Cortical compensation for afferent loss in older adults: Associations with GABA and speech recognition in noise

Abstract

Age-related deficits in auditory nerve (AN) function reduce afferent input to the auditory cortex. The extent to which the auditory cortex in older adults compensates for this loss of afferent input, also known as central gain, and the mechanisms underlying this compensation are not well understood. We took a neural systems approach to estimate central gain, measuring AN and cortical evoked responses within 50 older and 27 younger adults. Amplitudes were significantly smaller for older than for younger adults for AN responses but not for cortical responses. We used the relationship between AN and cortical response amplitudes in younger adults to predict cortical response amplitudes for older adults from their AN responses. Central gain in older adults was thus defined as the difference between their observed cortical responses and those predicted from the parameter estimates of younger adults. More central gain was associated with decreased cortical levels of GABA measured with 1H-MRS and poorer speech recognition in noise (SIN). Effects of central gain and GABA on SIN occur in addition to, and independent from, effects attributed to elevated hearing thresholds. Our results are consistent with animal models of central gain and suggest that reduced AN afferent input in some older adults may result in changes in cortical encoding and inhibitory neurotransmission, which contribute to reduced SIN. An advancement in our understanding of the changes that occur throughout the auditory system in response to the gradual loss of input with increasing age may provide potential therapeutic targets for intervention.