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Description
In humans, aging is associated with a loss of synapses between inner hair cells and auditory nerve fibers (cochlear synaptopathy (CS)). While CS is expected to affect afferent sound encoding, it may also influence efferent control of cochlear gain by reducing the input to the medial olivocochlear (MOC) system. To date, this possibility has not been extensively investigated. Here, we combined computational simulations and psychophysics to evaluate age-related CS effects in a forward-masking task designed to assess MOC-mediated gain reduction. We measured detection thresholds for a 2-kHz probe (10 ms) presented alone or preceded by a 400-ms noise precursor (20-ms gap) in young (yNH, n = 8) and older normal-hearing listeners (oNH, n = 7). Under these conditions, the precursor-induced threshold shift (dB) can be used to estimate cochlear gain reduction mediated by activation of the MOC system. Simulations were performed using a computational model of the auditory periphery with MOC feedback driven by inputs from the inferior colliculus and auditory nerve. Precursor bandwidth and temporal-envelope characteristics were systematically varied. Simulations predicted reduced threshold shifts in conditions simulating reduced auditory nerve input to the MOC system, particularly for narrowband precursors with weak temporal-envelope fluctuations. In contrast, behavioral results showed no differences between groups for narrowband precursors and significantly larger threshold shifts in oNH for wideband precursors (with and without temporal envelope modulations). These results suggest that forward-masking estimates of cochlear gain reduction reflect more than MOC-mediated gain reduction alone. Future work should combine behavioral paradigms with objective measures of MOC function to better isolate age-related changes in MOC control of cochlear gain.