Nicotinamide riboside protects noise-induced hearing loss by recovering the hair cell ribbon synapses.

OBJECTIVE: Nicotinamide riboside (NR) has been proved to protect the hearing. To achieve animal models of temporary threshold shift (TTS) and permanent threshold shift (PTS) respectively, evaluate the dynamic change of ribbon synapse before and after NR administration. METHODS: Mice were divided into control group, noise exposure (NE) group and NR group. The noise was exposed to NE and NR group, and NR was injected before noise exposure. Auditory brainstem response (ABR), ribbon synapse count and cochlear morphology were tested, as well as the concentration of hydrogen peroxide (H2O2) and ATP. RESULTS: Ribbon synapse count decrease with the intensity of noise exposure, and the cochlear morphology remains stable during TTS and was damaged during PTS. NR promotes the oxidation resistance to protect the synapse and the inner ear morphology. CONCLUSION: Our findings suggest that TTS mice are more vulnerable to noise, and NR can promote the recovery of the synapse count to protect the animals' hearing.

Gata3 is required for the functional maturation of inner hair cells and their innervation in the mouse cochlea.

KEY POINTS: The physiological maturation of auditory hair cells and their innervation requires precise temporal and spatial control of cell differentiation. The transcription factor gata3 is essential for the earliest stages of auditory system development and for survival and synaptogenesis in auditory sensory afferent neurons. We show that during postnatal development in the mouse inner ear gata3 is required for the biophysical maturation, growth and innervation of inner hair cells; in contrast, it is required only for the survival of outer hair cells. Loss of gata3 in inner hair cells causes progressive hearing loss and accounts for at least some of the deafness associated with the human hypoparathyroidism, deafness and renal anomaly (HDR) syndrome. The results show that gata3 is critical for later stages of mammalian auditory system development where it plays distinct, complementary roles in the coordinated maturation of sensory hair cells and their innervation. ABSTRACT: The zinc finger transcription factor gata3 regulates inner ear development from the formation of the embryonic otic placode. Throughout development, gata3 is expressed dynamically in all the major cochlear cell types. Its role in afferent formation is well established but its possible involvement in hair cell maturation remains unknown. Here, we find that in heterozygous gata3 null mice (gata3+/- ) outer hair cells (OHCs) differentiate normally but their numbers are significantly lower. In contrast, inner hair cells (IHCs) survive normally but they fail to acquire adult basolateral membrane currents, retain pre-hearing current and efferent innervation profiles and have fewer ribbon synapses. Targeted deletion of gata3 driven by otoferlin-cre recombinase (gata3fl/fl otof-cre+/- ) in IHCs does not affect OHCs or the number of IHC afferent synapses but it leads to a failure in IHC maturation comparable to that observed in gata3+/- mice. Auditory brainstem responses in gata3fl/fl otof-cre+/- mice reveal progressive hearing loss that becomes profound by 6-7 months, whilst distortion product otoacoustic emissions are no different to control animals up to this age. Our results, alongside existing data, indicate that gata3 has specific, complementary functions in different cell types during inner ear development and that its continued expression in the sensory epithelium orchestrates critical aspects of physiological development and neural connectivity. Furthermore, our work indicates that hearing loss in human hypoparathyroidism, deafness and renal anomaly (HDR) syndrome arises from functional deficits in IHCs as well as loss of function from OHCs and both afferent and efferent neurons.

The synaptic ribbon is critical for sound encoding at high rates and with temporal precision.

We studied the role of the synaptic ribbon for sound encoding at the synapses between inner hair cells (IHCs) and spiral ganglion neurons (SGNs) in mice lacking RIBEYE (RBEKO/KO). Electron and immunofluorescence microscopy revealed a lack of synaptic ribbons and an assembly of several small active zones (AZs) at each synaptic contact. Spontaneous and sound-evoked firing rates of SGNs and their compound action potential were reduced, indicating impaired transmission at ribbonless IHC-SGN synapses. The temporal precision of sound encoding was impaired and the recovery of SGN-firing from adaptation indicated slowed synaptic vesicle (SV) replenishment. Activation of Ca2+-channels was shifted to more depolarized potentials and exocytosis was reduced for weak depolarizations. Presynaptic Ca2+-signals showed a broader spread, compatible with the altered Ca2+-channel clustering observed by super-resolution immunofluorescence microscopy. We postulate that RIBEYE disruption is partially compensated by multi-AZ organization. The remaining synaptic deficit indicates ribbon function in SV-replenishment and Ca2+-channel regulation.

Evaluation of the Hair Cell Regeneration in Zebrafish Larvae by Measuring and Quantifying the Startle Responses.

The zebrafish has become an established model organism for the study of hearing and balance systems in the past two decades. The classical approach to examine hair cells is to use dye to conduct selective staining, which shows the number and morphology of hair cells but does not reveal their function. Startle response is a behavior closely related to the auditory function of hair cells; therefore it can be used to measure the function of hair cells. In this study, we developed a device to measure the startle response of zebrafish larvae. By applying various levels of stimulus, it showed that the system can discern a 10 dB difference. The hair cell in zebrafish can regenerate after damage due to noise exposure or drug treatment. With this device, we measured the startle response of zebrafish larvae during and after drug treatment. The results show a similar trend to the classical hair cell staining method. The startle response was reduced with drug treatment and recovered after removal of the drug. Together it demonstrated the capability of this behavioral assay in evaluating the hair cell functions of fish larvae and its potential as a high-throughput screening tool for auditory-related gene and drug discovery.

Hair cells use active zones with different voltage dependence of Ca2+ influx to decompose sounds into complementary neural codes.

For sounds of a given frequency, spiral ganglion neurons (SGNs) with different thresholds and dynamic ranges collectively encode the wide range of audible sound pressures. Heterogeneity of synapses between inner hair cells (IHCs) and SGNs is an attractive candidate mechanism for generating complementary neural codes covering the entire dynamic range. Here, we quantified active zone (AZ) properties as a function of AZ position within mouse IHCs by combining patch clamp and imaging of presynaptic Ca(2+) influx and by immunohistochemistry. We report substantial AZ heterogeneity whereby the voltage of half-maximal activation of Ca(2+) influx ranged over ∼20 mV. Ca(2+) influx at AZs facing away from the ganglion activated at weaker depolarizations. Estimates of AZ size and Ca(2+) channel number were correlated and larger when AZs faced the ganglion. Disruption of the deafness gene GIPC3 in mice shifted the activation of presynaptic Ca(2+) influx to more hyperpolarized potentials and increased the spontaneous SGN discharge. Moreover, Gipc3 disruption enhanced Ca(2+) influx and exocytosis in IHCs, reversed the spatial gradient of maximal Ca(2+) influx in IHCs, and increased the maximal firing rate of SGNs at sound onset. We propose that IHCs diversify Ca(2+) channel properties among AZs and thereby contribute to decomposing auditory information into complementary representations in SGNs.

The acquisition of mechano-electrical transducer current adaptation in auditory hair cells requires myosin VI.

KEY POINTS: The transduction of sound into electrical signals occurs at the hair bundles atop sensory hair cells in the cochlea, by means of mechanosensitive ion channels, the mechano-electrical transducer (MET) channels. The MET currents decline during steady stimuli; this is termed adaptation and ensures they always work within the most sensitive part of their operating range, responding best to rapidly changing (sound) stimuli. In this study we used a mouse model (Snell's waltzer) for hereditary deafness in humans that has a mutation in the gene encoding an unconventional myosin, myosin VI, which is present in the hair bundles. We found that in the absence of myosin VI the MET current fails to acquire its characteristic adaptation as the hair bundles develop. We propose that myosin VI supports the acquisition of adaptation by removing key molecules from the hair bundle that serve a temporary, developmental role. ABSTRACT: Mutations in Myo6, the gene encoding the (F-actin) minus end-directed unconventional myosin, myosin VI, cause hereditary deafness in mice (Snell's waltzer) and humans. In the sensory hair cells of the cochlea, myosin VI is expressed in the cell bodies and along the stereocilia that project from the cells' apical surface. It is required for maintaining the structural integrity of the mechanosensitive hair bundles formed by the stereocilia. In this study we investigate whether myosin VI contributes to mechano-electrical transduction. We report that Ca(2+) -dependent adaptation of the mechano-electrical transducer (MET) current, which serves to keep the transduction apparatus operating within its most sensitive range, is absent in outer and inner hair cells from homozygous Snell's waltzer mutant mice, which fail to express myosin VI. The operating range of the MET channels is also abnormal in the mutants, resulting in the absence of a resting MET current. We found that cadherin 23, a component of the hair bundle's transient lateral links, fails to be downregulated along the length of the stereocilia in maturing Myo6 mutant mice. MET currents of heterozygous littermates appear normal. We propose that myosin VI, by removing key molecules from developing hair bundles, is required for the development of the MET apparatus and its Ca(2+) -dependent adaptation.

Thyroid hormone is required for pruning, functioning and long-term maintenance of afferent inner hair cell synapses.

Functional maturation of afferent synaptic connections to inner hair cells (IHCs) involves pruning of excess synapses formed during development, as well as the strengthening and survival of the retained synapses. These events take place during the thyroid hormone (TH)-critical period of cochlear development, which is in the perinatal period for mice and in the third trimester for humans. Here, we used the hypothyroid Snell dwarf mouse (Pit1(dw)) as a model to study the role of TH in afferent type I synaptic refinement and functional maturation. We observed defects in afferent synaptic pruning and delays in calcium channel clustering in the IHCs of Pit1(dw) mice. Nevertheless, calcium currents and capacitance reached near normal levels in Pit1(dw) IHCs by the age of onset of hearing, despite the excess number of retained synapses. We restored normal synaptic pruning in Pit1(dw) IHCs by supplementing with TH from postnatal day (P)3 to P8, establishing this window as being critical for TH action on this process. Afferent terminals of older Pit1(dw) IHCs showed evidence of excitotoxic damage accompanied by a concomitant reduction in the levels of the glial glutamate transporter, GLAST. Our results indicate that a lack of TH during a critical period of inner ear development causes defects in pruning and long-term homeostatic maintenance of afferent synapses.

Selective Inner Hair Cell Dysfunction in Chinchillas Impairs Hearing-in-Noise in the Absence of Outer Hair Cell Loss.

Poorer hearing in the presence of background noise is a significant problem for the hearing impaired. Ototoxic drugs, ageing, and noise exposure can damage the sensory hair cells of the inner ear that are essential for normal hearing sensitivity. The relationship between outer hair cell (OHC) loss and progressively poorer hearing sensitivity in quiet or in competing background noise is supported by a number of human and animal studies. In contrast, the effect of moderate inner hair cell (IHC) loss or dysfunction shows almost no impact on behavioral measures of hearing sensitivity in quiet, when OHCs remain intact, but the relationship between selective IHC loss and hearing in noise remains relatively unknown. Here, a moderately high dose of carboplatin (75 mg/kg) that produced IHC loss in chinchillas ranging from 40 to 80 % had little effect on thresholds in quiet. However, when tested in the presence of competing broadband (BBN) or narrowband noise (NBN), thresholds increased significantly. IHC loss >60 % increased signal-to-noise ratios (SNRs) for tones (500-11,300 Hz) in competing BBN by 5-10 dB and broadened the masking function under NBN. These data suggest that IHC loss or dysfunction may play a significant role in listening in noise independent of OHC integrity and that these deficits may be present even when thresholds in quiet are within normal limits.

Re-Emergent Inhibition of Cochlear Inner Hair Cells in a Mouse Model of Hearing Loss.

Hearing loss among the elderly correlates with diminished social, mental, and physical health. Age-related cochlear cell death does occur, but growing anatomical evidence suggests that synaptic rearrangements on sensory hair cells also contribute to auditory functional decline. Here we present voltage-clamp recordings from inner hair cells of the C57BL/6J mouse model of age-related hearing loss, which reveal that cholinergic synaptic inputs re-emerge during aging. These efferents are functionally inhibitory, using the same ionic mechanisms as do efferent contacts present transiently before the developmental onset of hearing. The strength of efferent inhibition of inner hair cells increases with hearing threshold elevation. These data indicate that the aged cochlea regains features of the developing cochlea and that efferent inhibition of the primary receptors of the auditory system re-emerges with hearing impairment. SIGNIFICANCE STATEMENT: Synaptic changes in the auditory periphery are increasingly recognized as important factors in hearing loss. To date, anatomical work has described the loss of afferent contacts from cochlear hair cells. However, relatively little is known about the efferent innervation of the cochlea during hearing loss. We performed intracellular recordings from mouse inner hair cells across the lifespan and show that efferent innervation of inner hair cells arises in parallel with the loss of afferent contacts and elevated hearing threshold during aging. These efferent neurons inhibit inner hair cells, raising the possibility that they play a role in the progression of age-related hearing loss.

Insulin resistance due to dietary iron overload disrupts inner hair cell ribbon synapse plasticity in male mice.

To evaluate whether cochlear inner hair cells (IHCs) ribbon synapse plasticity would be interrupted by insulin resistance (IR) due to dietary iron overload, we established an IR model in C57Bl/6 male mice with an iron-enriched diet for 16 weeks. Glucose levels were measured at weeks 4, 8, 12, 16. Glucose tolerance test and insulin tolerance test were performed at week 16 after overnight fasting. Then, auditory brainstem responses (ABRs) measurements were performed for hearing threshold shifts. After ABR measurements, cochleae were harvested for assessment of the number of IHC ribbon synapses by immunostaining, the morphology of cochlear hair cells and spiral ganglion neurons (SGNs) by transmission electron microscopy or immunostaining. Here, we show that IR due to dietary iron overload decreased the number of ribbon synapses, and induced moderate ABR threshold elevations. Besides, additional components including outer hair cells (OHCs), IHCs, and SGNs were unaffected. Moreover, IR did not disrupt the expression of vesicular glutamate transporter 3 (VGLUT3), myosin VIIa and prestin in hair cells. These results indicate that IHC ribbon synapses may be more susceptible to IR due to dietary iron overload.

Filtering of acoustic signals within the hearing organ.

The detection of sound by the mammalian hearing organ involves a complex mechanical interplay among different cell types. The inner hair cells, which are the primary sensory receptors, are stimulated by the structural vibrations of the entire organ of Corti. The outer hair cells are thought to modulate these sound-evoked vibrations to enhance hearing sensitivity and frequency resolution, but it remains unclear whether other structures also contribute to frequency tuning. In the current study, sound-evoked vibrations were measured at the stereociliary side of inner and outer hair cells and their surrounding supporting cells, using optical coherence tomography interferometry in living anesthetized guinea pigs. Our measurements demonstrate the presence of multiple vibration modes as well as significant differences in frequency tuning and response phase among different cell types. In particular, the frequency tuning at the inner hair cells differs from other cell types, causing the locus of maximum inner hair cell activation to be shifted toward the apex of the cochlea compared with the outer hair cells. These observations show that additional processing and filtering of acoustic signals occur within the organ of Corti before inner hair cell excitation, representing a departure from established theories.

The recovery from AP adaptation in sensorineural hearing loss.

CONCLUSIONS: An abnormally slower action potential (AP) recovery from adaptation (decreased recovery, dR) was characteristically detected in many ears with sensorineural hearing loss (SNHL) but not Meniere's disease and idiopathic sudden SNHL. We assumed that this abnormal AP recovery from adaptation was attributed to an imbalance in the distributions of auditory neurons with high and low spontaneous firing rates. The significant difference of initial hearing level between dR and normal AP recovery groups (nR) was assumed to partially result from AP recovery being determined by the inner hair cell synapse, and not from outer hair cells. OBJECTIVE: This study aimed to detect the AP recovery pattern in SNHL. METHODS: Electrocochleography (ECochG) was performed transtympanically in 30 patients with SNHL. AP recovery was measured by a paired click stimulation paradigm as a function of inter-click intervals from 5 to 100 ms. RESULTS: The high prevalence of dR (9 of 30 ears) appears to be a characteristic ECochG finding in SNHL. Initial hearing level differed significantly between dR and nR groups.

Tonotopic morphometry of the lamina reticularis of the guinea pig cochlea with associated microstructures and related mechanical implications.

Morphometry of the lamina reticularis of the guinea pig cochlea was performed using scanning electron microscopy. Seventy-four geometrical parameters of the lamina reticularis, the bundles of stereocilia, and individual stereocilia, in all rows of hair cells and within the individual hair cells, were measured at ten equally spaced locations along the longitudinal direction of the cochlea. Variations of the parameters versus the longitudinal coordinate were statistically analyzed and fitted with polynomials (constant, linear, or quadratic). Our data show that a unique set of geometrical parameters of inner and outer hair cells is typical for every frequency-dependent position at the lamina reticularis. Morphology of the outer hair cell structures varies more than respective parameters of the inner hair cells. Mechanical modeling using the obtained geometrical parameters provides a novel glance at the mechanical characteristics with respect to the cochlear tonotopy.

A precedence effect model to simulate localization dominance using an adaptive, stimulus parameter-based inhibition process.

A number of precedence-effect models have been developed to simulate the robust localization performance of humans in reverberant conditions. Although they are able to reduce reverberant information for many conditions, they tend to fail for ongoing stimuli with truncated on/offsets, a condition human listeners master when localizing a sound source in the presence of a reflection, according to a study by Dizon and Colburn [J. Acoust. Soc. Am. 119, 2947-2964 (2006)]. This paper presents a solution for this condition by using an autocorrelation mechanism to estimate the delay and amplitude ratio between the leading and lagging signals. An inverse filter is then used to eliminate the lag signal, before it is localized with a standard localization algorithm. The current algorithm can operate on top of a basic model of the auditory periphery (gammatone filter bank, half-wave rectification) to simulate psychoacoustic data by Braasch et al. [Acoust. Sci. Tech. 24, 293-303 (2003)] and Dizon and Colburn. The model performs robustly with these on/offset truncated and interaural level difference based stimuli and is able to demonstrate the Haas effect.

Concurrent maturation of inner hair cell synaptic Ca2+ influx and auditory nerve spontaneous activity around hearing onset in mice.

Hearing over a wide range of sound intensities is thought to require complementary coding by functionally diverse spiral ganglion neurons (SGNs), each changing activity only over a subrange. The foundations of SGN diversity are not well understood but likely include differences among their inputs: the presynaptic active zones (AZs) of inner hair cells (IHCs). Here we studied one candidate mechanism for causing SGN diversity-heterogeneity of Ca(2+) influx among the AZs of IHCs-during postnatal development of the mouse cochlea. Ca(2+) imaging revealed a change from regenerative to graded synaptic Ca(2+) signaling after the onset of hearing, when in vivo SGN spike timing changed from patterned to Poissonian. Furthermore, we detected the concurrent emergence of stronger synaptic Ca(2+) signals in IHCs and higher spontaneous spike rates in SGNs. The strengthening of Ca(2+) signaling at a subset of AZs primarily reflected a gain of Ca(2+) channels. We hypothesize that the number of Ca(2+) channels at each IHC AZ critically determines the firing properties of its corresponding SGN and propose that AZ heterogeneity enables IHCs to decompose auditory information into functionally diverse SGNs.

Loudness adaptation accompanying ribbon synapse and auditory nerve disorders.

Abnormal auditory adaptation is a standard clinical tool for diagnosing auditory nerve disorders due to acoustic neuromas. In the present study we investigated auditory adaptation in auditory neuropathy owing to disordered function of inner hair cell ribbon synapses (temperature-sensitive auditory neuropathy) or auditory nerve fibres. Subjects were tested when afebrile for (i) psychophysical loudness adaptation to comfortably-loud sustained tones; and (ii) physiological adaptation of auditory brainstem responses to clicks as a function of their position in brief 20-click stimulus trains (#1, 2, 3 … 20). Results were compared with normal hearing listeners and other forms of hearing impairment. Subjects with ribbon synapse disorder had abnormally increased magnitude of loudness adaptation to both low (250 Hz) and high (8000 Hz) frequency tones. Subjects with auditory nerve disorders had normal loudness adaptation to low frequency tones; all but one had abnormal adaptation to high frequency tones. Adaptation was both more rapid and of greater magnitude in ribbon synapse than in auditory nerve disorders. Auditory brainstem response measures of adaptation in ribbon synapse disorder showed Wave V to the first click in the train to be abnormal both in latency and amplitude, and these abnormalities increased in magnitude or Wave V was absent to subsequent clicks. In contrast, auditory brainstem responses in four of the five subjects with neural disorders were absent to every click in the train. The fifth subject had normal latency and abnormally reduced amplitude of Wave V to the first click and abnormal or absent responses to subsequent clicks. Thus, dysfunction of both synaptic transmission and auditory neural function can be associated with abnormal loudness adaptation and the magnitude of the adaptation is significantly greater with ribbon synapse than neural disorders.

Developmental changes in the cochlear hair cell mechanotransducer channel and their regulation by transmembrane channel-like proteins.

Vibration of the stereociliary bundles activates calcium-permeable mechanotransducer (MT) channels to initiate sound detection in cochlear hair cells. Different regions of the cochlea respond preferentially to different acoustic frequencies, with variation in the unitary conductance of the MT channels contributing to this tonotopic organization. Although the molecular identity of the MT channel remains uncertain, two members of the transmembrane channel-like family, Tmc1 and Tmc2, are crucial to hair cell mechanotransduction. We measured MT channel current amplitude and Ca(2+) permeability along the cochlea's longitudinal (tonotopic) axis during postnatal development of wild-type mice and mice lacking Tmc1 (Tmc1-/-) or Tmc2 (Tmc2-/-). In wild-type mice older than postnatal day (P) 4, MT current amplitude increased ~1.5-fold from cochlear apex to base in outer hair cells (OHCs) but showed little change in inner hair cells (IHCs), a pattern apparent in mutant mice during the first postnatal week. After P7, the OHC MT current in Tmc1-/- (dn) mice declined to zero, consistent with their deafness phenotype. In wild-type mice before P6, the relative Ca(2+) permeability, P(Ca), of the OHC MT channel decreased from cochlear apex to base. This gradient in P(Ca) was not apparent in IHCs and disappeared after P7 in OHCs. In Tmc1-/- mice, P(Ca) in basal OHCs was larger than that in wild-type mice (to equal that of apical OHCs), whereas in Tmc2-/-, P(Ca) in apical and basal OHCs and IHCs was decreased compared with that in wild-type mice. We postulate that differences in Ca(2+) permeability reflect different subunit compositions of the MT channel determined by expression of Tmc1 and Tmc2, with the latter conferring higher P(Ca) in IHCs and immature apical OHCs. Changes in P(Ca) with maturation are consistent with a developmental decrease in abundance of Tmc2 in OHCs but not in IHCs.

Traveling waves on the organ of corti of the chinchilla cochlea: spatial trajectories of inner hair cell depolarization inferred from responses of auditory-nerve fibers.

Spatial magnitude and phase profiles for inner hair cell (IHC) depolarization throughout the chinchilla cochlea were inferred from responses of auditory-nerve fibers (ANFs) to threshold- and moderate-level tones and tone complexes. Firing-rate profiles for frequencies ≤2 kHz are bimodal, with the major peak at the characteristic place and a secondary peak at 3-5 mm from the extreme base. Response-phase trajectories are synchronous with peak outward stapes displacement at the extreme cochlear base and accumulate 1.5 period lags at the characteristic places. High-frequency phase trajectories are very similar to the trajectories of basilar-membrane peak velocity toward scala tympani. Low-frequency phase trajectories undergo a polarity flip in a region, 6.5-9 mm from the cochlear base, where traveling-wave phase velocity attains a local minimum and a local maximum and where the onset latencies of near-threshold impulse responses computed from responses to near-threshold white noise exhibit a local minimum. That region is the same where frequency-threshold tuning curves of ANFs undergo a shape transition. Since depolarization of IHCs presumably indicates the mechanical stimulus to their stereocilia, the present results suggest that distinct low-frequency forward waves of organ of Corti vibration are launched simultaneously at the extreme base of the cochlea and at the 6.5-9 mm transition region, from where antiphasic reflections arise.

Behavioral estimates of the contribution of inner and outer hair cell dysfunction to individualized audiometric loss.

Differentiating the relative importance of the various contributors to the audiometric loss (HL(TOTAL)) of a given hearing impaired listener and frequency region is becoming critical as more specific treatments are being developed. The aim of the present study was to assess the relative contribution of inner (IHC) and outer hair cell (OHC) dysfunction (HL(IHC) and HL(OHC), respectively) to the audiometric loss of patients with mild to moderate cochlear hearing loss. It was assumed that HL(TOTAL) = HL(OHC) + HL(IHC) (all in decibels) and that HL(OHC) may be estimated as the reduction in maximum cochlear gain. It is argued that the latter may be safely estimated from compression threshold shifts of cochlear input/output (I/O) curves relative to normal hearing references. I/O curves were inferred behaviorally using forward masking for 26 test frequencies in 18 hearing impaired listeners. Data suggested that the audiometric loss for six of these 26 test frequencies was consistent with pure OHC dysfunction, one was probably consistent with pure IHC dysfunction, 13 were indicative of mixed IHC and OHC dysfunction, and five were uncertain (one more was excluded from the analysis). HL(OHC) and HL(IHC) contributed on average 60 and 40 %, respectively, to the audiometric loss, but variability was large across cases. Indeed, in some cases, HL(IHC) was up to 63 % of HL(TOTAL), even for moderate losses. The repeatability of the results is assessed using Monte Carlo simulations and potential sources of bias are discussed.

Individual differences in behavioral estimates of cochlear nonlinearities.

Psychophysical methods provide a mechanism to infer the characteristics of basilar membrane responses in humans that cannot be directly measured. Because these behavioral measures are indirect, the interpretation of results depends on several underlying assumptions. Ongoing uncertainty about the suitability of these assumptions and the most appropriate measurement and compression estimation procedures, and unanswered questions regarding the effects of cochlear hearing loss and age on basilar membrane nonlinearities, motivated this experiment. Here, estimates of cochlear nonlinearities using temporal masking curves (TMCs) were obtained in a large sample of adults of various ages whose hearing ranged from normal to moderate cochlear hearing loss (Experiment 1). A wide range of compression slopes was observed, even for subjects with similar ages and thresholds, which warranted further investigation (Experiment 2). Potential sources of variance contributing to these individual differences were explored, including procedural-related factors (test-retest reliability, suitability of the linear-reference TMC, probe sensation levels, and parameters of TMC fitting algorithms) and subject-related factors (age and age-related changes in temporal processing, strength of cochlear nonlinearities estimated with distortion-product otoacoustic emissions, estimates of changes in cochlear function from damage to outer hair cells versus inner hair cells). Subject age did not contribute significantly to TMC or compression slopes, and TMC slopes did not vary significantly with threshold. Test-retest reliability of TMCs suggested that TMC masker levels and the general shapes of TMCs did not change in a systematic way when re-measured many weeks later. Although the strength of compression decreased slightly with increasing hearing loss, the magnitude of individual differences in compression estimates makes it difficult to determine the effects of hearing loss and cochlear damage on basilar membrane nonlinearities in humans.