Shearing Motion within Scala Media of Cochlear Models

Abstract

In 1953, Bekesy reported the observation of shearing motion within the cochlear duct, specifically as occurring between the organ of Corti and the tectorial membrane. The direction of this alternating shear motion for any given frequency depended upon the location re the place of maximal transversal displacement of the cochlear duct as a whole. Proximally to the maximum the shear was radially, and distally it was longitudinally directed. This phenomenon was studied with the aid of cochlear models. Not only were Bekesy's original observations confirmed, but the model experiments lend themselves to a simple explanation. Although the displacement of the cochlear duct is commonly represented as a wave pattern with a varying longitudinal curvature only, in reality there are curvatures in both the longitudinal as well as the radial directions because of the fixation of the partition along both of its rims. The shear motion then follows the direction of the dominant curvature in each region which, with reference to the place of maximal transversal displacement, is radially in the proximal region, and longitudinal in the distal region, with a zone of transition at the place of the maximum itself. (Note—directional terms were used here as apply to actual cochleae.) [This work was supported in part by the U. S. Air Force under Contract No. AF 41(657)-148, monitored by the School of Aviation Medicine, U. S. Air Force, Randolph Air Force Base, Texas and by a National Institute of Health Grant No. B-1909.]