The silhouette illusion — an optical illusion featuring a spinning dancer — plays tricks on our visual perception: some people see the dancer spinning clockwise, whereas others see her spinning counterclockwise. And, after staring at the silhouette long enough, many people start to see the dancer switch her spinning direction, from clockwise to counterclockwise, or the other way round.
Ever since the illusion was created by the Japanese Flash designer Nobuyuki Kayahara in 2003, scientists have been trying to explain why the dancer seems to move differently for some of us, and whether one's perceived spinning direction says anything about one's brain. But contrary to what many online quizzes may claim, seeing the dancer spin to the left or to the right does not mean you are right-brained or left-brained. In fact, surveys have found that most people initially see the dancer spin clockwise, and a 2010 study showed that this clockwise bias actually stems from our tendency to assume a viewpoint from above rather than below.
Now a new study suggests that the viewing-from-above factor is only one part of the story. The other reason for the clockwise bias is an attentional bias towards the right side of the body, according to the study, published in the British Journal of Psychology.
When the silhouette illusion is attributed a clockwise rotation, the dancer is perceived as standing on her left leg and extending her right leg forward. In contrast, in a counterclockwise rotation, the dancer is perceived as standing on her right leg and extending her left leg forward, noted the research team led by Chiara Lucafò, the University of Chieti in Italy.
"Besides the preference for taking a viewpoint from above, the tendency to perceive movements of the right rather than the left foot contributes — although to a lesser extent — to the observers' preference for interpreting the original spinning dancer as rotating clockwise," said study co-author Daniele Marzoli, of the University of Chieti.
Which way does it spin?
In the study involving 88 participants in Italy, the researchers looked at potential factors that could contribute to the perceived direction of the dancer's rotation, such as the viewpoint (from above or below), the leg that is being extended forward and is moving (the right leg or the left), the type of pirouette performed (either with the dancer turning away from the supporting leg versus turning towards the supporting leg), as well as the participants' own handedness and footedness.
For their experiments, the scientists used 136 animations based on the original version of the spinning dancer. These animations represented different combinations of starting frames, types of pirouette, and association between a perceived viewpoint, spinning direction and the leg being extended forward. The study participants then viewed the animations and specified the spinning direction they perceived.
The researchers then examined the results and found that people often perceived the dancer spinning clockwise, particularly when they had assumed viewing the scene from above. This confirmed the viewing-from-above bias found in previous studies.
But the results also showed that most of the time people perceived the dancer as having her right leg extended, regardless of their own handedness or footedness. This, again, went hand-in-hand with a clockwise rotation perception, suggesting that the tendency to perceive a right working leg also contributes to the perception of the dancer's rotation as clockwise.
Previous research has shown we have a tendency to interpret ambiguous human silhouettes as right-handed rather than left-handed. The new study suggests that this tendency may also extend to lower limbs.
The reason we pay more attention to the right side of the body may be due to the fact that most our interactions occur with right-handed and right-footed people, the researchers explained. (An example of right-footedness is the predominant use of one's right foot in sports such as soccer.)
"We hypothesize that such a bias could be adaptive in social life because it might imply an increased efficiency in monitoring both communicative and aggressive acts, the right limbs being more used than the left in both types of behavior," Marzoli told Braindecoder.
But sometimes this bias may backfire. One real-world example could be seen in sports: players tend to pay better attention to right-limb movements of their opponents and have a reduced ability to discriminate left-limb movements of left-handers, Marzoli said.
"Interestingly, this could explain the 'surprise effect' at the basis of the left-handers' advantage observed in a variety of interactive sports," including some combat sports, he said.