What you actually see is a deforming shadow – a black silhouette – but your brain makes sense of it instantly to see a young woman in full 3-D spinning on her vertical axis in one direction, clockwise or counterclockwise. But keep looking continuously at the picture a bit longer, and you will find out that her movement is ambiguous; indeed, you will probably see the dancer change – for just couple of seconds – direction.
Our brains draw inferences from the world. In terms of perception an example is that everyone has a blind spot on their retina, which means that there is a place that corresponds to that spot where you actually can’t see anything. But you don’t see that blind spot because your brain fills it in. So, that’s basically a hard wired kind of inference.
Optical illusions take advantage of this. When it comes to optical illusions it’s clear that we can’t change what we see, so these are hard wired inferences that our perceptual system makes.
Now, scientists say that what we perceive as a trick of the mind, or an optical illusion, is the need of human brain for symmetry. The conceptual importance of symmetry which has been explored by the Gestalt psychologists since early in the 20th century has been extended by Vilayanur S. Ramachandran and Diane Rogers- Ramachandran in the processing of motion.
The Gestaltists, emphasised that it is the relation of all elements in a scene, rather than the individual elements by themselves that influence our final perception, and they identified “laws” of perceptual organisation for determining, what is seen as figure, display or motion perception. The coupling of motion and direction is based partially on the object’s synchronicity in time (and speed). They tried -unsuccessfully – to explain visual illusions in terms of electromagnetic force fields on the surface of the brain. They never found these fields.
Thus, as the ballerina is spinning, each half of the display is synchronised to one direction, as expected, but across the axis of symmetry, both halves are synchronised to the opposite direction of movement. In other worlds, the two fields appeared to spin either toward or away from each other. Brain’s need of symmetry overrides our tendency to see synchronised and identical motion throughout the visual field.
Sources and further reading: Scientific American April/May 2009, New York Times