ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

2pPP32. Mechanisms underlying nonmonotonic Weber functions in forward-masked intensity discrimination.

Bart R. Clement

Robert S. Schlauch

Dept. of Commun. Disorders, Univ. of Minnesota, 164 Pillsbury Dr. S.E., Minneapolis, MN 55455

Nonmonotonic growth in threshold for intensity discrimination has been noted for pure tones preceded by intense sinusoidal maskers; thresholds are highest for 40--60 dB SPL standards, producing what has often been described as a ``midlevel hump.'' Both physiologic and cognitive factors have ben suggested as mechanisms underlying this phenomenon. Three subjects received four conditions designed to examine this issue. A 2IFC paradigm assessed intensity discrimination of 1000-Hz, 100-ms sinusoids. A no-masker condition provided a baseline. Three forward-masked conditions were implemented: An ipsilateral condition that typically produces a mid-level hump; a contralateral condition in which the hump is typically absent or greatly reduced; and a binaural condition presenting these two maskers simultaneously (contralateral stimulation was 13 dB greater than ipsilateral stimulation; listeners perceived only the contralateral masker). It was argued that if this condition produced a hump despite a perceptually absent ipsilateral masker, a sensory explanation would be suggested. Conversely, data mirroring the no-masker or contralateral results would be consistent with a cognitive hypothesis. Results suggest a strong sensory component, with only a small portion of the effect having a possible cognitive origin. Data will be compared to those of related literature. [Work supported by Bryng Bringelson Foundation.]