Sound Induced Changes in Electrically Evoked Cochlear Emissions

Abstract

We have previously proposed that the outer hair cells (OHC) interact with the hydromechanical elements of the cochlea to form a series of active filters which provide the high sensitivity and frequency selectivity observed in the responses of inner hair cells (IHC) and auditory nerve fibers (Mountain et ai, 1983). The active filtering is the result of a feedback loop consisting of the mechanical-to-electrical (forward) transduction process driving an electrical-to-mechanical (reverse) transduction process located in the OHC. The reverse transduction response may be due to haircell length changes or stereociliary motion, both of which have been demonstrated in vitro using injected current (Brownell et al.,1985; Ashmore, 1985; Crawford and Fettiplace, 1985). For acoustic frequencies well below the characteristic frequency of a given cochlear location the feedback is negative. This negative feedback serves to increase the apparent stiffness of the basilar membrane for low frequency stimuli. Support for this active-stiffness hypothesis has come from experiments in which we have injected sinusoidal current into scala media and measured acoustic response (emissions) in the ear canal of experimental animals (Mountain et al, 1983; Hubbard and Mountain, 1983; Mountain and Hubbard, 1989; Hubbard and Mountain, 1990).