Search

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. [ABSTRACT FROM AUTHOR]

Stimulus-frequency otoacoustic emissions (SFOAEs) are evoked by a single tone presented into the ear. The emissions are generated by reflection of a forward traveling wave (TW) on localized irregularities in the impedance along the basilar membrane (BM). The strongest wavelets are reflected from the place where the TW reaches its largest amplitude. Since the irregularities are localized in fixed places along the basilar membrane, the latency of the SFOAE phase can be used to estimate delay of cochlear filters. We used an iterative approach of Shera et al. [JASA (2005) 118:287-313] to obtain an analytical solution for SFOAEs in a nonlinear two-dimensional cochlear model. The solution allowed for decomposition of a reflection component and a component due to perturbation of the nonlinear force. The nonlinear force which in the smooth cochlea may reflect the forward TW backwards [Talmadge et al., JASA (2000) 108, 2911-2931], is perturbed due to irregularities. The perturbed nonlinear force in the model generates an SFOAE component with latency comparable with the latency of the component due to reflection. This means that although the component is evoked due to nonlinear force, it has a comparable (long) latency to the SFOAE component due to reflection. The amplitude of this component due to perturbation of the nonlinear force grows as the amplitude of irregularities increases. In addition, the component amplitude also grows with the tone intensity. The growth is determined by the amplitude of the nonlinear force. It is almost cubic at the lowest intensities and quickly saturates at levels above about 40 dB SPL (the intensity is determined by the position of the compressive nonlinearity in the input/output function of the basilar membrane displacement). An interesting result is that the nonlinear component partly cancels the reflection component because its phase is shifted by approximately 0.5 cycles, independent of stimulus intensity and frequency. As the level increases, the component due to perturbation reaches an amplitude comparable to the component due to coherent reflection, especially in the portions of SFOAEs with the longest latencies. The destructive interference between the components due to perturbation of the nonlinear force and coherent reflection emphasizes shorter latency wavelets in the overall SFOAEs as stimulus intensity increases and contributes to the saturation of SFOAE amplitude reported, for example, in human experimental data of Abdala and Kalluri [JASA (2017) 142, 812-824]. [ABSTRACT FROM AUTHOR]