SUPPRESSION IN AUDITORY-NERVE FIBERS OF CATS USING LOW-SIDE SUPPRESSORS .2. EFFECT OF SPONTANEOUS RATES

The responses of auditory nerve fibers with different spontaneous rate s were studied in anesthetized cats, using harmonically related charac teristic frequency (CF) tone and suppressor (SUP) tone (50-2000 Hz) as stimuli. The relative-response index, defined as the ratio of the max imum response level in the two-tone segment to the response level in t he CF-alone segment, at or near the intensity of maximum suppression ( i.e., where the two-tone rate was lowest), was dependent on fiber's sp ontaneous rate (SR). For all the SUP frequencies used, lower-SR fibers almost always showed values less than unity, while high-SR fibers alm ost always gave values near or greater than unity, The phase of maximu m suppression was not dependent upon fiber SR. In one experiment, a pa ir of low- and high-SR fibers with the same CF (12 kHz) were recorded consecutively in the same electrode penetration, and were studied with the same stimulus parameters. Their temporal responses showed dramati c temporal resemblances, with very similar phases of suppression and r esponse. But the relative-response indexes were different. The similar ities in the lower- and high-SR fibers support the idea that the basic response and suppression patterns in all fibers are formed at or befo re the inner hair cell (MC) stage, while the differences suggest that processes more central than the IHC receptor potential are important i n determining the magnitudes of suppression, particularly in the lower -SR fibers.