Here's a neat optical illusion, go to this website and stare at the dancer. Is she rotating clockwise or counter-clockwise? And can you change it at will?
She appears to rotate clockwise to me, but if I focus elsewhere, and let only my peripheral vision see the rotating dancer, she appears to rotate counter-clockwise!
Link - Thanks No Name!
i made a more primitive version of this if you look at the small circle it rotates one way if you look at the big one it will rotate the other way but in reality they are just moving left and right and increasing/decreasing in scale.
http://www.glossygames.com/sphere.html
This illusion is an example of what is called the ambiguous motion effect, which is caused by (as a couple of people have already mentioned) an absence of depth cues. Your brain will attempt to make sense of this image by selecting a random direction that you will percieve the figure to be rotating in, altough if you concentrate hard enough, you can change the percieved direction, or it can happen spontaneously. Most people probably percieve the image to be moving counter-clockwise because of the position of her feet or arms, which make one direction look a little more natural than the other.
As for the whole "right brain vs. left brain" claim, there is no such thing as dominance of either hemisphere of the brain over the other. This is just a theory that has been invented to try to explain individual differences in language ability, creativity etc..., and has no empirical evidence at all. Both sides of the brain are responsible for all of these abilities to a degree. While we may differ in our abilities and personalities, we all use both sides of our brains equally.
This is similar to the "we only use 10% of our brains" "theory", which is completely untrue. We absolutely use 100% of our brains, as MRIs would show. Have you ever heard of a person having a stroke and the doctor saying "oh not to worry, we only have tissue death in the 90% of the brain that the patient doesn't use!"?