Mechanical responses to acoustic clicks were recorded from various structures in the apical turns of chinchilla and guinea-pig cochleae using a displacement-sensitive laser interferometer. Responses from different longitudinal locations on the same structure (e.g., basilar membrane, tectorial membrane) provided evidence of both 'fast' and 'slow' travelling waves (TWs) in cochlear mechanics. Response components which were dominated by either type of wave were identified and separated in the time domain. The 'slow' response components exhibited (i) compressively nonlinear intensity dependence, at least under certain conditions, (ii) temporal and/or physiological lability, (iii) a radial amplitude profile which peaked near the center of the basilar membrane, and (iv) a radial phase profile which varied by 180 degrees from the center of the basilar membrane to the spiral limbus. In contrast, the 'fast' response component exhibited (i) linear intensity dependence, (ii) temporal stability/physiological invulnerability, (iii) a radial amplitude profile which differed from that of the slow component, and (iv) a uniform radial phase profile. The fast and slow response components were also affected differently by sealing the experimental openings that had to be made into the apical cochlea: the fast response component, in particular, was attenuated substantially when the experimental opening was well sealed. This observation leads us to suggest that the fast response components may not exist in truly intact cochleae. However, we also suggest (i) that the fast response components might be responsible for the 'plateau' response regions which have been described in previous studies of cochlear mechanics, and (ii) that frequency dependent interactions between the fast and slow components of a response might be responsible for the sharp sensitivity notches which have previously been observed in the apical cochlea.

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.

The equivalent rectangular bandwidths of auditory filters have been measured behaviourally in guinea-pigs with comb-filtered and notch-filtered noise masking and compared with physiological measurements on single fibres of the cochlear nerve in the same species. A high degree of correspondence between the physiological and behavioural data suggests that the bandwidth of auditory tuning is already determined at the level of the cochlea.