Pure-Tone and Speech-Evoked Responses in Normal and Noise-Exposed Squirrel Monkeys

Abstract

Monkeys were implanted with permanent recording electrodes having tips placed in the auditory nerve-cochlear nucleus region. AP input-output relationships and VDLs were obtained from computer-averaged evoked responses to pure-tone pulses and selected monosyllabic words before and after five consecutive daily 15-min exposures to 115 dB SPL of steady broad-band noise. Threshold shifts and sustained reduction in AP amplitudes occurred at higher frequencies. Response patterns evoked by speech appeared coded in the form of amplitude modulation, and words could be identified by the summed waveform associated with each stimulus. Response patterns for some words were altered by noise exposure. Results suggest that noise intensity-duration combinations currently accepted as nonhazardous to hearing (on the basis of TTS recovery) may produce persistent changes in hearing function for suprathreshold sounds, and that speech reception may be altered by noise stimulation followed by only minimal changes in pure-tone VDLs.