Perception, 2010, volume 39, pages 1286 ^ 1289
doi:10.1068/p6766
SHORT AND SWEET
The exploitation of Gestalt principles by magicians Anthony S Barnhart
Department of Psychology, Box 871104, Arizona State University, Tempe, Arizona 85287-1104, USA; e-mail:
[email protected] Received 2nd July 2010, in revised form 11 August 2010
Abstract. Magicians exploit a host of psychological principles in deceiving their audiences. Psychologists have recently attempted to pinpoint the most common psychological tendencies exploited by magicians. This paper highlights two co-occurring principles that appear to be the basis for many popular magic tricks: accidental alignment and good continuation.
In recent years magic tricks have received attention as a means of studying psychology and neuroscience. To this end, authors (Kuhn et al 2008; Macknik et al 2008) have cited lists of psychological tendencies most often exploited by magicians to successfully deceive audiences (eg inattentional blindness and priming), but two phenomena were notably absent in both analyses, specifically the concept of accidental alignment and the Gestalt grouping principle of good continuation. These two principles, often employed in tandem, are important psychological tendencies that magicians regularly exploit. In order to understand how magicians utilize these phenomena, imagine the following scenario. A magician presents one long length of rope. After folding the rope in half to identify its center, the rope is cut at what appears to be its center point. The resulting two ropes are then displayed as in figure 1a. Later, the ropes are `magically' restored to their original, uncut state. A video of the cut-and-restored rope trick can be seen in the supplementary materials (http:/dx.doi.org/10.1068/p6766). The cut-and-restored rope is one of the oldest themes in magic, and nearly every instantiation of the illusion relies on the audience's inclination to form faulty assumptions that guide their perceptions. In this case, the faulty assumption that the rope was cut at its center point leads audience members to perceive the arrangement in figure 1a as being two equal-length ropes continuing unimpeded, behind the visual obstruction of the magician's hand (see figure 1b for the assumed grouping). In actuality, before the ropes were cut, a subtle feat of sleight-of-hand was performed that caused the ropes to be cut at a different location, just inches from the end of the rope. The true grouping of the ropes can be seen in figure 1c.
(a)
(b)
(c)
Figure 1. (a) The audience's view of the cut rope. (b) The assumed grouping of the ropes (hand removed). (c) The true grouping of the ropes [see also the movie swf movie file (20 Mb)].
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Given an ambiguous grouping, as that imposed by the obstructing hand in figure 1a, viewers make spontaneous assumptions about the state of the grouping behind the obstruction. These assumptions are likely to be constructed from a few sources. First, when presented with an ambiguous input, the visual system is thought to select either the most likely (Helmholtz 1867/1962) or the simplest (Pomerantz and Kubovy 1986) available interpretation. Not only is an arrangement with two continuous, equal-length ropes more likely (the arrangement in figure 1c would be improbable in any situation other than a magic show), it is also a simpler arrangement than the two unequal lengths in figure 1c. The second factor that influences the audience's interpretation is their perception of the obstruction itself. The illusion of continuity is strengthened if spectators perceive the positioning of the magician's hand to be irrelevant, rather than purposeful. Specifically, the audience should assume that the magician's hand is not specially placed to occlude some anomaly in the ropes. Luckily, the likelihood principle again suggests that an accidental alignment is improbable. We regularly view occluded objects and have come to learn that accidental alignment of occluders with environmental anomalies is exceedingly rare. Rather , we typically assume that our perceptions are viewpointgeneral. When viewing some feature, we assume that a person standing in a different location is unlikely to have perceptions substantially different from our own (Cowie 1983). Yet for spectators at a magic show, these accidental alignments that are rare in the world are integral to the show. For example, the same two principles apply when the rope is later `restored' to its original state. What the audience perceives as a long, continuous rope is actually the short rope from the top of figure 1c positioned at the end of the longer rope, with the junction occluded by the magician's hand (see figure 2).
Figure 2. The short rope is held at the end of the long rope to create the illusion of restoration.
These two tendencies in conjunction (the assumption of non-accidental alignment and good continuation) reinforce the perceptual `filling-in' of objects behind an occluder. The automaticity of this filling-in process is the likely reason behind its common exploitation by magicians. Beyond the rope trick, other examples include the famous Chinese Linking Rings illusion (wherein a set of seemingly solid metal rings are linked together) and almost any illusion where an object like a spoon is bent, supposedly via psychokinesis. In the case of the rings, subtle methods of presentation facilitate the audience's faulty perception that the rings continue unbroken through the magician's hand. In most spoon-bending illusions, the fulcrum of the bend is covered by the fingertips, so the illusion only works if the audience assumes that the pieces of the spoon on either side of the hand were attached and continuous to begin with. These psychological principles are also employed in large-scale stage illusions. The classic `Sawing a Woman in Half' illusion is rendered ineffective if the audience perceives that the head protruding from one end of the box belongs to a different
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individual than the feet protruding from the opposite end. The trick only works if the audience `fills-in' the occluded body linking the head and feet. On a large scale, the illusion of continuity can be harder to elicit. As Shore and Enns (1997) demonstrated, completion of this kind is hindered as occluders increase in size. Magicians have developed novel ways to combat this problem. The `Zig Zag Girl' illusion, developed by Robert Harbin in the 1960s, employs a visual stimulus that makes the perception of good continuation irresistible. In this illusion, an assistant's midsection appears to be removed from her body. To facilitate the perception of continuity between the assistant's exposed head, hand, and foot, an abstract silhouette of the assistant is painted on the exterior of the box (see figure 3a). This imposes a specific interpretation of the orientation of the assistant's body within the box. With the silhouette removed (figure 3b), it is much easier to generate an alternative interpretation that can account for placement of the assistant's extremities. In this case, the assistant could position her body sideways within the contraption while still placing her face, hand, and foot through the openings. The availability of this alternative interpretation renders the illusion much less compelling.
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Figure 3. (a) The `Zig Zag Girl' illusion performed by Ken Levy (http://www.amazingken.com). The abstract silhouette facilitates the perception of good continuation. (b) The illusion becomes much less compelling with the silhouette removed.
The prominence of these two principles in magic suggests that many illusions are effective because they require non-Bayesian thinking in order to reach an accurate solution. Chater (1996) noted that the simplicity and likelihood principles (which explain good continuation) could be demonstrated within a Bayesian framework, and that the assumption of non-accidental alignment is based on perceptual priors. Similar non-Bayesian strategies would be necessary to avoid falling prey to Kuhn and Land's (2006) vanishing-ball illusion, which is driven by susceptibility to joint attention. The tendencies outlined here should be added to the growing list of heuristics that spectators use when attempting to make sense of magic illusions, with future research examining the automaticity of Bayesian assumptions in magic. Such a taxonomy could create a firm foundation for establishing a `science of magic' (Kuhn et al 2008; Macknik and Martinez-Conde 2009).
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Acknowledgments. This paper is dedicated to the memory of Sheila Levy. Special thanks to Ken Levy for permission to use his image. Thanks also to Gustav Kuhn, Steve Goldinger, and Mike McBeath for advice on a previous version of the manuscript. References Chater N, 1996 ``Reconciling simplicity and likelihood principles in perceptual organization'' Psychological Review 103 566 ^ 581 Cowie R I, 1983 ``The viewer's place in theories of vision'' Proceedings of the Eighth International Joint Conference on Artificial Intelligence volume 2 (San Francisco, CA: Morgan Kaufmann) pp 952 ^ 958 Helmholtz H von, 1867/1962 Treatise on Physiological Optics volume 3 (New York: Dover, 1962); English translation by J P C Southall for the Optical Society of America (1925) from the 3rd German edition of Handbuch der physiologischen Optik (Hamburg: Voss 1910; first published in 1867, Leipzig: Voss) Kuhn G, Amlani A A, Rensink R A, 2008 ``Towards a science of magic'' Trends in Cognitive Sciences 12 349 ^ 354 Kuhn G, Land M F, 2006 ``There's more to magic than meets the eye'' Current Biology 16 R950 ^ R951 Macknik S L, Martinez-Conde S, 2009 ``Real magic: Future studies of magic should be grounded in neuroscience'' Nature Reviews Neuroscience 10 241 Macknik S L, King M, Randi J, Robbins A, Teller, Thompson J, Martinez-Conde S, 2008 ``Attention and awareness in stage magic: Turning tricks into research'' Nature Reviews Neuroscience 9 871 ^ 879 Pomerantz J R, Kubovy M, 1986 ``Theoretical approaches to perceptual organization: Simplicity and likelihood principles'', in Handbook of Perception and Human Performance volume II Cognitive Processes and Performance Eds K R Boff, L Kaufman, J P Thomas (New York: John Wiley) pp 36 ^ 45 Shore D I, Enns J T, 1997 ``Shape completion time depends on the size of the occluded region'' Journal of Experimental Psychology: Human Perception and Performance 23 980 ^ 998
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