The mechanisms that govern visual perception are only partly understood by scientists, and in fact much of what we know about how the human visual system works stems from investigations into our susceptibility to visual illusions. While scientists have used knowledge of illusions to further our understanding of the mind, magicians have learned to master the art of deception for entertainment purposes. In new findings, researchers have taken advantage of the power of magicians to facilitate illusion, and have uncovered how particular types of information--in particular, cues from magicians--influence some aspects of the visual brain, whereas other components of the visual system are not similarly fooled by the social cues and expectations that mediate perception.
The findings are reported by Gustav Kuhn, of the University of Durham (UK), and Michael Land, of the University of Sussex (UK), and appear in the November 21st issue of the journal Current Biology, published by Cell Press.
Our perception of an event is often significantly modulated by our past experience and expectations. In the new work, the researchers used a magic trick (the "vanishing ball" illusion) to demonstrate how magicians can distort our perception in an everyday situation by manipulating our expectations; the researchers went on to investigate the mechanisms behind this deception.
The researchers found that when a magician performed the illusion, in which a ball was seen to disappear in the air after a fake throw, 68% of observers perceived the ball leaving the magician's hand, move upward, and disappear--this despite the fact that the ball did not leave the magician's hand. Experimental manipulations of the trick revealed that the observers' perception of the ball was determined by particular cues--namely the magician's head direction--that were indicative of the ball's intended location, rather than the location of the ball itself.
By measuring the actual eye movements of the observers, the researchers showed that rather than merely tracking the ball, most people looked at the magician's face prior to tracking the ball--consistent with the visual system utilizing so-called social cues, such as the magician's head direction and gaze, to help form a perception of the ball's location. However, once the ball was no longer physically present in the course of the illusion, observers did not look at the area where they claimed to have seen the ball, suggesting that the oculomotor system, which governs motor control of the eyes, was not fooled by the illusion.
The study's findings show that although people's visual perception was strongly influenced by expectations, the oculomotor system itself was largely driven by "bottom up," visual information from the ball itself. The findings are also consistent with recent evidence from other studies suggesting that two separate neural pathways exist for perception and visuomotor control.
The researchers include Gustav Kuhn of University of Durham, South Road in Durham, UK and Michael F. Land of University of Sussex in Brighton, UK.
Kuhn et al.: "There's more to magic than meets the eye." Publishing in Current Biology 16, R950-R951, November 21, 2006 DOI 10.1016/j.cub.2006.10.012. www.current-biology.com