This amazing optical illusion has been put forward as a visual personality test that can reveal whether you're right-brained or left-brained. That's totally bogus, of course, but it doesn't make the illusion any less amazing...and we'll explain how it works.
First of all, let's take a look at the illusion itself:
When you first looked at the silhouetted dancer, which way was she turning? Personally, I first saw her turning clockwise, but then when I stared at the shadow of her foot for a while, her direction suddenly reversed and I saw her moving counterclockwise. After that, I actually had a pretty hard time switching my perception back to clockwise. If you're having trouble perceiving how the dancer could be spinning in both directions, here's another video that adds some much-needed (and kinda sexy) depth cues to the original silhouette:
A couple years ago, this illusion went viral, with various sites claiming that people who saw her turn clockwise were right brain dominant and those who saw her turn counterclockwise were left brain dominant. That is, as you might imagine, complete crap, and the real reason is much simpler.
Researchers recently discovered that it all comes down to the angle from which you first look at the illusion. When they tested the illusion on 24 subjects, most people reported seeing her turn counterclockwise if they were looking up at her from below, while most said she was turning clockwise if looking down at her from above. According to chief researcher Niko Troje, that explains why more people report seeing her turn clockwise, because our visual system has a tendency to look down on things:
"Our visual system, if it has a choice, seems to prefer the view from above. It's a perceptual bias. It makes sense to assume that we are looking down onto objects that are located on the ground below us rather than floating in the air above us."
If you're struggling to reverse your own perception of the dancer, I can offer a couple pointers from my own experience. Like I said earlier, staring at the shadow of the dancer's foot is the best way to make her switch from clockwise to counterclockwise - I found it to be a pretty dramatic effect, as the dancer really appears to momentarily stop and then completely switch direction.
I had more trouble with switching back from counterclockwise to clockwise, but I found focusing on the dancer's head and upper body was a good start. Once I had looked at the dancer's head for a while, I let my eyes drift off of her. As soon as my eyes stopped concentrating on her, she started turning clockwise.
If you're still having trouble, watch the comparison video again, but this time sit far enough back from the screen that you can only barely see the lines on the silhouette. At that point, if you focus on one of the two silhouettes, both will seem to start turning in that direction. As you might be able to tell, this really is an excellent way to waste half an hour...