Two-tone rate suppression in auditory-nerve fibers: dependence on suppressor frequency and level

The growth of two-tone rate suppression with suppressor level was studied for auditory-nerve fibers in anesthetized cats. The level of a tone at the characteristic frequency (CF) was adjusted by an adaptive procedure (PEST) so that, when presented with a suppressor tone, the CF tone would produce a criterion discharge rate. Suppression (in dB) was defined as the CF-tone level that met criterion in the presence of a suppressor minus the level that met criterion in quiet. The growth of suppression with suppressor level was well characterized by a straight line whose slope (in dB-excitor/dB-suppressor) varied with suppressor frequency by as much as a factor of 10 in the same fiber. These slope differences were systematically related to the position of the suppressor frequency relative to the fiber CF: for below-CF suppressors, slopes ranged from 1 to 3 dB/dB, while, for above-CF suppressors, they were between 0.15 and 0.7 dB/dB. Slopes decreased rapidly with increasing suppressor frequency near the CF, but, for frequencies well below the CF, the slope reached a maximum that increased gradually with CF. These results resemble psychophysical data on the growth of masking and psychophysical suppression, and pose difficulties for existing models of two-tone suppression.