The Spinning Dancer and the Brain
This image, originally created by Nobuyuki Kayahara, is a great scientific personality test. If you see the dancer spinning clockwise, you’ve got excess spleen qi in your left frontal crockus. This means that you’re a vibrant personality whose passions are apparent to everyone around you, but sometimes you are indecisive. If you see her spinning counter-clockwise, the right ascension of your natal chart lies in your sagittal broab and there are Fire humours dribbling out your left nostril. You should see a doctor as soon as possible.
An Australian tabloid recently republished the dancer with a little spiel about how you can use her spin as an indication of whether you are right-brained or left-brained. Since then, she’s propagated all over the internets, and so has the accompanying spiel.
In contrast to my deeply insightful, completely factitious interpretations of the left frontal crockus, the tabloid’s claims about “left brain” and “right brain” personality types are vastly overblown. But the spinny dancer is captivating, and because we are watching her with our brains in addition to our eyeballs, she must surely tell us something about how our brains work… so what’s actually going on?
As it happens, my fiancé is a professional optical illusion geek. He spends his time making monkeys play video games, in order to figure out their visual processing systems. So I made him explain it to me.
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.
Your visual system has evolved to construct a reasonable mental image of the world with a limited amount of information, and it uses a dizzying array of assumptions to do so. In the natural world these assumptions are mostly valid, and there’s only one right way to interpret any given set of signals. Artists and sundry neuroscientists, however, can consciously exploit the assumptions your brain makes about the objects it’s looking at to produce images with two or more equally valid interpretations.
When presented with stimuli that have two valid, mutually contradictory interpretations, your brain just picks one. Then, sometimes, it picks the other. We still don’t understand why this happens, or what role conscious efforts might play in this shift in perception. Many people are able to make the dancer shift directions at will, but the strategies I’ve seen almost always invoke a change of focus – I shift my attention to her feet, or scroll up and down, others look at her hands or to her side. (I’ve also seen lots of people talk about staring at her nipples, but none who report that it helps them see her change directions.)
There is absolutely nothing special about what your brain is doing when it takes some funny black shapes and turns them into a dancer who spins in both directions. Bistable stimuli are resolved using the exact same neural circuitry as everything else, and you can prove it by sticking electrodes into monkeys’ heads (e.g., Grunewald et al., 2002). You could probably prove it by sticking electrodes into your own head, too, if you felt like it. Bistable images are useful tools for experiments, because they allow us to isolate the part of visual processing where the brain is actually making a decision about how to interpret an image from the parts that are purely determined by the action at the back of your eyeball.
Bistability is not just a visual phenomenon, either – there’s an audio version called the tritone paradox. * If humans had less wimpy olfactory processing, I’m sure we could figure out some bistable smell illusions, too – though as far as I know no one has tried to confuse dogs with this particular technique.
If this explanation hasn’t been technical enough for you, try this review paper by Parker and Krug.
* The way you perceive the tones in the tritone paradox is strongly related to your native language and the region in which you grew up. It’s harder to switch your perception of this paradox than it is to switch with most visual illusions, but after listening a couple times and paying attention to the overtones, I could hear the second pair of tones in the other direction.
There are lots of cool audio illusions to play with if you poke around that site. This one, which ostensibly induces your brain to pick out words related to things it has been dwelling on lately, is depressing – I default to hearing the words “no way” repeated over and over. But if I move my speakers around a little I also start to hear “burp”, “rainbow”, “wanker”, and the name of the guy in the office next door. I think I’ll stick to interpreting my dreams instead.