Suprathreshold hearing in middle age and relationship to cochlear synaptopathy

Animal models have demonstrated that the afferent synapses and nerve terminals innervating the cochlea are vulnerable to damage from acoustic overexposure and aging. This synaptopathy can occur without hair-cell loss, or more severely when accompanied by permanent audiometric shifts. In humans, postmortem temporal bone studies have shown that cochlear synaptopathy occurs throughout adulthood, decades before audiometric loss nominally occurs. However, effective non-invasive assays of synaptopathy have yet to be established in humans (or genetically heterogeneous animal cohorts). Moreover, whether synaptopathy contributes to age-related temporal perception deficits is debated. We are currently studying young and middle-aged humans with clinically “near-normal” audiograms using physiological and perceptual measures. Preliminary results suggest that although high-frequency audiometric shifts occur with age as previously known, middle age per se is associated with physiological effects consistent with cochlear synaptopathy. Further, we find that the effects of age and threshold elevation can oppose and obscure one another on certain perceptual measures. Finally, with speech-in-noise measures designed to degrade high-frequency envelope cues, perceptual deficits associated with middle age per se are apparent and consistent with synaptopathy. These results will be discussed in light of the ongoing debate about the prevalence and consequences of synaptopathy in humans.