This "right brain vs left brain test" from the Herald Sun is doing the rounds on the internet today. The article contains the so-called "spinning silhouette" optical illusion (below), and states that if you see the the dancer rotating in a clockwise direction "you use more of the right side of your brain and vice versa."

You've probably heard this left/ right brain dichotomy before. It goes something like this: the left hemisphere of the brain is logical, deductive, mathematical, etc., while the right hemisphere is artistic, visual and imaginative. The idea stems at least partly from the classic studies of split brain patients performed by Sperry and Gazzaniga in the 1960s.

There are some functional asymmetries in the brain, and it is true that certain regions of both hemispheres are specialized for particular functions. Speech illustrates this, but also shows that nothing is ever so simple when it comes to the brain: in most right-handed people, speech is processed in both hemispheres, but predominantly in the left. In some left-handers, speech is processed either predominantly in the right hemisphere or on both sides.



So the notion that someone is "left-brained" or "right-brained" is absolute nonsense. All complex behaviours and cognitive functions require the integrated actions of multiple brain regions in both hemispheres of the brain. All types of information are probably processed in both the left and right hemispheres (perhaps in different ways, so that the processing carried out on one side of the brain complements, rather than substitutes, that being carried out on the other).







When I first saw this illusion, I perceived the silhouette as spinning in a clockwise direction. But after staring at it for a while, it appeared to be rotating in the opposite direction. It took some time, but it happened eventually.

The effect can also be achieved by covering the silhouette and focusing on the shadow after you've looked at the illusion. When you uncover it, the image will suddenly appear to be rotating in the opposite direction.

Optical illusions can tell us much about the functioning of the brain's visual system. They work because the visual system reconstructs stimuli not according to how they actually are, but by making certain assumptions about their properties in order to "fill in the gaps".

It is unclear exactly how this illusion works, but it probably has something to do with the brain's representation of an ambiguous object. The silhouette is two-dimensional, but because almost all the objects we encounter are three-dimensional, the visual system reconstructs it as such. And the silhouette is not actually spinning - that is one of the assumptions made by the visual system. So, we perceive it as spinning in one direction one minute, and in the other the next.