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What: This optical illusion is a bit difficult to classify, but I'll place it in the “contrast effect” category. However, in this example, we have an animated circle moving from left to right, rather than two stationary objects. This effect was noticed by Ibn al - Haytham, who famously wrote a book on optics more than 1000 years ago. This particular effect was observed in relation to drying paint!

How To: If anything, you should be experiencing the illusion without an explanation. But in case you haven't grasped the mystery, the circle in the middle that moves from left to right remains the same color all the time... right? There are several settings I've enabled. You can change the colors of the left and right sides of the gradient, though the circle will always be a fixed color in the middle of the two. You can also change the size of the circle or add a border. I think the border makes it easier to see the reality, as it creates a small separation between the two colors, preventing your brain from blending the light and dark areas. Another interesting way to experience the illusion is to make the circle extremely large and then gradually shrink it until you start to notice the light/dark effect, and then find a point where you don't see it. As far as I can tell, I haven't found a way to completely “stop” experiencing it... I guess the brain will do what it does.

Explain it: Well, like many illusions, this one is open to debate. Some think it's a result of a form of bias (similar to other types of contrast effects), and some believe it's a physiological effect caused by the neurons in your brain's visual cortex. That said, if you're interested in delving deeper into a paper , I've linked one below written by MIT scientist Pawan Sinha that explores the effect in more detail, including many static examples of the effect.