Fast waves, slow waves and cochlear excitation

Abstract

In idealized cochlear models [especially, Peterson and Bogert, J. Acoust. Soc. Am. (1950)], intracochlear pressure is decomposed into two modes, the compression pressure (fast mode) and the traveling wave pressure (slow mode). Because the cochlear fluid is nearly incompressible, only the slow mode leads to significant motion. In the real cochlea, additional fast modes exist. These evanescent modes are similar to the traveling wave mode in driving significant fluid and tissue displacement. They are present in the region of the cochlear windows, where the anatomy is not the ideal symmetric structure of basic cochlear models. Evanescent modes also have emerged in experimental work in which cochleostomies are made in the apex. At high stimulus level fast modes are able excite hair cells, leading to auditory nerve responses. However, fast mode motions do not seem to be amplified by the cochlear amplifier. This observation supports the concept that the amplifier relies on traveling wave curvature, as has been proposed in cochlear models [for example, Yoon et al., Biophys. J. (2011)]. I will review experimental and theoretical results on the fast and slow waves, and propose experiments in which wavelength-based theories of amplification could be tested.