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Interesting Findings And World Unfolding Through My Eyes.

Wednesday, December 10, 2008

Magic Of Magicians...Trick Or Science?

Magicians consider the covert form of misdirection more elegant than the overt form. But neuroscientists want to know what kinds of neural and brain mechanisms enable a trick to work. If the artistry of magic is to be adapted by neuroscience, neuroscientists must understand what kinds of cognitive processes that artistry is tapping into.

Perhaps the first study to correlate the perception of magic with a physiological measurement was published in 2005 by psychologists Gustav Kuhn of Durham University in England and Benjamin W. Tatler of the University of Dundee in Scotland. The two investigators measured the eye movements of observers while Kuhn, who is also a magician, made a cigarette “disappear” by dropping it below a table. One of their goals was to determine whether observers missed the trick because they were not looking in the right place at the right time or because they did not attend to it, no matter which direction they were looking. The results were clear: it made no difference where they were looking.

A similar study of another magic trick, the “vanishing-ball illusion,” provides further evidence that the magician is manipulating the spectators’ attention at a high cognitive level; the direction of their gaze is not critical to the effect. In the vanishing-ball illusion the magician begins by tossing a ball straight up and catching it several times without incident. Then, on the final toss, he only pretends to throw the ball. His head and eyes follow the upward trajectory of an imaginary ball, but instead of tossing the ball, he secretly palms it. What most spectators perceive, however, is that the (unthrown) ball ascends—and then vanishes in midair.

The year after his study with Tatler, Kuhn and neurobiologist Michael F. Land of the University of Sussex in England discovered that the spectators’ gaze did not point to where they themselves claimed to have seen the ball vanish. The finding suggested the illusion did not fool the brain systems responsible for the spectators’ eye motions. Instead, Kuhn and Land concluded, the magician’s head and eye movements were critical to the illusion, because they covertly redirected the spectators’ attentional focus (rather than their gaze) to the predicted position of the ball. The neurons that responded to the implied motion of the ball suggested by the magician’s head and eye movements are found in the same visual areas of the brain as neurons that are sensitive to real motion. If implied and real motion activate similar neural circuits, perhaps it is no wonder that the illusion seems so realistic.

Kuhn and Land hypothesized that the vanishing ball may be an example of “representational momentum.” The final position of a moving object that disappears is perceived to be farther along its path than its actual final position—as if the predicted position was extrapolated from the motion that had just gone before.

More Tools of the Trickery Trade
Spectators often try to reconstruct magic tricks to understand what happened during the show—after all, the more the observer tries (and fails) to understand the trick, the more it seems as if it is “magic.” For their part, magicians often dare their audiences to discover their methods, say, by “proving” that a hat is empty or an assistant’s dress is too tight to conceal a second dress underneath. Virtually everything done is done to make the reconstruction as hard as possible, via misdirection.

But change blindness and inattentional blindness are not the only kinds of cognitive illusions magicians can pull out of a hat. Suppose a magician needs to raise a hand to execute a trick. Teller, half of the renowned stage magic act known as Penn & Teller, explains that if he raises his hand for no apparent reason, he is more likely to draw suspicion than if he makes a hand gesture—such as adjusting his glasses or scratching his head—that seems natural or spontaneous. To magicians, such gestures are known as “informing the motion.”

Unspoken assumptions and implied information are also important to both the perception of a trick and its subsequent reconstruction. Magician James Randi (“the Amaz!ng Randi”) notes that spectators are more easily lulled into accepting suggestions and unspoken information than direct assertions. Hence, in the reconstruction the spectator may remember implied suggestions as if they were direct proof.

Psychologists Petter Johansson and Lars Hall, both at Lund University in Sweden, and their colleagues have applied this and other magic techniques in developing a completely novel way of addressing neuroscientific questions. They presented picture pairs of female faces to naive experimental subjects and asked the subjects to choose which face in each pair they found more attractive. On some trials the subjects were also asked to describe the reasons for their choice. Unknown to the subjects, the investigators occasionally used a sleight-of-hand technique, learned from a professional magician named Peter Rosengren, to switch one face for the other—after the subjects made their choice. Thus, for the pairs that were secretly manipulated, the result of the subject’s choice became the opposite of his or her initial intention.

Intriguingly, the subjects noticed the switch in only 26 percent of all the manipulated pairs. But even more surprising, when the subjects were asked to state the reasons for their choice in a manipulated trial, they confabulated to justify the outcome—an outcome that was the opposite of their actual choice! Johansson and his colleagues call the phenomenon “choice blindness.” By tacitly but strongly suggesting the subjects had already made a choice, the investigators were able to study how people justify their choices—even choices they do not actually make.

The Pickpocket Who Picks Your Brain
Misdirection techniques might also be developed out of the skills of the pickpocket. Such thieves, who often ply their trade in dense public spaces, rely heavily on socially based misdirection—gaze contact, body contact and invasion of the personal space of the victim, or “mark.” Pickpockets may also move their hands in distinct ways, depending on their present purpose. They may sweep out a curved path if they want to attract the mark’s attention to the entire path of motion, or they may trace a fast, linear path if they want to reduce attention to the path and quickly shift the mark’s attention to the final position. The neuroscientific underpinnings of these maneuvers are unknown, but our research collaborator Apollo Robbins, a professional pickpocket, has emphasized that the two kinds of motions are essential to effectively misdirecting the mark. We have proposed several possible, testable explanations.

One proposal is that curved and straight hand motions activate two distinct control systems in the brain for moving the eyes. The “pursuit” system controls the eyes when they follow smoothly moving objects, whereas the “saccadic” system controls movements in which the eyes jump from one visual target to the next. So we have hypothesized that the pickpocket’s curved hand motions may trigger eye control by the mark’s pursuit system, whereas fast, straight motions may cause the saccadic system to take the lead. Then if the mark’s pursuit system locks onto the curved trajectory of the pickpocket’s hand, the center of the mark’s vision may be drawn away from the location of a hidden theft. And if fast, straight motions engage the mark’s saccadic system, the pickpocket gains the advantage that the mark’s vision is suppressed while the eye darts from point to point. (The phenomenon is well known in the vision sci­ences as saccadic suppression.)

Another possible explanation for the distinct hand motions is that curved motions may be perceptually more salient than linear ones and hence attract stronger attention. If so, only the attentional system of the brain—not any control system for eye motions—may be affected by the pickpocket’s manual misdirection. Our earlier studies have shown that the curves and corners of objects are more salient and generate stronger brain activity than straight edges. The reason is probably that sharp curves and corners are less predictable and redundant (and therefore more novel and informative) than straight edges. By the same token, curved trajectories may be less redundant, and therefore more salient, than straight ones.

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