As the circle moves left and right, it appears that its colors are changing... But are they?
What: This optical illusion is a little tricky to categorize, but I’ll put it in the category of “contrast effect” [1], although in this example we’ve got an animated circle shifting from left to right, as opposed to two fixed objects. The effect was noted by Ibn al-Haytham[2] who famously wrote a book on optics over 1000 years ago, this effect in particular was noted in regards to drying paint!
How To: If anything, you should be experiencing the illusion without explanation, but incase you haven’t figured out the magic, the circle in the middle that is moving from left to right stays the same color the entire time… that’s right?
There are multiple settings that I’ve enabled, you can change the colors of the left and right sides of the gradient, though the circle will always be a color that is fixed in the middle of the two. You can also change the size of the circle, or add a border. I find the border helps you see the reality a little easier, in that it provides a small break between the two colors, stopping your brain from blending the lights and the darks.
Another cool way to experience the illusion is to make the circle particularly large, and then slowly shrink it until you start to see the light/dark effect, and then find a point where you don’t see it. As far as I can tell, I haven’t been able to find a way to “stop” experiencing it at all… brain is gonna brain I guess.
Explain it: Well, like so many illusions, this one is debatable. Some believe that it’s a result of a form of bias (as it may be in other categories of contrast), and some feel that it’s a physiological effect caused by the neurons in your brain’s visual cortex. That said, if you’re interested in getting down and gritty with a paper [3], I’ve linked one below that goes into the effect in more detail from MIT scientist Pawan Sinha, which includes a lot of static examples of the effect.
I've researched these optical illusions in my spare time but am clearly not any kind of expert and my explainations are pretty smooth brained, if you find something mis-cited, earlier examples, or general mistakes please new let me know via toymaker@toms.toys, be kind!