Experiments on the origin of two-tone suppression

Two-tone suppression has been shown to exist in virtually every response of cochlear origin. For comparable stimulus conditions, however, suppression magnitude measured in responses of hair cells and auditory nerve fibers is considerably greater than suppression magnitude measured in basilar membrane displacement patterns, and tho mechanical suppression threshold appears to be higher than the auditory nerve-fiber suppression threshold. We recorded responses of single auditory nerve fibers in anesthetized cats and used discharge synchronization as an index of suppression magnitude. It was found that (a) when “notches” in input-output rate functions exist, suppressed responses cannot be made to occur to the right of the upper leg of the unsuppressed function; (b) unlike results obtained for cochlear microphonics, the discharge phase angle of a suppressed response in auditory nerve fibers is the same as the angle of the pure-tone response at the same intensity; and (c) for low-frequency suppressors, the rate of growth of suppression with increasing suppressor-tone intensity can easily exceed the presumed rate of growth of basilar membrane displacement. A possible explanation is that there are two generators for suppression. One generator operates at high intensities and originates in basilar membrane mechanics. The other generator operates at all suprathreshold intensities and originates either in the micromechanics of the subtectorial space or in the mechanical-to-neural transduction process occurring inside hair cells. [Work supported by NIH.]