You may have seen the "Spinning Dancer" illusion floating around the web, one instance of it residing here. It's also called the Left Brain vs. Right Brain test. If you are having trouble seeing her spin both ways, you're not alone. I've broken this down in my typical nerdy fashion.

For me, I stared for quite some time at the illusion, seeing only clockwise rotation. I simply could not figure out how to see the dancer spinning counter-clockwise. I found this particularly frustrating, as I associate myself much more with the left-brained traits than the right-brained ones. It certainly took some rather left-brained thinking to see the left-brained version.

Given my past experience writing computer graphics code that renders three dimensional objects on a two dimensional screen, I could tell by observing the dancing figure that it is not just the spin that causes you to see one or the other, it is the up-down oscillation of the dancer. This oscillation takes an otherwise simple left-to-right motion and gives the perception of an ellipse.

Because of this elliptical shape, the way you perceive the rotation of the silhouette depends less, I think, on whether you are left-or-right brained, but simply on whether your brain interprets your viewpoint to be above or below the dancer's centerline.

If you have a coffee mug or cup or other cylinder, hold it in front of you, below your eyeline. The top of your mug is an ellipse, with the low point being closest to you. Now lift the mug above your eyeline. The bottom of the mug is an ellipse, with the low point being furthest away from you.

Now look at the dancer again. If you see her spinning clockwise, then you perceive her left hand (the one at her side) to be dipping downwards as it passes in front of her. Her other, extended hand (right hand in this case) seems to dip slightly as well as it passes in front of her. Her "right" leg is also dipping slightly. If your mind interprets things this way, then your mind believes your line of sight is above the dancer's centerline.

In order to help you see her spin the other way, do not try to focus on her. Instead, tilt the top of your screen away from you and look slightly off to one side. Watch her in the periphery, and you might spot her spinning the other way. If you look back at her, she might suddenly go back to the way she was before, so it takes a few tries. For me, it actually helped to tilt my screen back, look slightly away, and hold my mug over her - to convince my mind that my line of sight was beneath the dancer's centerline.

Once you see this and are able to focus on her again without losing this perception, you'll notice something striking: she is still spinning backwards, but now her it's her left leg and left arm extended instead of her right.

Now, if you are having trouble seeing her spin clockwise, I suggest trying the opposite. Tilt your screen toward you. If need be, hold a mug underneath her foot, and watch her from the corner of your eye. Eventually, you will see things the other way around.

UPDATE:

I added eyes to the image to make it extraordinarily easy to see both permutations:


I suspect for two reasons now that the girl in the images is in fact turning counter clockwise. For one, the shadow at the base appears to be only possible if this is the case. Also, however, I did note one tiny "flaw" in the animation while adding the 'eyes' above:



The sliver of gray circled in red should have been erased to keep this a silhouette, but it was missed by the creators. This sliver of light can only be there if the extended leg is behind the pivot leg, meaning that the left leg is extended and she is turning counter-clockwise.

UPDATE 2:

Remarkably, even with the above "eyes", some people still cannot break the illusion of which way their mind wants to perceive the rotation. They claim to see the eyes "glowing through the back of her head" and then closing when she turns to face them. To this end, I've taken my attempt to break the illusion one step further, and pitted the two dancers side by side, shifted out of phase:



The above has proven to break the illusion more reliably, but still does not make it fool proof. The commitment of the mind to one permutation in this case astounds me. What that says about the way our brains work I think goes far beyond "left" vs. "right" brained thinking, and ventures into uncomfortable territory where our unwillingness to let go of certainty surfaces.

I've examined this far too long, and I'm convinced that the original rendering (before the silhouette) was a counter-clockwise rotation. Yet by default, I always see clockwise. It takes work to see things the other way. Amazing.

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