Suppression in auditory-nerve fibers of cats using low-side suppressors. I. Temporal aspects.

Two-tone suppression was studied in the auditory nerve fibers of anesthetized cats, using low-frequency suppressors (50-2000 Hz). The response to the characteristic-frequency (CF) tone was suppressed in a phase-specific manner, attaining one or two minimums in 1 cycle of the suppressor (SUP) tone. The suppression phase-lead (i.e., the phase of maximum suppression leading the phase of response to the SUP tone) was about 1/4 cycle for lower-frequency suppressors (50, 100 and 200 Hz), and was close to 1/2 cycle for higher-frequency suppressors (500, 1000 and 2000 Hz). Both the phase of suppression and the suppression phase-lead are independent of fiber spontaneous rate (SR). Some fibers also show a secondary (minor) suppression at higher SUP intensities, which is always about 1/2 cycle away from the first (major) one. Fibers with higher CFs (> 2 kHz) are more likely to show a secondary suppression than those with lower CFs. The threshold difference between the major and minor suppressions is CF-dependent: lower CF fibers usually show differences of 10 dB or greater, while higher CF fibers show smaller differences. The secondary suppression is suppressor-frequency-dependent, usually restricted to lower-frequency suppressors (< or = 200 Hz). No fibers showed a secondary suppression with a SUP frequency 1000 Hz or greater. The phases of suppressions (both the major and minor suppressions) are not affected by the intensity of the CF tone. Non-excitatory, low-frequency suppressors can also give rise to significant suppression. The threshold of synchronization to the SUP tone in the two-tone part was usually the lowest, while the SUP-alone rate threshold was highest. The threshold of synchronization in the SUP-alone segment and threshold of suppression were in between. In some low-SR fibers, complete suppression can be seen.