Temporary hearing loss from exposure to moderately intense tones in rhesus monkeys

The amount and duration of temporary threshold shift produced by exposure to moderately loud sounds were quantitatively examined in rhesus monkeys using a simple, behavioral reaction-time procedure. Subjects were exposed to pure tones (100 dB SPL) either at the threshold test frequency or half an octave below (-1/2 OCT) the test frequency, usually for a duration of 3-minutes. Many features of the results were comparable to those observed in human subjects following exposure to similar sounds: (1) both the magnitude and the time course of recovery from the hearing loss were a function of the frequency and duration of the exposure stimulus with higher-frequency and longer-lasting stimuli producing greater threshold shifts and longer recovery time courses; (2) recovery time courses were monotonic and approximately exponential; (3) at low- and mid-frequency regions of hearing, intense stimulation at the test frequency and at -1/2 OCT produced similar threshold shifts and durations of recovery, while for high-frequency hearing -1/2 OCT stimuli always yielded greater losses and longer recovery periods; (4) although the peak hearing loss was either at, or a half octave above, the exposure frequency, the overall threshold-shift pattern was always assymmetrically distributed toward higher frequencies; (5) alterations in functions relating response latency to stimulus intensity were demonstrable. These psychophysical experiments form a framework that will permit further investigations into the physiologic basis of temporary threshold shift in an animal model highly similar to man.