Micromechanics in the theory of cochlear mechanics

Abstract The consensus of empirical data suggests that the shear motion between the tectorial membrane and the reticular lamina, which is believed to control the stimulation of the hair cells, is more sharply tuned than the transversal vibration of the basilar membrane. Interaction between the length of basilar-membrane waves and the longitudinal coupling within the tectorial membrane may contribute to the required sharpening. However, such a mechanism is unlikely to be sufficient, acting alone. Simple calculations indicate that damping of the tectorial membrane coupled viscoelastically to the organ of Corti is low enough to allow the system to exhibit local radial responses above about 150 Hz. Some effects of such resonances are illustrated by means of simple mechanical models. In particular, it is demonstrated that radial vibration of the tectorial membrane nearly at right angles to the transversal vibration of the basilar membrane constitutes an inherently nonlinear system. The radial vibration can produce strong loading of the basilar membrane, affecting its tuning and introducing nonlinearities. Nonlinear longitudinal coupling within the tectorial membrane, if present, can produce quadratic and cubic distortion products and strong two-tone suppression by second tones at frequencies above and below the resonance frequency. Such a coupling is suggested by the finding that, in the alligator lizard, the part of auditory papilla endowed with a tectorial membrane produces both effects, but the part not so endowed does not.