FORWARD-MASKED AND BACKWARD-MASKED INTENSITY DISCRIMINATION MEASURED USING FORCED-CHOICE AND ADJUSTMENT PROCEDURES

A ''midlevel'' hump in the intensity jnd has been reported for pure to nes preceded [e.g., Zeng et al., Hear. Res. 55, 223-230 (1991)] or fol lowed [Plack and Viemeister, J. Acoust. Soc. Am. 92, 3097-3101 (1992)] by an intense masker where the signal-masker interval was 100 ms. The se previous studies used forced-choice procedures, in which subjects w ere required to indicate the more intense tone. Plack and Viemeister [ J. Acoust. Soc, Am. 92, 3097-3101 (1992)] have proposed that the task of judging the intensity of the short probe tone, when it is presented along with an intense masker, may lead to cognitive or central factor s influencing the results. The present experiments attempted to reduce these possible effects by measuring intensity jnd's using two additio nal paradigms. First, a ''multiple-look'' forced-choice method, in whi ch subjects listened to the stimulus pairs several times before respon ding, was used to obtain only forward-masked intensity jnd's. Second, the method of adjustment was used to obtain both forward- and backward -masked intensity jnd's. Both the standard forced-choice method and th e multiple-look forced-choice method yielded jnd data with a midlevel hump, when compared to jnd's measured without a masker. In contrast, j nd's obtained with the method of adjustment yielded jnd data with no m idlevel hump. The present results suggest that the traditional method of adjustment for intensity discrimination, where subjects adjust the signal level to a point of subjective equality, may measure a fundamen tally different quantity than that measured by forced-choice procedure s.