Answer: Visual

Most optical illusions are a novelty and often used as something to demonstrate the fallibility of human perception that might temporarily confuse the viewer but otherwise have no lasting effect.

The McCollough Effect, discovered by American psychologist Celeste McCollough in 1965, however, stands out among optical illusions because it has the ability to alter the way a person perceives color for anywhere from a few minutes to a few months. The way the optical illusion works is that the viewer looks at the perpendicular black grates on a white background seen here, then they look at what are called “induction” images that feature the same pattern but with red and green backgrounds. After observing the induction images for a period of time and then looking at the original black and white image, the viewer will see the colors of the induction images instead of the white background (compared to the induction images, the horizontal and vertical background colorings will be “reversed” when viewing the original image).

What’s fascinating about the effect is not that it occurs (the phenomenon of after-effect colors is well documented), but that it lasts so long. In most subjects the after-effect color doesn’t last for a few moments, as with many optical illusions, but for minutes on end. Further, a study conducted ten years after the McCollough Effect was originally discovered found that 15 minutes of induction image exposure could yield an effect that lasted 3.5 months after exposure.