The Bumpy Circle Illusion

The "bumpy circle illusion" is a plain and descriptive name for the circles you see above. These circles look like they aren't really circles, but more like irregular polygons of a sort. Japanese filmmaker Masaya Ishikawa stumbled upon this effect as he was working with Adobe Illustrator. He thought he may have been hallucinating from long hours of work, but other people see the bumpy circles, too. The black circles are perfectly round. What we are seeing is a trick your brain pulls to make sense of what the eye sees.

Ishikawa teamed up with Shuichiro Taya of Keio University to explore and explain the illusion. The result was a paper published in the Journal of Illusion. The illusion depends on the difference between the shades of the circles and the checkered background. The corner effect is also involved. The same picture will lose the bumpy circle illusion if the circles are a different shade or color, as you can see in a writeup at Real Clear Science. ​

(Image credit: Masaya Ishikawa)