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  • Title: Right-ear advantage drives the link between olivocochlear efferent 'antimasking' and speech-in-noise listening benefits.
    Author: Bidelman GM, Bhagat SP.
    Journal: Neuroreport; 2015 May 27; 26(8):483-7. PubMed ID: 25919996.
    Abstract:
    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.
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