Level effects in psychophysical two-tone suppression

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Abstract
Measurements of psychophysical 2-tone suppression in a number of human subjects are described. Levels of the stimulus components (suppressee, L1, and suppressor, L2) were the primary variables. The pulsation threshold was always used with the probe frequency fp fixed at the suppressee frequency f1. f1 was initially fixed at 1 kHz. The suppressor frequency f2 ranged from 0.2-1.4 kHz. At appropriate levels all subjects showed significant suppression. Suppression decreased to 0 as f2 approached f1. The amount of suppression depended on L1 and L2 in a way different from any of the current theories of 2-tone suppression. At higher overall levels suppression became increasingly prominent. The amount of 2-tone suppression in a given stimulus condition depended strongly on the subject. The maximum amount of suppression measured was about 35 dB. Suppression followed the same pattern at other frequencies f1 (0.5, 2 and 4 kHz). The equal f2/f1 ratios were quite similar. The 2-tone suppression effect decreased in a noisy environment. Within a 20-dB range of signal-to-noise ratios the effect of noise changed from negligible to the virtually complete elimination of 2-tone suppression.

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