Two-tone suppression in cochlear mechanics.

Mechanical responses to one- and two-tone stimuli were recorded from the basilar membrane (BM) in the hook region of the guinea-pig cochlea. The most sensitive or "best" frequencies (BFs) for the sites studied were approximately 25-30 kHz. Two-tone suppression (2TS) of the responses to near BF probe tones was noted using suppressor tones either above or below the BF. Rates of growth of 2TS were highest (approximately 1 dB/dB) when the suppressor tones were presented below the BF. Below-BF suppression thresholds (the suppressor intensities causing approximately 10% reduction in the probe-evoked responses) corresponded to BM displacements of approximately 1-5 nm. Above-BF suppression thresholds corresponded to much smaller displacements at the location studied. Both above- and below-BF suppressor tones changed the phase of the probe tone responses in the same way that increases in the probe tone intensity did (they evoked small phase-lags for below-BF probes, and small phase-leads for near- and above-BF probes). Low-frequency suppressor tones ( < approximately 7 kHz) evoked a frequency- and intensity-dependent mixture of phasic (ac) and tonic (dc) suppression. Peak (ac) suppression was observed around the times of peak BM displacement (not velocity). These findings are discussed in relation to those of other workers.