One the right you see the silhouette of a spinning figurine. Does she spin clockwise or opposite?
What to do
After you have decided which way she spins, mentally try to make her spin the other way. Yes, this is possible. But difficult. It may help to blink, or to look a special features. People differ largely here. While it may be very difficult to voluntarily flip the direction, it may also occur spontaneously.
Note: Silhouettes are ambiguous! She could turn either way and it would look exactly the same.
But it is very difficult to flip the spin direction. As an aid, press one of two rotation direction buttons ›↺‹ or ›↻‹. For two rotations this will temporarily add “eyes” (well, sort of) to the figurine. Thus the rotation direction will be “disambiguated”, as the somewhat sufflated term goes.
There is a wide divergence on the difficulty and on the methods to reverse the rotation, and I thank all correspondents for their input. One method that workes for many is to “grab” the tip of the toes of the extended leg and “pull it out of the screen” when it seems to turn behind the body. One visitor reported that by “defusioning” (e.g. looking into the far distance behind the screen), causing two versions of the silhouette to apper, he could even have them rotate differently! Amazing, but in keeping with what we know about the spatial spread of perceptual rivalry.
All silhouettes are ambiguous. Our brain tries to reconstruct the third dimension (space) from the flat image in our eyes, adding information which is usually realistic, but not really there. And in the case of a silhouette, there are two equally likely interpretations, leading to perceptual rivalry.
Actually, as some of you commented, and I agree: they are not completely identical in likelihood, because on left rotation the 3D arrangement is such that one looks from below – one looks at the sole of the foot. That may explain the statistical preference for rightward motion.
To unconfound the possible influence of the view angle I have added a fully symmetrical silhouette spinner on the right (with thanks to Andreas Karlsson). This movie is initially stopped to reduce visual clutter. [2008-10-03]
This illusion made the rounds in the Internet late 2007. And two aspects I find very obnoxious:
The original authors copyright was removed. Naughty! Here the material is reproduced with permission (granted Oct 2007, thanks again!).
Second, it was accompanied with misguided and erroneous comments about the right and left brain hemispheres, insinuating that a given spin direction was associated with dominance of one of the two hemispheres, topping this with totally exaggerated interpretations of hemispheric specialisation, which is strongly overstated in populist claims anyway.
During a recent presentation, I performed an informal ad-hoc experiment with the audience: I showed the silhouette for a few seconds, and then asked the audience which way they had seen the silhouette spinning.
Of the 61 women, 49 saw it turning clockwise, 12 anti-clockwise.
Of the 60 men, 50 saw it turing clockwise, 10 anti-clockwise.
Thus there was a close 50:50 sex distribution, and 20% of either saw anti-clockwise rotation.
I conclude: there is a strong preference for initial clockwise rotation, with out a difference between the sexes.