Metacontrast masking depends on luminance transients.

Metacontrast (reduction in brightness of a test gash by the subsequent presentation of a spatially-adjacent masking flash) is abolished if the test and mask flashes are changed in hue with no luminance transients. To obtain metaeontrast. the masking flash need only be a luminance transient of as little as 0.05 log unit. Metacontrast refers to the brightness reduction of a test flash that occurs if a spatially-adjacent masking flash is presented 5&125msec later (AIpern, 1953; Kahneman, 1968; Weisstein, 1972; Lefton, 1973). Previous studies of metacontrast used test and mask stimuli presented as luminance transients, either as luminance increments above a dim or dark background, or as luminance decrements presented against a higher luminance background field.’ We here report measures of metacontrast for conditions of hue substitution, a paradigm in which test and mask stimuli occur as changes in the spectral composition, but not the luminance, of a background field (Weingarten, 1972). With chromatic stimuli presented in hue substitution, we find that metacontrast is abolished-no change in the brightness of the test stimulus occurs. When the chromatic masking stimulus is presented as a luminance increment above the background field,