Adventure stories and horror movies ramp up the tension when hapless characters walk in circles. The Blair Witch Project, for example, wouldn't have been half as scary if those students had managed to walk in a straight line out of the forest. But is this navigation glitch real or just a handy plot device? A new study finds that people really do tend to walk in circles when they lack landmarks to guide them.
The idea for the study came from a German science television show called Kopfball (literally, "head ball," as in soccer), which tries to answer viewers' questions, says the study's first author, Jan Souman, a psychologist at the Max Planck Institute for Biological Cybernetics in Tübingen, Germany. The producers contacted Souman and his colleagues, who study perception and action, to find out if the common belief about walking in circles was true. "We didn't really know, but we thought it was an interesting question," Souman says. So the researchers collaborated with the program, resulting in an episode that aired in 2007.
In one experiment, 15 volunteers, blindfolded and outfitted with GPS receivers, tried to walk in a straight line across a large field. Most participants meandered this way and that, occasionally walking in circles as small as 20 meters in diameter. Previously, some researchers had proposed that walking in circles could result from subtle differences in the strength or length of the two legs that would bias a person to veer toward the left or right. But in Souman's experiment, most subjects showed no strong bias for leftward or rightward turns. Although subjects did have small differences in the strength of their left and right legs, the researchers found, these differences didn't correlate with their turning tendencies. And when the researchers exaggerated differences in leg length by adding a 12-millimeter-thick sole to the volunteers' left or right shoe, they found no systematic effect on the tendency to veer left or right, the team reports online today in Current Biology.
The findings suggest to Souman that the reason people occasionally walk in circles involves the brain. When people are blindfolded, the brain has to plot a course for straight ahead based on limited information, including input from the vestibular system, which informs the sense of balance, and movement sensors in the muscles and joints. "All those signals have very small errors," Souman says. In general, that leads people in a random, meandering path, Souman says. But occasionally, the errors in a particular direction build up, leading us to walk in circles.
Roberta Klatzky, a psychologist at Carnegie Mellon University in Pittsburgh, Pennsylvania, calls this proposed explanation "extremely simple and elegant." Previous studies, dating back to the 1920s, have found that people tend to veer from a straight path when they're unable to see, she says, but in pre-GPS days, researchers weren't able to track people long and far enough to see them walk in complete circles.
When people can see where they're going, however, it's a different story. Souman and his colleagues also tracked volunteers as they walked without blindfolds in two unfamiliar environments: a forest in Germany, and, thanks to a junket arranged by Kopfball, in the Sahara desert in Tunisia. In the forest, people again walked in circles--but only on cloudy days. When the sun was visible, they could walk in a more or less straight line for several hours. The same was true in the sunny Sahara. Visual cues such as the sun or the shadows it casts enable people to overcome the tendency to walk in circles, Souman says.