Age-related auditory nerve deficits propagate central gain throughout the auditory system: Associations with cortical microstructure and speech recognition.

There is growing evidence that perceptual difficulties associated with age-related hearing loss are not solely due to cochlear damage and are exacerbated by central nervous system changes. We examined electrophysiological (EEG) responses to clicks and diffusion kurtosis imaging in 49 older (29 female) and 26 younger (20 female) adults to determine the extent to which auditory nerve (AN) deficits in older adults contributed to functional and structural changes throughout the auditory system. Older adults exhibited smaller AN responses, similar brainstem responses, and larger auditory cortex (AC) responses, demonstrating progressive "central gain." Audiometric thresholds were not predictive of EEG measures. Reduced AN function predicted deficits in cortical microstructure (lower AC fractional anisotropy) in older adults, consistent with microstructure degeneration. These lower fractional anisotropy values in the AC of older adults also predicted larger AC responses and more central gain. Older adults exhibited significantly lower AC fractional anisotropy and kurtosis fractional anisotropy coupled with higher mean and radial diffusivity compared to younger adults. AC fractional anisotropy, radial diffusivity, axial diffusivity, and mean diffusivity were all significant predictors of speech-in-noise (SIN) recognition in older adults. The results suggest that reduced afferent input in older adults not only results in functional changes throughout the auditory system consistent with progressive gain but also contributes to deficits in AC structure beyond those explained by age alone, contributing to SIN recognition deficits. Understanding the complex effects of age, reduced AN input, central gain, and AC microstructure on SIN recognition may provide potential therapeutic targets for intervention.