Our goal is to understand how outer hair cells (OHCs) produce the high sensitivity of mammalian hearing. Great progress has been made in understanding the properties of isolated OHCs, but there are fundamental unanswered questions about how OHCs work in intact cochleas. Medial olivocochlear (MOC) efferent neurons innervate OHCs and provide an important tool for understanding OHC-based mechanisms. To study the mechanical effects of OHCs in vivo, we will measure basilar-membrane (BM) motion and auditory-nerve (AN) firing patterns and the changes produced in these responses by electrical stimulation of MOC efferents. Our recent work provides strong evidence for the existence of a second traveling wave along the cochlea, a wave that may couple OHC motility to BM motion. The proposed work will characterize the properties of this second wave and its relationship to the classic transverse traveling wave. This wil be done (1) at frequencies well below the local characteristic frequency (CF), where stiffness dominates the mechanical response and the effects of the two traveling waves can be studied without the complications produced by cochlear amplification, and (2) at frequencies near CF where the cochlear amplifier has its primary effects. Our work, and that of others, has shown that the """"""""slow effect"""""""" of MOC stimulation is due to a decrease in OHC stiffness. This stiffness change will be exploited to determine how OHC stiffness affects cochlear properties such as CF, and to test the OHC piezoelectric theory, a theory that may account for how OHC somatic motility can be effective at frequencies above the OHC membrane cut-off frequency. The results of the proposed work will help to flesh out a new picture of cochlear mechanics. Mechanics is a key area of cochlear function that is disrupted by many of the pathologies that affect hearing. Understanding cochlear mechanics, and the role of OHCs in cochlear mechanics, is essential for progress in almost all aspects of hearing.
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