Age-Related Compensation Mechanism Revealed in the Cortical Representation of Degraded Speech

Older adults understand speech with comparative ease in quiet, but signal degradation can hinder speech understanding much more than it does in younger adults. This difficulty may result, in part, from temporal processing deficits related to the aging process and/or high-frequency hearing loss that can occur in listeners who have normal- or near-normal-hearing thresholds in the speech frequency range. Temporal processing deficits may manifest as degraded neural representation in peripheral and brainstem/midbrain structures that lead to compensation, or changes in response strength in auditory cortex. Little is understood about the process by which the neural representation of signals is improved or restored by age-related cortical compensation mechanisms. Therefore, we used vocoding to simulate spectral degradation to compare the behavioral and neural representation of words that contrast on a temporal dimension. Specifically, we used the closure duration of the silent interval between the vowel and the final affricate /t∫/ or fricative /ʃ/ of the words DITCH and DISH, respectively. We obtained perceptual identification functions and electrophysiological neural measures (frequency-following responses (FFR) and cortical auditory-evoked potentials (CAEPs)) to unprocessed and vocoded versions of these words in young normal-hearing (YNH), older normal- or near-normal-hearing (ONH), and older hearing-impaired (OHI) listeners. We found that vocoding significantly reduced the slope of the perceptual identification function in only the OHI listeners. In contrast to the limited effects of vocoding on perceptual performance, vocoding had robust effects on the FFRs across age groups, such that stimulus-to-response correlations and envelope magnitudes were significantly lower for vocoded vs. unprocessed conditions. Increases in the P1 peak amplitude for vocoded stimuli were found for both ONH and OHI listeners, but not for the YNH listeners. These results suggest that while vocoding substantially degrades early neural representation of speech stimuli in the midbrain, there may be cortical compensation in older listeners that is not seen in younger listeners.