Cochlear coiling and low-frequency hearing

Surface gravity waves in a spiral channel can be used as an analogue for cochlear macromechanics (Manoussaki et a]. [1]). We found that in a vertical-walled channel with uniform cross section, as a low-frequency wave propagates inward from larger to smaller spiral radii, the wave amplitude near the outside wall grows while the amplitude near the inside wall decreases. This relative amplitude change induces a radial tilt of the free surface, the magnitude of which increases in inverse proportion to the spiral radius. The tilt, which can be interpreted in terms of energy redistribution, can be explained by a "whispering gallery effect," can develop dynamically on the cochlear partition and, by contributing to the bending of apical stereocilia (Cai et al. [2]), can augment low-frequency hearing by as much as 20 dB. We therefore hypothesized that cochlear spiral radii ratios (largest/smallest) could account for interspecies differences in low-frequency hearing. Preliminary analyses of spiral parameters obtained from published data on mammalian species, as well as from histological sections and 3D CT scans of the cochleae of low-frequency baleen whales and high-frequency dolphins and porpoises, tend to support our hypothesis: species with good low-frequency hearing have larger spiral ratios than do those with poor low-frequency hearing.