A detailed study of distortion product otoacoustic emission fine structure in normal -hearing adult ears
DPOAEs measured in the ear canal consist of two major components, one due to the nonlinearities at the overlap region between the primaries and the other due to a reflection from the DP tonotopic area. Interaction between these two components results in DPOAE fine structure (the pseudoperiodic variation of DPOAE level with frequency). The amplitudes of the two components were extracted using an IFFT of the amplitude fine-structure and phase. When primary ratio was varied (from 1.053 to 1.36), the amplitude of the component from the overlap region was band-pass in character with a peak near a ratio of 1.22. The pattern was very similar in all subjects. The characteristics of the reflected component from the DP area varied considerably between subjects. The ratio-function of the reflected component was low-pass in cochleae with low levels of activity, but was band-pass in more active cochleae. The differences in the ratio-functions of the two components meant that the reflected component from the DP region was larger than the generator component at narrow ratios. The overlap component was larger at mid ratios and the relative magnitudes of the components were dependent on the activity level of the cochlea at wide ratios. Both components saturated with increasing primary levels with saturation being more pronounced for the reflected component. ^ In addition to the two principal components, internal reflections within the cochlea produced components with longer latencies leading to a rich family of fine-structure shapes. The two-source interference model as proposed by Talmadge et al. (1998) was used as a framework for experimental design and result interpretation. Although the results were in general agreement with model predictions, they pointed towards more complex mechanisms involved in the generation of DPOAEs. Implications of these results for clinical applications of DPOAEs are also discussed. ^
Major Professor: Glenis R. Long, Purdue University.
Health Sciences, Audiology|Biophysics, General
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