Contralateral-probe measurements of the amplitude suppression and phase shift of one tone caused by another

Abstract

Observers are capable of adjusting, independently, the amplitude and phase of a probe tone at the frequency of a contralateral test tone so that the intracranial image is centered. This ability has been used to measure the amplitude suppression and phase shift of a test tone brought about by a suppressor tone. Suppression patterns are asymmetrical functions of frequency, with high-frequency tones causing greater amplitude reductions of lower-frequency tones than the reverse. With a low-frequency suppressor at a fixed sound-pressure level, the amount of suppression decreases rapidly as test-tone level is increased above masked threshold. A suppressor of higher frequency than the test tone produces a nearly constant amount of suppression over a large range of test-tone levels. Under all stimulus manipulations a phase lag of the suppressed tone is observed that co-varies with the magnitude of amplitude suppression. Some of the complexities of tone-on-tone masking may be understood as resulting from signal suppression prior to detection, the amount of suppression being dependent upon the frequency and intensity of the masker. [Work supported by NINCDS Grant NS03856.]