Numerical modeling and electro -acoustic stimulus response analysis for cochlear mechanics.

Abstract

A computational framework for cochlear system modeling is developed. It includes a hybrid finite element model that is developed from first principles and that is capable of incorporating various mechanical models of the microstructures and realistic geometry. Using this framework, the solution to a passive 3-d model of the cochlea with a simple geometry is compared to the WKB (Wentzel, Kramer, and Brillouin) solution that is used by most modelers. The disagreement between the two solutions is shown to be due to using the wrong wavenumber for the WKB solution. A matched WKB solution is then developed to include the transfer of energy from the traveling wave to a second evanescent wave as it passes through the <italic>in vacuo</italic> resonance of the basilar membrane. A generalized model for the multi-degree-of-freedom microstructures of the cochlear partition is also developed. Using this model in a two-duct formulation, electrically evoked otoacoustic emissions (EEOAE) are simulated. It is shown that standing waves are formed inside the cochlea by the anti-symmetric pressure component that determines the spectral characteristics of EEOAE. Electro-acoustic experiments are performed to formulate a model for the <italic> in vivo</italic> role of the outer hair cells (OHC). Using a nonlinear transducer model for the force generation by OHC, it is predicted that the OHC stiffness variations that arise from a change in the transmembrane potential could play a more important role than changes in OHC length (electromotility).