A goal of the proposed set of experiments is to perform a multidimensional analysis of distortion-product otoacoustic emissions (DPOAEs). This approach represents an examination of the """"""""complete parametric response"""""""", which will provide a more thorough understanding of the underlying cochlear mechanics and the interrelationships among the various DPOAEs. Such information promises to contribute toward developing more sensitive DPOAE tests for clinical applications. Within this conceptual framework, we intend to examine suppression/enhancement of DPOAEs above f2 using several tactics to help resolve the important issue of how these unanticipated phenomena are produced. Our expectation is that harmonics generated within the cochlea interact to produce difference-tone (f2-fl) emissions that are responsible for these complex effects as opposed to the notion that f3 produces these outcomes via a catalyst mechanism. Moreover, we plan to investigate in detail both lower- (LSB) and upper sideband (USB) DPOAEs, which have received little focused attention in the field, although a great deal of information about cochlear nonlinear ties can be revealed by these particular emissions. We propose to use STC techniques to determine if LBS vs USB DPOAEs have unique sites of generation. Additionally, we believe that their behavior following noise exposure will provide a sensitive index of cochlear abnormality. Further, by carefully examining DPOAEs generated with fixed f2/fl-ratio sweeps, detailed contour-surface plots of the generation of wave vs. place fixed DPOAEs will be obtained. The phase characteristics of these plots provide a means of examining the fundamental 'building blocks' of OAEs based upon nonlinear distortion and linear-coherent reflection processes. This approach will be used to examine changes in nonlinear DPOAE generation and cochlear reflection in normal vs. impaired ears. Finally, according to our own preliminary findings and the published studies of other investigators, growth of suppression appears to be altered following cochlear insult. We plan to develop 'suppression' DP-grams to reveal growth of suppression as a function of frequency so that abnormalities in suppression can be related to changes in cochlear function across a specific damaged cochlear region. The proposed studies will be conducted in both normal and noise-exposed rabbits and, whenever feasible, in both normal-hearing human subjects and patients with noise-induced hearing loss.
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