Like tricking a dog into chasing a stick that is not thrown, a stage magician can create the illusion she has tossed a ball into the air when actually she has palmed it. Researchers report that the illusion, which they found could be rather convincing, results simply from watching the magician's face and not from glancing where the palmed ball would have traveled. "People claim they're looking at the ball but really they're making use of social cues," says a co-author of the report, psychologist and magician Gustav Kuhn of the University of Durham in England.
A magician performing the trick tosses a ball in the air twice and then pantomimes a third throw [see video at right]. "It's one of these standard tricks in magic. I knew that it was quite powerful," Kuhn says. To study the source of its power, Kuhn and his colleague Michael Land of the University of Sussex in Brighton, England, videotaped Kuhn doing the trick in two ways: on the final fake throw, he would either look up where the ball should have flown or he would look down at his hand.
In both cases, some people who saw the video claimed they perceived the ball flying off the top of the screen after the fake throw, based on their answers to a questionnaire, the researchers report in the November 21 Current Biology. Participants were nearly twice as likely to experience the illusion when Kuhn looked up on the last throw than when he looked at his hand. "That tells us it's due to expectations," Kuhn explains. His gaze was crucial in causing the illusion because it cued the expectations of the watchers, he says.
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Participants' eyes, in contrast, apparently were not fooled. The researchers tracked the gazes of those viewing the videos: during the first two throws viewers' eyes followed the ball to the top of the screen, whereas on the fake throw their eyes were fixated primarily on Kuhn's face.
The experiment had some shortcomings, however, cautions cognitive neuroscientist Amir Raz of the Vancouver Coastal Health Research Institute in Canada, who also studies magic tricks. Even when done correctly the trick only fooled 19 of 38 test subjects, which makes interpreting the results potentially more complicated, he points out. And eye tracking is not the best way to monitor attention, he says. "You can still attend to the sleeve without looking at it directly," he explains.
To Kuhn, the result illustrates how expectations can trump raw visual inputs. "It happens much more than we think," he says. "We feel that we're aware of everything that's going on around us, but we actually perceive what we expect. Magicians have really learned to exploit this." As have dog owners.