Stanley Sheft
William A. Yost
Parmly Hearing Inst., Loyola Univ., 6525 N. Sheridan Rd., Chicago, IL 60626
Thresholds were measured for detecting amplitude modulation of a 2049-Hz probe carrier in the presence of a two-tone masker complex (909 and 4186 Hz). The probe and masker were modulated in-phase at 10 Hz. The effect of prior stimulation on masked probe AM detection was evaluated by either preceding the probe with a 200-ms, 2049-Hz tone burst (precursor) or adding an unmodulated carrier fringe preceding the onset of probe modulation. Precursor and masker onsets were synchronous, with the interval between precursor offset and probe onset ((Delta)t) either 0, 40, or 160 ms. The precursor was either unmodulated, modulated at 10 Hz in-phase with the masker, modulated at 10 Hz antiphasic to the masker, or modulated at 20 Hz. With a (Delta)t of 160 ms, precursor modulation characteristics had little effect. For (Delta)t's of 0 and 40 ms, modulation of the precursor lowered probe AM detection thresholds with performance generally best with 10-Hz precursor AM. In the fringe conditions, the unmodulated probe carrier preceded the masker onset, with the probe- and masker-modulation onsets synchronous. Fringe duration was either 50, 100, or 200 ms. The addition of the fringe improved performance only with the 50-ms fringe. Results will be discussed in terms of perceptual grouping of stimulus events. [Work supported by NSF.]