Local wave dispersion in the cochlea

Fine tuning in the cochlea is attributed to interactions in the gap between the tectorial membrane (TM) and the reticular lamina (RL). Acoustically driven traveling waves are controlled by outer-hair-cell (OHC) activity triggered in response to TM–RL relative motions. Variable geometry makes the dispersion of these waves location-specific. Here, such local dispersion diagrams (complex wave number, k, frequency, ω, axial position, z) for the cochlea with emphasis on the manifestation of dispersion within the TM–RL gap are constructed. The model employs a two-mode cochlea cross section that includes fluid loading both within and outside the gap. The two degrees of freedom represent basilar and tectorial membrane motion. Insights are sought into the allowed wave types, their localization along the cochlea, and the nature of the vibration modes. Future, more sophisticated models follow from these insights. Here, it is found that the gap presents a complex impedance to the traveling waves. The dissipative component of the gap impedance is strongly dependent on location z, for a given k and ω. This greatly affects the TM–RL relative phase, and hence, for a given frequency, a place principle.