The long term goal of the proposed research is to confirm that outer hair cells are responsible for electromechanical amplification in the cochlear and to clarify the underlying mechanisms of this amplification. The specific goals of the proposal are to determine which properties of the organ of Corti are responsible for the cochlear frequency map, to establish how the OHCs amplify the traveling wave, to determine how the motions of the cochlear partition is coupled to IHC stereocilia motion, and to establish how otacoustic emissions relate to the cochlea partition motion. To achieve these goals, the investigators will measure the mechanical impedance of the cellular and extracellular elements of the organ of Corti, to image the motion of the elements of the organ of Corti in response to electrical and mechanical stimulation in excised cochleas, and to simultaneously measure basilar membrane motion, cochlear microphonics, and otacoustic emissions in live animals. The results of the proposed experiments together with previous experiments by the investigators and others will be interpreted with the aid of computational models. Two types of models will be used: micromechanical and macromechanical. The computational models will be physically based as possible and will include specific descriptions of the mechanical properties of the organ of Corti, basilar membrane, and tectorial membrane, as well as hair-cell mechanoelectric transduction and OHC motility.