We have discovered a new cochlear motion that plays a major role in the neural coding of sound in the apical half of the cochlea, the region important for human speech. This new motion is not contained in the transverse vibration of the classic traveling wave that is shown by basilar-membrane (BM) motion in the cochlear base. Stimulation of medial olivocochlear (MOC) efferents that innervate outer hair cells (OHCs) inhibits this new motion, implying that it is produced, or amplified, by OHCs. Understanding what is actually moving in the new motion, the properties of this new motion, and its effect on neural coding, promises to have a profound impact on our view of cochlear mechanics. To achieve this, we propose three Aims: (1) To find the mechanical manifestation of the new motion and understand how it is produced or amplified by OHCs, we will measure the motion of structures in the organ of Corti of the apical turn, and determine how these motions are affected by MOC stimulation and bias tones. (2) To characterize the properties of the new motion and understand how it shapes the neural coding of sound throughout the cochlea, we will measure auditory-nerve (AN) responses, determine which response features are produced by the new motion vs. by the classic transverse wave, and determine the properties of these response features. (3) To reveal interactions of classic transverse vibration with other motions, interactions that may be the basis of cochlear amplification, we will measure MOC effects on basal-turn BM and AN responses in parallel experiments. The results of the proposed work will flesh out a new picture of cochlear mechanics especially for speech frequencies. Mechanics is a key area of cochlear function that is disrupted by many inherited and acquired pathologies that affect hearing. The lack of a correct conceptual framework for cochlear mechanics hinders progress in understanding, diagnosing, and treating these pathologies. Understanding cochlear mechanics, and the role of OHCs in mechanics, is essential for progress in almost all aspects of hearing.
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