The Society for Neuroscience (SFN) had its annual meeting this past November in San Diego, California. This is typically an enormous event, as 20,000-plus neuroscientists from around the world converge in one of the best cities in the country. San Diego is home to a spacious and well-designed convention center that regularly hosts meetings similar in size to SFN and, with events like Comic-Con, often surpasses that attendance. With such a large group of scientists presenting research, there’s bound to be some great work on almost anything a curious mind can imagine. While the projects presented range in their aims from understanding some key aspect of brain functioning impaired by disease to demonstrating the latest in backyard neuroscience technologies like the Roboroach, there was also some tantalizing information about basic processes in perception.
Magicians are masters of perceptual psychology — some illusions take advantage of the principles upon which our sensory systems process incoming signals. Many of these principles were identified, studied, and described by the famous Gestalt psychologists. For instance, the principle of closure describes how our visual systems tend to complete otherwise-incomplete structures, like a circle that isn’t entirely closed and the panda from the WWF logo (World Wildlife Fund, that is). The principle of multistability is a particularly powerful one. This is an effect in which alternative percepts of ambiguous images such as the Necker cube and the Rubin vase pop back and forth from one view to the other. Meaning, in the case of the Rubin vase, you see a vase, then you see two faces, then a vase again, and so on.
Another fun example of multistability is the three-legged blivet. The object in the illusion (basically a hair pick with a funky middle leg) is drawn in such a way that the middle leg transitions perceptually as the vase. Incidentally, an object of the sort seen in the illusion was not among the definitions of blivet provided by wiktionary.org. Furthermore, while the definition provided by urbandictionary.com also did not include such a description, it was a lot more fun to read.
Being inclined to see something that is not there, like the missing part of an incomplete circle, is a pretty useful predisposition in a magician’s fan base. If our brains trick us regularly, it shouldn’t be that difficult to get one over on us. We can also be fooled into seeing motion when there is nothing more than two blinking lights.
Such apparent motion can be found in illusions like the phi phenomenon, in which two stationary lights are positioned at a fixed distance from each another in a darkroom (this is done on a computer nowadays). Simply turning the lights on and off, one after the other at the proper rate, produces the illusion that a dot of light is racing back and forth between the points where the lights originate. If any number of things can grab our attention, even for a moment, this gives magicians all the time they need in order to imperceptibly execute their illusions.
This sort of fabrication is not limited to the visual world. At SFN, Daniel Harper and Dr. Mark Hollins from the University of North Carolina demonstrated that the thermo-tactile illusion known as the thermal-grill effect is not as strong when subjects are allowed to experience cold touch prior to the illusion. In this illusion, two separate grills (or coils) are intertwined. Neither grill possesses an extreme temperature — when touched singly, one is warm, and the other is cool. However, when subjects touch the intertwined grills, they experience a burning pain. The researchers found that prior exposure to the cool grill reduces the perceived pain. Because thermal sense is relative, prior exposure renders the cool grill less cool perceptually, changing the relationship between the two grills when experienced together and thereby reducing the perceived level of burning.
We can also experience similar illusions in higher-order cognitive processes, such as speech perception and interpretation. While synesthetes are typically thought of as special because of their highly integrated sensory systems, there is a degree of crossover in all of us. Sight and sound can be two dimensions of a single stimulus. Evolved with this in mind, the parts of the brain (e.g., the superior and inferior colliculus) are particularly excited by sights that identify the origin of a sound in the environment. The McGurk effect demonstrates the interdependence of auditory and visual input quite nicely. If a subject is watching a video of a person mouthing a sound (e.g., “ga-ga”), but a different phoneme (e.g., “ba-ba”) is played over the video, the subject watching/hearing this actually perceives a third phoneme (“da-da”). So, in addition to magicians manipulating what we see and feel, they can also potentially trick us into hearing things that were never said.
We can even be tricked into some rather ridiculous things, like feeling the stimulation applied to a prosthetic limb is actually happening to our own limbs — or that a tool has become part of our bodies (referred to as the rubber-hand and tool-ownership illusions, respectively). Probably most comical (to me, at least) is the child-size-body illusion, in which subjects come to feel that they actually inhabit such a body. This is accomplished similarly to the rubber-hand and tool illusions in that the subject experiences synchronized visual and tactile stimulation in order to perceive the effect. Interestingly, SFN presenters Bergouignan, van der Hoort and Ehrsson found that establishing this illusion could boost the recall of childhood memories, sort of like a context effect in test taking.
Many of these somatic, or body-based, illusions may have to do with how the brain maps our bodies. For instance, tool use establishes the device into the neural representation of the hand using the tool so that it can be effectively controlled. Or when one wears a really tall hat, the hat becomes assimilated into the overall body representation such that we adjust our movements to account for additional height, ducking under doorways and keeping head movements to a minimum.
While locations, drugs, and emotions have all been shown to permit state- and/or location-dependent memories, it is conceivable to think that relevant body maps throughout development may be tied to memories that were formed during those times. That recall of a specific memory would be easier when a particular map is expressed is an intriguing thought. By establishing these kinds of illusions, the possibilities are endless for stage magicians (especially mean-spirited ones) to make utter fools of all of us in front of large crowds.
Understanding the psychological and neural processes responsible for the perceptual effects described above allows for insight into how these predispositions can lend themselves to magic illusions. Many of the presenters at SFN are currently attempting to do just that, though probably not because they are very interested in magic or having child-size bodies. In no way was an attempt made to reveal the methods by which magicians operate.
There is a television show called “Breaking the Magician’s Code: Magic’s Biggest Secrets Finally Revealed” that does exactly that. Explained are illusions such as sawing a woman in half and making a woman disappear behind one of three very large playing cards. In all cases, some diversion of attention or trick of the mind is core to the execution of the illusion itself — though it remains unclear to me why so many of these illusions involve dismembering or otherwise eliminating the magician’s assistant! In any event, I have no real inside knowledge on magic tricks, aside from what I’ve learned at SFN and watching the Science Channel.