The speed of auditory low-side suppression

The nonlinear cochlear phenomenon of two-tone suppression is known to be very fast, but precisely how fast is unknown. We studied the timing of low-side suppression in the auditory nerve of the cat using multitone complexes as auditory stimuli. An evalution of the group delays of the responses to these complexes allowed us to measure the timing of the responses with sub-millisecond accuracy for a large number of fibers with characteristic frequencies (CFs) between 2 and 40 kHz. In particular, we measured the delays with which the same below-CF tone complexes affected the response either as an excitor (when presented alone) or as a suppressor (when combined with a CF probe). For CFs <10 kHz, we found that the delay of suppression was larger than the delay of excitation by several hundred microseconds. The difference between the delay of suppression and that of excitation decreased with increasing CF, becoming negligible for CFs >15 kHz. The results are analyzed in terms of traveling-wave delays and a purported cochlear gain control. The data suggest that suppression originates from a gain-control mechanism with an integration time in the order of two cycles of CF.