pathways resulting in a listener's inability to process sound correctly -remains poorly understood. In this study, we focus on the pathway from the cochlear nucleus to the medial nucleus of the trapezoid body (MNTB), which depends on heavily myelinated axons for microsecond-level temporal precision required for sound localization. Using a combination of auditory brainstem response recordings (ABR), advanced light and electron microscopy, and behavioral testing with prepulse inhibition of the acoustic startle response (PPI) we identified a correlation between oligodendrocyte loss, abnormal myelination in MNTB afferents, altered ABR wave III morphology indicating MNTB dysfunction, and deficits in spatial hearing behaviors in aging Mongolian gerbils. These findings provide a mechanistic explanation of how demyelination contributes to age-related dysfunction in the auditory brainstem's sound localization pathway.

A novel therapeutic pathway to the human cochlear nerve

Traditional approaches to the human cochlear nerve have been impeded by its bony encasement deep inside the skull base. We present an innovative, minimally invasive, therapeutic pathway for direct access to the nerve to deliver novel regenerative therapies. Neuroanatomical studies on 10 cadaveric human temporal bones were undertaken to identify a potentially safe therapeutic pathway to the cochlear nerve. Simulations based on three-dimensional delineation of anatomical structures obtained from synchrotron phase-contrast imaging were analyzed. This enabled the identification of an approach to the nerve in the fundus of the internal auditory meatus by trephining the medial modiolar wall of the cochlea via the round window for a median depth of 1.48 mm (range 1.21-1.91 mm). The anatomical access was validated on 9 additional human temporal bones using radio-opaque markers and contrast injection with micro-computed tomography surveillance. We thus created an effective conduit for the delivery of therapeutic agents to the cochlear nerve.

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.

Pharmacological Approaches to Hearing Loss

Hearing disorders pose significant challenges to individuals experiencing them and their overall quality of life, emphasizing the critical need for advanced pharmacological approaches to address these conditions. Current treatment options often focus on amplification devices, cochlear implants, or other rehabilitative therapies, leaving a substantial gap regarding effective pharmacological interventions. Advancements in our understanding of the molecular and cellular mechanisms involved in hearing disorders induced by noise, aging, and ototoxicity have opened new avenues for drug development, some of which have led to numerous clinical trials, with promising results. The development of optimal drug delivery solutions in animals and humans can also enhance the targeted delivery of medications to the ear. Moreover, large genome studies contributing to a genetic understanding of hearing loss in humans combined with advanced molecular technologies in animal studies have shown a great potential to increase our understanding of the etiologies of hearing loss. The auditory system exhibits circadian rhythms and temporal variations in its physiology, its vulnerability to auditory insults, and its responsiveness to drug treatments. The cochlear clock rhythms are under the control of the glucocorticoid system, and preclinical evidence suggests that the risk/benefit profile of hearing disorder treatments using chronopharmacological approaches would be beneficial. If translatable to the bedside, such approaches may improve the outcome of clinical trials. Ongoing research into the molecular and genetic basis of auditory disorders, coupled with advancements in drug formulation and delivery as well as optimized timing of drug administration, holds great promise of more effective treatments. SIGNIFICANCE STATEMENT: Hearing disorders pose significant challenges to individuals and their overall quality of life, emphasizing the critical need for advanced pharmacological approaches to address these conditions. Ongoing research into the molecular and genetic basis of auditory disorders, coupled with advancements in drug delivery procedures and optimized timing of drug administration, holds the promise of more effective treatments.

Gap detection ability declines with central auditory neurodegeneration following age-related cochlear synaptopathy

Age-related hearing impairment (ARHI) is commonly associated with decreased auditory temporal resolution caused by auditory neurodegeneration. Age-related deterioration in gap detection ability, resulting in poor temporal auditory processing, is often attributed to pathophysiological changes in both the peripheral and central auditory systems. This study aimed to investigate whether the gap detection ability declines in the early stages of ageing and to determine its usefulness in detecting peripheral and central auditory degeneration. The study used 1-month-old (1 M), 6-month-old (6 M) and 12-month-old (12 M) mice to examine changes in gap detection ability and associated auditory pathophysiology. Although hearing thresholds did not significantly differ between the groups, the amplitude of auditory brainstem response (ABR) wave I decreased significantly in an age-dependent manner, consistent with age-related cochlear synaptopathy. The relative ABR amplitude ratio of waves 2 and 5 to wave 1 was significantly increased in 12 M mice, indicating that the central auditory system had increased in relative neuroactivity. A significant increase in gap detection thresholds was observed in 12 M mice compared to 1 M mice. Although cochlear synaptopathy and central hyperactivity were positively correlated with gap detection thresholds, central hyperactivity strongly influenced gap detection ability. In the cochlear nucleus and auditory cortex, the inhibitory synaptic expression of GAD65 and the expression of parvalbumin were significantly decreased in 12 M mice, consistent with central hyperactivity. Evaluating gap detection performance may allow the identification of decreased auditory temporal resolution in the early stages of ARHI, which is strongly associated with auditory neurodegeneration.

Noise-induced cochlear synaptopathy in C57BL/6 N mice as a function of trauma strength: ribbons are more vulnerable than postsynapses

Noise-induced cochlear synaptopathy is characterized by irreversible loss of synapses between inner hair cells (IHCs) and spiral ganglion neurons (SGNs) despite normal hearing thresholds. We analyzed hearing performance and cochlear structure in C57BL/6 N mice exposed to 100, 106, or 112 dB SPL broadband noise (8-16 kHz) for 2 h. Auditory brainstem responses (ABRs) were assessed before, directly after, and up to 28 days post-trauma. Finally, the number, size, and pairing of IHC presynaptic (CtBP2-positive) ribbons and postsynaptic AMPA receptor scaffold (Homer1-positive) clusters were analyzed along the cochlea. Four weeks after the 100 dB SPL trauma, a permanent threshold shift (PTS) was observed at 45 kHz, which after the higher traumata extended toward middle to low frequencies. Loss in ABR wave I amplitudes scaled with trauma strength indicating loss of functional IHC synaptic connections. Latencies of wave I mostly increased with trauma strength. No trauma-related OHC loss was found. The number of synaptic pairs was reduced in the midbasal and basal cochlear region in all trauma conditions, with ribbon loss amounting up to 46% of control. Ribbons surviving the trauma were paired, whereas 4-6 unpaired postsynapses/IHC were found in the medial, midbasal, and basal regions irrespective of trauma strength, contrasting findings in CBA/CaJ mice. Our data confirm the susceptibility of ribbon synapses and ABR wave I amplitudes to a noise trauma of 100 dB SPL or larger. Notably, peripheral dendrites bearing IHC postsynapses were less vulnerable than presynaptic ribbons in C57BL/6 N mice.

Models optimized for real-world tasks reveal the task-dependent necessity of precise temporal coding in hearing

Neurons encode information in the timing of their spikes in addition to their firing rates. Spike timing is particularly precise in the auditory nerve, where action potentials phase lock to sound with sub-millisecond precision, but its behavioral relevance remains uncertain. We optimized machine learning models to perform real-world hearing tasks with simulated cochlear input, assessing the precision of auditory nerve spike timing needed to reproduce human behavior. Models with high-fidelity phase locking exhibited more human-like sound localization and speech perception than models without, consistent with an essential role in human hearing. However, the temporal precision needed to reproduce human-like behavior varied across tasks, as did the precision that benefited real-world task performance. These effects suggest that perceptual domains incorporate phase locking to different extents depending on the demands of real-world hearing. The results illustrate how optimizing models for realistic tasks can clarify the role of candidate neural codes in perception.

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.

The role of hidden hearing loss in tinnitus: Insights from early markers of peripheral hearing damage

Since the presence of tinnitus is not always associated with audiometric hearing loss, it has been hypothesized that hidden hearing loss may act as a potential trigger for increased central gain along the neural pathway leading to tinnitus perception. In recent years, the study of hidden hearing loss has improved with the discovery of cochlear synaptopathy and several objective diagnostic markers. This study investigated three potential markers of peripheral hidden hearing loss in subjects with tinnitus: extended high-frequency audiometric thresholds, the auditory brainstem response, and the envelope following response. In addition, speech intelligibility was measured as a functional outcome measurement of hidden hearing loss. To account for age-related hidden hearing loss, participants were grouped according to age, presence of tinnitus, and audiometric thresholds. Group comparisons were conducted to differentiate between age- and tinnitus-related effects of hidden hearing loss. All three markers revealed age-related differences, whereas no differences were observed between the tinnitus and non-tinnitus groups. However, the older tinnitus group showed improved performance on low-pass filtered speech in noise tests compared to the older non-tinnitus group. These low-pass speech in noise scores were significantly correlated with tinnitus distress, as indicated using questionnaires, and could be related to the presence of hyperacusis. Based on our observations, cochlear synaptopathy does not appear to be the underlying cause of tinnitus. The improvement in low-pass speech-in-noise could be explained by enhanced temporal fine structure encoding or hyperacusis. Therefore, we recommend that future tinnitus research takes into account age-related factors, explores low-frequency encoding, and thoroughly assesses hyperacusis.

Parameters influencing auditory fatigue among professionals working in the amplified music sector: noise exposure and individual factors

OBJECTIVE

Hearing disorders are common among music professionals, as they are frequently exposed to sound levels exceeding 100 dB(A). By assessing auditory fatigue, situations that are deleterious for hearing could be identified, allowing the deployment of preventive measures before permanent impairment occurs. However, little is known about the factors contributing to auditory fatigue. The objective is to determine the exposure parameters most influencing auditory fatigue during occupational exposure to amplified music.

DESIGN

Auditory fatigue was defined as variations of both pure tone auditory (ΔPTA) and efferent reflex thresholds (ΔER) during the workday. Noise exposure was monitored and information on the volunteers was gathered using a questionnaire.

STUDY SAMPLE

The population consisted of 43 adult volunteers exposed to amplified music (sound, light or stage technicians, security agents, barmen) and 24 unexposed administrative agents.

RESULTS

ΔPTA and ΔER were positively correlated with the energy of noise exposure and its stability over time, i.e a steady noise tends to create more auditory fatigue.

CONCLUSION

In addition to a global decrease of music levels and a systematic use of hearing protection, our results advocate for the provision of quiet periods within noise exposures as they reduce auditory fatigue accumulation and long-term risks for hearing.

Auditory brainstem response to paired clicks as a candidate marker of cochlear synaptopathy in humans

OBJECTIVE

This study aimed to evaluate whether auditory brainstem response (ABR) using a paired-click stimulation paradigm could serve as a tool for detecting cochlear synaptopathy (CS).

METHODS

The ABRs to single-clicks and paired-clicks with various inter-click intervals (ICIs) and scores for word intelligibility in degraded listening conditions were obtained from 57 adults with normal hearing. The wave I peak amplitude and root mean square values for the post-wave I response within a range delayed from the wave I peak (referred to as the RMSpost-w1) were calculated for the single- and second-click responses.

RESULTS

The wave I peak amplitudes did not correlate with age except for the second-click responses at an ICI of 7 ms, and the word intelligibility scores. However, we found that the RMSpost-w1 values for the second-click responses significantly decreased with increasing age. Moreover, the RMSpost-w1 values for the second-click responses at an ICI of 5 ms correlated significantly with the scores for word intelligibility in degraded listening conditions.

CONCLUSIONS

The magnitude of the post-wave I response for the second-click response could serve as a tool for detecting CS in humans.

SIGNIFICANCE

Our findings shed new light on the analytical methods of ABR for quantifying CS.

The curvature quantification of wave I in auditory brainstem responses detects cochlear synaptopathy in human beings

PURPOSE

Patients with age-related hearing loss complain often about reduced speech perception in adverse listening environment. Studies on animals have suggested that cochlear synaptopathy may be one of the primary mechanisms responsible for this phenomenon. A decreased wave I amplitude in supra-threshold auditory brainstem response (ABR) can diagnose this pathology non-invasively. However, the interpretation of the wave I amplitude in humans remains controversial. Recent studies in mice have established a robust and reliable mathematic algorithm, i.e., curve curvature quantification, for detecting cochlear synaptopathy. This study aimed to determine whether the curve curvature has sufficient test-retest reliability to detect cochlear synaptopathy in aging humans.

METHODS

Healthy participants were recruited into this prospective study. All subjects underwent an audiogram examination with standard and extended high frequencies ranging from 0.125 to 16 kHz and an ABR with a stimulus of 80 dB nHL click. The peak amplitude, peak latency, curvature at the peak, and the area under the curve of wave I were calculated and analyzed.

RESULTS

A total of 80 individuals with normal hearing, aged 18 to 61 years, participated in this study, with a mean age of 26.4 years. Pearson correlation analysis showed a significant negative correlation between curvature and age, as well as between curvature and extended high frequency (EHF) threshold (10-16 kHz). Additionally, the same correlation was observed between age and area as well as age and EHF threshold. The model comparison demonstrated that the curvature at the peak of wave I is the best metric to correlate with EHF threshold.

CONCLUSION

The curvature at the peak of wave I is the most sensitive metric for detecting cochlear synaptopathy in humans  and may be applied in routine diagnostics to detect early degenerations of the auditory nerve.

Hearing loss-related altered neuronal activity in the inferior colliculus

Hearing loss is well known to cause plastic changes in the central auditory system and pathological changes such as tinnitus and hyperacusis. Impairment of inner ear functions is the main cause of hearing loss. In aged individuals, not only inner ear dysfunction but also senescence of the central nervous system is the cause of malfunction of the auditory system. In most cases of hearing loss, the activity of the auditory nerve is reduced, but that of the successive auditory centers is increased in a compensatory way. It has been reported that activity changes occur in the inferior colliculus (IC), a critical nexus of the auditory pathway. The IC integrates the inputs from the brainstem and drives the higher auditory centers. Since abnormal activity in the IC is likely to affect auditory perception, it is crucial to elucidate the neuronal mechanism to induce the activity changes of IC neurons with hearing loss. This review outlines recent findings on hearing-loss-induced plastic changes in the IC and brainstem auditory neuronal circuits and discusses what neuronal mechanisms underlie hearing-loss-induced changes in the activity of IC neurons. Considering the different causes of hearing loss, we discuss age-related hearing loss separately from other forms of hearing loss (non-age-related hearing loss). In general, the main plastic change of IC neurons caused by both age-related and non-age-related hearing loss is increased central gain. However, plastic changes in the IC caused by age-related hearing loss seem to be more complex than those caused by non-age-related hearing loss.

Decreasing the physical gap in the neural-electrode interface and related concepts to improve cochlear implant performance

Cochlear implants (CI) represent incredible devices that restore hearing perception for those with moderate to profound sensorineural hearing loss. However, the ability of a CI to restore complex auditory function is limited by the number of perceptually independent spectral channels provided. A major contributor to this limitation is the physical gap between the CI electrodes and the target spiral ganglion neurons (SGNs). In order for CI electrodes to stimulate SGNs more precisely, and thus better approximate natural hearing, new methodologies need to be developed to decrease this gap, (i.e., transitioning CIs from a far-field to near-field device). In this review, strategies aimed at improving the neural-electrode interface are discussed in terms of the magnitude of impact they could have and the work needed to implement them. Ongoing research suggests current clinical efforts to limit the CI-related immune response holds great potential for improving device performance. This could eradicate the dense, fibrous capsule surrounding the electrode and enhance preservation of natural cochlear architecture, including SGNs. In the long term, however, optimized future devices will likely need to induce and guide the outgrowth of the peripheral process of SGNs to be in closer proximity to the CI electrode in order to better approximate natural hearing. This research is in its infancy; it remains to be seen which strategies (surface patterning, small molecule release, hydrogel coating, etc.) will be enable this approach. Additionally, these efforts aimed at optimizing CI function will likely translate to other neural prostheses, which face similar issues.

The Not-So-Slight Perceptual Consequences of Slight Hearing Loss in School-Age Children: A Scoping Review

PURPOSE

This study aimed to conduct a scoping review of research exploring the effects of slight hearing loss on auditory and speech perception in children.

METHOD

A comprehensive search conducted in August 2023 identified a total of 402 potential articles sourced from eight prominent bibliographic databases. These articles were subjected to rigorous evaluation for inclusion criteria, specifically focusing on their reporting of speech or auditory perception using psychoacoustic tasks. The selected studies exclusively examined school-age children, encompassing those between 5 and 18 years of age. Following rigorous evaluation, 10 articles meeting these criteria were selected for inclusion in the review.

RESULTS

The analysis of included articles consistently shows that even slight hearing loss in school-age children significantly affects their speech and auditory perception. Notably, most of the included articles highlighted a common trend, demonstrating that perceptual deficits originating due to slight hearing loss in children are particularly observable under challenging experimental conditions and/or in cognitively demanding listening tasks. Recent evidence further underscores that the negative impacts of slight hearing loss in school-age children cannot be solely predicted by their pure-tone thresholds alone. However, there is limited evidence concerning the effect of slight hearing loss on the segregation of competing speech, which may be a better representation of listening in the classroom.

CONCLUSION

This scoping review discusses the perceptual consequences of slight hearing loss in school-age children and provides insights into an array of methodological issues associated with studying perceptual skills in school-age children with slight hearing losses, offering guidance for future research endeavors.

Predictors of Speech-in-Noise Understanding in a Population of Occupationally Noise-Exposed Individuals

Understanding speech in noise is particularly difficult for individuals occupationally exposed to noise due to a mix of noise-induced auditory lesions and the energetic masking of speech signals. For years, the monitoring of conventional audiometric thresholds has been the usual method to check and preserve auditory function. Recently, suprathreshold deficits, notably, difficulties in understanding speech in noise, has pointed out the need for new monitoring tools. The present study aims to identify the most important variables that predict speech in noise understanding in order to suggest a new method of hearing status monitoring. Physiological (distortion products of otoacoustic emissions, electrocochleography) and behavioral (amplitude and frequency modulation detection thresholds, conventional and extended high-frequency audiometric thresholds) variables were collected in a population of individuals presenting a relatively homogeneous occupational noise exposure. Those variables were used as predictors in a statistical model (random forest) to predict the scores of three different speech-in-noise tests and a self-report of speech-in-noise ability. The extended high-frequency threshold appears to be the best predictor and therefore an interesting candidate for a new way of monitoring noise-exposed professionals.

Comprehensive Characterization of Hearing Loss and Tinnitus in Military-Affiliated and Non-Military-Affiliated Individuals

PURPOSE

Military-affiliated individuals (MIs) are at a higher risk of developing hearing loss and tinnitus. While these disorders are well-studied in MIs, their impact relative to non-military-affiliated individuals (non-MIs) remains understudied. Our study compared hearing, speech-in-noise (SIN) perception, and tinnitus characteristics between MIs and non-MIs.

METHOD

MIs (n = 84) and non-MIs (n = 193) underwent hearing threshold assessment and Quick Speech-in-Noise Test. Participants with tinnitus completed psychoacoustic tinnitus matching, numeric rating scale (NRS) for loudness and annoyance, and Tinnitus Functional Index. Comorbid conditions such as anxiety, depression, and hyperacusis were assessed. We used a linear mixed-effects model to compare hearing thresholds and SIN scores between MIs and non-MIs. A multivariate analysis of variance compared tinnitus characteristics between MIs and non-MIs, and a stepwise regression was performed to identify predictors of tinnitus severity.

RESULTS

MIs exhibited better hearing sensitivity than non-MIs; however, their SIN scores were similar. MIs matched their tinnitus loudness to a lower intensity than non-MIs, but their loudness ratings (NRS) were comparable. MIs reported greater tinnitus annoyance and severity on the relaxation subscale, indicating increased difficulty engaging in restful activities. Tinnitus severity was influenced by hyperacusis and depression in both MIs and non-MIs; however, hearing loss uniquely contributed to severity in MIs.

CONCLUSIONS

Our findings suggest that while MIs may exhibit better or comparable listening abilities, they were significantly more affected by tinnitus than non-MIs. Furthermore, our study highlights the importance of assessing tinnitus-related distress across multiple dimensions, facilitating customization of management strategies for both MIs and non-MIs.

Effects of Temporal Processing on Speech-in-Noise Perception in Middle-Aged Adults

Auditory temporal processing is a vital component of auditory stream segregation, or the process in which complex sounds are separated and organized into perceptually meaningful objects. Temporal processing can degrade prior to hearing loss, and is suggested to be a contributing factor to difficulties with speech-in-noise perception in normal-hearing listeners. The current study tested this hypothesis in middle-aged adults-an under-investigated cohort, despite being the age group where speech-in-noise difficulties are first reported. In 76 participants, three mechanisms of temporal processing were measured: peripheral auditory nerve function using electrocochleography, subcortical encoding of periodic speech cues (i.e., fundamental frequency; F0) using the frequency following response, and binaural sensitivity to temporal fine structure (TFS) using a dichotic frequency modulation detection task. Two measures of speech-in-noise perception were administered to explore how contributions of temporal processing may be mediated by different sensory demands present in the speech perception task. This study supported the hypothesis that temporal coding deficits contribute to speech-in-noise difficulties in middle-aged listeners. Poorer speech-in-noise perception was associated with weaker subcortical F0 encoding and binaural TFS sensitivity, but in different contexts, highlighting that diverse aspects of temporal processing are differentially utilized based on speech-in-noise task characteristics.

Ototoxicity: a high risk to auditory function that needs to be monitored in drug development

Hearing loss constitutes a major global health concern impacting approximately 1.5 billion people worldwide. Its incidence is undergoing a substantial surge with some projecting that by 2050, a quarter of the global population will experience varying degrees of hearing deficiency. Environmental factors such as aging, exposure to loud noise, and the intake of ototoxic medications are implicated in the onset of acquired hearing loss. Ototoxicity resulting in inner ear damage is a leading cause of acquired hearing loss worldwide. This could be minimized or avoided by early testing of hearing functions in the preclinical phase of drug development. While the assessment of ototoxicity is well defined for drug candidates in the hearing field - required for drugs that are administered by the otic route and expected to reach the middle or inner ear during clinical use - ototoxicity testing is not required for all other therapeutic areas. Unfortunately, this has resulted in more than 200 ototoxic marketed medications. The aim of this publication is to raise awareness of drug-induced ototoxicity and to formulate some recommendations based on available guidelines and own experience. Ototoxicity testing programs should be adapted to the type of therapy, its indication (targeting the ear or part of other medications classes being potentially ototoxic), and the number of assets to test. For multiple molecules and/or multiple doses, screening options are available: in vitro (otic cell assays), ex vivo (cochlear explant), and in vivo (in zebrafish). In assessing the ototoxicity of a candidate drug, it is good practice to compare its ototoxicity to that of a well-known control drug of a similar class. Screening assays provide a streamlined and rapid method to know whether a drug is generally safe for inner ear structures. Mammalian animal models provide a more detailed characterization of drug ototoxicity, with a possibility to localize and quantify the damage using functional, behavioral, and morphological read-outs. Complementary histological measures are routinely conducted notably to quantify hair cells loss with cochleogram. Ototoxicity studies can be performed in rodents (mice, rats), guinea pigs and large species. However, in undertaking, or at the very least attempting, all preclinical investigations within the same species, is crucial. This encompasses starting with pharmacokinetics and pharmacology efficacy studies and extending through to toxicity studies. In life read-outs include Auditory Brainstem Response (ABR) and Distortion Product OtoAcoustic Emissions (DPOAE) measurements that assess the activity and integrity of sensory cells and the auditory nerve, reflecting sensorineural hearing loss. Accurate, reproducible, and high throughput ABR measures are fundamental to the quality and success of these preclinical trials. As in humans, in vivo otoscopic evaluations are routinely carried out to observe the tympanic membrane and auditory canal. This is often done to detect signs of inflammation. The cochlea is a tonotopic structure. Hair cell responsiveness is position and frequency dependent, with hair cells located close to the cochlea apex transducing low frequencies and those at the base transducing high frequencies. The cochleogram aims to quantify hair cells all along the cochlea and consequently determine hair cell loss related to specific frequencies. This measure is then correlated with the ABR & DPOAE results. Ototoxicity assessments evaluate the impact of drug candidates on the auditory and vestibular systems, de-risk hearing loss and balance disorders, define a safe dose, and optimize therapeutic benefits. These types of studies can be initiated during early development of a therapeutic solution, with ABR and otoscopic evaluations. Depending on the mechanism of action of the compound, studies can include DPOAE and cochleogram. Later in the development, a GLP (Good Laboratory Practice) ototoxicity study may be required based on otic related route of administration, target, or known potential otic toxicity.

Electrocochleographic frequency-following responses as a potential marker of age-related cochlear neural degeneration

Auditory nerve (AN) fibers that innervate inner hair cells in the cochlea degenerate with advancing age. It has been proposed that age-related reductions in brainstem frequency-following responses (FFR) to the carrier of low-frequency, high-intensity pure tones may partially reflect this neural loss in the cochlea (Märcher-Rørsted et al., 2022). If the loss of AN fibers is the primary factor contributing to age-related changes in the brainstem FFR, then the FFR could serve as an indicator of cochlear neural degeneration. In this study, we employed electrocochleography (ECochG) to investigate the effects of age on frequency-following neurophonic potentials, i.e., neural responses phase-locked to the carrier frequency of the tone stimulus. We compared these findings to the brainstem-generated FFRs obtained simultaneously using the same stimulation. We conducted recordings in young and older individuals with normal hearing. Responses to pure tones (250 ms, 516 and 1086 Hz, 85 dB SPL) and clicks were recorded using both ECochG at the tympanic membrane and traditional scalp electroencephalographic (EEG) recordings of the FFR. Distortion product otoacoustic emissions (DPOAE) were also collected. In the ECochG recordings, sustained AN neurophonic (ANN) responses to tonal stimulation, as well as the click-evoked compound action potential (CAP) of the AN, were significantly reduced in the older listeners compared to young controls, despite normal audiometric thresholds. In the EEG recordings, brainstem FFRs to the same tone stimulation were also diminished in the older participants. Unlike the reduced AN CAP response, the transient-evoked wave-V remained unaffected. These findings could indicate that a decreased number of AN fibers contributes to the response in the older participants. The results suggest that the scalp-recorded FFR, as opposed to the clinical standard wave-V of the auditory brainstem response, may serve as a more reliable indicator of age-related cochlear neural degeneration.

A novel model of sensorineural hearing loss induced by repeated exposure to moderate noise in mice: the preventive effect of resveratrol

Sensorineural hearing loss (SNHL) induced by noise has increased in recent years due to personal headphone use and noisy urban environments. The study shows a novel model of gradually progressive SNHL induced by repeated exposure to moderate noise (8-kHz octave band noise, 90-dB sound pressure level) for 1 hr exposure per day in BALB/cCr mice. The results showed that the repeated exposure led to gradually progressive SNHL, which was dependent on the number of exposures, and resulted in permanent hearing loss after 5 exposures. Repeated exposure to noise causes a loss of synapses between the inner hair cells and the peripheral terminals of the auditory nerve fibers. Additionally, there is a reduction in the expression levels of c-fos and Arc, both of which are indicators of cochlear nerve responses to noise exposure. Oral administration of resveratrol (RSV, 50 mg/kg/day) during the noise exposure period significantly prevented the noise exposure-induced synapse loss and SNHL. Furthermore, the study found that RSV treatment prevented the noise-induced increase in the gene expression levels of the proinflammatory cytokine interleukin-1β in the cochlea. These results demonstrated the potential usefulness of RSV in preventing noise-induced SNHL in the animal model established as gradually progressive SNHL.

How 'hidden hearing loss' noise exposure affects neural coding in the inferior colliculus of rats

Exposure to brief, intense sound can produce profound changes in the auditory system, from the internal structure of inner hair cells to reduced synaptic connections between the auditory nerves and the inner hair cells. Moreover, noisy environments can also lead to alterations in the auditory nerve or to processing changes in the auditory midbrain, all without affecting hearing thresholds. This so-called hidden hearing loss (HHL) has been shown in tinnitus patients and has been posited to account for hearing difficulties in noisy environments. However, much of the neuronal research thus far has investigated how HHL affects the response characteristics of individual fibres in the auditory nerve, as opposed to higher stations in the auditory pathway. Human models show that the auditory nerve encodes sound stochastically. Therefore, a sufficient reduction in nerve fibres could result in lowering the sampling of the acoustic scene below the minimum rate necessary to fully encode the scene, thus reducing the efficacy of sound encoding. Here, we examine how HHL affects the responses to frequency and intensity of neurons in the inferior colliculus of rats, and the duration and firing rate of those responses. Finally, we examined how shorter stimuli are encoded less effectively by the auditory midbrain than longer stimuli, and how this could lead to a clinical test for HHL.

Hidden hearing loss: Fifteen years at a glance

Hearing loss affects approximately 18% of the population worldwide. Hearing difficulties in noisy environments without accompanying audiometric threshold shifts likely affect an even larger percentage of the global population. One of the potential causes of hidden hearing loss is cochlear synaptopathy, the loss of synapses between inner hair cells (IHC) and auditory nerve fibers (ANF). These synapses are the most vulnerable structures in the cochlea to noise exposure or aging. The loss of synapses causes auditory deafferentation, i.e., the loss of auditory afferent information, whose downstream effect is the loss of information that is sent to higher-order auditory processing stages. Understanding the physiological and perceptual effects of this early auditory deafferentation might inform interventions to prevent later, more severe hearing loss. In the past decade, a large body of work has been devoted to better understand hidden hearing loss, including the causes of hidden hearing loss, their corresponding impact on the auditory pathway, and the use of auditory physiological measures for clinical diagnosis of auditory deafferentation. This review synthesizes the findings from studies in humans and animals to answer some of the key questions in the field, and it points to gaps in knowledge that warrant more investigation. Specifically, recent studies suggest that some electrophysiological measures have the potential to function as indicators of hidden hearing loss in humans, but more research is needed for these measures to be included as part of a clinical test battery.

Cochlear Ribbon Synapses in Aged Gerbils

In mammalian hearing, type-I afferent auditory nerve fibers comprise the basis of the afferent auditory pathway. They are connected to inner hair cells of the cochlea via specialized ribbon synapses. Auditory nerve fibers of different physiological types differ subtly in their synaptic location and morphology. Low-spontaneous-rate auditory nerve fibers typically connect on the modiolar side of the inner hair cell, while high-spontaneous-rate fibers are typically found on the pillar side. In aging and noise-damaged ears, this fine-tuned balance between auditory nerve fiber populations can be disrupted and the functional consequences are currently unclear. Here, using immunofluorescent labeling of presynaptic ribbons and postsynaptic glutamate receptor patches, we investigated changes in synaptic morphology at three different tonotopic locations along the cochlea of aging gerbils compared to those of young adults. Quiet-aged gerbils showed about 20% loss of afferent ribbon synapses. While the loss was random at apical, low-frequency cochlear locations, at the basal, high-frequency location it almost exclusively affected the modiolar-located synapses. The subtle differences in volumes of pre- and postsynaptic elements located on the inner hair cell's modiolar versus pillar side were unaffected by age. This is consistent with known physiology and suggests a predominant, age-related loss in the low-spontaneous-rate auditory nerve population in the cochlear base, but not the apex.

Spatial patterns of noise-induced inner hair cell ribbon loss in the mouse mid-cochlea

In the mammalian cochlea, moderate acoustic overexposure leads to loss of ribbon-type synapse between the inner hair cell (IHC) and its postsynaptic spiral ganglion neuron (SGN), causing a reduced dynamic range of hearing but not a permanent threshold elevation. A prevailing view is that such ribbon loss (known as synaptopathy) selectively impacts the low-spontaneous-rate and high-threshold SGN fibers contacting predominantly the modiolar IHC face. However, the spatial pattern of synaptopathy remains scarcely characterized in the most sensitive mid-cochlear region, where two morphological subtypes of IHC with distinct ribbon size gradients coexist. Here, we used volume electron microscopy to investigate noise exposure-related changes in the mouse IHCs with and without ribbon loss. Our quantifications reveal that IHC subtypes differ in the worst-hit area of synaptopathy. Moreover, we show relative enrichment of mitochondria in the surviving SGN terminals, providing key experimental evidence for the long-proposed role of SGN-terminal mitochondria in synaptic vulnerability.

The effects of acute and chronic noise trauma on stimulus-evoked activity across primary auditory cortex layers

Exposure to intense noise environments is a major cause of sensorineural hearing loss and auditory perception disorders, such as tinnitus and hyperacusis, which may have a central origin. The effects of noise-induced hearing loss on the auditory cortex have been documented in many studies. One limitation of these studies, however, is that the effects of noise trauma have been mostly studied at the granular layer (i.e, the main cortical recipient of thalamic input), while the cortex is a very complex structure, with six different layers each having its own pattern of connectivity and role in sensory processing. The present study aims to investigate the effects of acute and chronic noise trauma on the laminar pattern of stimulus-evoked activity in the primary auditory cortex of the anesthetized guinea pig. We show that acute and chronic noise trauma are both followed by an increase in stimulus-evoked cortical responses, mostly in the granular and supragranular layers. The cortical responses are more monotonic as a function of the intensity level after noise trauma. There was minimal change, if any, in local field potential (LFP) amplitude after acute noise trauma, while LFP amplitude was enhanced after chronic noise trauma. Finally, LFP and the current source density analysis suggest that acute but more specifically chronic noise trauma is associated with the emergence of a new sink in the supragranular layer. This result suggests that supragranular layers become a major input recipient. We discuss the possible mechanisms and functional implications of these changes.NEW & NOTEWORTHY Our study shows that cortical activity is enhanced after trauma and that the sequence of cortical column activation during stimulus-evoked response is altered, i.e. the supragranular layer becomes a major input recipient. We speculate that these large cortical changes may play a key role in the auditory hypersensitivity (hyperacusis) that can be triggered after noise trauma in human subjects.

Mechanisms of age-related hearing loss at the auditory nerve central synapses and postsynaptic neurons in the cochlear nucleus

Sound information is transduced from mechanical vibration to electrical signals in the cochlea, conveyed to and further processed in the brain to form auditory perception. During the process, spiral ganglion neurons (SGNs) are the key cells that connect the peripheral and central auditory systems by receiving information from hair cells in the cochlea and transmitting it to neurons of the cochlear nucleus (CN). Decades of research in the cochlea greatly improved our understanding of SGN function under normal and pathological conditions, especially about the roles of different subtypes of SGNs and their peripheral synapses. However, it remains less clear how SGN central terminals or auditory nerve (AN) synapses connect to CN neurons, and ultimately how peripheral pathology links to structural alterations and functional deficits in the central auditory nervous system. This review discusses recent progress about the morphological and physiological properties of different subtypes of AN synapses and associated postsynaptic CN neurons, their changes during aging, and the potential mechanisms underlying age-related hearing loss.

Oxidative Stress Plays an Important Role in Glutamatergic Excitotoxicity-Induced Cochlear Synaptopathy: Implication for Therapeutic Molecules Screening

The disruption of the synaptic connection between the sensory inner hair cells (IHCs) and the auditory nerve fiber terminals of the type I spiral ganglion neurons (SGN) has been observed early in several auditory pathologies (e.g., noise-induced or ototoxic drug-induced or age-related hearing loss). It has been suggested that glutamate excitotoxicity may be an inciting element in the degenerative cascade observed in these pathological cochlear conditions. Moreover, oxidative damage induced by free hydroxyl radicals and nitric oxide may dramatically enhance cochlear damage induced by glutamate excitotoxicity. To investigate the underlying molecular mechanisms involved in cochlear excitotoxicity, we examined the molecular basis responsible for kainic acid (KA, a full agonist of AMPA/KA-preferring glutamate receptors)-induced IHC synapse loss and degeneration of the terminals of the type I spiral ganglion afferent neurons using a cochlear explant culture from P3 mouse pups. Our results demonstrated that disruption of the synaptic connection between IHCs and SGNs induced increased levels of oxidative stress, as well as altered both mitochondrial function and neurotrophin signaling pathways. Additionally, the application of exogenous antioxidants and neurotrophins (NT3, BDNF, and small molecule TrkB agonists) clearly increases synaptogenesis. These results suggest that understanding the molecular pathways involved in cochlear excitotoxicity is of crucial importance for the future clinical trials of drug interventions for auditory synaptopathies.

Targeted therapeutic hypothermia protects against noise induced hearing loss

Introduction

Exposure to occupational or recreational loud noise activates multiple biological regulatory circuits and damages the cochlea, causing permanent changes in hearing sensitivity. Currently, no effective clinical therapy is available for the treatment or mitigation of noise-induced hearing loss (NIHL). Here, we describe an application of localized and non-invasive therapeutic hypothermia and targeted temperature management of the inner ear to prevent NIHL.

Methods

We developed a custom-designed cooling neck collar to reduce the temperature of the inner ear by 3-4°C post-injury to deliver mild therapeutic hypothermia.

Results

This localized and non-invasive therapeutic hypothermia successfully mitigated NIHL in rats. Our results show that mild hypothermia can be applied quickly and safely to the inner ear following noise exposure. We show that localized hypothermia after NIHL preserves residual hearing and rescues noise-induced synaptopathy over a period of months.

Discussion

This study establishes a minimally-invasive therapeutic paradigm with a high potential for rapid translation to the clinic for long-term preservation of hearing health.

Normal behavioral discrimination of envelope statistics in budgerigars with kainate-induced cochlear synaptopathy

Cochlear synaptopathy is a common pathology in humans associated with aging and potentially sound overexposure. Synaptopathy is widely expected to cause "hidden hearing loss," including difficulty perceiving speech in noise, but support for this hypothesis is controversial. Here in budgerigars (Melopsittacus undulatus), we evaluated the impact of long-term cochlear synaptopathy on behavioral discrimination of Gaussian noise (GN) and low-noise noise (LNN) signals processed to have a flatter envelope. Stimuli had center frequencies of 1-3kHz, 100-Hz bandwidth, and were presented at sensation levels (SLs) from 10 to 30dB. We reasoned that narrowband, low-SL stimuli of this type should minimize spread of excitation across auditory-nerve fibers, and hence might reveal synaptopathy-related defects if they exist. Cochlear synaptopathy was induced without hair-cell injury using kainic acid (KA). Behavioral threshold tracking experiments characterized the minimum stimulus duration above which animals could reliably discriminate between LNN and GN. Budgerigar thresholds for LNN-GN discrimination ranged from 40 to 60ms at 30dB SL, were similar across frequencies, and increased for lower SLs. Notably, animals with long-term 39-77% estimated synaptopathy performed similarly to controls, requiring on average a ∼7.5% shorter stimulus duration (-0.7±1.0dB; mean difference ±SE) for LNN-GN discrimination. Decision-variable correlation analyses of detailed behavioral response patterns showed that individual animals relied on envelope cues to discriminate LNN and GN, with lesser roles of FM and energy cues; no difference was found between KA-exposed and control groups. These results suggest that long-term cochlear synaptopathy does not impair discrimination of low-level signals with different envelope statistics.