Exploring auditory frequency selectivity using otoacoustic emissions

Abstract

Frequency selectivity—the ability of the auditory system to separate one stimulus out from others on the basis of frequency—originates in the cochlea where outer hair cells (OHCs) provide sharp mechanical tuning at low signal levels. Whether humans have sharper cochlear tuning compared to common laboratory species has been a matter of scientific debate. Whereas cochlear tuning can be measured directly in animal models, similar tests cannot be performed in humans due to their invasiveness. However, cochlear tuning can be gauged from measurements of other OHC-dependent phenomena, such as otoacoustic emissions (OAEs). Here, we compare frequency selectivity derived from stimulus-frequency (SF)OAE suppression tuning curves measured in both humans and chinchillas. SFOAE suppression tuning curves have previously been shown to be as sharply tuned as compound-action-potential tuning curves in chinchillas and behavioral tuning curves in humans (when using simultaneous masking). These earlier findings support the ideas that: (1) OAE tuning curves reflect aspects of cochlear tuning and (2) Sharp frequency selectivity is primarily established at the level of the cochlea. After correcting for interspecies differences in the apical-basal transition frequency and in cochlear lengths, we find that SFOAE tuning curves are twice as sharp in humans as in chinchillas.