Age-related changes in auditory function of rhesus monkeys (Macaca mulatta).

The purpose of this study was to investigate the changes in auditory function associated with aging in rhesus monkeys (Macaca mulatta) as a model for age-related changes in humans. One advantage of using monkeys from the Wisconsin Regional Primate Research Center was that lifestyle factors such as diet and excessive noise exposure were controlled. Twenty younger (mean: 10 years, 9 months, S.D.=+/-6 months) and 20 older (mean: 25 years, 11 months, S.D.=+/-11 months) monkeys were used in this study. Cochlear function in these monkeys was measured with distortion product otoacoustic emissions (DPOAEs); neural function was measured with auditory brainstem responses (ABRs) and middle latency responses (MLRs). Older monkeys had (1) significantly smaller DPOAEs, (2) significantly smaller ABR peak amplitudes, and (3) significantly longer ABR peak latencies compared to younger monkeys. Overall, these results suggest that older monkeys have decreased cochlear and neural function to the level of the brainstem as compared to younger monkeys. The decrease in DPOAE level and the increase in age accounted for approximately 70% of the increase in peak IV latency.