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Human hearing sensitivity is easily compromised with overexposure to excessively loud sounds, leading to permanent hearing damage. Consequently, finding activities and/or experiential factors that distinguish "tender" from "tough" ears (i.e., acoustic vulnerability) would be important for identifying people at higher risk for hearing damage. To regulate sound transmission and protect the inner ear against acoustic trauma, the auditory system modulates gain control to the cochlea via biological feedback of the medial olivocochlear (MOC) efferents, a neuronal pathway linking the lower brainstem and cochlear outer hair cells. We hypothesized that a salient form of auditory experience shown to have pervasive neuroplastic benefits, namely musical training, might act to fortify hearing through tonic engagement of these reflexive pathways. By measuring MOC efferent feedback via otoacoustic emissions (cochlear emitted sounds), we show that dynamic ipsilateral and contralateral cochlear gain control is enhanced in musically-trained individuals. Across all participants, MOC strength was correlated with the years of listeners' training suggested that efferent gain control is experience dependent. Our data provide new evidence that intensive listening experience(s) (e.g., musicianship) can strengthen the ipsi/contralateral MOC efferent system and sound regulation to the inner ear. Implications for reducing acoustic vulnerability to damaging sounds are discussed., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Auditory filter theory posits a tradeoff in time-frequency analysis: high temporal precision is achievable only at the expense of poorer frequency resolution and vice versa. Here, we examined the hierarchy of brain mechanisms of these spectrotemporal tradeoffs through a series of physiological and behavioral measures aimed to tap temporal and spectral acuity at different levels of the auditory neuroaxis (cochlea→brainstem→percept). Cochlear and behavioral frequency selectivity was measured by stimulus-frequency otoacoustic emissions (SFOAE) and psychophysical tuning curves; temporal acuity was measured physiologically and behaviorally by paired click recovery of auditory brainstem responses (ABRs) and gap detection thresholds (GDTs), respectively. Comparison of physiological and behavioral estimates of temporal acuity and frequency tuning showed high consistency between measurement domains with temporal thresholds of ∼3-4 ms and filter tuning Q3≈10 across brain and behavioral measures. Cochlear SFOAE estimates of tuning inversely predicted listeners' temporal acuity estimated from both brainstem ABRs and behavioral GDTs. The high predictive power of cochlear responses on temporal thresholds and similarity between time-frequency tradeoffs measured at progressively higher levels of the processing hierarchy (brainstem, behavior) suggest that the temporal resolution of human hearing established in the cochlea might be inherited at progressively higher levels of the hearing pathway.

The mammalian cochlea functions as a filter bank that performs a spectral, Fourier-like decomposition on the acoustic signal. While tuning can be compromised (e.g., broadened with hearing impairment), whether or not human cochlear frequency resolution can be sharpened through experiential factors (e.g., training or learning) has not yet been established. Previous studies have demonstrated sharper psychophysical tuning curves in trained musicians compared to nonmusicians, implying superior peripheral tuning. However, these findings are based on perceptual masking paradigms, and reflect engagement of the entire auditory system rather than cochlear tuning, per se. Here, by directly mapping physiological tuning curves from stimulus frequency otoacoustic emissions (SFOAEs)-cochlear emitted sounds-we show that estimates of human cochlear tuning in a high-frequency cochlear region (4 kHz) is further sharpened (by a factor of 1.5×) in musicians and improves with the number of years of their auditory training. These findings were corroborated by measurements of psychophysical tuning curves (PTCs) derived via simultaneous masking, which similarly showed sharper tuning in musicians. Comparisons between SFOAE and PTCs revealed closer correspondence between physiological and behavioral curves in musicians, indicating that tuning is also more consistent between different levels of auditory processing in trained ears. Our findings demonstrate an experience-dependent enhancement in the resolving power of the cochlear sensory epithelium and the spectral resolution of human hearing and provide a peripheral account for the auditory perceptual benefits observed in musicians. Both local and feedback (e.g., medial olivocochlear efferent) mechanisms are discussed as potential mechanisms for experience-dependent tuning., (Copyright © 2016 Elsevier B.V. All rights reserved.)

The mammalian cochlea receives feedback from the brainstem medial olivocochlear (MOC) efferents, whose putative 'antimasking' function is to adjust cochlear amplification and enhance peripheral signal detection in adverse listening environments. Human studies have been inconsistent in demonstrating a clear connection between this corticofugal system and behavioral speech-in-noise (SIN) listening skills. To elucidate the role of brainstem efferent activity in SIN perception, we measured ear-specific contralateral suppression of transient-evoked otoacoustic emissions (OAEs), a proxy measure of MOC activation linked to auditory learning in noisy environments. We show that suppression of cochlear emissions is stronger with a more basal cochlear bias in the right ear compared with the left ear. Moreover, a strong negative correlation was observed between behavioral SIN performance and right-ear OAE suppression magnitudes, such that lower speech reception thresholds in noise were predicted by larger amounts of MOC-related activity. This brain-behavioral relation was not observed for left ear SIN perception. The rightward bias in contralateral MOC suppression of OAEs, coupled with the stronger association between physiological and perceptual measures, is consistent with left-hemisphere cerebral dominance for speech-language processing. We posit that corticofugal feedback from the left cerebral cortex through descending MOC projections sensitizes the right cochlea to signal-in-noise detection, facilitating figure-ground contrast and improving degraded speech analysis. Our findings demonstrate that SIN listening is at least partly driven by subcortical brain mechanisms; primitive stages of cochlear processing and brainstem MOC modulation of (right) inner ear mechanics play a critical role in dictating SIN understanding.

Musicianship confers enhancements to hearing at nearly all levels of the auditory system from periphery to percept. Musicians' superior psychophysical abilities are particularly evident in spectral discrimination and noise-degraded listening tasks, achieving higher perceptual sensitivity than their nonmusician peers. Greater spectral acuity implies that musicianship may increase auditory filter selectivity. This hypothesis was directly tested by measuring both forward- and simultaneous-masked psychophysical tuning curves. Sharper filter tuning (i.e., higher Q10) was observed in musicians compared to nonmusicians. Findings suggest musicians' pervasive listening benefits may be facilitated, in part, by superior spectral processing/decomposition as early as the auditory periphery.

The existence of efferent feedback from cortical and subcortical brain centers to the hair cells of the cochlea has been recognized for many years, but the role that efferent neurons play in hearing is not completely known. Stimulation of medial olivocochlear (MOC) efferent neurons suppresses sound-evoked basilar membrane responses and changes the tuning of single auditory nerve fibers in animal models. Both of these effects are linked to a MOC-induced reduction in the gain of the cochlear amplification provided by outer hair cells. To non-invasively examine the link between cochlear suppression and tuning in humans, stimulus-frequency otoacoustic emissions (SFOAEs) were recorded in conditions with and without contralateral acoustic stimulation (CAS) from 28 normal-hearing participants. SFOAEs were measured using clusters of closely-spaced probe-tone frequencies centered near 1.4 and 2.0kHz. An index of cochlear tuning, QERB, was calculated based on measures of SFOAE group delay at both 1.4 and 2.0kHz. A statistically significant (p<0.01) decrease in SFOAE levels acquired during CAS was detected only for the SFOAE cluster centered at 2kHz. No statistically significant differences in QERB were found between conditions with and without CAS at 1.4 and 2.0kHz. These findings suggest that in humans, tuning based on SFOAE group delay estimates is not appreciably altered at cochlear locations with MOC efferent-induced reductions in cochlear gain., (Copyright © 2013 Elsevier Ireland Ltd. All rights reserved.)

Several lines of evidence suggest that medial olivocochlear (MOC) efferent neurons modify cochlear output to improve signal detection in noise. In animal models, stimulation of MOC efferents reduces the amount of compression in basilar membrane (BM) growth functions. Linearization of BM growth functions may assist in extending the neural response to the signal above that of noise, leading to a decrease in masking. In order to test this hypothesis, effects of MOC efferent neurons on BM compression were studied indirectly in humans by examining the effects of contralateral noise on distortion-product otoacoustic emission (DPOAE) input-output functions at 1.0 and 2.0kHz. Compression threshold estimates from a three-segment linear regression model applied to the DPOAE functions were derived in order to determine correlations with psychophysical measurements of masking of tones at 1.0 and 2.0kHz. Contralateral noise shifted the DPOAE compression threshold to a significantly higher level at 1.0kHz, but not at 2.0kHz. A significant negative correlation between the change in DPOAE compression threshold and the amount of masking at 1.0kHz was observed, but no correlation between these variables was detected at 2.0kHz. The results of this experiment at the lower test frequency indicated that contralateral noise linearized DPOAE input-output functions, and individuals with larger DPOAE compression threshold shifts tended to exhibit less masking. Under certain conditions, decreases in cochlear compression induced by MOC efferent neurons may lead to unmasking of tones presented in noise., (Copyright © 2010 Elsevier Ireland Ltd. All rights reserved.)

The purpose of this study was to examine if a pre-determined exposure level and duration of MP3 player music would result in significant changes in cochlear function when measured with audiometric and physiological methods. Distortion-product otoacoustic emissions (DPOAEs), synchronized spontaneous otoacoustic emissions (SSOAEs), and hearing thresholds were measured in 20 normal-hearing adults before and after a 30-minute MP3 player music exposure. DPOAEs were acquired with 65/45 dB SPL primary tones (f(2)=0.842-7.996 kHz) with a frequency resolution of 8 points/octave. A probe microphone system recorded ear-canal music levels and was used to equalize levels at approximately 85 dBC across individuals during the music presentation. Comparison of pre- and post-exposure measurements revealed no significant differences in hearing thresholds, but DPOAE levels in half-octave bands centered from 1.4-6.0 kHz were significantly reduced following the music exposure. Post-exposure shifts in SSOAE frequency and level were highly variable in individuals identified with SSOAEs. The results for the exposure conditions explored in this study indicate that changes in otoacoustic emissions may precede the development of music-induced hearing threshold shifts.