Perspectives

Nature Reviews Neuroscience | AOP, published online 30 July 2008; doi:10.1038/nrn2473 PERSPECTIVES involve higher-level cognitive functions, SCIENCE AND SOCIETY such as attention and causal inference (most coin and card tricks used by magicians fall Attention and awareness in stage into this category). The application of all these devices by magic: turning tricks into research the expert magician gives the impression of a ‘magical’ event that is impossible in the physical realm (see TABLE 1 for a classifica- Stephen L. Macknik, Mac King, James Randi, Apollo Robbins, Teller, tion of the main types of magic effects and John Thompson and Susana Martinez-Conde their underlying methods). This Perspective addresses how cognitive and visual illusions Abstract | Just as vision scientists study visual art and illusions to elucidate the are applied in magic, and their underlying workings of the visual system, so too can cognitive scientists study cognitive neural mechanisms. We also discuss some of illusions to elucidate the underpinnings of cognition. Magic shows are a the principles that have been developed by manifestation of accomplished magic performers’ deep intuition for and magicians and pickpockets throughout the understanding of human attention and awareness. By studying magicians and their centuries to manipulate awareness and attention, as well as their potential applications techniques, neuroscientists can learn powerful methods to manipulate attention to research, especially in the study of the and awareness in the laboratory. Such methods could be exploited to directly study brain mechanisms that underlie attention the behavioural and neural basis of consciousness itself, for instance through the and awareness. This Perspective therefore use of brain imaging and other neural recording techniques. seeks to inform the cognitive neuroscientist that the techniques used by magicians can be powerful and robust tools to take to the Magic is one of the oldest and most wide- powers. The devices used by magicians can laboratory. The study of the artistic intui- spread forms of performance art1 (FIG. 1). It is include one or more of the following: visual tions that magicians have developed about also a discipline with a long legacy of infor- illusions (after-images), optical illusions attention and awareness might further lead mal experimentation. This informal research (‘smoke and mirrors’), cognitive illusions to significant new scientific insights into by magicians aims to determine what condi- (inattentional blindness), special effects (explo- their neural bases. tions allow for the maximum manipulation sions, fake gunshots, et cetera), and secret of human attention and perception. Much devices and mechanical artifacts (gimmicks). Visual illusions in magic as early filmmakers experimented with Visual illusions — and other sensory Visual illusions are often used by neuro- editing techniques to determine which illusions — are phenomena in which the scientists to dissociate the neural activity technique would communicate their intent subjective perception of a stimulus does not that matches the perception of a stimulus most effectively, magicians have explored the match the physical reality of the stimulus. from the neuronal activity that matches the techniques that most effectively divert atten- Visual illusions occur because neural circuits physical reality. Those neurons, circuits and tion or exploit the shortcomings of human in the brain amplify, suppress, converge brain areas with activity that matches the vision and awareness. As such, magic is a and diverge visual information in a fashion physical stimulus rather than the subjective rich and largely untapped source of insight that ultimately leaves the observer with a perception can be excluded from the neural into perception and awareness. Insofar as the subjective perception that is different from correlates of consciousness. Visual illusions understanding of behaviour and perception the reality. For example, lateral inhibitory are also used by magicians to fool their audi- goes, there are specific cases in which the circuits in the early visual system enhance ences, often to enhance cognitive illusions. magician’s intuitive knowledge is superior to the contrast of edges and corners so that Here we discuss a few categories of visual that of the neuroscientist. In this Perspective, these visual features seem to be more salient illusions that have contributed to magic we underline potential areas in which neuro- than they truly are2–6. Unlike visual illusions, tricks, as well as their neural bases. scientists stand to reap great benefits from optical illusions do not result from brain collaboration with the magic community processes: they manipulate the physical Spoon bending. In this illusion the magician (BOX 1 highlights one such potential area of properties of light, such as reflection (using bends a spoon, apparently by using the collaboration). mirrors) and refraction (a pencil looks power of the mind. In one part of the trick, Using completely natural means, magi- broken when it is placed upright in a glass the magician holds the spoon horizontally cians create effects (magic tricks) that seem of water owing to the different refraction and shakes it up and down. This shows that to be outside the laws of nature. One should indices of air and water). Cognitive illusions the neck of the spoon has apparently become note that, unlike so-called psychics, magi- can be distinguished from visual illusions flexible7. The apparent rubberiness of the cians do not claim to possess supernatural in that they are not sensory in nature: they spoon is an example of the Dancing Bar NATURE REVIEWS | NEUROSCIENCE ADVANCE ONLINE PUBLICATION | 1 PERSPECTIVES on stage that consequently seems to both expand and contract. Motion after-effects, more commonly known as The Waterfall Illusion, are the oldest-recorded visual illusions. First reported in his Parva Naturalia, Aristotle noticed that if one fixates a moving stream of water and then looks away, the rocks at the side of the stream will seem to move in the opposite direction to the water. This effect is caused by neural adaptation — that is, by the decrease in responsiveness of a neural system to a constant stimulus. In the Trizonal Space Warp illusion, adaptation to expanding and contracting motion occurs in three different parts of the visual field. The above illusions are examples of magic tricks that could have been used to help elucidate the underpinnings of visual perception. There might be other fundamental visual processes that could be discovered by studying magic (BOX 1). Further, we propose that there are cognitive processes that will be better understood as we learn more from magicians, as discussed in the next section. Cognitive illusions in magic Figure 1 | The Conjurer, by Hieronymus Bosch. A magician performs for the crowd in medieval Inattentional blindness and change blindness. Europe, while pickpockets steal the spectators’ belongings. The paintingNa istur in ethe Re vieMuséews | Neur Municipaloscienc ine St.-Germain-en-Laye, France. Attended objects can seem to be more salient or to have higher contrast than unattended objects19–22. These perceptual effects have (or Rubber Tree) illusion8, in which an woman is revealed to be actually wearing a well-documented neural correlates in the oscillating bar (or rubber tree) seems to red dress. Here is how it works: when the red visual system23. Magicians use the general bend when it is bounced rapidly. The neural light shuts off there is a short period of dark- term ‘misdirection’ to refer to the diversion basis of this illusion lies in the fact that ness in which the audience is left with a brief of the spectator’s attention away from a end-stopped neurons (that is, neurons that positive after-image of the red-dressed (actu- secret action. Thus, misdirection can be respond both to motion and to the termina- ally white-dressed but red-lit) woman. This defined as drawing the audience’s attention tions of a stimulus’ edges, such as corners short after-image persists for enough time away from the ‘method’ (the secret behind or the ends of lines) in the primary visual to allow the white dress to be rapidly the ‘effect’) and towards the effect (what the cortex (area V1) and the middle temporal removed while the room is still dark. When spectator perceives)7,24. Misdirection can be visual area (area MT, also known as area V5) the white lights come back, the red dress applied in an overt or a covert manner. Here respond differently from non-end-stopped that the assistant was always wearing below we use the term ‘overt misdirection’ to indi- neurons to oscillating stimuli8–11. This dif- the white dress is now visible. cate cases in which the magician redirects ferential response results in an apparent This same illusion is the basis for percep- the spectator’s gaze away from the method. spatial mislocalization between the ends of a tual stability during the viewing of motion In the more subtle ‘covert misdirection’, the stimulus and its centre, making a solid object pictures (the image seems to be stable when magician draws the spectator’s attentional look like it flexes in the middle. in fact it is flickering). On a neural level, spotlight (which can be thought of as the both turning on and turning off a stimulus spectator’s focus of suspicion) away from the The Retention-of-Vision Vanish. Persistence generate responses in visual neurons that method without redirecting the spectator’s of vision is an effect in which an image result in the perceptual visibility of the gaze. Thus, in covert misdirection the spec- seems to persist for longer than its presenta- stimulus15. The neural response that is tators can be looking directly at the method tion time12–14. Thus, an object that has been generated by turning off a stimulus is called behind the trick and yet be unaware of it removed from the visual field will still seem the after-discharge, and it has the perceptual because their attention is focused elsewhere.

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