Two identical center circles appear weirdly different based on their surrounding circles.
What: The Titchener Circles illusion (also called the Ebbinghaus Illusion) is one of those classic brain betrayals that really gets you. Hermann Ebbinghaus discovered this back in 1898, although it got its more popular name from Edward Titchener who put it in his textbook in 1901 and everyone just… ran with that. Even after 125+ years, researchers note that “the origin of this well-known phenomenon… is still puzzling” [1].
The effect is beautifully simple: those two orange circles in the center are exactly the same size, but your brain is absolutely convinced they’re different. The one surrounded by large circles looks smaller, while the one surrounded by small circles looks larger. Even when you know they’re identical, the illusion just won’t quit!
How To: Look back and forth between the two center circles and try to convince yourself they’re the same size (good luck with that). The effect should be pretty obvious right away, but you can mess around with the controls to see how different settings change the strength of the illusion.
For me, when you make the surrounding circles really different in size, the effect gets even more dramatic. You can also try different colors to see how contrast plays into it… though fair warning, if you remove the contrast entirely the illusion will start to fall apart.
If you’re having trouble believing they’re the same size, go ahead and measure them on your screen. I’ll wait. Still looks wrong though, doesn’t it?
Explain it: This illusion is all about relative size perception, which is a fancy way of saying your brain doesn’t really do absolute measurements… it just compares things to what’s around them. The center circle surrounded by large circles has a smaller ratio to its neighbors, so your brain says “small!” even though it’s not.
It’s kind of like how a gray square looks darker on a white background and lighter on a black background, except this is happening with size instead of color. Your visual system is taking shortcuts, using context to make quick judgments about the world.
The really interesting bit is that this illusion works across species too. Recent research has shown that fish and birds also fall for the Ebbinghaus illusion [2], which suggests it might be a fundamental feature of how visual systems work rather than something unique to human cognition. (Using your brain to study your brain is hard… but using a fish’s brain helps!)
Fun fact: There’s still some debate about exactly why this works. The classic explanation is all about relative size and context, but researchers are still figuring out the details even after 125+ years!
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!