In human perception, contingent aftereffects are illusory percepts that are apparent on a test stimulus after exposure to an induction stimulus for an extended period.
The McCollough effect is one of a family of contingent aftereffects related to the processing of color and orientation.
One can induce the aftereffect by exposure to a magenta and black vertical grating alternating with a green and black horizontal grating.
Furthermore, the orientation-color contingencies present in the illusion are the reverse of those present in the adapting stimulus (i.e., the magenta-vertical and green-horizontal adaptation gratings produced illusory magenta on the horizontal test gratings and illusory green on the vertical test grating).
There are also color-contingent motion aftereffects,[1] and other varieties of these phenomena.