Economists are silly
Both Tyler Cowen and Steven Levitt have posted a link to this article which features the following animation/optical illusion. They both seem a little taken aback, so here’s a quick explanation.
The spinning silhouette is a parallel projection of a twirling dance. Since the it’s not a perspective projection, her limbs do not get larger when they move forward. For any given frame of the animation, there is no way to tell if the dancer is facing forwards or backwards. Similarly, when her arms or legs cross, you have no way of knowing which is in front.
The ambiguity of the image forces your brain to make an arbitrary choice as to which limbs are in front. It then sticks with this choice from frame to frame. The result is that the dancer either spins clockwise or counterclockwise, although getting it to switch directions is tricky.
I can get the dancer to change directions by staring at her leg then blinking when it swings all the way to the right (or left). As I blink, I imagine her changing direction, and if I time it right, she’s changed direction when I open my eyes. I’ve seen this illusion before, and I’ve always like the idea that getting her to switch directions make the illusion interactive, even though the animation doesn’t change one bit.
Anyway, for some reason both Steven and Tyler appear a bit too eager to believe the article’s completely baseless claim that which way you see the dancer spins depends on whether you are left or right brain dominate. I’m not sure what left and right have to do with clockwise and counterclockwise, but regardless both of these economists/bloggers have asked their readers to leave a comment stating which way they see the dancer spinning along with their profession.
Here’s my concern with the survey. If you tell someone the dancer is spinning one way before they see the image, they will have a bias. Their brain will likely interpret the image as spinning the way they were told it should. Tyler does not provide such a bias, but Steven (who now has about 600 comments) says he sees the image as spinning clockwise but his wife sees the image as spinning counter-clockwise. I expect for Freakonomics fans, this is a bias towards clockwise. Perhaps tomorrow he will provide a tally of his results.
Update: Steven just posted a tally of the first 219 respondents. There was a very strong bias towards clockwise, as predicted.