Contribution of the reticular lamina motion to the traveling wave: a WKB approach

Optical coherence tomography (OCT) experiments showed that, in the peak region, at low stimulus levels, the motion of the reticular lamina (RL) may be larger than that of the basilar membrane (BM), suggesting that the contribution of the RL motion to the development of a slow traveling wave (TW) could be a relevant one. In this study, a transmission-line cochlear model with two mechanical degrees of freedom at each tonotopic place is used, in which the outer hair cell (OHC) force is represented by a low-pass filtered elastic term proportional to the OHC elongation. The hydrodynamic effects of fluid focusing and viscous damping in the peak region are also included in the model. In the simulations, the contribution of the RL motion to the traveling wave is due to the volume change of the Organ of Corti (OoC), which adds up to the antisymmetric volume change of the two scalae associated with the BM motion. Including the RL motion in the computation of the local wavenumber in the Wentzel-Kramers-Brillouin (WKB) framework implies a change of both the focusing factor and of the real part of the admittance, dependent on the phase of the relative RL-BM motion. To make the RL contribution on focusing the most effective, the BM and the RL should be approximately in phase in the peak region, which, in the model, is consistent with a dominant gain effect of fluid focusing over OHC internal force in the peak region. The negative/positive sign of the RL-BM local phase shift would imply an additional damping/antidamping effect, which is more difficult to predict, and model-dependent.