Understanding Cochlear Amplification and Otoacoustic Emissions

University Of Southern California, Los Angeles CA

Investigators

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Abstract

Project Summary Realizing the full potential of otoacoustic emissions (OAEs) as noninvasive probes of cochlear function requires understanding the physical and physiological mechanisms that generate and shape these sounds. To address important unresolved issues of cochlear mechanics while improving our understanding of OAE generation, we propose three aims involving innovative theoretical modeling rigorously tested by experimental measurements. The first Aim seeks to understand how the active, nonlinear cochlea analyzes frequency sweeps, an ecologically important and readily manipulable class of dynamic sounds. Through an innovative mix of behavior, physiology, and theoretical modeling we will test our hypothesis that up-down differences in behavioral masking, cochlear suppression, and the generation of reflection-source OAEs all share a similar, non-monotonic dependence on sweep rate and direction, consistent with the involvement of temporal suppression. As an important control, we will evaluate possible contributions from feedback-based auditory reflexes. We will interpret our results using models of cochlear wave amplification and OAE generation to test the hypothesis that the measured response patterns arise via mechanisms involving interactions between traveling-wave dispersion and nonlinear suppression. The second Aim explores the micromechanics of cochlear wave amplification and its consequences for the generation of distortion-product OAEs (DPOAEs). We study DPOAE generation in tightly controlled models that incorporate a variable mix of both local (classical) and nonlocal (push-pull) amplification, the latter as suggested by the prevailing interpretation of the oblique geometry of the outer hair cells within the organ of Corti. The project will determine how the known properties of DPOAEs constrain the nature of the active forces responsible for boosting the sensitivity of hearing. The third Aim studies contributions to cochlear gain arising not directly via active forces within the organ of Corti but via hydrodynamic effects (“pressure focusing”) that depend on fluid coupling within the geometry of the cochlear duct. Our methods allow us to vary the hydrodynamic coupling independent of cochlear micromechanics, enabling us to evaluate the robustness of a promising new method for understanding the functional role of the many details of cytoarchitecture. Completion of these Aims will significantly enhance our understanding of OAE generation and its relationship to cochlear amplification and nonlinearity. The knowledge we gain is also directly relevant to our long-term goal of improving the power of OAE-based diagnostics and other technological applications—such as hearing aids and preprocessors for speech-recognition devices—that benefit from the knowledge of cochlear amplification, nonlinearity, and signal processing.