Auditory brainstem response forward-masking recovery functions in older humans with normal hearing

We investigated the auditory brainstem response (ABR) recovery from forward masking using toneburst maskers and probes. Two subject groups matched for hearing thresholds were evaluated: normal-hearing young adults (21-40 year s) and older subjects (63-77 years) with normal audiometric thresholds. Sti muli consisted of 1, 4 and 8 kHz tonebursts, with 2-4 cycle rise/fall time and no plateau. Forward maskers were tonebursts of the same frequency, with a 5 ms rise/fall time and a 20 ms plateau time. Probes were presented at 4 0 dB above threshold, and the forward masker was adjusted to a level that j ust eliminated the ABR to the 40 dB sensation level toneburst when the prob e onset occurred at masker offset. Forward-masker intervals varied from 2 t o 64 ms. ABR wave V latencies were similar for the young and old age groups regardless of toneburst frequency. Under forward-masking conditions, wave V latency was prolonged for the shorter intervals, and recovered to baselin e latency by 64 ms. The forward-masker recovery functions were nearly ident ical for the two age groups for the 1 kHz toneburst. In contrast, there wer e clear differences in the recovery functions for the two age groups for th e 4 and 8 kHz tonebursts. Specifically, the mean latency shift was greater for the aged group for forward-masker intervals of 16 ms or less. The two a ge groups showed identical latency shifts for longer forward-masker interva ls. These data demonstrate prolonged recovery from forward masking in older human subjects. As these subjects had audiometric thresholds within normal limits, one plausible interpretation of this finding is that the prolonged recovery time is a manifestation of an aging effect on the central auditor y nervous system rather than the periphery. (C) 1999 Elsevier Science B.V. All rights reserved.