Student Theses and Dissertations

Date of Award

2009

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

RU Laboratory

Hudspeth Laboratory

Abstract

The role of the ear is to detect sound. Many ears, however, can also generate sounds and transmit them into the environment. These sounds, known as spontaneous otoacoustic emissions, are the most striking proof of the existence of an active process within the ear. This active process originates in hair cells, the sensory receptors of the inner ear; by augmenting the energy contained in sound, it improves both the sensitivity and the frequency selectivity of hearing. Under certain conditions, the active process generates spontaneous oscillations, which then produce spontaneous emissions. Little is known about the process by which the spontaneous oscillations of a collection of hair cells within the ear are transformed into a particular sound that can be measured externally. Here, we suggest that interactions between tonotopically arranged hair cells play a critical role in the formation of spontaneous otoacoustic emissions. We have created a model based on the basilar papilla of the tokay gecko, a species notable both for the unusual anatomy of its inner ear and for the prominence of its spontaneous emissions. In this model, the papilla is represented by a tonotopic array of nonlinear van der Pol oscillators, with connections along the array mediated by either viscous or elastic elements. We find that a model incorporating elastic interactions, but not one incorporating viscous interactions, reproduces the appearance of spontaneous emissions. The model also suggests a prominent role for disorder along the length of the papilla. Finally, the model explains what may be the most distinctive feature of spontaneous emissions: the repulsion of spectral peaks within the emission by pure-tone external stimuli. To test our model, we also recorded spontaneous otoacoustic emissions from the tokay gecko. Previous recordings of emissions have primarily focused on the behavior of single peaks within the emission spectrum. We chose instead to focus on the relationships between multiple emission peaks. Upon applying single-frequency tones at a range of amplitudes and frequencies into the ears of geckos we have observed novel behaviors previously predicted by our model. We conclude that interactions along the basilar papilla, mediated by elastic elements, play an important role in the formation of spontaneous otoacoustic emissions.

Comments

A Thesis Presented to the Faculty of The Rockefeller University In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

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