Despite great progress in understanding many aspects of cochlear function, knowledge in the key area of cochlear micromechanics is rudimentary and knowledge of apical macromechanics is almost absent. In the conventional view, basilar-membrane motion bends inner-hair-cell (IHC) stereocilia by a single vibrational pattern and there is a single traveling wave along the basilar-membrane. However, this view does not fit with our recent work which shows there are multiple excitation drives or with mechanical and neural data showing multiple group delays in the apex. Several lines of evidence point to a new conception of cochlear micromechanics in which the organ of Corti vibrates in modes, each with its own resonant frequency and each providing an excitation drive to IHC stereocilia. With the cochlear partition allowed to move in multiple, overlapping motions, there can be multiple traveling waves. Recordings from single auditory-nerve fibers reveal the multiple resonances and are ideally suited for tracking these resonances along the cochlea. The proposed work will (1) distinguish the excitation drives that affects the responses of individual auditory-nerve fibers, map them along the cochlea, and determine their traveling wave velocities, (2) determine how these resonances are affected by efferent stimulation and low-frequency """"""""bias"""""""" tones, and (3) test the hypothesis that a profound cochlear nonlinearity control the transition between certain modes. The proposed experiments will test the hypothesis suggested by our preliminary results that there ate two overlapping traveling waves. Our results will provide data that will flesh out a new picture of cochlear mechanics and provide a rich source of data for the formation of new experiments, and new models for the biophysical and cell-biological basis of the cochlear resonances. Since auditory-nerve-fiber experiment do not invade the cochlea, our characterization of the resonant modes present in an intact, normally-functioning cochlea will provide a gold standard that can be used to determine the normally of the modes seen and any invasive experiment. Obtaining an overall functional characterization at this pivotal mechanical level is essential for understanding how outer -hair-cell motility and other cellular and structure properties of the cochlea produce the cochlear amplifier and lead to output of the cochlea.
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