Debate about whether the cochlear amplifier is more influential at the base of the cochlea as compared to the apex is ongoing. Greater nonlinearity at higher frequencies is an expected outcome of this increasing nonlinearity. Direct measurements of cochlear mechanics and auditory nerve activity in laboratory animals and psychophysical measures in humans have yielded conflicting results to date. Distortion product otoacoustic emission (DPOAE) input output functions (I/O function) offer a noninvasive assay to probe this question in human subjects. Two past studies using DPOAE I/O functions in human subjects have revealed increasing nonlinearity with increasing frequency. These results were limited in the frequency and input stimulus range explored. Employing hardware with reduced system distortion at high frequencies and high levels along with an improved calibration technique, our study extended these findings further in frequency and compared DPOAE I/O functions at multiple frequencies between 0.5 and 14 kHz up to input levels of 95 dB SPL.
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