Behavioural measurement of level-dependent shifts in the vibration pattern on the basilar membrane

Physiological data suggest that the travelling wave on the basilar membrane evoked by a sinusoid of fixed frequency moves towards the base with increasing level. We describe two psychoacoustic experiments that attempted to provide evidence for and quantify the extent of such a shift in humans. In experiment 1, masking patterns were measured in forward masking using a fixed 6-kHz tone presented at 65 or 85 dB sound pressure level. The threshold for detecting a brief sinusoidal signal was measured as a function of signal frequency for several time delays of the signal relative to the end of the masker. A background noise was included to reduce 'off-frequency listening'. As the signal delay was increased, the signal level at the peaks of the masking patterns decreased and the signal frequency at the peak of the patterns moved progressively towards higher frequencies. The pattern of results was consistent with the idea of a basalward shift of the travelling wave with increasing level. The estimated shift corresponds to about 0.25 octaves for a 40-dB change in level. Experiment 2 also used forward masking. The signal was a 4-kHz tone presented at 10 dB sensation level. For three fixed masker levels (65, 85 and 95 dB), 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 sometimes showed two peaks. One occurred just below the signal frequency and the frequency at the peak was hardly affected by masker level. The second peak fell at a lower frequency, and this frequency tended to decrease with increasing masker level. The gap threshold patterns tended to spread markedly towards lower frequencies with increasing masker level. The shift with level provides further evidence for a basalward spread of the travelling wave with increasing level.