THE CRAIK-OBRIEN-CORNSWEET ILLUSION IN COLOR - QUANTITATIVE CHARACTERIZATION AND COMPARISON WITH LUMINANCE

The strength of the Craik-O'Brien-Cornsweet illusion was measured for different values of spatial and temporal stimulus parameters, in the t raditional achromatic version, and in an isoluminant colour version. I t was found that the illusion is much weaker with isoluminant colour s timuli than with achromatic luminance stimuli. The illusion depends on the spatial parameters of the stimulus in a way that yields an approx imate scale invariance: The strength of the illusion is similar for di fferent stimulus sizes, as long as the ratio of the width of the trans ition region around the edge, where luminance or colour change, to the total stimulus width is preserved. In both the achromatic and the chr omatic case, the strength of the illusion decreases with increasing pr esentation time. The similarity of the differences between brightness and colour effects on one hand and the differences in sensitivity for colour and luminance changes in humans on the other suggests that a la ck of gradient detection underlies the Craik-O'Brien-Cornsweet illusio n.