Peripheral origins of the upward spread of masking.

Abstract

Recent physiological studies have shown that the basilar membrane (BM) response to tones at characteristic frequency (CF) is highly compressive at medium levels, with a ratio of about 5:1, while the response to tones well below CF is linear. A psychophysical measurement of these characteristics was attempted using forward masking, to avoid peripheral interaction of stimuli, with a 4‐ms, 6‐kHz signal presented 2 ms after the offset of a 100‐ms, 3‐kHz masker, in order to measure thresholds at high signal levels. The growth of masking was very compressive at medium signal levels; increasing the signal level from 50 to 70 dB SPL resulted in a mean increase in masker level of only 3.6 dB. Growth at lower and higher signal levels was more linear. Thresholds were also measured for a number of other masker frequencies. As expected, the difference between masker and signal growth rate decreased as the masker frequency approached signal frequency. The results are quantitatively consistent with the hypothesis that BM nonlinearity governs the upward spread of masking. Furthermore, the technique may provide a simple way of estimating BM nonlinearity in humans. [Supported by the Wellcome Trust and the Royal Society.]