How do biological systems ensure robustness of function despite developmental and environmental variation? Our sense of hearing boasts exquisite sensitivity, precise frequency discrimination, and a broad dynamic range. Experiments and modeling imply, however, that the auditory system achieves this performance for only a narrow range of parameter values. Although the operation of some systems appears to require precise control over parameter values, I describe how the function of the ear might instead be made robust to parameter perturbation. The sensory hair cells of the cochlea are physiologically vulnerable: small changes in parameter values could compromise hair cells' ability to detect stimuli. Most ears, however, remain highly sensitive despite differences in their physical properties. I propose that, rather than exerting tight control over parameters, the auditory system employs a homeostatic mechanism that increases the robustness of its operation to variation in parameter values. To slowly adjust the response to sinusoidal stimulation, the homeostatic mechanism feeds back to its adaptation process a rectified version of the hair bundle's displacement. When homeostasis is enforced, the range of parameter values for which the sensitivity, tuning sharpness, and dynamic range exceed specified thresholds can increase by more than an order of magnitude. Certain characteristics of the hair cell's behavior might provide a means to determine through experiment whether such a mechanism operates in the auditory system. This homeostatic strategy constitutes a general principle by which many biological systems might ensure robustness of function.

A thesis presented to the faculty of The Rockefeller University in partial fulfillment of the requirements for the degree of Doctor of Philosophy