Level dependence of distortion-product otoacoustic emissions and intracochlear distortion products in mice

Distortion-product (DP) otoacoustic emissions (DPOAEs) provide a noninvasive window onto cochlear mechanics and outer hair cell (OHC) function. While the growth of DPOAEs with stimulus level has been used to infer cochlear sensitivity and characterize aspects of the underlying nonlinearity, the relationship between the level dependence of DPOAEs and intracochlear DPs remains poorly understood. Here, this relationship was assessed by comparing DPOAEs with DPs measured from the apex of the live mouse cochlea using optical coherence tomography. With the f 2 stimulus frequency fixed at the characteristic frequency of the measurement location (∼9 kHz), the dependence of 2f 1 − f 2 DPOAEs and intracochlear DPs on the f 1 stimulus frequency and the levels of both stimulus tones was thoroughly examined. For f 2/f 1 ratios eliciting the largest DPOAE amplitudes, the growth rates of DPs and intracochlear DPs were often similar at low stimulus levels. However, at small f 2/f 1 ratios and/or high stimulus levels, DPOAEs could exhibit sharp notches and/or growth rates that were markedly different from those observed for the intracochlear DPs. The results are consistent with DPOAEs being strongly shaped by wave interference between spatially distributed sources. These interference effects and their stimulus dependence must be considered when interpreting DPOAE growth patterns.