Psychophysical suppression as a function of signal frequency: noise and tonal maskers

Physiological studies have suggested that the basal region of the cochlea is more nonlinear than the apical region. To evaluate this possibility psychophysically, suppression was investigated across signal frequency (250, 500, 1000, 2000, and 4000 Hz) in a forward-masking paradigm using both noise and tonal maskers/suppressors. Masker duration was 200 ms, signal duration was 20 or 40 ms, and signal delay was 0 or 20 ms; the longer delay was necessary to eliminate potential confusion effects observed with the (narrow-band) noise masker. When using a noise masker (spectrum level of 40 dB), suppression was determined by comparing the threshold in the presence of a broadband masker with that in the presence of a critical band (ERB) masker. When using a tonal masker (masker level of 50 dB SPL, suppressor level of 70 dB SPL, with the suppressor frequency being 1.2 times the masker/signal frequency), suppression was determined by comparing the threshold in the presence of the masker plus suppressor with that in the presence of the masker alone. The magnitude of suppression was determined either by the measured change in signal threshold or by the inferred change in masker level. Regardless of how suppression was quantified, for both masker types, the amount of suppression increased as signal frequency increased up to about 1000 Hz, but then reached an asymptote or decreased somewhat as signal frequency increased to 4000 Hz. The magnitude of suppression was much larger with a noise masker than with a tonal masker, which could be a result of the different number of components in the masker which might serve as a suppressor.