A real study of magicians' fake movements

Magicians trick us with their sleights of hand, reaching for objects that aren't there and pretending to drop others that they've really kept hold of. This ability is all the more remarkable because research has shown how poor the rest of us are at faking reaching gestures and other movements. Now Cristiana Cavina-Pratesi and her colleagues have used motion-tracking technology to investigate how the magicians do it.

First off, ten magicians and ten controls reached for and picked up a wooden block, or mimed reaching and picking up an imaginary block situated next to the real one. Just as the participants began reaching, their sight was completely obscured by shutter glasses - this was to simulate the way that magicians often look away from where they're reaching. The participants' grasps were performed either with forefinger and thumb or little-finger and thumb, and markers were worn on these digits so they could be monitored with a motion-tracking system.

Just as has been found in earlier research, the controls' pantomime grasping movements were quite distinct from the real thing - the 'maximum grip aperture' (the maximum gap between thumb and finger) was smaller, as was a metric called the 'grip overshoot', calculated from the position of the thumb and fingers during the actual grasp. In contrast, the magicians' maximum grip aperture and grip overshoot were the same whether they actually grasped a real wooden block, or mimed grasping an imaginary one next to it.

Having confirmed that magicians' fake movements really are like the real thing, a second experiment, involving batteries rather than wooden blocks, made things harder. This time, the miming condition was performed without a real, to-be-grasped object anywhere in sight. The seven magicians and seven controls performed their real grasps as before, but when the miming grasps were performed, the batteries were hidden away. Curiously, under these conditions, the magicians were no better at faking than the controls.

The researchers said this suggests that 'the talent of magicians lies in their ability to use visual input from real objects to calibrate a grasping action toward a separate spatial location (that of the imagined object).'

How do they develop this ability? Cavina-Pratesi's team think it reflects a flexibility in the magicians' occipito-parietal system (located towards the back of the brain). 'This flexibility,' they said, 'might exploit mechanisms similar to those underlying people's ability to adapt to spatially displacing prisms through repeated target-directed movements.' They're referring here to the human ability to adapt to prism glasses that distort the visual world. At first the glasses are disorientating, but most people are able to adapt quickly. The researchers said future brain imaging studies will help reveal exactly what's going on in the magicians' brains as they perform their trickery.
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Cavina-Pratesi, C., Kuhn, G., Ietswaart, M., and Milner, A. (2011). The Magic Grasp: Motor Expertise in Deception. PLoS ONE, 6 (2) DOI: 10.1371/journal.pone.0016568