Basilar-membrane nonlinearity estimated by pulsation threshold

The pulsation threshold technique was used to estimate the basilar-membrane (BM) response to a tone at characteristic frequency (CF). A pure-tone signal was alternated with a pure-tone masker. The frequency of the masker was 0.6 times that of the signal. For signal levels from around 20 dB above absolute threshold to 85 dB SPL, the masker level was varied to find the level at which a transition occurred between the signal being perceived as "pulsed" or "continuous" (the pulsation threshold). The transition is assumed to occur when the masker excitation is somewhat greater than the signal excitation at the place on the BM tuned to the signal. If it is assumed further that the response at this place to the lower-frequency masker is linear, then the shape of the masking function provides an estimate of the BM response to the signal. Signal frequencies of 0.25, 0.5, 1, 2, 4, and 8 kHz were tested. The mean slopes of the masking functions for signal levels between 50 and 80 dB SPL were 0.76, 0.50, 0.34, 0.32, 0.35, and 0.41, respectively. The results suggest that compression on the BM increases between CFs of 0.25 and 1 kHz and is roughly constant for frequencies of 1 kHz and above. Despite requiring a subjective criterion, the pulsation threshold measurements had a reasonably low variability. However, the estimated compression was less than in an earlier study using forward masking. The smaller amount of compression observed here may be due to the effects of off-frequency listening.