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Abstract

The strength of the Craik-O'Brien-Cornsweet illusion was measured for different values of spatial and temporal stimulus parameters, in the
traditional achromatic version, and in an isoluminant colour version. It was found that the illusion is much weaker with isoluminant colour stimuli than with
achromatic luminance stimuli. The illusion depends on the spatial parameters of the stimulus in a way that yields an approximate scale invariance: The
strength of the illusion is similar for different stimulus sizes, as long as the ratio of the width of the transition region around the edge, where luminance or
colour change, to the total stimulus width is preserved. In both the achromatic and the chromatic case, the strength of the illusion decreases with
increasing presentation 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 lack of gradient detection underlies the Craik-O'Brien-Cornsweet illusion.