You’ve likely seen the spinning dancer illusion, which tricks the brain into believing the dancer is spinning either way without actually changing rotation. Now, this new animation by psychophysiologist Marcel de Heer shows that a single moving image can appear to move in three different ways. It’s a little psychedelic, but strong visual contrast is part of the illusion. Watch:

According to de Heer, most people will see [the stingrays] swimming up and down, with their tail always away from us. But in the middle of the animation, as the stingrays turn sideways, their true motion is revealed. The one on the left was actually rotating clockwise while the one on the right was turning counterclockwise. Did you see them spinning or swimming?

When we look around us, our visual system assesses the angle of objects to infer the perspective of the scene. But because of the stingrays’ shape and two-dimensional striped pattern, it’s hard to determine the point of view. This leads to ambiguous motion perception with several plausible alternatives. If you watch this animation for long enough, the stingrays’ motion may even flip between the different possibilities.

[source]