Two-tone suppression and power balance in a 2D nonlinear cochlear model

Two-tone suppression (TTS) is the nonlinear phenomenon in which cochlear responses in the region most sensitive to a probe tone are reduced if a second tone (suppressor) of different frequency is simultaneously presented. TTS occurs due to saturation of mechanoelectrical transducers (MET) in the outer hair cells. The nonlinear dependence of MET channel conductance is the main cause of nonlinearity in cochlear amplification. Although the cochlear amplifier primarily acts locally, it influences distant fluid coupled parts of the basilar membrane (BM). The question therefore arises, how the fluid coupling contributes to the TTS. To answer this question, TTS was analysed using a 2D nonlinear cochlear model in two cases: for a suppressor of higher frequency and for a suppressor of lower frequency than the probe tone. It is shown that the BM-BM hydrodynamical coupling redistributed the excess of the power generated by the OHC electromechanical feedback force. It is also shown that the short-range part of the BM-BM coupling plays an important role in power balance along the BM. Moreover, it turned out from the model simulations that the suppression by a low-frequency suppressor causes a basal shift of the amplitude maximum and affects the phase of the BM stationary response. In contrast, the suppression by a high-frequency suppressor does not significantly affect the BM response phase; it flattens the amplitude of the BM stationary response, which leads to an apical shift of the amplitude maximum.