Read an Excerpt
Sleights of Mind
What the Neuroscience of Magic Reveals About Our Everyday Deceptions
By Stephen L. Macknik, Susana Martinez-Conde, Sandra Blakeslee Henry Holt and Company
Copyright © 2010 Stephen L. Macknik and Susana Martinez-Conde
All rights reserved.
ISBN: 978-1-4299-5108-1
CHAPTER 1
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 showbiz.
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 assistant, 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 assistant 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.
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 relationships 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 documented. 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 fashion, 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 massive. 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.
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.
(Continues...)
Excerpted from Sleights of Mind by Stephen L. Macknik, Susana Martinez-Conde, Sandra Blakeslee. Copyright © 2010 Stephen L. Macknik and Susana Martinez-Conde. Excerpted by permission of Henry Holt and Company.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
