Like all the best optical illusions, you'll fall for this one even when you know how it works.
The results of the Neural Correlate Society's annual optical illusion contest have been posted online. Many of the illusions are unlike anything we've seen before – but 2014's first place winner looks rather familiar. It's a play on the Ebbinghaus illusion, a classic bit of visual trickery that reveals how our brains gauge an object's size based on the objects that surround it. Even if you've never heard of the Ebbinghaus illusion, you've almost certainly seen it. My guess is that few among you needs reminding that the orange circles in the image below are, despite appearances, the same size:
The winner of this year's Best Illusion of the Year Contest, submitted by Christopher D. Blair, Gideon P. Caplovitz, and Ryan E.B. Mruczek, of the University of Nevada Reno, puts the Ebbinghaus illusion in motion. The effect is dramatic – stronger, even, than the original Ebbinghaus:
The Dynamic Ebbinghaus takes a classic, static size illusion and transforms it into a dynamic, moving display. A central circle, which stays the same size, appears to change size when it is surrounded by a set of circles that grow and shrink over time. Interestingly, this effect is relatively weak when looking directly at a stationary central circle. But if you look away from the central circle or move your eyes, or if the entire stimulus move across the screen, then the illusory effect is surprisingly strong – at least twice as large as the classic, static Ebbinghaus illusion.
The Dynamic Ebbinghaus isn't the only optical illusion to confound our brains with motion. In some optical illusions, movement can actually lead to temporary blindness.