Gain of cochlear amplifier affects the shape of the distortion-product otoacoustic emission growth function

Distortion-product otoacoustic emissions (DPOAEs) seem to be very useful in the detection of mild hearing loss in which the hearing threshold declines by about 20–30 dB. We simulated this amount of hearing loss in a nonlinear cochlear model, which simulates the transversal displacement of the basilar membrane (BM) modeled as an array of fluid-coupled oscillators with undamping feedback force. This force transformed by the second-order Boltzmann function is proportional to the displacement of a second array of oscillators driven by the BM acceleration. The model predicts nonlinearly growing DPOAE amplitude for stimulus intensities following the “scissor” paradigm (L1 = 0.4L2 + 39 dB SPL). Because the decline in cochlear amplification affects the DPOAEs evoked by low-intensity stimuli more than high-intensity stimuli, the cochlear gain changes the shape of the DPOAE growth function. This change in the shape could provide important information for objective detection of hearing loss. We measured the growth of DPOAE amplitude in young normally hearing subjects (<30 years). The shape of their DPOAE growth functions was very similar despite relatively large changes in the DPOAE amplitude, which supports our simulations. DPOAE growth functions measured in several older subjects with slightly worse audiograms also confirm the simulated results.