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McCollough Effect Depends upon Perceived Organization

McCollough (1965) discovered an aftereffect of long duration, in which white gratings of a test figure are seen as tinted with a hue roughly complementary to the hue of induction gratings of the same orientation. Subsequent studies have sought to isolate the stimulus dimensions, or combinations of them, upon which the effect may be made contingent (Fidell 1970; May and Matteson 1976; Stromeyer 1972; Stromeyer and Mansfield 1970; Wyatt 1974). We report observations which show that the McCollough effect is contingent on subjective changes in the organization of a test pattern. As the organization flips from one state to the other, the colour aftereffect switches in and out. Unlike most test patterns used to measure the McCollough effect, figure 1 has no contours separating regions of vertical and horizontal gratings. The pattern can be seen as either (i) a set of concentric square channels, or (ii) four large triangles converging to a point in the centre of the pattern. When seen in the latter way, the pattern appears to be made up of four distinct regions, two striped horizontally and two vertically, but, seen in the former way, it is regionless. We induced the McCollough effect many times, with vertical green-horizontal red gratings, or the reverse. Whether the effect was made strong or weak, our invariable observation was as follows. When the pattern appeared as separate triangular regions we observed the expected McCollough effect, white stripes tinted with the hue complementary to that of the induction grating of the same orientation. But, when the pattern appeared as concentric square channels, and regionless, the appearance of