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. Lifestyle factors such as diet and excessive noise exposure are controlled in monkeys from the breeding colony of the Wisconsin Regional Primate Research Center. Twenty younger (mean age: 10 years, 9 months) and twenty older (mean age: 25 years, 11 months) monkeys were used in this study. Cochlear function in these monkeys was measured with distortion product otoacoustic emissions (DPOAEs) whereas neural function was measured with auditory brainstem responses (ABRs) and middle latency responses (MLRs). Older monkeys had significantly smaller DPOAE amplitudes compared to younger monkeys. Older monkeys had significantly longer peak IV latencies and significantly smaller peak IV amplitudes than did younger monkeys. Also, older monkeys had fewer measurable peak IVs at lower stimulus levels whereas almost all younger monkeys had measurable peak IVs at the lowest stimulus level. The older monkeys had smaller peak Pa amplitudes than the younger monkeys although peak Pa latency was similar between the two age groups. There were no significant gender differences for DPOAE amplitude, peak IV latency, peak IV amplitude, or peak Pa measures. Based on DPOAE, ABR, and MLR measures, older monkeys have decreased cochlear and neural function compared to younger monkeys. The decrease in DPOAE amplitude and age explained approximately 70% of the variability associated with changes in peak IV latency. The remaining 30% could be attributed to atherosclerosis, high blood pressure, high cholesterol, or some other unknown health condition.