To figure out how an optical illusion works, sometimes you just have to stare at it for a while. But when it comes to the McCollough effect, do so at your own risk.
The phenomenon was discovered by Celeste McCollough in 1965. McCollough found that staring at colorful gratings for too long can affect vision for months to come. Turns out, looking at these gratings for more than 10 minutes can have the effect last over three months.
Here's how it works.
Read more: This mind-boggling optical illusion quiz will leave your head spinning — how many can you solve?
Look at the red and black stripes on the left for a few seconds. Then switch and look at the green and black stripes on the right for a few seconds. Continue alternating between the colors for at least three minutes.
Now, look at the black and white stripes. The white horizontal stripes should appear green, and the white vertical stripes should look red.
This phenomenon is a kind of afterimage, which Encyclopedia Britannica defines as a "visual illusion in which retinal impressions persist after the removal of a stimulus, believed to be caused by the continued activation of the visual system." In other words, the strong colors make an impression on the visual system and cause it to think that it's still looking at the colors even when it's not, making the black and white stripes appear tinged with red and green.
The same way a camera's bright flash can linger in your vision, a study in the Journal of Experimental Psychology found that the McCollough effect can last more than three months if the subject looks at the colorful gratings for extended periods of time. So if you don't see the effect after a few minutes of looking back and forth, exercise caution before trying it for longer.
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