Are You Left-Brained or Right? She Can Tell… Or Not? | Zoli’s Blog
"It’s an optical illusion.
The image is not objectively “spinning” in one direction or the other. It is a two-dimensional image that is simply shifting back and forth. But our brains did not evolve to interpret two-dimensional representations of the world but the actual three-dimensional world. So our visual processing assumes we are looking at a 3-D image and is uses clues to interpret it as such. Or, without adequate clues it may just arbitrarily decide a best fit - spinning clockwise or counterclockwise. And once this fit is chosen, the illusion is complete - we see a 3-D spinning image
By looking around the image, focusing on the shadow or some other part, you may force your visual system to reconstruct the image and it may choose the opposite direction, and suddenly the image will spin in the opposite direction."
Article de wikipedia sur les hémisphères cérébraux.