Effects of excitatory and non-excitatory suppressor tones on two-tone rate suppression in auditory nerve fibers.

Recordings were obtained from individual auditory nerve fibers in anesthetized chinchillas. Rate versus level functions were obtained for best frequency (BF) tones alone and for simultaneously-gated tone pairs comprising a BF tone and a second tone at a fixed intensity that produced evidence of two-tone rate suppression. Care was taken in selecting a range of suppressor tone levels that included excitatory (i.e., the suppressor tone evoked a rate change by itself) and non-excitatory (i.e., no suppressor tone-evoked rate increase) suppressor tone levels. Addition of a suppressor tone produced a shift of the dynamic range portion of the BF rate versus level function to higher test intensities. A parallel shift of the dynamic range portion of the rate versus level function was associated with a non-excitatory suppressor tone. The shift produced by an excitatory suppressor tone was characterized by a decrease in slope. Results indicated that the magnitude of shift increased monotonically as suppressor tone intensity was raised and that there was a gradual transition from a non-excitatory response to an excitatory response. The rate of shift (i.e., dB of shift per dB change in suppressor tone intensity) did not differ for non-excitatory versus excitatory responses, but was substantially greater for below-BF suppressor tones (1.38 dB/dB) than for above-BF suppressor tones (0.54 dB/dB). The rate of shift did not, however, appear to be related systematically to suppressor tone frequency separation from BF. Above- and below-BF suppression was noted for fibers over the range of best frequencies tested (110 Hz to 16.4 kHz).