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. [...]
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. [...]
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.
