Peter Vishton's favorite version of his lab's tests involves substituting an overside disk on one side of the Ebbinghaus array. The oversized disk compensates for the “shrinkage” of the Ebbinghaus effect. It creates the visual illusion, for most people, that both disks are the same size.
“When the subject says that the circles look to be about the same size, you’d think they would use the same size grip when they reach for the two,” he said. “But even if the one on the left looks to be the same size as the one on the right, somewhere in your head there’s an accurate representation of just how big each one of them is.”
A small percentage of the population is immune to the Ebbinghaus illusion, Vishton said, but for most of us, that one circle just looks smaller than the other, even if we’re familiar with the illusion if not its name. We continue to be fooled, even as we know we’re being fooled.
“Most people have heard of this before. Every intro psych class talks about pictorial illusions so they’ve at least seen this once,” he said. “I still see the illusion. I think I’ve seen this maybe more than anyone else on the planet. I’ve looked at this thousands and thousands of times now and that disk still looks bigger to me than the one over there.”