Shearing Motion in Scala Media of Cochlear Models

In 1953, Bekesy first reported the occurrence of shearing motion within the cochlear duct and furthermore the notion that it is this mode of motion which constitutes the adequate stimulus for the hair cells. There were two modes of shear motion: radially directed in the region proximal to the place of maximal amplitude of the traveling-wave pattern, and longitudinally directed distal to that point. This phenomenon was studied in cochlear models. The envelopes over both modes of shear motion were found to be much smaller than that over the traveling-wave pattern. A simple explanation presented itself. The direction of the shearing motion in each domain coincides with the dominant curvature produced by the traveling waves in that section of the cochlear duct. Earlier, it had been shown that under the effect of Bekesy's eddies, i.e., at higher driving amplitudes, the displacement pattern of the cochlear partition becomes asymmetrical. This asymmetry produces a dc-shift in both domains of shear motion: in an outward direction within the domain of radial shear and in an apical direction within the domain of longitudinal shear. However, when the round window was driven, both directions of the dc-components were reversed. The latter finding aided in establishing the causal relationship mentioned in the foregoing.