Longitudinal elasticity of the cochlear partition and distortion product otoacoustic emissions: How dead ears produce sounds

Sounds generated by the inner ear are known as otoacoustic emissions and are produced by active motile hair cells in the cochlear sensory epithelium. Damage to these cells impairs the sensitivity of the ear and compromises its ability to generate sounds. Consequently, otoacoustic emissions hold substantial clinical significance in that they provide an objective means of diagnosing cochlear dysfunction and damage. Despite the well-established notion that otoacoustic emissions are produced actively in functional living ears, they have also been recorded in necrotic, hypoxic, and pharmacologically traumatized cochlea where the active hair cells are presumably no longer functioning. In a sense, these findings are paradoxical. It should not be possible to elicit a response associated with healthy cochleae from those suffering from trauma How is it possible for the dead ear to produce sounds?; One possible answer centers on the sensory epithelium that houses the sensory elements of the cochlea Currently, the sensory epithelium is considered to be composed of tuned resonant unconnected sections. Based on recent experimental evidence and the behavioral features of otoacoustic emissions, this view is challenged in this thesis. The hypothesis presented holds that each section is coupled to its neighbors elastically through connective epithelial tissue. The consequence of such coupling is the introduction of a cubic partial differential nonlinearity into the cochlear dynamic equations. Such a model is capable of producing cubic distortions as large as 7 dB(SPL) while sharing a number of similarities to active cochlear function. The non-linearity was found to be weakly compressive, capable of subdividing the traveling wave, and capable of altering the resonance properties of the sensory epithelium. The emissions produced maintained the dominance of the 2f₁ − f₂ tone above the other cubic distortions while demonstrating level asymmetry, fine harmonic structure, and chaotic period doubling behavior. The emissions were also produced at multiple sites and increased in level for larger tissue elastic moduli. The many shared similarities and differences with real emissions provide a deeper understanding of the emission process and its origins as a whole, and challenge the widely accepted view that stiff longitudinal coupling within the cochlea can be ignored.