Influence of cochlear amplifier gain and operating point on distortion-product otoacoustic emissions in a cochlear...

A cochlear model composed of fluid-coupled oscillators was used to predict cubic difference tone (CDT) and quadratic difference tone (QDT) distortion-product otoacoustic emissions (DPOAEs). The model contains a nonlinear feedback force term with either a symmetrically saturating first-order Boltzmann function (BF) or an asymmetrically saturating second-order BF. We directly varied the force gain and also explored how changes in the force operating point (OP), which inherently influence the force amplitude, affect CDT and QDT DPOAEs. CDT DPOAEs evoked by low-level stimuli were more sensitive to the gain manipulations than high-level CDT DPOAEs. However, direct force gain reduction decreased high-level CDT DPOAEs more than the OP shift, which could even increase high-level DPOAEs in the case of the second-order BF. The effects of the gain decline on QDT DPOAEs are more variable across levels than for CDT DPOAEs due to the larger role of distributed sources in QDT DPOAE generation. The presented results suggest that DPOAE growth functions could be used to detect and distinguish between changes in cochlear mechanics due to OP shift and direct gain changes.