As a matter of fact, this optical illusion from Japanese designer Nobuyuki Kayahara comes from the total lack of 3rd dimension data in the image. Nobody can formally determine the rotation direction of the dancer. But the brain must take a decision. So, it chooses, but can be easily perturbated and changes its mind. Then, the dancer starts spinning in the other direction (some people may not be able to see the change, probably because their brain has some kind of preference).