SUMMARY

The cochlea – a key sensory organ for sound perception in mammals – maps frequencies spatially and amplifies low-level sounds. Sound waves in the ear canal enter the cochlea and propagate along two fluid ducts and a set of structures between them, the organ of Corti, vibrating maximally at a frequency-dependent location. Recent \emph{in vivo} experiments show that the vibratory responses of two structures in the organ of Corti vary in the frequency range of level-dependent nonlinearity they exhibit. These findings motivate the goals of this research, which are to use a cochlear model to identify the underlying mechanisms of the observations, link them to a variety of experimentally observed cochlear phenomena, and provide physical insight that is inaccessible from the experiments alone. A physiologically based, nonlinear, finite element model of the gerbil cochlea with coupled acoustical, mechanical, and electrical domains was calibrated using experiments. From there, preliminary work studied the effect of the model's mechanical parameters on the responses of the sub-structures of the organ of Corti provides insight on the experimentally observed discrepancies. This work proposes the use of the model to improve understanding of other observed phenomena that are linked to cochlear amplification.
The first of these phenomena is the link between perturbations in the resting electrical potential in the top fluid duct and nonlinearity in cochlear responses. The connection between this electrical potential and the nonlinear mechanoelectrical transduction current that drives amplification has been identified experimentally, and the model will be used to explore its implications.
Another phenomenon linked to cochlear amplification is that of spontaneous otoacoustic emissions - sharp spectral peaks that exist without external acoustic stimuli and that are measurable non-invasively in the ear canal. These are present in most humans, and understanding how they are generated is an important step to understanding overall cochlear function. A linear theory of how they arise is coherent reflection, whereby standing waves formed by reflections in the cochlea are globally amplified. A key test of the physiologically motivated model is to compare its predictions of emissions to those from the coherent reflection theory and to experiments. Model parameters will then be adjusted to assess which are key in predicting emissions that are in line with experiment.