Part 2 (1/2)
Another great illusionist is the Dutch lithographer and woodcut artist Maurits Cornelis (better known as M. C.) Escher. Early in his career, Escher carved realistic scenes based on his observations and travels. Later, he turned to his imagination, rendering some of the most brilliant visual illusions in the history of art. When he was in high school, one of Steve's favorite posters was an Escher print of the never-ending staircase (Ascending and Descending, 1960), in which a group of robed monks perpetually climb or descend an impossible staircase situated at the top of a temple. It was impossible because it circled around on itself and never ended. So how could it be drawn if it was physically impossible? Escher must have cheated somewhere in the print and failed to depict the proper structure of a real staircase. But Steve couldn't find it, no matter how closely he looked. He realized he should examine the structure as a whole to see if there was a small systematic warp along the entire structure that allowed for the illusion.
And that's when Steve found that he couldn't look at the structure globally. He could only really see one area of the staircase at a time. His vision could process the details of the staircase when he centered his gaze on a specific part. But when he did that, every other area of the staircase, in his visual periphery, was left in a blur. And he realized that that was how Escher must have done it: since you can see only one local area at any given time, small, gradual errors along the entire structure could not be seen with the naked eye.
This effect challenges our hard-earned perception that the world around us follows certain inviolable rules. It also reveals that our brains construct the feeling of a global percept by sewing together multiple local percepts. As long as the local relation between surfaces and objects follows the rules of nature, our brains don't seem to mind that the global percept is impossible.
Susana's formal introduction to visual illusions came in 1997 when she arrived at Harvard University to study under David Hubel and Margaret Livingstone. At the time, Harvard was the mecca for the study of illusions, and in fact this is where she met Steve. Not only were Livingstone and Hubel leading the field in the study of illusions in the brain, but a number of Harvard psychologists were discovering an array of completely new phenomena.
As part of her postdoctoral training, Susana decided to choose a visual illusion and investigate its effects. Leafing through an art book, she found the perfect playground for her curiosity: op art, a field that explores many aspects of visual perception, such as the relations between geometrical shapes, variations on ”impossible” figures that cannot occur in reality, and illusions involving brightness, color, and shape perception.11 Susana settled on op artist Victor Vasarely, whose Nested Squares series exhibited an odd illusion: the corners of the squares looked brighter than their straight-edged sides. But the effect wasn't just about the lightness of the corners, because if Vasarely reversed the order of the nested squares from white-to-black (center to exterior) to black-to-white, now the corners were darker than the sides. So it seemed to be an illusion concerning contrast, and not lightness per se.
Susana searched the vision research literature and found that only a couple of people had discussed this effect previously and n.o.body had investigated its neural bases. And no one had looked at shapes other than squares. Squares are a special type of shape in which all of the corners are convex (all point away from the center of the square). But n.o.body had examined the effect for nonsquare shapes with concave corners or for shapes with corner angles other than 90 degrees. Susana realized there were many aspects of this illusion that she could study perceptually, followed by physiological research in the brain.
After several years, first as a trainee at Harvard and later as the director of her own research team, Susana learned one of the most fundamental secrets of the visual system. The previous dogma in the field had been that neurons in the first few stages of the visual system were most sensitive to the edges of object surfaces. Susana's results showed instead that neurons of the visual system are more sensitive to the corners, curves, and discontinuities in the edges of surfaces, as opposed to the straight edges that had previously been thought to reign.
Vasarely's Utem (1981). Nested squares of increasing or decreasing luminance produce illusory diagonals that look brighter or darker than the rest of the squares. (Courtesy of Michele Vasarely) Op artists were also interested in kinetic or motion illusions. In these eye tricks, stationary patterns give rise to the powerful but subjective perception of illusory motion. An example is Enigma by Isia Leviant.
Reinterpretation of Enigma (Created by and courtesy of Jorge Otero-Millan, Martinez-Conde Laboratory, Barrow Neurological Inst.i.tute) This static image of regular patterns elicits powerful illusory motion in most of us and has generated an enormous amount of interest in the visual sciences since it was created in 1981. However, the origin of the illusion-is it the brain, the eye, or a combination of both?-remains, appropriately, an enigma.
In 2006 we designed an experiment to probe this question. We asked observers to say when illusory motion sped up or slowed down as they looked at the image. At the same time, we recorded their eye movements with high precision. Before they reported ”faster” motion periods, their rate of microsaccades-tiny eye movements that occur during visual fixation of an image-increased. Before ”slower” or ”no” motion periods, the rate of microsaccades decreased. The experiment proved that there is a direct link between the production of microsaccades and the perception of illusory motion in Enigma. The illusion starts in the eye, not the brain.
Another of our favorite visual illusions is Mona Lisa's smile. Her expression is often called ”enigmatic” or ”elusive” but, as our mentor Margaret Livingstone at Harvard University observed, the illusory nature of her smile is explained when you consider exactly how the visual system works. When you look directly at the Mona Lisa's mouth, her smile is not apparent. But when you gaze away from her mouth, her smile appears, beckoning you. Look at her mouth, and the smile disappears again. In fact, her smile can be seen only when you look away from her mouth. This is due to the fact, mentioned earlier, that each eye has two distinct regions for seeing the world. The central area, the fovea, is where you read fine print and pick out details. The peripheral area, surrounding the fovea, is where you see gross details, motion, and shadows. When you look at a face, your eyes spend most of the time focused on the other person's eyes. Thus, when your center of gaze is on Mona Lisa's eyes, your less accurate peripheral vision is on her mouth. And because your peripheral vision is not interested in detail, it readily picks up shadows from Mona Lisa's cheekbones that enhance the curvature of her smile. But when your eyes go directly to her mouth, your central vision does not integrate the shadows from her cheeks with her mouth. The smile is gone.
Mona Lisa (Leonardo da Vinci).
The Best Illusion of the Year contest, mentioned in the introduction, has been a huge success. You would think that after generations of talented, dedicated, sometimes obsessively driven visual artists and scientists tinkering and laboring at their easels, drafting tables, scratch pads, darkrooms, and PC graphics programs, this particular vein of ore would be all mined out. But it isn't.
Consider the Leaning Tower illusion discovered by McGill University scientists Frederick Kingdom, Ali Yoonessi, and Elena Gheorghiu, which took first prize in 2007.
The two images of the Leaning Tower of Pisa are identical, but to you it seems that the tower on the right leans more. This is because your visual system treats the two images as if they were part of a single scene. Normally, two neighboring towers will rise skyward at the same right angle, with the result that their image outlines converge as they recede from view. This is one of the ironclad laws of perspective, so invariant that your visual system automatically takes it into account. Since the outlines don't converge in the images above, your visual system is forced to a.s.sume that the two side-by-side towers must be diverging. And this is what you ”see.”
Mona Lisa up close. The three panels are simulations of how your visual system sees Mona Lisa's smile in the far periphery, the near periphery, and the center of gaze. The smile is more p.r.o.nounced in the left and middle panels. (”Blurring and deblurring” by Margaret S. Livingstone, Harvard Medical School) The Leaning Tower illusion. (F. A. A. Kingdom, A. Yoonessi, and E. Gheorghiu, McGill University) This illusion is so basic, so simple, it is almost beyond belief that no one ever reported it before 2007. It just goes to show that there is still plenty of low-hanging fruit just waiting to be discovered in the world of illusions. Each new illusion adds depth and definition to perceptual and cognitive theory, bolstering certain hypotheses while weakening others or inspiring new ones. Some suggest new experiments. Each inches us just that much closer to understanding perception and awareness.
The illusion of s.e.x (Richard Russell).
The only difference between these two faces is their degree of contrast. Yet one appears female and the other male. That's because female faces tend to have more contrast between the eye and mouth (think how makeup exaggerates these features) and the rest of the face than males. Richard Russell, the Harvard University neuroscientist who created the illusion, has found that increasing the contrast of a face (more makeup!) makes it more feminine. Conversely, reducing contrast makes it look more masculine.
Next, the Rotating Snakes illusion, which was presented at the 2005 contest.
The perception of motion need not arise from actual action in the world. Rather, the perception of motion occurs when dedicated motion processing neurons in your brain are activated by specific patterns of light intensity changes in your retina.
The Rotating Snakes illusion (Akiyos.h.i.+ Kitaoka).
Some stationary patterns generate the illusory perception of motion. For instance, in this illusion invented by the scientist Akiyos.h.i.+ Kitaoka, the ”snakes” appear to twist. But nothing is really moving other than your eyes. If you hold your gaze steady on one of the black dots in the center of each ”snake,” the motion will slow down or even stop. Because holding the eyes still stops the illusory motion, eye movements must make the snakes twist. This is supported by the fact that the illusory effect is usually stronger if you move your eyes around the image.
Finally, there is the Standing Wave of Invisibility illusion, which we hope to turn into a totally new magic trick and someday in the future unveil at the Magic Castle. This is the illusion Steve discovered while working on his thesis in graduate school. He wondered what is required for an object to be visible. You might think that visibility should require only that light fall on your retina. But it can be more complicated. Illusions of invisibility show that a stimulus can be projected onto your retina and nevertheless be wholly or partly invisible.
A cla.s.sic example is visual masking. In this illusion, a visual target-for instance, a black bar against a white background-is rendered invisible when two ab.u.t.ting black bars appear a tenth of a second after the target. What's cool is that a target that is seen initially by the brain can be erased by a mask that enters the brain afterward.
Steve's graduate thesis showed how the illusion works in the brain. As it turns out, the target causes two responses in your visual pathway. One, the onset response, occurs after the target turns on. A second, the after discharge, occurs after the target turns off. Other labs had ignored the after discharge because it occurs after the stimulus turns off. But Steve showed that if you inhibit the after discharge, the stimulus disappears. The same also happens if you inhibit the onset response but not the after discharge. So both the onset response to a stimulus and the after discharge contribute to the neural representation of a stimulus. He realized that if this was true, we should be able to predict a new and very powerful illusion in which a flickering target is perpetually rendered invisible by inhibiting both the onset response and the after discharge of each flicker. It worked!12 We called the new illusion the Standing Wave of Invisibility, and it unites our interest in visual illusions and magic. It is this illusion that we plan to turn into a new stage effect to wow magicians with the power of neuroscience on their own turf. To make this happen we are going to need the help of a magic studio that specializes in electrically engineered lighting effects. For now the trick is on our ”to do” list.
Welcome to the Show but Please Leave on Your Blinders.
Cognitive Illusions.
Apollo Robbins is sweeping his hands around the body of the fellow he has just chosen from the audience. ”What I'm doing now is fanning you,” the master pickpocket from Las Vegas informs his mark, ”just checking to see what you have in your pockets.” Apollo's hands move in a flurry of gentle strokes and pats over the man's clothes. More than two hundred scientists are watching him like hawks, trying to catch a glimpse of fingers trespa.s.sing into a pocket. But to all appearances this is a perfectly innocent and respectful frisking. ”I have a lot of intel on you now,” Apollo continues. ”You scientists carry a lot of things.”13 Apollo is demonstrating his kleptic arts to a roomful of neuroscientists who have come to Las Vegas for the 2007 Magic of Consciousness symposium. The idea behind this evening is to show these researchers that magicians have much to teach them about the subjects of their life's work: attention, perception, and even the holy grail, consciousness. Magicians and neuroscientists share a pa.s.sion for understanding the nuts and bolts of the human mind, but we have been developing our respective arts and theories more or less in dependently of each other for generations. Starting tonight, if all goes as planned, our two communities are going to pay close attention to each other's discoveries.
Apollo has dared everyone in the auditorium to try to catch him pilfering this man's belongings up on stage in plain view. We watch intently just like everyone else, but none of us really stand a chance. This is Apollo Robbins, the infamous ”Gentleman Thief” who once pickpocketed ex-president Jimmy Carter's Secret Service detail, relieving them of their watches, wallets, badges, confidential itinerary, and the keys to Carter's limo. He can keep the joke on us for as long as he feels like it, but at least we know one thing he doesn't. As soon as we see who Apollo has plucked randomly from the crowd, we exchange amused glances. This man isn't a scientist at all, as Apollo a.s.sumes, but the New York Times science reporter George Johnson, who will be explaining to the wider world what transpires here tonight. George is a man of great humor and intelligence, but he is quite shy. His awkwardness makes for great theater.
The fanning continues as Apollo engages in his highly honed rapid-fire patter. ”You have so many things in your pockets I'm not sure where to begin. Here, was this yours?” he asks, thrusting something into George's hand. George frowns down at it. ”You had a pen in here,” Apollo says, opening George's breast pocket, ”but that's not what I was looking for. What's in that pocket over there?” George looks over. ”There was a napkin or a tissue, maybe? You have so many things it's confusing to me. You know, to be honest I'm not sure that I've pickpocketed a scientist before. I've never had to do indexing as I went through someone's pockets.”
Patter, it turns out, is one of the most important tools in the magician's toolkit for attention management. There are only a dozen or two (depending on whom you ask) main categories of magic effects in the magician's repertoire; the apparent wide variety of tricks is all in the presentation and details. Sleight of hand is of course critical to a pickpocket, but so is patter-the smooth and confident stream of commentary that can be used to hold, direct, or divide attention. Apollo tells George one thing while doing two other things with his hands. This means that in the best-case scenario, George has only a one in three chance of noticing when something of his gets s.n.a.t.c.hed. His real chances are actually far below one in three: in the psychic sparring ring of attention management, Apollo is a tenth-degree black belt. By continually touching George in various places-his shoulder, wrist, breast pocket, outer thigh-he jerks George's attention around the way a magnet draws a compa.s.s needle. While George is trying to keep track of it all, Apollo is delicately dipping his other hand into George's pockets, using his fast-driving voice to help keep George's attention riveted on Apollo's cognitive feints and jabs and away from the pockets being picked.
SPOILER ALERT! THE FOLLOWING SECTION DESCRIBES MAGIC SECRETS AND THEIR BRAIN MECHANISMS!.
Apollo steals George's pen, notes, digital recorder, some receipts, loose cash, wallet, and, very early on, his watch. One cla.s.sic way to lift somebody's watch is to first grab his wrist over the watchband and squeeze. This creates a lingering sensory afterimage. You know about visual afterimages from chapter 1-the red dress, the vanis.h.i.+ng coin-but afterimages can occur in any sensory system. Apollo is exploiting the same principle, only in this case the afterimage is tactile. The afterimage renders the touch neurons in George's skin and spinal cord less sensitive to the watch's removal and creates a conveniently lasting perception of the watch long after it has disappeared. George simply doesn't notice his watch is missing because his skin tells him it is still there. We notice the watch when we see Apollo folding his arms behind his back, buckling it onto his own wrist as his patter leads George down some new garden path of attention.
END OF SPOILER ALERT.
On Adaptation.
At one point or another in your life you surely tore your living s.p.a.ce apart in search of your gla.s.ses-”They can't have just disappeared!?”-only to realize that you were wearing them. When you first put them on an hour ago, the touch receptors in the skin of your face and head gave you a rich sensory impression of their location, their weight, their tightness against your temples. But since then they have become an in effective stimulus and you feel nothing.
Or try to touch the elastic band of your sock without looking, while you keep your legs and feet still. Chances are you will miss it by at least a couple of inches. This same elastic band was very noticeable against your skin when you first put your socks on this morning. But because nothing has changed since, it has become undetectable to your touch sensors. Or put your hand on a table and hold it completely still. At first you will feel it; after a short time, you no longer notice it.
Adaptation is a critical and ubiquitous process in the nervous system, not just in sensory processing but in all brain systems. It saves energy by reducing the metabolism in neurons that do not receive new information.
A few times during the fleecing, Apollo holds a pilfered object high up behind George's head for the audience to see. This makes everyone laugh but George, who smiles and looks around sheepishly, wondering what the joke is. Then, to more laughter, Apollo returns all of George's belongings one by one. ”If you're recording, I think we have evidence,” he warns as he hands over the digital recorder. Proffering a folded stack of bills, he says, ”I presume this is your gratuity money?” Finally he turns to George and says, ”We all pitched in to buy you a watch, very similar to the one you were wearing when you got here.” He unstraps George's watch from his own wrist and pa.s.ses it over. George gasps and then rolls his eyes.
How could George be so inattentive? Why can some joking thief manipulate his attention like a matador leading a bull? It's truly amazing that this can happen to a professionally trained observer like George while he's onstage (and therefore has heightened attention) and has been told what is about to happen to him. It makes you wonder, what is attention? Can you look directly at something and literally not see it?
Magicians are masterminds of human cognition. They control very sophisticated cognitive processes, such as attention, memory, and causal inference, with a bewildering combination of visual, auditory, tactile, and social manipulations. The cognitive illusions they create, unlike the visual illusions discussed so far, are not sensory in nature. Rather, they involve higher-level brain functions. By toying with your cognition-even if they don't know which neural circuits they are tapping-magicians make it impossible for you to follow the physics of what is actually happening. They leave you with the impression that there is only one explanation for what just happened: pure magic.
Possibly the best definition of attention was put forth in 1890 by William James, author of The Principles of Psychology and the philosopher king of modern psychology. He wrote: ”Everyone knows what attention is. It is the taking possession by the mind, in clear and vivid form, of one out of what seem several simultaneously possible objects or trains of thought. Focalization, concentration, of consciousness are of its essence. It implies withdrawal from some things in order to deal effectively with others.”
James elegantly describes the phenomenon of attention, but he says nothing about how it is generated by your brain or how it is modulated in everyday experience. In William James's day, attention could be studied only in terms of introspection-the reflective looking inward on your own thoughts and feelings.
For the next one hundred years, researchers groped in the dark for new and better ways to understand attention. In experiments, subjects wore headphones that piped different words into their left ear and right ear and were asked to listen to just one side, to see if attention could be divided. Some scientists studied radar operators and combat pilots to see how well they could split attention. Others examined the ”c.o.c.ktail party effect,” which enables you, in a noisy ballroom filled with loud inebriated people, to hear your name spoken from across the room.
But such studies were observational, meaning the brain was still a black box. Neuroscientists could examine the brain's mechanisms of attention in animals, or in human patients undergoing neurosurgery for diseases such as epilepsy, but there was simply no way to probe the inner cogs and wheels of the brain's attentional circuitry in healthy humans. That changed in the 1990s with the advent of modern brain imaging techniques that allow us to peer into the black box and look for the location of neural correlates of attention. Now we can also begin to figure out how magicians twiddle your attentional circuits with such consummate skill.
Already neuroscientists have learned that attention refers to a number of different cognitive processes. You can pay attention to your TV show voluntarily, which is one process (top-down attention), or your baby's crying can draw your attention away from the TV, which is a different process (bottom-up attention). You can look right at what you are paying attention to (overt attention), or you can look at one thing while secretly paying attention to something else (covert attention). You can draw somebody's gaze to a specific object by looking at it ( joint attention), or you can simply not pay attention to anything in particular. Some of the brain mechanisms controlling these processes are beginning to be understood. For example, you have a ”spotlight of attention,” meaning that you have a limited capacity for attention. This restricts how much information you can take in from a region of visual s.p.a.ce at any given time. When you attend to something, it is as if your mind aims a spotlight onto it. You actively ignore virtually everything else that is happening around your spotlight, giving you a kind of ”tunnel vision.” Magicians exploit this feature of your brain to maximum effect.