Acoustic stimulation of the human round window by laser-induced nonlinear optoacoustics

The feasibility of low frequency pure tone generation in the inner ear by laser-induced nonlinear optoacoustic effect at the round window was demonstrated in three human cadaveric temporal bones (TB) using an integral pulse density modulation (IPDM). Nanosecond laser pulses with a wavelength in the near-infrared (NIR) region were delivered to the round window niche by an optical fiber with two spherical lenses glued to the end and a viscous gel at the site of the laser focus. Using IPDM, acoustic tones with frequencies between 20 Hz and 1 kHz were generated in the inner ear. The sound pressures in scala tympani and vestibuli were recorded and the intracochlear pressure difference (ICPD) was used to calculate the equivalent sound pressure level (eq. dB SPL) as an equivalent for perceived loudness. The results demonstrate that the optoacoustic effect produced sound pressure levels ranging from 140 eq. dB SPL at low frequencies ≤ 200 Hz to 90 eq. dB SPL at 1 kHz. Therefore, the produced sound pressure level is potentially sufficient for patients requiring acoustic low frequency stimulation. Hence, the presented method offers a potentially viable solution in the future to provide the acoustic stimulus component in combined electro-acoustic stimulation with a cochlear implant.

Swept Along: Measuring Otoacoustic Emissions Using Continuously Varying Stimuli

At the 2004 Midwinter Meeting of the Association for Research in Otolaryngology, Glenis Long and her colleagues introduced a method for measuring distortion-product otoacoustic emissions (DPOAEs) using primary-tone stimuli whose instantaneous frequencies vary continuously with time. In contrast to standard OAE measurement methods, in which emissions are measured in the sinusoidal steady state using discrete tones of well-defined frequency, the swept-tone method sweeps across frequency, often at rates exceeding 1 oct/s. The resulting response waveforms are then analyzed using an appropriate filter (e.g., by least-squares fitting). Although introduced as a convenient way of studying DPOAE fine structure by separating the total OAE into distortion and reflection components, the swept-tone method has since been extended to stimulus-frequency emissions and has proved an efficient and valuable tool for probing cochlear mechanics. One day-a long time coming-swept tones may even find their way into the audiology clinic.

Otoacoustic emissions in African mole-rats

African mole-rats display highly derived hearing that is characterized by low sensitivity and a narrow auditory range restricted to low frequencies < 10 kHz. Recently, it has been suggested that two species of these rodents do not exhibit distortion product otoacoustic emissions (DPOAE), which was interpreted as evidence for a lack of cochlear amplification. If true, this would make them unique among mammals. However, both theoretical considerations on the generation of DPOAE as well as previously published experimental evidence challenge this assumption. We measured DPOAE and stimulus-frequency otoacoustic emissions (SFOAE) in three species of African mole-rats (Ansell's mole-rat - Fukomys anselli; Mashona mole-rat - Fukomys darlingi; naked mole-rat - Heterocephalus glaber) and found unexceptional otoacoustic emission values. Measurements were complicated by the remarkably long, narrow and curved external ear canals of these animals, for which we provide a morphological description. Both DPOAE and SFOAE displayed the highest amplitudes near 1 kHz, which corresponds to the region of best hearing in all tested species, as well as to the frequency region of the low-frequency acoustic fovea previously described in Ansell's mole-rat. Thus, the cochlea in African mole-rats shares the ability to generate evoked otoacoustic emission with other mammals.

Signaling pathways regulating the immune function of cochlear supporting cells and their involvement in cochlear pathophysiology

The inner ear, including the cochlea, used to be regarded as an immune-privileged site because of its immunologically isolated environment caused by the blood-labyrinthine barrier. Cochlear resident macrophages, which originate from the yolk sac or fetal liver during the embryonic stage and are maintained after birth, are distributed throughout various regions of the cochlear duct. Intriguingly, these cells are absent in the organ of Corti, where hair cells (HCs) and supporting cells (SCs) are located, except for a limited number of ionized calcium-binding adapter molecule 1 (Iba1)-positive cells. Instead, SCs exert glial functions varying from a quiescent to an emergency state. Notably, SCs acquire the nature of macrophages and begin to secrete inflammatory cytokines during viral infection in the organ of Corti, which is ostensibly unprotected owing to the lack of general resident macrophages. This review provides an overview of both positive and negative functions of SCs enabled to acquire macrophage phenotypes upon viral infection focusing on the signaling pathways that regulate these functions. The former function protects HCs from viral infection by inducting type I interferons, and the latter function induces HC death by necroptosis, leading to sensorineural hearing loss. Thus, SCs play contradictory roles as immune cells with acquired macrophage phenotypes; thereby, they are favorable and unfavorable to HCs, which play a pivotal role in hearing function.

Connexins 30 and 43 expression changes in relation to age-related hearing loss

Age-related hearing loss (ARHL), also known as presbycusis, is the number one communication disorder for aging adults. Connexin proteins are essential for intercellular communication throughout the human body, including the cochlea. Mutations in connexin genes have been linked to human syndromic and nonsyndromic deafness; thus, we hypothesize that changes in connexin gene and protein expression with age are involved in the etiology of ARHL. Here, connexin gene and protein expression changes for CBA/CaJ mice at different ages were examined, and correlations were analyzed between the changes in expression levels and functional hearing measures, such as ABRs and DPOAEs. Moreover, we investigated potential treatment options for ARHL. Results showed significant downregulation of Cx30 and Cx43 gene expression and significant correlations between the degree of hearing loss and the changes in gene expression for both genes. Moreover, dose-dependent treatments utilizing cochlear cell lines showed that aldosterone hormone therapy significantly increased Cx expression. In vivo mouse treatments with aldosterone also showed protective effects on connexin expression in aging mice. Based on these functionally relevant findings, next steps can include more investigations of the mechanisms related to connexin family gap junction protein expression changes during ARHL; and expand knowledge of clinically-relevant treatment options by knowing what specific members of the Cx family and related inter-cellular proteins should be targeted therapeutically.

Frequency selectivity in monkey auditory nerve studied with suprathreshold multicomponent stimuli

Data from non-human primates can help extend observations from non-primate species to humans. Here we report measurements on the auditory nerve of macaque monkeys in the context of a controversial topic important to human hearing. A range of techniques have been used to examine the claim, which is not generally accepted, that human frequency tuning is sharper than traditionally thought, and sharper than in commonly used animal models. Data from single auditory-nerve fibers occupy a pivotal position to examine this claim, but are not available for humans. A previous study reported sharper tuning in auditory-nerve fibers of macaque relative to the cat. A limitation of these and other single-fiber data is that frequency selectivity was measured with tonal threshold-tuning curves, which do not directly assess spectral filtering and whose shape is sharpened by cochlear nonlinearity. Our aim was to measure spectral filtering with wideband suprathreshold stimuli in the macaque auditory nerve. We obtained responses of single nerve fibers of anesthetized macaque monkeys and cats to a suprathreshold, wideband, multicomponent stimulus designed to allow characterization of spectral filtering at any cochlear locus. Quantitatively the differences between the two species are smaller than in previous studies, but consistent with these studies the filters obtained show a trend of sharper tuning in macaque, relative to the cat, for fibers in the basal half of the cochlea. We also examined differences in group delay measured on the phase data near the characteristic frequency versus in the low-frequency tail. The phase data are consistent with the interpretation of sharper frequency tuning in monkey in the basal half of the cochlea. We conclude that use of suprathreshold, wide-band stimuli supports the interpretation of sharper frequency selectivity in macaque nerve fibers relative to the cat, although the difference is less marked than apparent from the assessment with tonal threshold-based data.

Deep Learning Models for Predicting Hearing Thresholds Based on Swept-Tone Stimulus-Frequency Otoacoustic Emissions

OBJECTIVES

This study aims to develop deep learning (DL) models for the quantitative prediction of hearing thresholds based on stimulus-frequency otoacoustic emissions (SFOAEs) evoked by swept tones.

DESIGN

A total of 174 ears with normal hearing and 388 ears with sensorineural hearing loss were studied. SFOAEs in the 0.3 to 4.3 kHz frequency range were recorded using linearly swept tones at a rate of 2 Hz/msec, with stimulus level changing from 40 to 60 dB SPL in 10 dB steps. Four DL models were used to predict hearing thresholds at octave frequencies from 0.5 to 4 kHz. The models-a conventional convolutional neural network (CNN), a hybrid CNN-k-nearest neighbor (KNN), a hybrid CNN-support vector machine (SVM), and a hybrid CNN-random forest (RF)-were individually built for each frequency. The input to the DL models was the measured raw SFOAE amplitude spectra and their corresponding signal to noise ratio spectra. All DL models shared a CNN-based feature self-extractor. They differed in that the conventional CNN utilized a fully connected layer to make the final regression decision, whereas the hybrid CNN-KNN, CNN-SVM, and CNN-RF models were designed by replacing the last fully connected layer of CNN model with a traditional machine learning (ML) regressor, that is, KNN, SVM, and RF, respectively. The model performance was evaluated using mean absolute error and SE averaged over 20 repetitions of 5 × 5 fold nested cross-validation. The performance of the proposed DL models was compared with two types of traditional ML models.

RESULTS

The proposed SFOAE-based DL models resulted in an optimal mean absolute error of 5.98, 5.22, 5.51, and 6.06 dB at 0.5, 1, 2, and 4 kHz, respectively, superior to that obtained by the traditional ML models. The produced SEs were 8.55, 7.27, 7.58, and 7.95 dB at 0.5, 1, 2, and 4 kHz, respectively. All the DL models outperformed any of the traditional ML models.

CONCLUSIONS

The proposed swept-tone SFOAE-based DL models were capable of quantitatively predicting hearing thresholds with satisfactory performance. With DL techniques, the underlying relationship between SFOAEs and hearing thresholds at disparate frequencies was explored and captured, potentially improving the diagnostic value of SFOAEs.

Neural Fluctuation Contrast as a Code for Complex Sounds: The Role and Control of Peripheral Nonlinearities

The nonlinearities of the inner ear are often considered to be obstacles that the central nervous system has to overcome to decode neural responses to sounds. This review describes how peripheral nonlinearities, such as saturation of the inner-hair-cell response and of the IHC-auditory-nerve synapse, are instead beneficial to the neural encoding of complex sounds such as speech. These nonlinearities set up contrast in the depth of neural-fluctuations in auditory-nerve responses along the tonotopic axis, referred to here as neural fluctuation contrast (NFC). Physiological support for the NFC coding hypothesis is reviewed, and predictions of several psychophysical phenomena, including masked detection and speech intelligibility, are presented. Lastly, a framework based on the NFC code for understanding how the medial olivocochlear (MOC) efferent system contributes to the coding of complex sounds is presented. By modulating cochlear gain control in response to both sound energy and fluctuations in neural responses, the MOC system is hypothesized to function not as a simple feedback gain-control device, but rather as a mechanism for enhancing NFC along the tonotopic axis, enabling robust encoding of complex sounds across a wide range of sound levels and in the presence of background noise. Effects of sensorineural hearing loss on the NFC code and on the MOC feedback system are presented and discussed.

Tripolar configuration and pulse shape in cochlear implants reduce channel interactions in the temporal domain

The present study investigates effects of current focusing and pulse shape on threshold, dynamic range, spread of excitation and channel interaction in the time domain using cochlear implant stimulation. The study was performed on 20 adult guinea pigs using a 6-channel animal cochlear implant, recording was performed in the auditory midbrain using a multielectrode array. After determining the best frequencies for individual recording contacts with acoustic stimulation, the ear was deafened and a cochlear implant was inserted into the cochlea. The position of the implant was controlled by x-ray. Stimulation with biphasic, pseudomonophasic and monophasic stimuli was performed with monopolar, monopolar with common ground, bipolar and tripolar configuration in two sets of experiments, allowing comparison of the effects of the different stimulation strategies on threshold, dynamic range, spread of excitation and channel interaction. Channel interaction was studied in the temporal domain, where two electrodes were activated with pulse trains and phase locking to these pulse trains in the midbrain was quantified. The results documented multifactorial influences on the response properties, with significant interaction between factors. Thresholds increased with increasing current focusing, but decreased with pseudomonophasic and monophasic pulse shapes. The results documented that current focusing, particularly tripolar configuration, effectively reduces channel interaction, but that also pseudomonophasic and monophasic stimulation and phase duration intensity coding reduce channel interactions.

Drug-induced hearing loss: Listening to the latest advances

Sensorineural hearing loss (SNHL) is the most common type of hearing loss. Causes include degenerative changes in the sensory hair cells, their synapses and/or the cochlear nerve. As human inner ear hair cells have no capacity for regeneration, their destruction is irreversible and leads to permanent hearing loss. SNHL can be genetically inherited or acquired through ageing, exposure to noise or ototoxic drugs. Ototoxicity generally refers to damage to the structures and functions of the inner ear following exposure to specific drugs. Ototoxicity can be multifactorial, causing damage to cochlear hair cells or cells with homeostatic functions that modulate cochlear hair cell function. Clinical strategies to limit ototoxicity include identifying patients at risk, monitoring drug concentrations, performing serial hearing assessments and switching to less ototoxic therapy. This review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, using the PubMed® database. The search terms "ototoxicity", "hearing loss" and "drugs" were combined. We included studies published between September 2013 and June 2023, and focused on medicines and drugs used in hospitals. The review highlighted a number of articles reporting the main drug classes potentially involved: namely, immunosuppressants, antimalarials, vaccines, antibiotics, antineoplastic agents, diuretics, nonsteroidal anti-inflammatory drugs and analgesics. The presumed ototoxic mechanisms were described, together with the therapeutic and preventive options developed over the last ten years.

Neuromorphic Engineering in Wetware: Discriminating Acoustic Frequencies through Their Effects on Chemical Waves

Some features of the human nervous system can be mimicked not only through software or hardware but also through liquid solutions of chemical systems maintained under out-of-equilibrium conditions. We describe the possibility of exploiting a thin layer of the Belousov-Zhabotinsky (BZ) reaction as a surrogate for the cochlea for sensing acoustic frequencies. Experiments and simulations demonstrate that, as in the human ear where the cochlea transduces the mechanical energy of the acoustic frequencies into the electrochemical energy of neural action potentials and the basilar membrane originates topographic representations of sounds, our bioinspired chemoacoustic system, based on the BZ reaction, gives rise to spatiotemporal patterns as the representation of distinct acoustic bands through transduction of mechanical energy into chemical energy. Acoustic frequencies in the range 10-2000 Hz are partitioned into seven distinct bands based on three attributes of the emerging spatiotemporal patterns: (1) the types and frequencies of the chemical waves, (2) their velocities, and (3) the Faraday waves' wavelengths.

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

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

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.

Optogenetics Reveals Roles for Supporting Cells in Force Transmission to and From Outer Hair Cells in the Mouse Cochlea

Outer hair cells (OHCs) of the organ of Corti (OoC), acting as bidirectional cellular mechanoelectrical transducers, generate, receive, and exchange forces with other major elements of the cochlear partition, including the sensory inner hair cells (IHCs). Force exchange is mediated via a supporting cell scaffold, including Deiters' (DC) and outer pillar cells (OPC), to enable the sensitivity and exquisite frequency selectivity of the mammalian cochlea and to transmit its responses to the auditory nerve. To selectively activate DCs and OPCs in male and female mice, we conditionally expressed in them a hyperpolarizing halorhodopsin (HOP), a light-gated inward chloride ion pump, and measured extracellular receptor potentials (ERPs) and their DC component (ERPDCs) from the cortilymph, which fills the OoC fluid spaces, and compared the responses with similar potentials from HOP-/- littermates. The compound action potentials (CAP) of the auditory nerve were measured as an indication of IHC activity and transmission of cochlear responses to the CNS. HOP light-activated hyperpolarization of DCs and OPCs suppressed cochlear amplification through changing the timing of its feedback, altered basilar membrane (BM) responses to tones at all measured levels and frequencies, and reduced IHC excitation. HOP activation findings reported here complement recent studies that revealed channelrhodopsin activation depolarized DCs and OPCs and effectively bypassed, rather than blocked, the control of OHC mechanical and electrical responses to sound and their contribution to timed and directed electromechanical feedback to the mammalian cochlea. Moreover, our findings identify DCs and OPCs as potential targets for the treatment of noise-induced hearing loss.

A Clinically Viable Medial Olivocochlear Reflex Assay Using Transient-Evoked Otoacoustic Emissions

OBJECTIVES

The contralateral medial olivocochlear reflex (MOCR) strength may indicate various auditory conditions in humans, but a clinically viable assay and equipment are needed for quick, accurate, and reliable measurements. The first experiment compared an earlier version of the assay, which used a nonlinear-mode chirp stimulus, with a new assay using a linear-mode click stimulus, designed to give reliable MOCR measurements in most normal-hearing ears. The second experiment extended the improved assay on a purpose-built binaural hardware platform that used forward-pressure level (FPL) calibration for both the stimulus and the contralateral MOCR elicitor.

DESIGN

Transient-evoked otoacoustic emission (TEOAE) tests were measured with and without a 60-dB SPL MOCR-evoking contralateral broadband noise. The normalized MOCR strength (MOCR%) was derived from the TEOAE responses for each trial pair using the complex pressure difference weighted by the TEOAE magnitude. Experiment 1 compared MOCR% within-subject and across-day using two TEOAE stimuli: nonlinear-mode chirps (50 dB SPL, bandpass 1-5 kHz, 14 ms window delayed by 2 ms) and linear-mode clicks (50 dB SPL, bandpass 0.5-2.5 kHz, 13 ms window delayed by 5 ms). TEOAE responses were analyzed in the 0.5 to 2.5 kHz band. Thirty adult participants with normal hearing (30 ears) completed the study. The TEOAE stimulus was calibrated in situ using spectral flattening, and the contralateral noise was calibrated in a coupler. Twelve TEOAE trial pairs were collected for each participant and condition. Experiment 2 used a purpose-built binaural system. The TEOAE stimuli were linear-mode clicks (50 dB SPL, bandpass 1-3 kHz, 13 ms window delayed by 5 ms), analyzed in the 1 to 3 kHz band over ~12 trial pairs. After a probe refit, an additional trial pair was collected for the two early-stopping signal-to-noise ratio criteria (15 and 20 dB). They were evaluated for single-trial reliability and test time. Nineteen adult participants with normal hearing (38 ears) completed the study. The TEOAE clicks and contralateral elicitor noise were calibrated in situ using FPL and delivered with automated timing.

RESULTS

MOCR% for linear-mode clicks was distinguishable from measurement variability in 98% to 100% of participants' ears (both experiments), compared with only 73% for the nonlinear-mode chirp (experiment 1). MOCR detectability was assessed using the MOCR% across-subject/within-subject variance ratio. The ratio in experiment 1 for linear-mode clicks was higher (8.0) than for nonlinear-mode chirps (6.4). The ratio for linear-mode clicks (8.9) in experiment 2 was slightly higher than for the comparable linear-mode stimulus (8.0) in experiment 1. TEOAEs showed excellent reliability with high signal-to-noise ratios in both experiments, but reliability was higher for linear-mode clicks than nonlinear-mode chirps. MOCR reliability for the two stimuli was comparable. The FPL pressure response retest reliability derived from the SPL at the microphone was higher than the SPL retest reliability across 0.4 to 8 kHz. Stable results required 2 to 3 trial pairs for the linear-mode click (experiments 1 and 2) and three for the nonlinear-mode chirp (experiment 1), taking around 2 min on average.

CONCLUSIONS

The linear-mode click assay produced measurable, reliable, and stable TEOAE and MOCR results on both hardware platforms in around 2 min per ear. The stimulus design and response window ensured that any stimulus artifact in linear mode was unlikely to confound the results. The refined assay is ready to produce high-quality data quickly for clinical and field studies to develop population norms, recognize diagnostic patterns, and determine risk profiles.

Influence of umbilical cord pH on the outcome of hearing screening with otoacoustic emissions in healthy newborns

The effect of hypoxia on the functioning of the outer hair cells of the cochlea, which are responsible for the response to otoemissions used in neonatal hearing screening, is well known. The aim of this study is to determine the influence of mild to moderate variations in umbilical cord pH at birth on the outcome of hearing screening with otoemissions in healthy newborns without hearing risk factors. The sample is composed of 4536 healthy infants. The results show no significant differences in the hearing screening outcome between the asphyctic (<7.20) and normal pH group. Nor is a figure below 7.20 detected in the sample that is related to an alteration in the screening. When broken down into subgroups with known factors of variation in the screening result, such as gender or lactation, no significant differences in response were detected. Apgar ≤7 is significantly related to pH<7.20. In conclusion, mild-moderate asphyxia associated with delivery of healthy newborns, without auditory risk factors, does not alter the outcome of otoemission screening.

In situ regeneration of inner hair cells in the damaged cochlea by temporally regulated co-expression of Atoh1 and Tbx2

Cochlear inner hair cells (IHCs) are primary sound receptors, and are therefore a target for developing treatments for hearing impairment. IHC regeneration in vivo has been widely attempted, although not yet in the IHC-damaged cochlea. Moreover, the extent to which new IHCs resemble wild-type IHCs remains unclear, as is the ability of new IHCs to improve hearing. Here, we have developed an in vivo mouse model wherein wild-type IHCs were pre-damaged and nonsensory supporting cells were transformed into IHCs by ectopically expressing Atoh1 transiently and Tbx2 permanently. Notably, the new IHCs expressed the functional marker vGlut3 and presented similar transcriptomic and electrophysiological properties to wild-type IHCs. Furthermore, the formation efficiency and maturity of new IHCs were higher than those previously reported, although marked hearing improvement was not achieved, at least partly due to defective mechanoelectrical transduction (MET) in new IHCs. Thus, we have successfully regenerated new IHCs resembling wild-type IHCs in many respects in the damaged cochlea. Our findings suggest that the defective MET is a critical barrier that prevents the restoration of hearing capacity and should thus facilitate future IHC regeneration studies.

Speech Perception in Noise and Medial Olivocochlear Reflex: Effects of Age, Speech Stimulus, and Response-Related Variables

PURPOSE

The role of the medial olivocochlear system in speech perception in noise has been debated over the years, with studies showing mixed results. One possible reason for this could be the dependence of this relationship on the parameters used in assessing the speech perception ability (age, stimulus, and response-related variables).

METHODS

The current study assessed the influence of the type of speech stimuli (monosyllables, words, and sentences), the signal-to-noise ratio (+5, 0, -5, and -10 dB), the metric used to quantify the speech perception ability (percent-correct, SNR-50, and slope of the psychometric function) and age (young vs old) on the relationship between medial olivocochlear reflex (quantified by contralateral inhibition of transient evoked otoacoustic emissions) and speech perception in noise.

RESULTS

A linear mixed-effects model revealed no significant contributions of the medial olivocochlear reflex to speech perception in noise.

CONCLUSION

The results suggest that there was no evidence of any modulatory influence of the indirectly measured medial olivocochlear reflex strength on speech perception in noise.

Cochlear metabolomics, highlighting novel insights of purine metabolic alterations in age-related hearing loss

Aging is an inevitable phase in mammals that leads to health impairments, including hearing loss. Age-related hearing loss (AHL) leads to psychosocial problems and cognitive decline in the elderly. In this study, mean thresholds of auditory brainstem responses (ABR) and distortion-product otoacoustic emissions (DPOAE) increased at multiple frequencies in aged rats (14 months old) compared to young rats (2 months old). Using untargeted ultra-high performance liquid chromatography-mass spectroscopy (LC-MS), we quantified molecular metabolic markers in the cochlea of aged rats with hearing loss. A total of 137 different metabolites were identified in two groups, highlighting several prominent metabolic pathways related to purine metabolism; glycine, serine, and threonine metabolism; arginine and proline metabolism; and pyrimidine metabolism. In addition, the beneficial effects of purine supplementation were demonstrated in a mimetic model of senescent marginal cells (MCs). Overall, altered metabolic profiling is both the cause and manifestation of pathology, and our results suggest that cellular senescence and dysfunctional cochlear metabolism may contribute to the progression of AHL. These findings are seminal in elucidating the pathophysiological mechanisms underlying AHL and serve as a basis for future clinical predictions and interventions in AHL.

Case reopened: A temporal basis for harmonic pitch templates in the early auditory system?a)

A fundamental assumption of rate-place models of pitch is the existence of harmonic templates in the central nervous system (CNS). Shamma and Klein [(2000). J. Acoust. Soc. Am. 107, 2631-2644] hypothesized that these templates have a temporal basis. Coincidences in the temporal fine-structure of neural spike trains, even in response to nonharmonic, stochastic stimuli, would be sufficient for the development of harmonic templates. The physiological plausibility of this hypothesis is tested. Responses to pure tones, low-pass noise, and broadband noise from auditory nerve fibers and brainstem "high-sync" neurons are studied. Responses to tones simulate the output of fibers with infinitely sharp filters: for these responses, harmonic structure in a coincidence matrix comparing pairs of spike trains is indeed found. However, harmonic template structure is not observed in coincidences across responses to broadband noise, which are obtained from nerve fibers or neurons with enhanced synchronization. Using a computer model based on that of Shamma and Klein, it is shown that harmonic templates only emerge when consecutive processing steps (cochlear filtering, lateral inhibition, and temporal enhancement) are implemented in extreme, physiologically implausible form. It is concluded that current physiological knowledge does not support the hypothesis of Shamma and Klein (2000).

Study on damage of the macrostructure of the cochlea under the impact load

Due to the tiny and delicate structure of the cochlea, the auditory system is the most sensitive to explosion impact damage. After being damaged by the explosion impact wave, it usually causes long-term deafness, tinnitus, and other symptoms. To better understand the influence of impact load on the cochlea and basilar membrane (BM), a three-dimensional (3D) fluid-solid coupling finite element model was developed. This model accurately reflects the actual spatial spiral shape of the human cochlea, as well as the lymph environment and biological materials. Based on verifying the reliability of the model, the curve of impact load-amplitude response was obtained, and damage of impact load on the cochlea and the key macrostructure-BM was analyzed. The results indicate that impact wave at middle frequency has widest influence on the cochlea. Furthermore, impact loading causes tears in the BM and destroys the cochlear frequency selectivity.

Subcortical auditory model including efferent dynamic gain control with inputs from cochlear nucleus and inferior colliculus

An auditory model has been developed with a time-varying, gain-control signal based on the physiology of the efferent system and subcortical neural pathways. The medial olivocochlear (MOC) efferent stage of the model receives excitatory projections from fluctuation-sensitive model neurons of the inferior colliculus (IC) and wide-dynamic-range model neurons of the cochlear nucleus. The response of the model MOC stage dynamically controls cochlear gain via simulated outer hair cells. In response to amplitude-modulated (AM) noise, firing rates of most IC neurons with band-enhanced modulation transfer functions in awake rabbits increase over a time course consistent with the dynamics of the MOC efferent feedback. These changes in the rates of IC neurons in awake rabbits were employed to adjust the parameters of the efferent stage of the proposed model. Responses of the proposed model to AM noise were able to simulate the increasing IC rate over time, whereas the model without the efferent system did not show this trend. The proposed model with efferent gain control provides a powerful tool for testing hypotheses, shedding insight on mechanisms in hearing, specifically those involving the efferent system.

Spread of activation and interaction between channels with multi-channel optogenetic stimulation in the mouse cochlea

For individuals with severe to profound hearing loss resulting from irreversibly damaged hair cells, cochlear implants can be used to restore hearing by delivering electrical stimulation directly to the spiral ganglion neurons. However, current spread lowers the spatial resolution of neural activation. Since light can be easily confined, optogenetics is a technique that has the potential to improve the precision of neural activation, whereby visible light is used to stimulate neurons that are modified with light-sensitive opsins. This study compares the spread of neural activity across the inferior colliculus of the auditory midbrain during electrical and optical stimulation in the cochlea of acutely deafened mice with opsin-modified spiral ganglion neurons (H134R variant of the channelrhodopsin-2). Monopolar electrical stimulation was delivered via each of four 0.2 mm wide platinum electrode rings at 0.6 mm centre-to-centre spacing, whereas 453 nm wavelength light was delivered via each of five 0.22 × 0.27 mm micro-light emitting diodes (LEDs) at 0.52 mm centre-to-centre spacing. Channel interactions were also quantified by threshold changes during simultaneous stimulation by pairs of electrodes or micro-LEDs at different distances between the electrodes (0.6, 1.2 and 1.8 mm) or micro-LEDs (0.52, 1.04, 1.56 and 2.08 mm). The spread of activation resulting from single channel optical stimulation was approximately half that of monopolar electrical stimulation as measured at two levels of discrimination above threshold (p<0.001), whereas there was no significant difference between optical stimulation in opsin-modified deafened mice and pure tone acoustic stimulation in normal-hearing mice. During simultaneous micro-LED stimulation, there were minimal channel interactions for all micro-LED spacings tested. For neighbouring micro-LEDs/electrodes, the relative influence on threshold was 13-fold less for optical stimulation compared electrical stimulation (p<0.05). The outcomes of this study show that the higher spatial precision of optogenetic stimulation results in reduced channel interaction compared to electrical stimulation, which could increase the number of independent channels in a cochlear implant. Increased spatial resolution and the ability to activate more than one channel simultaneously could lead to better speech perception in cochlear implant recipients.

Electrophysiological status indexed by early changes in impedance after cochlear implantation: A literature review

Cochlear implantation is a major treatment option for severe-to-profound hearing loss. By insertion into the cochlea and stimulation of the cochlear nerve, cochlear implantation can improve the performance of hearing and speech performance of the implantees. The microenvironment of the cochlea is innate and gets disturbed in response to the insertion of a foreign body. However, real-time changes inside the cochlea in terms of electrophysiology at the molecular level can never be investigated in vivo in human beings. Thus, impedance is a good guide that reflects the electrophysiology inside the cochlea. Because the initial measurement of impedance cannot be performed earlier than the traditional interval of 1 month postoperatively, early changes in impedance have not been explored until recently; however, surgeons are now trying the initial switch-on earlier than 1 month after implantation. This review discusses the scenario of electrophysiological variation after early switch-on in <1 day postimplantation. Evidence has shown that fluctuations in impedance after implantation depend on the interplay between cell cover formation, fibrosis, electrode design, and electrical stimulation. Further studies addressing the correlation between impedance and clinical parameters are required to develop reliable biomarkers for better performance of cochlear implantation.

Contribution of macrophages to neural survival and intracochlear tissue remodeling responses following cochlear implantation

BACKGROUND

Cochlear implants (CIs) restore hearing to deafened patients. The foreign body response (FBR) following cochlear implantation (post-CI) comprises an infiltration of macrophages, other immune and non-immune cells, and fibrosis into the scala tympani, a space that is normally devoid of cells. This FBR is associated with negative effects on CI outcomes including increased electrode impedances and loss of residual acoustic hearing. This study investigates the extent to which macrophage depletion by an orally administered CSF-1R specific kinase (c-FMS) inhibitor, PLX-5622, modulates the tissue response to CI and neural health.

MAIN TEXT

10- to 12-week-old CX3CR1 + /GFP Thy1 + /YFP mice on C57BL/6J/B6 background was fed chow containing 1200 mg/kg PLX5622 or control chow for the duration of the study. 7 days after starting the diet, 3-channel cochlear implants were implanted in the ear via the round window. Serial impedance and neural response telemetry (NRT) measurements were acquired throughout the study. Electric stimulation began 7 days post-CI until 28 days post-CI for 5 h/day, 5 days/week, with programming guided by NRT and behavioral responses. Cochleae harvested at 10, 28 or 56 days post-CI were cryosectioned and labeled with an antibody against α-smooth muscle actin (α-SMA) to identify myofibroblasts and quantify the fibrotic response. Using IMARIS image analysis software, the outlines of scala tympani, Rosenthal canal, modiolus, and lateral wall for each turn were traced manually to measure region volume. The density of nuclei, CX3CR1 + macrophages, Thy1 + spiral ganglion neuron (SGN) numbers, and the ratio of the α-SMA + volume/scala tympani volume were calculated. Cochlear implantation in control diet subjects caused infiltration of cells, including macrophages, into the cochlea. Fibrosis was evident in the scala tympani adjacent to the electrode array. Mice fed PLX5622 chow showed reduced macrophage infiltration throughout the implanted cochleae across all time points. However, scala tympani fibrosis was not reduced relative to control diet subjects. Further, mice treated with PLX5622 showed increased electrode impedances compared to controls. Finally, treatment with PLX5622 decreased SGN survival in implanted and contralateral cochleae.

CONCLUSION

The data suggest that macrophages play an important role in modulating the intracochlear tissue response following CI and neural survival.

Associations Between Physiological Correlates of Cochlear Synaptopathy and Tinnitus in a Veteran Population

PURPOSE

Animal models and human temporal bones indicate that noise exposure is a risk factor for cochlear synaptopathy, a possible etiology of tinnitus. Veterans are exposed to high levels of noise during military service. Therefore, synaptopathy may explain the high rates of noise-induced tinnitus among Veterans. Although synaptopathy cannot be directly evaluated in living humans, animal models indicate that several physiological measures are sensitive to synapse loss, including the auditory brainstem response (ABR), the middle ear muscle reflex (MEMR), and the envelope following response (EFR). The purpose of this study was to determine whether tinnitus is associated with reductions in physiological correlates of synaptopathy that parallel animal studies.

METHOD

Participants with normal audiograms were grouped according to Veteran status and tinnitus report (Veterans with tinnitus, Veterans without tinnitus, and non-Veteran controls). The effects of being a Veteran with tinnitus on ABR, MEMR, and EFR measurements were independently modeled using Bayesian regression analysis.

RESULTS

Modeled point estimates of MEMR and EFR magnitude showed reductions for Veterans with tinnitus compared with non-Veterans, with the most evident reduction observed for the EFR. Two different approaches were used to provide context for the Veteran tinnitus effect on the EFR by comparing to age-related reductions in EFR magnitude and synapse numbers observed in previous studies. These analyses suggested that EFR magnitude/synapse counts were reduced in Veterans with tinnitus by roughly the same amount as over 20 years of aging.

CONCLUSION

These findings suggest that cochlear synaptopathy may contribute to tinnitus perception in noise-exposed Veterans.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.24347761.

Ultrastructure of noise-induced cochlear synaptopathy

Acoustic overexposure can eliminate synapses between inner hair cells (IHCs) and auditory nerve fibers (ANFs), even if hair-cell function recovers. This synaptopathy has been extensively studied by confocal microscopy, however, understanding the nature and sequence of damage requires ultrastructural analysis. Here, we used focused ion-beam scanning electron microscopy to mill, image, segment and reconstruct ANF terminals in mice, 1 day and 1 week after synaptopathic exposure (8-16 kHz, 98 dB SPL). At both survivals, ANF terminals were normal in number, but 62% and 53%, respectively, lacked normal synaptic specializations. Most non-synapsing fibers (57% and 48% at 1 day and 1 week) remained in contact with an IHC and contained healthy-looking organelles. ANFs showed a transient increase in mitochondrial content (51%) and efferent innervation (34%) at 1 day. Fibers maintaining synaptic connections showed hypertrophy of pre-synaptic ribbons at both 1 day and 1 week. Non-synaptic fibers were lower in mitochondrial content and typically on the modiolar side of the IHC, where ANFs with high-thresholds and low spontaneous rates are normally found. Even 1 week post-exposure, many ANF terminals remained in IHC contact despite loss of synaptic specializations, thus, regeneration efforts at early post-exposure times should concentrate on synaptogenesis rather than neurite extension.

Apical-basal distribution of different subtypes of spiral ganglion neurons in the cochlea and the changes during aging

Sound information is transmitted from the cochlea to the brain mainly by type I spiral ganglion neurons (SGNs), which consist of different subtypes with distinct physiological properties and selective expression of molecular markers. It remains unclear how these SGN subtypes distribute along the tonotopic axis, and whether the distribution pattern changes during aging that might underlie age-related hearing loss (ARHL). We investigated these questions using immunohistochemistry in three age groups of CBA/CaJ mice of either sex, including 2-5 months (young), 17-19 months (middle-age), and 28-32 months (old). Mouse cochleae were cryo-sectioned and triple-stained using antibodies against Tuj1, calretinin (CR) and calbindin (CB), which are reportedly expressed in all type I, subtype Ia, and subtype Ib SGNs, respectively. Labeled SGNs were classified into four groups based on the expression pattern of stained markers, including CR+ (subtype Ia), CB+ (subtype Ib), CR+CB+ (dual-labeled Ia/Ib), and CR-CB- (subtype Ic) neurons. The distribution of these SGN groups was analyzed in the apex, middle, and base regions of the cochleae. It showed that the prevalence of subtype Ia, Ib and dual-labeled Ia/Ib SGNs are high in the apex and low in the base. In contrast, the distribution pattern is reversed in Ic SGNs. Such frequency-dependent distribution is largely maintained during aging except for a preferential reduction of Ic SGNs, especially in the base. These findings corroborate the prior study based on RNAscope that SGN subtypes show differential vulnerability during aging. It suggests that sound processing of different frequencies involves distinct combinations of SGN subtypes, and the age-dependent loss of Ic SGNs in the base may especially impact high-frequency hearing during ARHL.

Histological Correlates of Auditory Nerve Injury from Kainic Acid in the Budgerigar (Melopsittacus undulatus)

PURPOSE

Loss of auditory nerve afferent synapses with cochlear hair cells, called cochlear synaptopathy, is a common pathology in humans caused by aging and noise overexposure. The perceptual consequences of synaptopathy in isolation from other cochlear pathologies are still unclear. Animal models provide an effective approach to resolve uncertainty regarding the physiological and perceptual consequences of auditory nerve loss, because neural lesions can be induced and readily quantified. The budgerigar, a parakeet species, has recently emerged as an animal model for synaptopathy studies based on its capacity for vocal learning and ability to behaviorally discriminate simple and complex sounds with acuity similar to humans. Kainic acid infusions in the budgerigar produce a profound reduction of compound auditory nerve responses, including wave I of the auditory brainstem response, without impacting physiological hair cell measures. These results suggest selective auditory nerve damage. However, histological correlates of neural injury from kainic acid are still lacking.

METHODS

We quantified the histological effects caused by intracochlear infusion of kainic acid (1 mM; 2.5 µL), and evaluated correlations between the histological and physiological assessments of auditory nerve status.

RESULTS

Kainic acid infusion in budgerigars produced pronounced loss of neural auditory nerve soma (60% on average) in the cochlear ganglion, and of peripheral axons, at time points 2 or more months following injury. The hair cell epithelium was unaffected by kainic acid. Neural loss was significantly correlated with reduction of compound auditory nerve responses and auditory brainstem response wave I.

CONCLUSION

Compound auditory nerve responses and wave I provide a useful index of cochlear synaptopathy in this animal model.

Oncomodulin regulates spontaneous calcium signalling and maturation of afferent innervation in cochlear outer hair cells

Cochlear outer hair cells (OHCs) are responsible for the exquisite frequency selectivity and sensitivity of mammalian hearing. During development, the maturation of OHC afferent connectivity is refined by coordinated spontaneous Ca2+ activity in both sensory and non-sensory cells. Calcium signalling in neonatal OHCs can be modulated by oncomodulin (OCM, β-parvalbumin), an EF-hand calcium-binding protein. Here, we investigated whether OCM regulates OHC spontaneous Ca2+ activity and afferent connectivity during development. Using a genetically encoded Ca2+ sensor (GCaMP6s) expressed in OHCs in wild-type (Ocm+/+ ) and Ocm knockout (Ocm-/- ) littermates, we found increased spontaneous Ca2+ activity and upregulation of purinergic receptors in OHCs from Ocm-/- cochlea immediately following birth. The afferent synaptic maturation of OHCs was delayed in the absence of OCM, leading to an increased number of ribbon synapses and afferent fibres on Ocm-/- OHCs before hearing onset. We propose that OCM regulates the spontaneous Ca2+ signalling in the developing cochlea and the maturation of OHC afferent innervation. KEY POINTS: Cochlear outer hair cells (OHCs) exhibit spontaneous Ca2+ activity during a narrow period of neonatal development. OHC afferent maturation and connectivity requires spontaneous Ca2+ activity. Oncomodulin (OCM, β-parvalbumin), an EF-hand calcium-binding protein, modulates Ca2+ signals in immature OHCs. Using transgenic mice that endogenously expressed a Ca2+ sensor, GCaMP6s, we found increased spontaneous Ca2+ activity and upregulated purinergic receptors in Ocm-/- OHCs. The maturation of afferent synapses in Ocm-/- OHCs was also delayed, leading to an upregulation of ribbon synapses and afferent fibres in Ocm-/- OHCs before hearing onset. We propose that OCM plays an important role in modulating Ca2+ activity, expression of Ca2+ channels and afferent innervation in developing OHCs.

Inter-aural separation during hearing by bilateral bone conduction stimulation

Cross-head transmission inherent in bone conduction (BC) hearing is one of the most important factors that limit the performance of BC binaural hearing compared to air conduction (AC) binaural hearing. In AC, cross-head transmission is imperceptible leading to a clear understanding of the nature and position of the sound source(s). In this study, the prominence of cross-head transmission in BC hearing is addressed using the fact that ipsilateral cochlear excitation can be canceled by controlled bilateral BC stimulation. A cancellation experiment was conducted on twenty participants with normal hearing at thirteen third-octave frequencies between 250 and 4000 Hz. Both stationary and transient BC stimulation at the mastoid was used. The technique employed multiple stages of masking enabling adjustments of the stimulation level and phase until the tones got canceled in the ipsilateral ear. In addition, the ear canal sound pressure was obtained for ipsilateral and contralateral BC stimulation in isolation, and with bilateral BC stimulation at perceptual cancellation. The inter-aural level differences of both the types of stimulations were found to be the same. Crosstalk was found to be the lowest around 2 kHz and the highest around 1 kHz. The unwrapped inter-aural phase difference from stationary signal cancellation showed an overall increase with frequency starting at around no difference (35°) at 250 Hz to reach 607° at 4 kHz. Cycle-adjusted inter-aural time difference was very low (61 µs) at 250 Hz and increased to 1.1 ms at 800 Hz before falling to 0.6 ms at 4 kHz. It was also found that the ear canal sound pressure was not cancelled at the same phase as the sound in the cochlea.

The effect of coffee on contralateral suppression of transient evoked otoacoustic emissions

Background: Coffee is a popular non-alcoholic beverage consumed by humans across the world. It contains caffeine, which is a type of stimulant of the central nervous system. In the auditory system, it has a positive effect on auditory brainstem response and perception of speech in noise. Further, caffeine has an inhibitory effect in the cochlea, but studies have rarely investigated its effect on otoacoustic emissions (OAEs) in humans. OAEs are low-intensity sounds produced by the cochlea, which could be recorded in the ear canal. The present study was carried out to investigate the effect of coffee on transient evoked otoacoustic emission (TEOAE) and contralateral suppression of TEOAE. Method: A total of 52 young adults participated in the study. A cross-over study design was used for the present investigation. The TEOAE and contralateral suppression of TEOAE were recorded before and after consumption of coffee and milk. The contralateral suppression of TEOAE was measured by presenting white noise to the contralateral ear at 40, 50, and 60 dB sound pressure level (SPL). Results: The mean amplitude of TEOAE before and after consumption of coffee was similar in both ears. Further, the mean contralateral suppression of TEOAE was slightly larger after consumption of coffee in both ears. However, the mean difference was not significant in both the ears. Conclusions: Based on the findings of present study, coffee has no significant effect on the amplitude of TEOAE and contralateral suppression of TEOAE.

Tissue-Engineered Cochlear Fibrosis Model Links Complex Impedance to Fibrosis Formation for Cochlear Implant Patients

Cochlear implants are a life-changing technology for those with severe sensorineural hearing loss, partially restoring hearing through direct electrical stimulation of the auditory nerve. However, they are known to elicit an immune response resulting in fibrotic tissue formation in the cochlea that is linked to residual hearing loss and suboptimal outcomes. Intracochlear fibrosis is difficult to track without postmortem histology, and no specific electrical marker for fibrosis exists. In this study, a tissue-engineered model of cochlear fibrosis is developed following implant placement to examine the electrical characteristics associated with fibrotic tissue formation around electrodes. The model is characterized using electrochemical impedance spectroscopy and an increase in the resistance and a decrease in capacitance of the tissue using a representative circuit are found. This result informs a new marker of fibrosis progression over time that is extractable from voltage waveform responses, which can be directly measured in cochlear implant patients. This marker is tested in a small sample size of recently implanted cochlear implant patients, showing a significant increase over two postoperative timepoints. Using this system, complex impedance is demonstrated as a marker of fibrosis progression that is directly measurable from cochlear implants to enable real-time tracking of fibrosis formation in patients, creating opportunities for earlier treatment intervention to improve cochlear implant efficacy.

Cochlear aging disrupts the correlation between spontaneous rate- and sound-level coding in auditory nerve fibers

The spiking activity of auditory nerve fibers (ANFs) transmits information about the acoustic environment from the cochlea to the central auditory system. Increasing age leads to degeneration of cochlear tissues, including the sensory hair cells and stria vascularis. Here, we aim to identify the functional effects of such age-related cochlear pathologies of ANFs. Rate-level functions (RLFs) were recorded from single-unit ANFs of young adult (n = 52, 3-12 months) and quiet-aged (n = 24, >36 months) Mongolian gerbils of either sex. RLFs were used to determine sensitivity and spontaneous rates (SRs) and were classified into flat-saturating, sloping-saturating, and straight categories, as previously established. A physiologically based cochlear model, adapted for the gerbil, was used to simulate the effects of cochlear degeneration on ANF physiology. In ANFs tuned to low frequencies (<3.5 kHz), SR was lower in those of aged gerbils, while an age-related loss of low-SR fibers was evident in ANFs tuned to high frequencies. These changes in SR distribution did not affect the typical SR versus sensitivity correlation. The distribution of RLF types among low-SR fibers, however, shifted toward that of high-SR fibers, specifically showing more fast-saturating and fewer sloping-saturating RLFs. A modeled striatal degeneration, which affects the combined inner hair cell and synaptic output, reduced SR but left RLF type unchanged. An additional reduced basilar membrane gain, which decreased sensitivity, explained the changed RLF types. Overall, the data indicated age-related changes in the characteristics of single ANFs that blurred the established relationships between SR and RLF types.NEW & NOTEWORTHY Auditory nerve fibers, which connect the cochlea to the central auditory system, change their encoding of sound level in aged gerbils. In addition to a general shift to higher levels, indicative of decreased sensitivity, level coding was also differentially affected in fibers with low- and high-spontaneous rates. Loss of low-spontaneous rate fibers, combined with a general decrease of spontaneous rate, further blurs the categorization of auditory nerve fiber types in the aged gerbil.

Auditory brainstem mechanisms likely compensate for self-imposed peripheral inhibition

Feedback networks in the brain regulate downstream auditory function as peripheral as the cochlea. However, the upstream neural consequences of this peripheral regulation are less understood. For instance, the medial olivocochlear reflex (MOCR) in the brainstem causes putative attenuation of responses generated in the cochlea and cortex, but those generated in the brainstem are perplexingly unaffected. Based on known neural circuitry, we hypothesized that the inhibition of peripheral input is compensated for by positive feedback in the brainstem over time. We predicted that the inhibition could be captured at the brainstem with shorter (1.5 s) than previously employed long duration (240 s) stimuli where this inhibition is likely compensated for. Results from 16 normal-hearing human listeners support our hypothesis in that when the MOCR is activated, there is a robust reduction of responses generated at the periphery, brainstem, and cortex for short-duration stimuli. Such inhibition at the brainstem, however, diminishes for long-duration stimuli suggesting some compensatory mechanisms at play. Our findings provide a novel non-invasive window into potential gain compensation mechanisms in the brainstem that may have implications for auditory disorders such as tinnitus. Our methodology will be useful in the evaluation of efferent function in individuals with hearing loss.

Gradients of glucose metabolism regulate morphogen signalling required for specifying tonotopic organisation in the chicken cochlea

In vertebrates with elongated auditory organs, mechanosensory hair cells (HCs) are organised such that complex sounds are broken down into their component frequencies along a proximal-to-distal long (tonotopic) axis. Acquisition of unique morphologies at the appropriate position along the chick cochlea, the basilar papilla, requires that nascent HCs determine their tonotopic positions during development. The complex signalling within the auditory organ between a developing HC and its local niche along the cochlea is poorly understood. Using a combination of live imaging and NAD(P)H fluorescence lifetime imaging microscopy, we reveal that there is a gradient in the cellular balance between glycolysis and the pentose phosphate pathway in developing HCs along the tonotopic axis. Perturbing this balance by inhibiting different branches of cytosolic glucose catabolism disrupts developmental morphogen signalling and abolishes the normal tonotopic gradient in HC morphology. These findings highlight a causal link between graded morphogen signalling and metabolic reprogramming in specifying the tonotopic identity of developing HCs.

Input compensation of dolphin and sea lion auditory brainstem responses using frequency-modulated up-chirps

Frequency-modulated "chirp" stimuli that offset cochlear dispersion (i.e., input compensation) have shown promise for increasing auditory brainstem response (ABR) amplitudes relative to traditional sound stimuli. To enhance ABR methods with marine mammal species known or suspected to have low ABR signal-to-noise ratios, the present study examined the effects of broadband chirp sweep rate and level on ABR amplitude in bottlenose dolphins and California sea lions. "Optimal" chirps were designed based on previous estimates of cochlear traveling wave speeds (using high-pass subtractive masking methods) in these species. Optimal chirps increased ABR peak amplitudes by compensating for cochlear dispersion; however, chirps with similar (or higher) frequency-modulation rates produced comparable results. The optimal chirps generally increased ABR amplitudes relative to noisebursts as threshold was approached, although this was more obvious when sound pressure level was used to equate stimulus levels (as opposed to total energy). Chirps provided progressively less ABR amplitude gain (relative to noisebursts) as stimulus level increased and produced smaller ABRs at the highest levels tested in dolphins. Although it was previously hypothesized that chirps would provide larger gains in sea lions than dolphins-due to the lower traveling wave speed in the former-no such pattern was observed.

Distortion Product Otoacoustic Emissions in Mice Above and Below the Eliciting Primaries

Normal hearing is associated with cochlear nonlinearity. When two tones (f1 and f2) are presented, the intracochlear response contains additional components that can be recorded from the ear canal as distortion product otoacoustic emissions (DPOAEs). Although the most prominent intermodulation distortion component is at 2f1-f2, other cubic distortion products are also generated. Because these measurements are noninvasive, they are used in humans and in animal models to detect hearing loss. This study evaluated how loss of sensitivity affects DPOAEs with frequencies above and below the stimulating primaries, i.e., for upper sideband (USB) components like 2f2-f1 and for lower sideband (LSB) components like 2f1-f2. DPOAEs were recorded in several mouse mutants with varying degrees of hearing loss associated with structural changes to the tectorial membrane (TM), or with loss of outer hair cell (OHC) somatic electromotility due to lack of prestin or to the expression of a non-functional prestin. In mice with changes in sensitivity, magnitude reductions were observed for 2f1-f2 relative to controls with mice lacking prestin showing the greatest changes. In contrast, 2f2-f1 was minimally affected by reductions in cochlear gain due to changes in the TM or by the loss of OHC somatic electromotility. In addition, TM mutants with spontaneous otoacoustic emissions (SOAEs) generated larger responses than controls at 2f2-f1 when its frequency was similar to that for the SOAEs. Although cochlear pathologies appear to affect USB and LSB DPOAEs in different ways, both 2f1-f2 and 2f2-f1 reflect nonlinearities associated with the transducer channels. However, in mice, the component at 2f2-f1 does not appear to receive enhancement due to prestin's motor action.

On the Tonotopy of the Low-Frequency Region of the Cochlea

It is generally assumed that frequency selectivity varies along the cochlea. For example, at the base of the cochlea, which is a region sensitive to high-frequency sounds, the best frequency of a cochlear location increases toward the most basal end, that is, near the stapes. Response phases also vary along cochlear locations. At any given frequency, there is a decrease in phase lag toward the stapes. This tonotopic arrangement in the cochlea was originally described by Georg von Békésy in a seminal series of experiments on human cadavers and has been confirmed in more recent works on live laboratory animals. Nonetheless, our knowledge of tonotopy at the apex of the cochlea remains incomplete in animals with low-frequency hearing, which is relevant to human speech. The results of our experiments on guinea pig, gerbil, and chinchilla cochleas, regardless of the sex of the animal, show that responses to sound differ at locations across the apex in a pattern consistent with previous studies of the base of the cochlea.SIGNIFICANCE STATEMENT Tonotopy is an important property of the auditory system that has been shown to exist in many auditory centers. In fact, most auditory implants work on the assumption of its existence by assigning different frequencies to different stimulating electrodes based on their location. At the level of the basilar membrane in the cochlea, a tonotopic arrangement implies that high-frequency stimuli evoke largest displacements at the base, near the ossicles, and low-frequency sounds have their greatest effects at the apex. Although tonotopy has been confirmed at the base of the cochlea on live animals at the apex of the cochlea, however, it has been less studied. Here, we show that a tonotopic arrangement does exist at the apex of the cochlea.

Hearing as adaptive cascaded envelope interpolation

The human auditory system is designed to capture and encode sounds from our surroundings and conspecifics. However, the precise mechanisms by which it adaptively extracts the most important spectro-temporal information from sounds are still not fully understood. Previous auditory models have explained sound encoding at the cochlear level using static filter banks, but this vision is incompatible with the nonlinear and adaptive properties of the auditory system. Here we propose an approach that considers the cochlear processes as envelope interpolations inspired by cochlear physiology. It unifies linear and nonlinear adaptive behaviors into a single comprehensive framework that provides a data-driven understanding of auditory coding. It allows simulating a broad range of psychophysical phenomena from virtual pitches and combination tones to consonance and dissonance of harmonic sounds. It further predicts the properties of the cochlear filters such as frequency selectivity. Here we propose a possible link between the parameters of the model and the density of hair cells on the basilar membrane. Cascaded Envelope Interpolation may lead to improvements in sound processing for hearing aids by providing a non-linear, data-driven, way to preprocessing of acoustic signals consistent with peripheral processes.

Auditory Deprivation during Development Alters Efferent Neural Feedback and Perception

Auditory experience plays a critical role in hearing development. Developmental auditory deprivation because of otitis media, a common childhood disease, produces long-standing changes in the central auditory system, even after the middle ear pathology is resolved. The effects of sound deprivation because of otitis media have been mostly studied in the ascending auditory system but remain to be examined in the descending pathway that runs from the auditory cortex to the cochlea via the brainstem. Alterations in the efferent neural system could be important because the descending olivocochlear pathway influences the neural representation of transient sounds in noise in the afferent auditory system and is thought to be involved in auditory learning. Here, we show that the inhibitory strength of the medial olivocochlear efferents is weaker in children with a documented history of otitis media relative to controls; both boys and girls were included in the study. In addition, children with otitis media history required a higher signal-to-noise ratio on a sentence-in-noise recognition task than controls to achieve the same criterion performance level. Poorer speech-in-noise recognition, a hallmark of impaired central auditory processing, was related to efferent inhibition, and could not be attributed to the middle ear or cochlear mechanics.SIGNIFICANCE STATEMENT Otitis media is the second most common reason children go to the doctor. Previously, degraded auditory experience because of otitis media has been associated with reorganized ascending neural pathways, even after middle ear pathology resolved. Here, we show that altered afferent auditory input because of otitis media during childhood is also associated with long-lasting reduced descending neural pathway function and poorer speech-in-noise recognition. These novel, efferent findings may be important for the detection and treatment of childhood otitis media.

A Survey of U.S. Audiologists' Usage of and Attitudes Toward Otoacoustic Emissions

PURPOSE

Otoacoustic emissions (OAEs) provide information on outer hair cell function and have multiple clinical applications. Two types of OAEs, transient-evoked OAEs (TEOAEs) and distortion product OAEs (DPOAEs), are currently utilized in clinical practice. However, it remains unknown how confident U.S. clinicians are in performing and interpreting TEOAEs and DPOAEs. Additionally, the extent to which U.S. audiologists incorporate OAEs for different clinical applications and populations has not been thoroughly investigated. To fill these gaps in knowledge, this study characterized the attitudes toward and usage of TEOAEs and DPOAEs in a sample of U.S. audiologists.

METHOD

This study utilized an online survey distributed to U.S. audiologists through multiple channels from January to March 2021. A total of 214 completed surveys were included in the analysis. Results were analyzed descriptively. Associations between variables and comparisons between users of DPOAEs only and users of TEOAEs and DPOAEs were also examined.

RESULTS

DPOAEs were reportedly utilized more frequently and with greater confidence than TEOAEs. The most common clinical application of both OAE types was a cross-check. Significant associations were found between responses to DPOAE questions and the clinician's setting and patient age. There were some significant differences between users of DPOAEs only and users of TEOAEs and DPOAEs.

CONCLUSIONS

Results suggest that U.S. audiologists utilize OAEs for multiple clinical purposes and that there are appreciable differences in terms of attitudes toward and usage of DPOAEs versus TEOAEs. Future work could investigate the reasons that underlie these differences to further improve clinical implementation of OAEs.

Preservation of developmental spontaneous activity enables early auditory system maturation in deaf mice

Intrinsically generated neural activity propagates through the developing auditory system to promote maturation and refinement of sound processing circuits prior to hearing onset. This early patterned activity is induced by non-sensory supporting cells in the organ of Corti, which are highly interconnected through gap junctions containing connexin 26 (Gjb2). Although loss of function mutations in Gjb2 impair cochlear development and are the most common cause of congenital deafness, it is not known if these variants disrupt spontaneous activity and the developmental trajectory of sound processing circuits in the brain. Here, we show in a new mouse model of Gjb2-mediated congenital deafness that cochlear supporting cells adjacent to inner hair cells (IHCs) unexpectedly retain intercellular coupling and the capacity to generate spontaneous activity, exhibiting only modest deficits prior to hearing onset. Supporting cells lacking Gjb2 elicited coordinated activation of IHCs, leading to coincident bursts of activity in central auditory neurons that will later process similar frequencies of sound. Despite alterations in the structure of the sensory epithelium, hair cells within the cochlea of Gjb2-deficient mice were intact and central auditory neurons could be activated within appropriate tonotopic domains by loud sounds at hearing onset, indicating that early maturation and refinement of auditory circuits was preserved. Only after cessation of spontaneous activity following hearing onset did progressive hair cell degeneration and enhanced auditory neuron excitability manifest. This preservation of cochlear spontaneous neural activity in the absence of connexin 26 may increase the effectiveness of early therapeutic interventions to restore hearing.

Sensitivity to Frequency Modulation is Limited Centrally

Modulations in both amplitude and frequency are prevalent in natural sounds and are critical in defining their properties. Humans are exquisitely sensitive to frequency modulation (FM) at the slow modulation rates and low carrier frequencies that are common in speech and music. This enhanced sensitivity to slow-rate and low-frequency FM has been widely believed to reflect precise, stimulus-driven phase locking to temporal fine structure in the auditory nerve. At faster modulation rates and/or higher carrier frequencies, FM is instead thought to be coded by coarser frequency-to-place mapping, where FM is converted to amplitude modulation (AM) via cochlear filtering. Here, we show that patterns of human FM perception that have classically been explained by limits in peripheral temporal coding are instead better accounted for by constraints in the central processing of fundamental frequency (F0) or pitch. We measured FM detection in male and female humans using harmonic complex tones with an F0 within the range of musical pitch but with resolved harmonic components that were all above the putative limits of temporal phase locking (>8 kHz). Listeners were more sensitive to slow than fast FM rates, even though all components were beyond the limits of phase locking. In contrast, AM sensitivity remained better at faster than slower rates, regardless of carrier frequency. These findings demonstrate that classic trends in human FM sensitivity, previously attributed to auditory nerve phase locking, may instead reflect the constraints of a unitary code that operates at a more central level of processing.SIGNIFICANCE STATEMENT Natural sounds involve dynamic frequency and amplitude fluctuations. Humans are particularly sensitive to frequency modulation (FM) at slow rates and low carrier frequencies, which are prevalent in speech and music. This sensitivity has been ascribed to encoding of stimulus temporal fine structure (TFS) via phase-locked auditory nerve activity. To test this long-standing theory, we measured FM sensitivity using complex tones with a low F0 but only high-frequency harmonics beyond the limits of phase locking. Dissociating the F0 from TFS showed that FM sensitivity is limited not by peripheral encoding of TFS but rather by central processing of F0, or pitch. The results suggest a unitary code for FM detection limited by more central constraints.

The Effects of Burr-Assisted Rhinoplasty on Hearing

AIM

The aim of this study is to examine the effects of the burr used for hump reduction and osteoplasty on cochlear function.

MATERIALS AND METHODS

The design of this study was prospective, randomized, and controlled. Twenty patients who underwent burr-assisted septorhinoplasty were included in the study group. The control group consisted of 20 patients who underwent septoplasty. Pure tone audiometry, distortion product otoacoustic emission test, and tympanometry were performed to determine the auditory functions.

RESULTS

No significant difference was observed between the bone conduction thresholds of the right and left ears in both groups, except for a single frequency (1000 Hz in the left ear) in the control group. There was no significant difference between the 2 groups' air conduction thresholds at frequencies of 500, 1000, 2000, 4000, 6000, and 8000 Hz preoperatively and postoperatively. In addition, the study and control group did not differ significantly in signal-to-noise ratio measurements at frequencies of 500, 1000, 2000, and 4000 Hz. The comparison of preoperative and postoperative otoacoustic emission measurement results of the study group revealed a statistically significant difference only at the frequencies of 2000 Hz in the right ear and 500 Hz in the left ear. Despite those differences, otoacoustic emissions were still present at those frequencies postoperatively.

CONCLUSIONS

Our study showed that using burrs during rhinoplasty slightly impacts hearing, but it does not cause significant hearing loss. Burr-assisted rhinoplasty appears to be a safe operation regarding the auditory functions.

Reprogramming by drug-like molecules leads to regeneration of cochlear hair cell-like cells in adult mice

Strategies to overcome irreversible cochlear hair cell (HC) damage and loss in mammals are of vital importance to hearing recovery in patients with permanent hearing loss. In mature mammalian cochlea, co-activation of Myc and Notch1 reprograms supporting cells (SC) and promotes HC regeneration. Understanding of the underlying mechanisms may aid the development of a clinically relevant approach to achieve HC regeneration in the nontransgenic mature cochlea. By single-cell RNAseq, we show that MYC/NICD "rejuvenates" the adult mouse cochlea by activating multiple pathways including Wnt and cyclase activator of cyclic AMP (cAMP), whose blockade suppresses HC-like cell regeneration despite Myc/Notch activation. We screened and identified a combination (the cocktail) of drug-like molecules composing of small molecules and small interfering RNAs to activate the pathways of Myc, Notch1, Wnt and cAMP. We show that the cocktail effectively replaces Myc and Notch1 transgenes and reprograms fully mature wild-type (WT) SCs for HC-like cells regeneration in vitro. Finally, we demonstrate the cocktail is capable of reprogramming adult cochlea for HC-like cells regeneration in WT mice with HC loss in vivo. Our study identifies a strategy by a clinically relevant approach to reprogram mature inner ear for HC-like cells regeneration, laying the foundation for hearing restoration by HC regeneration.

Dissection of Adult Mouse Stria Vascularis for Single-Nucleus Sequencing or Immunostaining

Endocochlear potential, which is generated by the stria vascularis, is essential to maintain an environment conducive to appropriate hair cell mechanotransduction and ultimately hearing. Pathologies of the stria vascularis can result in a decreased hearing. Dissection of the adult stria vascularis allows for focused single-nucleus capture and subsequent single-nucleus sequencing and immunostaining. These techniques are used to study stria vascularis pathophysiology at the single-cell level. Single-nucleus sequencing can be used in the setting of transcriptional analysis of the stria vascularis. Meanwhile, immunostaining continues to be useful in identifying specific populations of cells. Both methods require proper stria vascularis dissection as a prerequisite, which can prove to be technically challenging.

Medial Olivocochlear Reflex Strength in Ears With Low-to-Moderate Annual Noise Exposure

PURPOSE

Studies in lower mammals demonstrate enhancement of the medial olivocochlear reflex (MOCR) following noise exposure. A similar effect may occur in humans, and there is some evidence of an individual's acoustic history affecting the MOCR. The current work evaluates the relationship between an individual's annual noise exposure history and their MOCR strength. Given the potential role of the MOCR as a biological hearing protector, it is important to identify factors associated with MOCR strength.

METHOD

Data were collected from 98 normal-hearing young adults. Annual noise exposure history was estimated using the Noise Exposure Questionnaire. MOCR strength was assayed using click-evoked otoacoustic emissions (CEOAEs) measured with and without noise presented to the contralateral ear. MOCR metrics included the MOCR-induced otoacoustic emission (OAE) magnitude shift and phase shift. A CEOAE signal-to-noise ratio (SNR) of at least 12 dB was required for estimation of the MOCR metrics. Linear regression was applied to evaluate the relationship between MOCR metrics and annual noise exposure.

RESULTS

Annual noise exposure was not a statistically significant predictor of the MOCR-induced CEOAE magnitude shift. However, annual noise exposure was a statistically significant predictor of the MOCR-induced CEOAE phase shift-the MOCR-induced phase shift decreased with increasing noise exposure. Additionally, annual noise exposure was a statistically significant predictor of OAE level.

CONCLUSIONS

Findings contrast with recent work that suggests MOCR strength increases with annual noise exposure. Compared with previous work, data for this study were collected using more stringent SNR criteria, which is expected to increase the precision of the MOCR metrics. Additionally, data were collected for a larger subject population with a wider range of noise exposures. Whether findings generalize to other exposure durations and levels is unknown and requires future study.

The effects of broadband elicitor duration on a psychoacoustic measure of cochlear gain reduction

Physiological and psychoacoustic studies of the medial olivocochlear reflex (MOCR) in humans have often relied on long duration elicitors (>100 ms). This is largely due to previous research using otoacoustic emissions (OAEs) that found multiple MOCR time constants, including time constants in the 100s of milliseconds, when elicited by broadband noise. However, the effect of the duration of a broadband noise elicitor on similar psychoacoustic tasks is currently unknown. The current study measured the effects of ipsilateral broadband noise elicitor duration on psychoacoustic gain reduction estimated from a forward-masking paradigm. Analysis showed that both masker type and elicitor duration were significant main effects, but no interaction was found. Gain reduction time constants were ∼46 ms for the masker present condition and ∼78 ms for the masker absent condition (ranging from ∼29 to 172 ms), both similar to the fast time constants reported in the OAE literature (70-100 ms). Maximum gain reduction was seen for elicitor durations of ∼200 ms. This is longer than the 50-ms duration which was found to produce maximum gain reduction with a tonal on-frequency elicitor. Future studies of gain reduction may use 150-200 ms broadband elicitors to maximally or near-maximally stimulate the MOCR.