Look at this graphic…is the dancer spinning clockwise, or counter-clockwise…?

Actually…she can be spinning in either direction.  It depends on how your brain initially puts together the visual cues it finds. This from The NeuroLogica Blog, where Steven Novella debunks the notion that this optical illusion reveals left brain/right brain dominance…

Take a look at the spinning girl below. Do you see it spinning clockwise or counter-clockwise? I see it spinning counter-clockwise, and I had a hard time getting it to switch direction. Give it a try.

These kinds of optical illusions are always fun. What they reveal is how our brain processes visual information in order to create a visual model of the world. The visual system evolved to make certain assumptions that are almost always right (like, if something is smaller is it likely farther away). But these assumptions can be exploited to created a false visual construction, or an optical illusion.

The spinning girl is a form of the more general spinning silhouette 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. 

The trick, once you’ve settled on a direction for the dancer’s movement, is changing your mind about it at will.  It’s not easy at first…at least it wasn’t for me.  Initially I saw her spinning counter-clockwise.  It took effort, but after a while I found that if I view the image in my peripheral vision, I can train my eye, while not looking directly at the image, to see her spinning in the opposite direction until it "takes".  Then when I look directly at her she’s now spinning in that direction.  At first it took a while and a lot of effort, but after some practice I could make her switch pretty quickly.

But…now I have a headache…