Sleights Of Mind Part 1

Sleights of Mind.

Stephen Macknik.

Introduction.

CLARKE'S THIRD LAW: "Any sufficiently advanced technology is indistinguishable from magic."

NIVEN'S LAW: "Any sufficiently advanced magic is indistinguishable from technology."

AGATHA HETERODYNE ("GIRL GENIUS") PARAPHRASE OF NIVEN'S LAW: "Any sufficiently a.n.a.lyzed magic is indistinguishable from science!"

Have you ever wondered how magic effects work? Coins materialize out of thin air. Cards move through a deck as if pulled by an invisible force. Beautiful women are cut in half. Spoons bend. Fish, elephants, even the Statue of Liberty disappear before your eyes. How does a mentalist actually read your mind? How can you not see the gorilla in the room? Really, how can someone catch a bullet in his teeth? How do they do it?

Don't bother to ask a conjurer. When joining an organization of professional magicians, the initiate may be asked to take an oath: "As a magician I promise never to reveal the secret of any illusion to a nonmagician, unless that person also swears to uphold the magicians' oath. I promise never to perform any illusion for any nonmagician without first practicing the effect until I can do it well enough to maintain the illusion of magic." It is a code. A brotherhood. The magician who breaks this code risks being blackballed by his fellow magicians.

So what are we, a couple of muggles, doing writing a book on magic? Zipped lips aside, hasn't most everything about magic been revealed? Enter "magic" in the Amazon Books search box and 75,000 results pop up. Log in to YouTube and you can see just about every magic trick ever devised-often demonstrated by darling seven-year-olds in their bedrooms with Mom or Dad wielding the videocam. Visit Craigslist and choose from myriad charming descriptions of local amateur magicians. What's left to say?

Actually, plenty. This is the first book ever written on the neuroscience of magic, or, if you will, neuromagic, a term we coined as we began our travels in the world of magic.1 Much has been said about the history of magic, tricks of the trade, the latest props, and psychological responses to magical effects. But neuroscience probes more deeply. We want to pop the hood on your brain as you are suckered in by sleights of hand. We want to explain at a fundamental level why you are so thoroughly vulnerable to sleights of mind. We want you to see how deception is part and parcel of being human. That we deceive each other all the time. And that we survive better and use fewer brain resources while doing so because of the way our brains produce attention.

Like so much that happens in science, we fell into magic by accident. We are neuroscientists at the Barrow Neurological Inst.i.tute in Phoenix, Arizona. The BNI is the oldest stand-alone neurological inst.i.tute in the United States and currently the largest neurosurgical service in North America, performing more than six thousand craniotomies per year. Each of us runs a research laboratory in the inst.i.tute. Stephen is director of the laboratory of behavioral neurophysiology. Susana is director of the laboratory of visual neuroscience. Incidentally, we are married. Both of us are primarily interested in how the brain, as a device that is made up of individual cells called neurons, can produce awareness, the feeling of our first-person experience.2 Somehow, when neurons are hooked up to each other in specific circuits, awareness is achieved. It's the ultimate scientific question, and neuroscience is on the verge of answering it.

Our foray into illusions began a decade ago when, as young scientists seeking to make a name for ourselves, we tried to rustle up some public enthusiasm for our specialty of visual neuroscience. In 2005, after accepting faculty appointments at BNI, we organized the annual meeting of the European Conference on Visual Perception, which was held in Susana's hometown of A Coruna, Spain. We wanted to showcase visual science in a new way that would intrigue the public and the media. We were fascinated with how science can explain something about the visual arts-for example, Margaret Livingstone's work on why the Mona Lisa's smile is so ineffably enigmatic. We also knew that visual illusions are fundamentally important to understanding how the brain turns raw visual information into perception.

The idea we came up with was simple: we would create the Best Illusion of the Year contest. We asked the scientific and artistic communities to contribute new visual illusions and received more than seventy entries. The audience (a mixture of scientists, artists, and the public) viewed the ten best illusions and then chose the top three. The contest, now in its seventh year, has been a huge success. Our Internet audience doubles every year, and our Web site (http://illusionoftheyear.com) currently has about 5 million page views each year.

Because of our success with the illusion contest, the a.s.sociation for the Scientific Study of Consciousness asked us to chair its 2007 annual conference. The a.s.sC is a society of neuroscientists, psychologists, and philosophers united in the aim to understand how conscious experience emerges from the interactions of mindless, individually nonconscious brain cells.

As our opening move, we proposed holding the conference in our hometown of Phoenix, but the a.s.sociation's board nixed that right away because the city is an inferno midyear. Instead, they suggested...Las Vegas. Hmmm. Las Vegas is every bit as blisteringly hot in June as Phoenix, and if you take the lap dancing, gambling, and showgirls into account it is probably several degrees hotter due to friction. So apparently our colleagues in consciousness studies were looking for a bit of real excitement to spice up their thought experiments.

So Vegas it was. We flew there in October 2005 to do some scouting. On the flight over we asked ourselves: How could we raise the visibility of consciousness research to the public? We didn't want to do another contest. The answer began to germinate the moment our plane dipped its wings on approach to the Las Vegas airport. Out the window we could see, all at once, the Statue of Liberty, the Eiffel Tower, an erupting volcano, the s.p.a.ce Needle, the Sphinx, Camelot, and the Great Pyramid. Soon we were driving up and down the Strip, checking out hotels for our meeting s.p.a.ce. We pa.s.sed Aladdin's castle, the Grand Ca.n.a.l of Venice, and Treasure Island. It seemed too strange to be real. Then, bingo: the theme for our conference appeared. Festooned on billboards, taxicabs, and buses were huge images of magicians: Penn & Teller, Criss Angel, Mac King, Lance Burton, David Copperfield. They stared out at us with mischievous eyes and beguiling smiles. And then it hit us that these tricksters were like scientists from Bizarro World-doppelgangers who had outpaced us real scientists in their understanding of attention and awareness and had flippantly applied it to the arts of entertainment, pickpocketing, mentalism, and bamboozlement (as well as to unique and unsettling patterns of facial hair).

We knew as vision scientists that artists have made important discoveries about the visual system for hundreds of years, and visual neuroscience has gained a great deal of knowledge about the brain by studying their techniques and ideas about perception. It was painters rather than scientists who first worked out the rules of visual perspective and occlusion, in order to make pigments on a flat canvas seem like a beautiful landscape rich in depth. We realized now that magicians were just a different kind of artist: instead of form and color, they manipulated attention and cognition.

Magicians basically do cognitive science experiments for audiences all night long, and they may be even more effective than we scientists are in the lab. Now, before our in-boxes fill up with flames from angry colleagues, let us explain. Cognitive neuroscience experiments are strongly susceptible to the state of the observer. If the experimental subject knows what the experiment is about, or is able to guess it, or sometimes even if she incorrectly thinks she has figured it out, the data are often corrupted or impossible to a.n.a.lyze. Such experiments are fragile and clunky. Extraordinary control measures must be put in place to keep the experimental data pure.

Now compare this with magic shows. Magic tricks test many of the same cognitive processes we study, but they are incredibly robust. It doesn't matter in the slightest that the entire audience knows it is being tricked; it falls for each trick every time it is performed, show after show, night after night, generation after generation. We thought, if only we could be that deft and clever in the lab! If only we were half so skilled at manipulating attention and awareness, what advances we could make!

The idea rapidly took shape: we would bring scientists and magicians together so scientists could learn the magicians' techniques and harness their powers.

But there was just one problem: we were clueless about magic. We didn't know any magicians. Neither of us had ever even seen a real magic show. Fortunately, our colleague Daniel Dennett got us our big break. Dennett is a fellow scientist and philosopher who also happens to be a good friend of James the Amaz!ng Randi, a famous magician and skeptic who has spent decades debunking claims of the paranormal. Randi wrote back, enthusiastically endorsing our idea. He told us that he knew three more magicians who would be perfect for our purposes: Teller (from the magic duo Penn & Teller), Mac King, and Johnny Thompson. All of them lived in Las Vegas and all were personally interested in cognitive science. Apollo Robbins, "the Gentleman Thief," a friend of Teller, joined our group a few months later. Much of this book is based on our interactions with these talented magicians.

Thus began our journey of discovery about the neural underpinnings of magic. We have spent the last few years traveling the world, meeting magicians, learning tricks, and inventing the science of neuromagic. We developed our own magic show and decided to audition at the world's most prestigious magic club, the Magic Castle in Hollywood, California, as bona fide magicians. (For how we did, see chapter 11.) Magic tricks work because humans have a hardwired process of attention and awareness that is hackable. By understanding how magicians hack our brains, we can better understand how the same cognitive tricks are at work in advertising strategy, business negotiations, and all varieties of interpersonal relations. When we understand how magic works in the mind of the spectator, we will have unveiled the neural bases of consciousness itself.3 So pull up a seat, because Sleights of Mind is the story of the greatest magic show on earth: the one that is happening right now in your brain.

The Woman in the Chameleon Dress.

Visual Illusions and Magic.

Johnny Thompson, the Polish magician, known as the Wizard from Warsaw with a routine of countless corny jokes-"Since I'm part Polish, Irish, and Sicilian, I could have been a drunken janitor who doubles as a hit man"-sweeps onstage in his immaculate tuxedo. Renowned as the Great Tomsoni-"you can call me Great"-Johnny has the affable air of a master conjurer who is about to lead you up (or is it down?) an M. C. Escher staircase of trickery. He has a strong chin, a prominent nose, huge ears, and one of the most wondrous combovers in the world of s...o...b..z.

Imagine for a moment that you are in the audience. The lights dim and Johnny flings his arm toward a bright spotlight enveloping his beautiful a.s.sistant, who is clad in a tiny white dress. The Great Tomsoni announces that he will magically change her dress from white to red.

As your eyes focus on the woman, her image is burned deeply into your retinas and brain. Johnny claps his hands. The spotlight dims ever so briefly and then flares up in a dazzling blaze of red light. The woman is suddenly awash in red.

Wait a minute! Switching the color of an ordinary spotlight is not exactly mind-blowing magic. Johnny stands at the side of the stage, looking pleased with his little joke. Yes, he admits, it was a cheap trick, his favorite kind, he explains. But you have to agree, he did turn her dress red-along with the rest of her. Please, indulge him and direct your attention once more to his gorgeous a.s.sistant as he switches the lights back on for the next trick.

Johnny claps his hands. The lights dim again. You're wondering why you bought tickets to such a lame magic show when suddenly the stage explodes in a supernova of whiteness. And what do you see? Inexplicably, this time the woman's dress really has turned red. Bright crimson red. She does a couple of turns so you can observe the magical transformation.

The Great Tomsoni has done it again.

Johnny has just created a spectacular illusion based on fundamental properties of your brain's visual system. Visual illusions-which we study for a living-are a particularly palpable demonstration of the systematic illusion spinning that is happening all the time in your brain, at all levels of perception, awareness, and thought. By definition, visual illusions are subjective visual perceptions that do not match the reality of the world all around you.

When you experience a visual illusion, you may see something that is not there, fail to see something that is there, or see something different from what is there. Your perceptions contradict the physical properties of what you are looking at. You can immediately appreciate why visual illusions are useful to magicians. And for scientists, they are indispensable tools for explaining the neural circuits and computations by which your brain constructs its everyday experiences.

The spooky truth is that your brain constructs reality, visual and otherwise. What you see, hear, feel, and think is based on what you expect to see, hear, feel, and think. In turn, your expectations are based on all your prior experiences and memories. What you see in the here and now is what proved useful to you in the past. You know that shadows fall a certain way depending on time of day, that faces are normally viewed in an upright position, and that gravity exerts a predictable influence on all things. When these predictions are violated, your brain may take more time to process the data, or you may focus your attention on the violation. But when everything sails smoothly along, with no surprises, your visual system will miss much of what is going on around you. This is how you drive home without remembering what happened between your office and the driveway.

A fundamental theme of this book is that the brain mechanisms that elicit perceived illusions, automatic reactions, and even consciousness itself essentially define who you are. They evolved along with your bipedal gait and hairless monkey physique. They are the products of an evolutionary path that made it possible for your ancestors to make it through numerous bottlenecks of human history, survive the ice age, and go on to invent agriculture, language, writing, and ever more sophisticated tools.4 You are the result of this epic journey, the likes of which the world has never seen before. Without these innate sensory, motor, and cognitive skills you could not download apps on your smart phone, drive a car, negotiate the interpersonal relations.h.i.+ps required to graduate from high school, or even hit a baseball. The reason you can do these things is that, essentially, you are a prediction machine, and you effortlessly and correctly predict almost every event that is about to occur in your life.

Magicians understand at a deeply intuitive level that you alone create your experience of reality, and, like Johnny, they exploit the fact that your brain does a staggering amount of outright confabulation in order to construct the mental simulation of reality known as "consciousness." This is not to say that objective reality isn't "out there" in a very real sense. But all you get to experience is a simulation. The fact that consciousness feels like a solid, robust, fact-rich transcript of reality is just one of the illusions your brain creates for itself. Think about it. The same neural machinery that interprets actual sensory inputs is also responsible for your dreams, delusions, and failings of memory. The real and the imagined share a physical source in your brain.

In coming chapters we will argue, and hopefully convince you, that a surprising proportion of your perceptions are fundamentally illusory. You think you see curvy lines but, when you measure them with a yardstick, the lines are straight. You think you are paying attention, but the pickpocket deftly removes your watch in front of your face. You believe you are aware of your surroundings, but at any given moment you're blocking out 95 percent of all that is happening. Magicians use these various perceptual pitfalls and brain processes against you in a form of mental jujitsu. The samurai invented jujitsu as a way to continue fighting if their swords broke in battle. Striking an armored opponent would be futile, so jujitsu is based on the principle of using an attacker's own energy against him rather than opposing it. Magicians have a similar MO. Their arts are founded on the principle of using your mind's own intrinsic properties against you. They reveal your brain for the liar that it is.

For the red dress trick, Johnny is hacking into your visual system. Comprised of eye and brain, this system should not be compared to an expensive video camera that takes pixel-rich images of the world. Rather, it is a highly evolved kludge of circuits that relies on approximations, guesses, predictions, and other shortcuts to literally construct what might be happening in the world at any given moment.

So, what do we know about those circuits? Exactly what aspects of the brain give rise to visual illusions? How can we probe the visual system to understand the ultimate source of illusions? Full disclosure: often we cannot. Throughout this book, we will be making a distinction between psychological principles and their neural correlates. Take, for instance, post-traumatic stress disorder, or PTSD. The psychological principle that too much stress can lead to PTSD is well doc.u.mented. But that does not tell you anything about the brain mechanisms involved. To get at the neural correlates of PTSD, you need a neuroscientist to delve into the brain to ferret out the details of what's going on physically inside its circuits.

As for visual illusions, a psychological principle refers to an illusion that occurs when physical reality does not match perception. If your eyes see depth when you look at a painting on a canvas, it must be due to how edges and contours in the image interact in your mind. But that does not tell you anything about how the brain produces the illusion. A psychological principle treats the brain as a black box. It is a "dry" description of perceptions and their presumed underpinnings. A neural correlate is a direct measure of brain activity and anatomy, and it tells you what parts of the brain are used to process the percept, which circuits within those brain areas give rise to an illusion, or even minute details such as what neurotransmitters are involved. It is "wet" biology. We know more about psychological principles than we do about neural correlates, but the gap is beginning to close. Arguably the most exciting breakthroughs in science today are happening in the field of neuroscience.

To understand what neuroscientists are up against when explaining visual illusions, you need to know some nuts and bolts of how your visual system is put together. Your eyes tell you only part of what you are able to "see." The rest is done by your brain in a labyrinth of stages.

The first layer of your visual system consists of photoreceptors in your eyes that convert light into electrochemical signals. It is also in this layer where a cardinal attribute of your brain originates: the ability to detect contrast. This property forms the basis of all cognition, including your capacity to see, hear, feel, think, and pay attention. Without it, the world would have no boundaries and your brain could make no sense of itself or anything outside itself.

Naturally, magicians have stumbled onto methods that capitalize on contrast detection, including a stunning illusion called Black Art, which we'll describe later in this chapter.

Information from your retina is funneled into a bundle of fibers called the optic nerve, which carries electrochemical patterns into your brain. Everything you perceive enters your brain as patterns. You don't really "see" anything; rather, you process patterns related to objects, people, scenes, and events to build up representations of the world. This information makes a brief stop in the center of your brain, the thalamus, before ascending to your primary visual cortex-the forebrain's first visual area, and the first of thirty or so cortical regions that, in hierarchical fas.h.i.+on, extract more detailed information about the visual world. This is where you first detect the different orientations of lines, edges, and corners in a visual scene.

Moving up the hierarchy, you have neurons that fire in response to contours, curves, motion, colors, and even specific features such as hands and faces. You have neurons that are binocular-they respond to stimulation from both eyes as opposed to one eye alone. Some fire when a target moves left to right; others fire only when a target moves right to left. Still others respond only to up-down or down-up movement. Some respond best to moving edges, or to moving edges of a particular orientation. Thus you go from detecting points of light in photoreceptors to detecting the presence of contrast, edges, and corners, to building entire objects, including an awareness of their color, size, distance, and relation to other objects.

In this process, your visual system makes inferences and guesses from the get-go. You perceive a three-dimensional world despite the fact that a simple two-dimensional image falls on each retina. Your visual circuits amplify, suppress, converge, and diverge visual information. You perceive what you see as something different from reality. Perception means resolving ambiguity. You reach the most plausible interpretation of retinal input by integrating local cues. Consider the full moon rising on the horizon. It looks ma.s.sive. But hours later, when the moon is high overhead, and is in fact closer to you by one-half the diameter of the earth, it looks much smaller. What could explain this? The disc that falls on your retina is not smaller for the overhead moon than for the rising moon. So why does the overhead moon seem smaller? One answer is that you inferred the larger size of the rising moon because you see it next to trees, hills, or other objects on the horizon. Your brain literally enlarges it based on context. This is also why a gray piece of paper can appear dark if surrounded by white or the same sheet can appear bright if it is surrounded by black.

The neurons of the early visual system reside inside the eyes, in the lateral geniculate nucleus (center of the brain) and the primary visual cortex (back of the brain). The wires that connect these brain areas into the visual pathway are in the optic nerve, optic chiasm, optic tract, and optic radiations. (Courtesy of the Barrow Neurological Inst.i.tute) Alas, you simply cannot trust your eyes.

You also make up a lot of what you see. You "fill in" parts of visual scenes that your brain cannot process. You have to do this because of the sheer limitations in the numbers of neurons and neuronal connections underlying your sensory and mental processes. For example, your optic nerve contains all the fibers that send visual information to your brain. Each optic nerve is made up of about a million neural wires connecting each retina to your brain. The individual wires are called axons, and each represents one "pixel" of your visual image. Each eye is thus roughly equivalent to a one-megapixel camera. Sounds like a lot, but consider that even your cell phone camera probably has better resolution than that. So how can it be that you have such a rich and detailed perception of the world, when in fact your visual system's resolution is equivalent to a cheap digital camera? The short answer is that the richness of your visual experience is an illusion created by the filling-in processes of your brain.

Visibility and Light.

You might think that visibility should merely require that light fall on your retina. But it is more complicated than that. Not all of the light used by your brain is visible to you. For instance, like all humans, you are bad at accurately estimating the physical light level of your surrounding environment. You don't consciously know how big your pupil is at any given time. Part of the reason for this is that the irises adjust for light level and help to make differently lit environments accessible for neural processing. In low light, your irises open to allow in more photons, and in high light your irises close to keep your retina from becoming blinded by glare. That's why a light level expert such as a photographer must use an objective light level measuring device called a photometer, rather than her own subjective visual estimates of light level, before she can determine the best f-stop to use with her camera lens. But this seems almost like circular reasoning. How can it be that we are unable to accurately quantify the amount of light coming into our eyes due to the change in our irises, yet it must be the brain that controls our irises to optimize the photon density reaching the retina? The answer is that the neural control of the iris does indeed accurately estimate changes in light level, but it does so with circuits that are not connected to the visual circuits that result in conscious awareness. Thus you are only conscious of certain aspects of the scene, such as the relative luminance of objects in the scene, whereas other bits of visual information, such as a quantified measure of overall light level, are handled unconsciously.

Magicians are constantly exploiting these features of your visual system in their tricks. They use illusions of depth in card tricks. They use context to mislead your perceptions. They count on your filling in the missing pieces of a scene. They draw on edge-detecting neurons to convince you they can bend spoons. And they can even draw on specific properties of your visual system to make you momentarily blind-which gets us back to Johnny.

Spoiler Alerts.

Some magicians believe that the secrets behind tricks and illusions must never be revealed. But most agree that some exposure of magic is necessary for the art to thrive, as long as the secrets are revealed carefully, and only to those people who need to know. Jack Delvin, president of the Magic Circle, a leading international society of magic and illusion, puts it like this: "The door to magic is closed, but it's not locked." That is, there are no real secrets in magic; it's all there for everybody to discover. But you have to want it enough to seek it. You have to practice like a demon to gain entry to the club, lest you accidentally reveal secrets through poor performance. And it would be unacceptable for somebody to accidentally run across a secret while reading a magazine or overhearing a conversation-or reading a book.

Because it is necessary to reveal some secrets in order to discuss the neuroscience of magic, we have marked each section of the book in which we reveal secrets. The heading is "spoiler alert." If you don't want to know the magical secrets, or to learn how your brain is being hacked by them, you can skip those portions. Or you can join us in exploring why and how you are so easily fooled.

SPOILER ALERT! THE FOLLOWING SECTION DESCRIBES MAGIC SECRETS AND THEIR BRAIN MECHANISMS!.

The Great Tomsoni's red dress trick reveals a deep intuitive understanding of neural processes taking place in your brain. Here is how he did it.

As Johnny introduces his a.s.sistant, her skintight white dress lures you into a.s.suming that nothing-certainly not another dress-could possibly be hiding under the white one. Of course that reasonable a.s.sumption is wrong.

The woman's slinky, seductive body also helps to focus your attention right where Johnny wants it-on her. The more you stare at her, the less likely you are to notice the hidden devices in the floor and the better adapted your retinal neurons become to the brightness of the spotlight s.h.i.+ning on her.

All during Johnny's patter after his little "joke," your eyes and brain are undergoing a neural adaptation. When the spotlight is turned off, visual neurons that have become adapted will fire a rebound response known as an after discharge. This response causes a ghostly image of the object to linger for a moment.

You see illusory afterimages such as this every day. Think about a camera flash. It goes off and you are left with a temporary bright white spot in your field of vision that fades to dark. For a fleeting instant, the photoreceptors in the portion of your retina that registered the flash "think" that the whole world has suddenly gone bright and white. They adjust to that brightness level instantly. If the flash is bright enough, it may take seconds, sometimes minutes, for your retinas to completely readapt to the true lighting levels.

Adaptation of motion neurons in your brain also explains the waterfall illusion. If you stare at a waterfall for a minute or more and then s.h.i.+ft your gaze to the rocks or foliage next to the flowing water, the stationary objects will appear to flow upward. The illusion occurs because the neurons in your brain that detect downward motion have become adapted to the steady stimulation of falling water, making these neurons relatively less active. Neighboring neurons that detect upward motion are not adapted to the motion and, despite having been at rest, are relatively more active. Since your visual system is set up to see contrast-in this case, neurons adapted to downward motion versus unadapted neurons-your brain makes the net conclusion that something is moving upward. Thus, when you look at the stationary rocks, they magically appear to flow upward for a few seconds.

So now do you see why Johnny's trick works? The neurons of your retina that are selective for the color red adapt to the red-lit dress by reducing their activity. Red photoreceptors are more sensitive to this color than are blue or green photoreceptors. Thus the red-sensitive neurons in your visual system are more adapted and will have a bigger after discharge. In the split second after Johnny dims the lights, you perceive a burst of red as an afterimage in the shape of a woman. It lingers in your brain for about a tenth of a second.

During that split second, a trapdoor in the stage opens briefly, and the white dress, held only lightly in place with Velcro and attached to invisible cables leading under the stage, is ripped from her body. Then the lights come back up to reveal a genuine red dress.

Two other factors help to make the trick work. First, the lighting is so bright just before the dress comes off that, when it dims, you are effectively blinded. You cannot see the rapid motions of the cables and the white dress as they disappear underneath the stage. The same temporary blindness can overtake you when you walk from a sunny street into a dimly lit shop. Second, Johnny performs the real trick only after you think it is already over. That gains him an important cognitive advantage: surprise. You are not looking for a trick at the critical moment, and so you slightly relax your scrutiny.

Afterimages linger in all your sensory systems. When you were a child, you may have learned how to create a muscle memory afterimage by pressing the backs of your wrists outward against a doorframe for a count of thirty, after which your arms seemed to levitate. Indeed, sensory afterimages abound in day-to-day life, and insofar as you are consciously aware of them, they are usually only minor, fleeting impressions or annoyances. But to magicians they are gold.

END OF SPOILER ALERT.

As vision scientists, we are constantly amazed by the clever ways magicians finagle your brain's visual circuitry. Recall what we said about your ability to detect contrast: without it, the world would have no boundaries and your brain could make no sense of itself or anything outside itself.

Well, magicians know all about contrast detection. They stumbled onto it more than a hundred years ago with the invention of black art. This is not the abracadabra of ancient wizards and witches but a stage method for producing stunning visual illusions that was discovered by accident in 1875 by a German actor and director, Max Auzinger. The story goes that Auzinger was preparing a dungeon scene for a play and, to make it as fearsome as possible, he lined the room with black velvet. At a critical moment, a black Moor was to appear from a dungeon window and recite his lines. But when the actor playing the Moor put his head in the window, no one could see him. The only things visible were two rows of white teeth floating in air below two white eyeb.a.l.l.s.

Auzinger immediately grasped the implications of the illusion. By manipulating black sheets against a black background, he could make objects and people appear and disappear onstage. He could build a magic act that no one had ever seen. Soon his show, "The Black Cabinet," starring himself as Ben Ali Bey, was touring the continent to rave reviews.

Today a black art act, Omar Pasha, is just as popular and, equipped with modern materials and lighting techniques, is no doubt more spectacular than shows mounted a century ago. Owned and performed by Mich.e.l.le and Ernest Ostrowsky with their son, Louis-Olivier, the show features a character who appears with fluorescent props on a jet-black stage bathed in black light. Black light-what scientists call ultraviolet light-vibrates at a shorter wavelength than visible violet light, and it is called "black" because it is invisible. Fluorescence occurs when one wavelength of light is converted into another. Thousands of substances glow or fluoresce under the influence of black light because the invisible light is converted to visible light, making fluorescent substances glow with seemingly unnatural brightness. Vaseline is electric blue. Fluorite s.h.i.+nes bright purple, yellow, blue, pink, or green. Other materials glow red or orange, depending on the chemicals in them.

In the summer of 2009, we saw Omar Pasha in a live performance.5 Here's how it goes. As the curtain rises, a man wearing a white turban decorated with red brocade, a white silk brocade tunic, silk pantaloons, a red sash and red cape, white gloves, and red shoes with those little red curlicues on the toes, like one of Santa's elves, bows low. He does not smile-ever. It is Omar Pasha (Ernest Ostrowsky), who resembles a cross between the seductive French president Nicolas Sarkozy and the swashbuckling actor Errol Flynn. Ravel's Bolero throbs in the background. The stage floor, side walls, curtains-all that you can see-are pitch-black except for Omar Pasha, who is bathed in black light.

For his first trick, Omar removes a large felt pen from his turban and, with a few broad strokes, draws what looks like a five-foot-high music stand with a gold crossbar on top. The object seems to pop out of nowhere. Then he draws three red candles rising from the crossbar. Now it's a candelabra-only it's not a drawing. It's a real three-dimensional object. Omar picks it up and bows slightly, inviting applause. Next, he lights one candle and holds it at arm's length from his right side. He holds a second candle at arm's length to his left. He glances at the flame in a beckoning way. The flame then floats up over his head and descends to the other candle, lighting it. Omar, looking pleased, nods in the direction of the third candle, which is still on the stand. The solo flame takes off again, arcing high over his head, and lands on the third candlestick. After taking a bow, he presses the three candles between his palms and they disappear. The candelabra floats across the stage and parks itself on a table.

For his second trick, Omar picks up a white silk sheet lying crumpled on the floor and snaps it with a flourish. A chair appears out of thin air. Omar walks behind the chair and covers it with the sheet, showing the profile of the empty chair. He then picks up the sheet, filling it with air like a spinnaker, and when it falls, you see that a turbaned young man is now seated in the chair. Omar blindfolds him and grabs a saber. Stepping in front of the man, Omar makes a slas.h.i.+ng motion across the man's neck. When he steps to the side, the man is headless. Omar holds the severed head for a few seconds and then places it in the outstretched hand of the headless man. It sure looks real. Then Omar steps in front of the man a second time, and when he moves aside the head is restored. Oh, good, no harm done.

For his third trick, Omar reaches out into s.p.a.ce, and a rolled-up poster appears in his hand. He unrolls it, revealing the drawing of a beautiful young woman. He then hangs the poster in midair and unrolls it to reveal the woman herself, who steps through the frame and onto the stage.

Omar covers the woman and the young man in sheets. He makes some magic moves with his hands and the sheets grow and shrink in a ghostly manner. The short sheet with the woman in it grows to man size, and the tall man-sized sheet shrinks to woman size. You know what happens next. The transportation effect is the cleanest version of this particular trick we have ever seen.

Now for the finale. Omar again covers the young man in a silky sheet and invites him to take a few steps forward. Standing behind him, Omar waves his hands, grabs the sheet, and rips it off. The man has vanished. Next, he sets a hula hoop on the ground and steps through it to show it is unenc.u.mbered. The young woman steps into it. Omar pulls up the hoop and we actually see her disappear into the ether as the hoop rises and obliterates her body. Finally, he picks up a sheet from the floor and covers himself. As the Bolero reaches its crescendo, he disappears from beneath the sheet while it is twice snapped by an invisible hand. The six-minute show is over.

All of these amazing effects are rooted in contrast detection. Your eyes cannot detect anything without some sort of change being present. One way to explain this is through a familiar experience-gazing up at the night sky filled with stars. Imagine you're lying on your back on a warm summer evening under a moonless firmament. All of those points of light are so far away that the area each star activates on your retina is smaller than the area of a single photoreceptor. This means that from your brain's perspective, a star is the smallest thing you can see.

Now imagine you are looking up at the blue sky on a clear day. All those stars are still there, s.h.i.+ning brightly, but you cannot see them. During the day, you are star blind. The reason has to do with the comparative amounts of light reaching your eyes during the day and night. At night, a typical star produces 10 percent more light than the surrounding scattered light from the atmosphere. It's a tiny amount but enough to enable your visual system to discern the star. This contrast between foreground and background is the fundamental signal used by your brain to create your mind's image of the star. Without that contrast, your brain's neurons would have nothing to talk about with each other. During the daytime, the blue sky is 10 million times brighter than the blackest night sky. The star that was perfectly visible at night cannot be detected by your visual system because the surrounding sky is so bright that the star's minute contribution of brightness simply cannot be detected as contrast. In the words of Henry Wadsworth Longfellow, "the sky is filled with stars invisible by day."

SPOILER ALERT! THE FOLLOWING SECTION DESCRIBES MAGIC SECRETS AND THEIR BRAIN MECHANISMS!.

Omar Pasha created his gorgeous illusions by covering the stage completely with black velvet. When the show began, various objects onstage-the music stand, candles, chair-were also draped in black velvet. With no contrast for our visual system to work with, the objects were invisible to us. He also used black light and fluorescent paints to further decrease the visibility of the black background against the glowing objects onstage.

The entire act is mute. If Omar spoke, his teeth would glow a ghoulish purple. If he did not wear gloves, his fingernails would fluoresce. His eyes s.h.i.+ne mysteriously. As Omar prances around the stage, he removes one black covering after another, rendering objects visible. When he replaces the covers, they become invisible. a.s.sistants wrapped in black velvet easily move onto and off the stage without our catching a glimpse of them. Hands in black velvet gloves make flames float through the air. The head is chopped off with the a.s.sistance of a black velvet hood. The woman disappears from within a hoop attached to black sheets.

It's not that the black velvet is invisible. Had Omar stuck his white-gloved hand behind the candlestick before it had been revealed, you would have seen a blackened candlestick silhouetted against the glove. No, the trick lies in the contrast, or lack thereof, between the black cloth covering the various objects and the black background of the set and stage.

Magicians are not the only ones who manipulate contrast to make things invisible. Animals do it all the time. It's called camouflage.

Every animal who ever used camouflage is decreasing its contrast as compared to the background, making itself as invisible as possible. With stars in the night sky and with Omar Pasha, reducing contrast means reducing the amount of light against a black background. But another way to decrease contrast is to make yourself the same color, texture, or brightness as the background-like a chameleon, or a stick bug, or a soldier in camouflage fatigues. Contrast is the difference between one object and its surroundings. If there's no difference in color, luminance, or texture, there's no visible contrast, no matter how much light is on the subject.

END OF SPOILER ALERT.

Jamy Ian Swiss-with his neatly trimmed Vand.y.k.e, combed-back hair, and diamond stud in his left ear-is a magician's magician and czar of close-up card tricks. New Yorker writer Adam Gopnik calls Jamy the Yo-Yo Ma of mentalism. Penn and Teller call him "James Bond with a deck of cards for a pistol." Jamy refers to himself as "an honest liar." Being a magician, he says, is the most honest living he has ever made-he promises to deceive you, and then he does.

Boy, does he ever. He lays out four cards facedown, rolls up his sleeves, and waves his elegant hands over the cards, as if stirring up magical currents. His fingers snap and one of the cards is inexplicably faceup-the ace of spades. Another snap. A second card has mysteriously turned over-the ace of hearts. Snap. Snap. Two more aces are faceup. Your brain snaps, too. How could he possibly do that?

Jamy pulls out a deck of cards and shows you a card on top-say, the three of diamonds. "Did you ever see anybody wave their hand over a deck of cards and have it change into another card?" he says as he makes the motion. The three of diamonds turns into the jack of clubs. "You just wave your hand over it," he says, repeating the movement, "and then it changes"-the jack of clubs turns into the six of hearts-"just like that." He continues, "Sometimes you just give the cards a little snap..." and he turns card after card into a different card. You cannot see him do anything remotely suspicious-cards seem to float through solid matter under the spell of his agile fingers.