Distortion product otoacoustic emissions in rhesus (Macaca mulatta) monkey ears: normative findings.

Distortion product otoacoustic emissions (DPOAEs) in rhesus monkeys were characterized and the optimal parameters for their generation were determined. Robust DPOAEs were readily measurable from the ear canals of six rhesus monkeys (n = 12 ears). The nonmonotonic behavior of the f2/f1 ratio functions in rhesus monkeys was found to be similar to other animals and humans. The optimal mean f2/f1 ratio of 1.21 and the effect of the primary frequency and level on the optimal f2/f1 ratios were also similar to human measurements. The contour of the rhesus monkey DPOAE 'audiograms' and their behavior were also comparable to human measurements with slight differences in peak frequencies. The rhesus monkey DPOAE input/output (I/O) functions were generally monotonic with a slope approaching unity with increasing frequency. Therefore, our study shows that many basic DPOAE characteristics are remarkably similar in the two species and emphasizes the appropriateness of the rhesus monkey as a model for DPOAE research. Detailed studies of the behavior of DPOAEs can be carried out in a model that is phylogenetically close to human both in hearing and in the gross structure and histology of the inner ear.