Adopting Cochlear Place-Specific Stimulus Properties to Improve the Accuracy of Distortion Product Otoacoustic Emission MeasurementsPublic
The nonlinear attributes of cochlear function are fascinating. Responses are compressed with surprisingly little distortion, different stimuli interact in competitive ways, and tones are created either with or without acoustic stimulation. Moreover, because of their dynamism, these nonlinear phenomena have provided an invaluable means for noninvasively probing and understanding cochlear mechanisms using distortion product otoacoustic emissions (DPOAEs). DPOAEs are faint acoustic signals recorded using a sensitive microphone sealed in the ear canal. These DPOAEs originate as a byproduct of the active cochlear process and thus provide a window into the mechanisms underlying peripheral auditory function. Because of the benefits associated with exploiting DPOAE measurements, DPOAEs have been incorporated in clinical settings, primarily for the purposes of screening for hearing loss. However, variability in measurement between tests and when compared to behavioral hearing sensitivity has resulted in DPOAEs being limited in their clinical use. Since the level of the DPOAE recorded in the ear canal depends on the frequency and level characteristics of the stimuli used to elicit it, the choice of stimulus parameters may influence the success with which DPOAEs can be used to assess cochlear function and predict auditory status. DPOAEs are elicited when the nonlinear interaction between the two stimulus tones generates a distortion waveform at a frequency different from the stimulus tones. Changing the stimulus frequency relationship changes the number of distortion generators (i.e., outer hair cells), which changes the characteristics of the distortion produced that eventually travels back to the ear canal. Because the spatial properties of the traveling waves are determined by local cochlear mechanics, which change from base to apex, the frequency and level characteristics between the stimulus tones need to be adjusted as a function of frequency to maintain optimal interaction between them. Therefore, the timely and accurate detection of cochlear pathology remains suboptimal as current protocols: 1) do not consider the changing local cochlear mechanical properties when using fixed stimulus parameters, and 2) do not assess the entire length of the cochlea through the base, where high frequencies are encoded and where environmental and age-related cochlear decline first manifests. The purpose of this work was to develop a measurement protocol guided by cochlear mechanical properties to derive physiologically motivated and locally appropriate stimulus parameters up to the highest frequencies of human hearing. This dissertation explored and quantified DPOAE responses across a wide range of stimulus parameter settings in young and middle-aged adults with either audiometrically normal hearing sensitivity or sensorineural hearing loss. We hypothesized that DPOAE stimulus parameters that are optimized to the region around the cochlear place of stimulation would evoke emissions that accurately reflect cochlear health and, therefore, would be closely related to behavioral thresholds. DPOAE responses were then used to explore more complex ideas and theoretical frameworks regarding human DPOAE generation and propagation. Overall, stimulus frequency ratio and levels that were adjusted in a frequency-specific manner maximized DPOAE generation and improved test performance for measuring normal cochlear function and screening for hearing loss.
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