Three years ago I wrote a post about a popular illusion – the spinning girl or silhouette illusion. This is a popular online illusion, and also remains my most popular post. (Original illusion by Nobuyuki Kayahara here.) The popularity of this illusion seems to be tied to the fact that it is used in many online quizzes, with the claim that the direction in which you see the girl spin will tell you which side of your brain is dominant. In my prior post I primarily addressed that claim – explaining that the “left brain – right brain” thing is all nonsense, and which way the girl appears to spin tells you nothing about your personality or talents. (Briefly – while many neurological functions are lateralized to one side of the brain or the other, both hemispheres are massively connected and work together to form your abilities and personality.)

The real question prompted by this illusion is why do we perceive it as rotating one way or the other, and is there a preference. It turns out, most people will see the girl spinning clockwise. You can get her to switch and spin the opposite way to your original perception – but when first looking at the illusion most people will see her spinning clockwise.

Psychology professors Nikolaus F Troje and Matthew McAdam wrote a paper explaining this preference, and not surprisingly it has nothing to do with hemispheric dominance. It is purely a visual phenomenon – they write:

Here, we show that this rotational bias is in fact due to the visual system’s preference for viewpoints from above rather than from below.

Our visual system has many such biases and preferences. In effect, our brains process visual information with many default assumptions that are true most of the time. Many optical illusions are based upon creating a special situation in which one or more of these assumptions are false. For example, our visual system has a bias for lighting from above, assumes that smaller objects are farther away, and assumes that if one object overlaps another it must be relatively closer.

Troje and McAdam did some experiments with 24 subjects, playing with the apparent camera angle of the image. They found that there does not appear to be any rotational bias (preference for clockwise or counter-clockwise). The only bias they documented was the viewed from above (VFA) bias.

They also point out that silhouettes are inherently ambiguous – they do not provide clues for how to construct a 3-D image from the 2-D information. However, in this illusion there are subtle and conflicting clues for direction. They write:

Tracing the end of the outstretched hand shows that the camera elevation with respect to the hand is only 6.0 deg, which is a little less than the value for the foot. That means that the figure was rendered with a perspective camera positioned about 75 meters away from the figure (assuming a vertical distance of 1mbetween foot and hand). According to this perspective cue, the view from above and therefore the clockwise rotation is the ‘true’ rotation. However, another cue to the rotation of the figure is provided by the shadow the feet cast on the ground. The ellipse circumscribed by the shadow of the outstretched foot—at least assuming that the ground is horizontal and that we are looking at it from above—clearly suggests counter-clockwise rotation.

Their experiment and explanation is elegant, but it turns out the answer to this illusion was already available online. In addition to my post debunking the “left brain-right brain” claim, psychologist Michael Bach had also explained in 2007 the viewed from above bias as the true explanation. While I appreciate the extra detail and data provided by the new study, the answer was available online all along. (It should be noted that Troje and McAdam reference Bach in their paper.) Yet interestingly Bach’s explanation did not get much attention, while this new paper is being credited as finally debunking the silhouette illusion as personality test.

It is good news that if you search on the various relevant search terms for the illusion, my blog post ranks highly (it is the first hit when you search on “the spinning girl”, and now the Troje and McAdam paper dominate the rankings as well. Hopefully this was all a good opportunity to teach the public a little bit about neuroscience and visual perception, and to make a dent in the popularity of the left brain-right brain myth that is so common.

This is also another episode that points out the frustrating disconnect between pop-psychology and actual psychology. So much of what passes for psychology in the public domain is nonsense, or is simply false or outdated information. Yet there are many interesting and useful psychological experiments out there that the public is not aware of. Psychologist Richard Wiseman discusses this briefly on the SGU, and says this was the motivation for his book, 59 Seconds, in which he debunks a lot of pop-psychology but then gives useful knowledge from the actual research.

I don’t know if the internet and “web 2.0” is helping or hurting this phenomenon. At least for those who are interested, the real information is usually available, and often long before it finds its way into the official literature. In this case it took three years to catch up – but better late than never.