The cortilymph wave: Its relation to the traveling wave, auditory-nerve responses, and low-frequency downward glides

Abstract

In the cochlear base, recent data show that amplification of the traveling wave does not come from outer-hair-cell (OHC) forces acting on the basilar membrane (BM). Instead, traveling wave amplification is hypothesized to come from OHCs producing cyclic cortilymph flow along the organ-of-Corti (OoC) tunnels (the “cortilymph wave”), which changes OoC cross-section area and adds energy to the scala-media-fluid traveling wave. This hypothesis accounts for amplification of cochlear-motion in the base but may not work in the low-frequency apex. One base-to-apex difference is the OHC-membrane resistance-capacitance (RC) low-pass filter. Measurements in live animals found the OHC-RC corner frequency, Fc, was ∼3 kHz. At tone frequencies >>Fc, the RC filter delays the cortilymph wave from the traveling wave by ¼ cycle, which provides the correct timing for OoC-area-change traveling-wave amplification. However, at frequencies <<Fc, the RC-filter delay is shorter, so traveling-wave amplification may be different in the low-frequency cochlear apex. A source of data for understanding low-frequency cochlear mechanics is auditory-nerve (AN) data. AN rate-vs-level functions show two components separated by a phase jump. The frequency relative to the fiber characteristic frequency (CF) where the jump is a phase reversal varies across fibers. This variation is hypothesized to be due to a variable phase relationship of the cortilymph and traveling waves. It is further hypothesized (1) that low-CF AN-tuning-curve “side lobes” have short group delays because they are excited by a cortilymph wave that flows ahead of the traveling wave, and (2) low-CF AN-fiber impulse-response instantaneous-frequency-versus-time profiles (glides) are downward mostly due to a cortilymph wave driven from more basal OHCs. A long downward glide may indicate the presence of a cortilymph wave. Cochlear output is driven by both the traveling wave and the cortilymph wave.