I missed this last October, butaccording to the Herald-Sun (in what is till one of their most-emailed stories), whether you see the dancer spin clockwise or anticlockwise indicates which hemisphere of your brain is dominant. I’m curious as to which way Hoyden readers stack up, and whether the list of strengths for each matches your own perceptions of your mentalities.
Now, the Hun says that people can change the perceived direction of spin with a little bit of concentration, so what does that mean then? I started off seeing her spin clockwise, and it took me a while to reverse that spin. Now I can fairly reliably reverse the perceived direction of the spin any time I choose with the old magic-eye squinting defocussing trick. How about you lot? Go and give it a try before reading the rest of this story.
If you find that you too can actually reverse the spin easily with a bit of practise, I’m not at all surprised. This illusion in truth has nothing at all to do with any cerebral-hemisphere-dominance, the whole right-brain left-brain theory being crock: a gross oversimplification and unjustified extrapolation of results from studying so-called “split-brain” patients who had had their cross-hemispheric connections (via the corpus callosum) surgically severed.
So how does the illusion work? Maria Brumm at Gabbro B-sides explains:
This much is true: you process some visual stimuli on the right side of your brain, and some on the left. You also have two optic nerves, one from the back of each eyeball. On their way to the brain these nerves meet up in a location called the optic chiasm. From the optic chiasm, information about the left side of your field of view, no matter which eye it’s coming from, is sent to the right hemisphere of your brain to be processed. Information about the right side of your field of view is sent to the left hemisphere. Therefore, if you want to see what your right brain makes of the dancer, you just need to look over to her right and watch her from your peripheral vision. Looking to her left will show you the left-brained view.
After a little practice, I can get the dancer to switch between clockwise and counter-clockwise spins – from either side of my visual field. This means both sides of my brain see both directions of spin just fine. This effect doesn’t have anything to do with differences in visual processing by the right and left sides of the brain.
Actually, the spinning dancer is an example of something called bistable perception. As an object that can be seen in either of two ways, it’s in the same class of illusion as the Necker cube and the face-vase.
There’s more to learn about bistable perception and optical illusions in Maria’s post, particularly if you follow the links she’s embedded which I haven’t included in the above quote.
Via Maria, I also learned that the silhouette illusion was created by Nobuyuki Kayahara, which is more than the Hun saw fit to tell anyone.