McCollough effect
![(click to enlarge) A test image for the McCollough effect. On first looking at this image, the vertical and horizontal lines should look black and white, colorless. After induction (see images below), the space between vertical lines should look reddish and the space between horizontal lines should look greenish.](/Images/godic/202501/26/Grid_for_McCollough_effect.svg1809.png")
![One induction image for the McCollough effect. Stare at the center of this image for a few seconds, then at the center of the second image below for a few seconds. Then return to this image. Keep looking between the two colored images for at least three minutes.](/Images/godic/202501/26/Red_grid_for_McCollough_effect1809.png")
![A second induction image for the McCollough effect. Stare at the center of this image for a few seconds, then at the center of the image above for a few seconds. Then return to this image. Keep looking between the two colored images for at least three minutes.](/Images/godic/202501/26/Green_grid_for_McCollough_effect1809.png")
The McCollough effect is a phenomenon of human visual pe****tion 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 often lasting an hour or more.