Today’s optical illusion animation is called “The Mutually Interfering Shapes” and was originally created by Maarten Wijntjes, Robert Volcic and Tomas Knapen in 2008 for Illusion of The Year contest hosted by the Vision Sciences Society. This is how authors themselves described the effect:
“A circle’s a circle and a square’s a square, right? Wrong! Just look at the center of displayed animation and you’ll see why – Two dots are moving around bouncing off one another. Do you see a square and a circle? They really are! The illusion becomes even stronger when tracing the inner square closely: you’ll see the outer dot moving along four arcs instead of one circle. If you now follow the outer dot, the square suddenly seems curved inward!”
So, what did you see? Does it appear to you that the blue dot is following tracks of the perfectly circular path, or is it following a path more similar to four arcs instead?