McCollough effect

The McCollough effect is a phenomenon of human visual perception in which colorless gratings appear colored contingent on the orientation of the gratings. It is an aftereffect requiring a period of induction to produce it. For example, if someone alternately looks at a red horizontal grating and a green vertical grating for a few minutes, a black-and-white horizontal grating will then look greenish and a black-and-white vertical grating will then look pinkish. The effect is remarkable for it often lasts up to three months or more.

The effect was discovered by American psychologist Celeste McCollough in 1965.

The effect is inducted by looking at a test image similar to that below. It contains oppositely oriented gratings of lines, horizontal and vertical. Next, the subject stares alternately at two induction images similar to the ones directly beneath the top image. One image should show one orientation of grating (here horizontal) with a colored background (red) and the other should show the other orientation of grating (here vertical) with a different, preferably oppositely colored background (green). Each image should be gazed at by the subject for several seconds at a time, and the two images should be gazed at for a total of several minutes for the effect to become visible. The subject should stare approximately at the center of each image, allowing the eyes to move around a little. After several minutes, the subject should look back to the test image; the gratings should appear tinted by the opposite color to that of the induction gratings (i.e., horizontal should appear greenish and vertical pinkish).

The McCollough effect is remarkable, as it is long-lasting. McCollough originally reported that these aftereffects may last for an hour or more. They can last much longer than that, however: Jones and Holding (1975) found that 15 minutes of induction can lead to an effect lasting 3.5 months.

The effect is different from colored afterimages, which appear superimposed on whatever is seen and which are quite brief. It depends on retinal orientation (tilting the head to the side by 45 degrees makes the colors in the above example disappear; tilting the head by 90 degrees makes the colors reappear such that the gravitationally vertical grating now looks green), and inducing the effect with one eye leads to no effect being seen with the other eye. However, there is some evidence of binocular interactions.

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