Behavioural measurement of level-dependent shifts in the vibration pattern on the basilar membrane at 1 and 2 kHz

Physiological data suggest that the peak of the travelling wave on the basilar membrane evoked by a high-frequency sinusoid moves towards the base with increasing level. Previously, we used a forward-masking technique to provide evidence for a similar effect in humans at 4 and 6.5 kHz. In the present study, we used a similar technique to determine whether level-dependent shifts occur for mid-range frequencies. The signal was a brief 1-kHz or 2-kHz tone presented at 10 dB SL (approximately 30 dB SPL). For three fixed masker levels (75, 85 and 95 dB SPL), we measured the duration of the gap between the masker and signal required to give 79.4% correct detection of the signal (called the 'gap threshold') as a function of masker frequency; the longer the gap threshold, the more effective is the masker. The gap-threshold patterns nearly always showed a single peak close to the signal frequency. The gap-threshold patterns spread markedly towards lower frequencies with increasing masker level, but the frequency at the peak did not change systematically with level. We conclude that, for mid-range frequencies, the peak of the travelling wave does not shift significantly with increasing level over the range 30-95 dB SPL, but the envelope of the travelling wave becomes more shallow on its basal side.