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Your Brain: The Missing Manual by Matthew MacDonald

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Chapter 4. Perception

Your brain is a reality-construction machine. It takes the vast oceans of information that flood your senses, and transforms them into a highly subjective inner world.

This inner world has a few things in common with outside reality, but less than you’d think. It’s run by a processing system that’s quick to jump to conclusions, confidently ignorant of its mistakes, and easily fooled. This processing system sees what it expects to see, hears what it expects to hear, and petulantly refuses to be corrected on even the simplest point. You may enjoy this world or you may not. However, you’ll never get a chance to step out of your head and take a clear look at what’s really happening outside.

That’s where this chapter fits in. Here, you’ll explore some of the ways that the brain shapes outside reality. You’ll learn about the quirks of the eyes, ears, and other senses, and the automatic assumptions that are deeply ingrained in your brain. Occasionally, this knowledge will help you “unfool” yourself—in other words, it lets you anticipate your brain’s hiccups and work around them. Other times you’ll learn enough to fool someone else, which is just as good (and makes a solid foundation for a career in politics, advertising, or real estate). Either way, this chapter gives you an opportunity to pull back the curtain and steal another quick look at the strange machine that runs your life.

The Doors of Perception

It’s tempting to divide the brain’s information processing into two neat categories: conscious (what you know you see and hear) and subconscious (what your brain deals with automatically, behind the scenes). After all, you don’t consciously perceive the inner ear signals that ensure you stay balanced while navigating an intricate dance routine, but you are acutely aware of the crushing heel that your dance partner just placed on your big toe.

However, if you dig a little deeper into the brain’s jelly-like matter you’ll quickly find that it’s a little bit like sharing an apartment with a group of freewheeling friends—there’s a lot more going on than you realize (and a fair bit more than you’d probably consent to). Basic avenues of perception that you take for granted, like seeing, hearing, and touch, are actually colored by layers and layers of the brain’s automatic preprocessing. In essence, your brain expects the world to behave in certain ways, and it subtly shapes your perception according to these biases.

Furthermore, this isn’t just a story about any one sense. It most obviously affects vision, but its effects are equally apparent with sound, touch, taste, and more complex combinations. These automatic assumptions happen at the lower levels of the brain (for example, through specialized neurons that deal with particular optical phenomena) and higher ones (for example, in the folds of the cerebral cortex, where deep thinking takes place).

Although this automatic processing sounds a bit suspicious, you’d be ill advised to turn it off (and short of heavy quantities of illegal pharmaceuticals, there’s no way you could). Most people don’t want to spend minutes thinking about shapes, illuminations, and perspective simply to follow their favorite sitcom. Similarly, they don’t want to go through a painstaking process of logical deduction to determine if the object they’re looking at is a person and, furthermore, if it is in fact their spouse (as memorably described in Oliver Sacks’ The Man Who Mistook his Wife for a Hat [Summit Books, 1985]).

That’s not to say it isn’t worthwhile to learn more about the automatic processing of your brain. Using the insight you pick up in this chapter, you’ll be able to:

  • Defend yourself against accidental mistakes. A little bit of knowledge can help make certain that you aren’t tripped up by faulty brain assumptions (or at least figure out what went wrong after the fact). This is a theme you’ll revisit throughout this book, including the next chapter, when you’ll discover the ways the brain can mangle memories despite the best intentions of the rememberer.

  • Defend yourself against out-and-out trickery. Magicians, pickpockets, and psychics often rely on the well known quirks of human perception—the assumptions, omissions, and unusual glitches the brain encounters while processing the outside world. Once you know what to expect, you’ll be able to unravel a few tricks (or get better at pulling them off yourself).

  • Dazzle your friends with party tricks. What über-geek doesn’t need a trusty optical illusion to break the ice at a party? And if your interests are more practical, wagering possibilities abound (“Are you willing to bet this line is longer than that one?”).

Optical Illusions

One of the most fascinating ways to size up the workings of the brain is by exploring optical illusions, the strange images that aren’t quite what they seem to be. To a certain extent, all optical illusions work by exploiting a chink in the brain’s visual processing systems—an automatic assumption that doesn’t always hold true, an interpretive technique that can run astray, an attempt to compensate for another shortcoming, and so on. However, there’s an amazing diversity in the way these illusions work. You can easily line up a dozen different optical illusions and find that each one relies on a different trick to short-circuit the brain.

Some of the simplest illusions work by overstimulating some part of the brain’s visual processing system. Conceptually, their effects are like the afterimage you get when you stare foolishly into the sun (against your mother’s advice).

One example of this phenomenon is found in the grid of squares shown below. When you stare at it, you’ll see gray shaded areas flash into existence where the white lines intersect, even though there’s nothing there.

Figure 4-1. 

As with many optical illusions, it’s difficult to pinpoint exactly what goes wrong in your brain when you look at the grid. However, part of the brain’s strategy when picking out shapes involves emphasizing edges and contrasts. In high-contrast images like this grid and the slanted lines shown below, the effect can be pumped up to dizzying proportions.

Figure 4-2. 

In order to perceive a scene, your brain takes the information from your eyes and pushes it through a long, complex pipeline. (Actually, the pipeline metaphor isn’t quite correct, because it implies that operations take place sequentially. In reality, your brain has many visual modules working at the same time, sometimes collaborating to arrive at an insight, other times competing to decide the best interpretation of what you see.) The illusions shown here kick in at a low level, before your brain has a chance to process the full details of the scene in front of you. Although they make for fun eye candy, they don’t teach us very much. They’re also short on practical payoff, unless you’re planning to disorient friends and colleagues with bursts of random patterns.

In this chapter, you’ll see a variety of optical illusions, including some that game the system early on (like the grid illusion) and others that mislead neurons further down. You’ll also consider illusions that cross over into your other senses, such as hearing and touch. All of these illusions emphasize the same sorry fact—namely, your brain is a very unreliable spectator.


One of the oldest known illusions is based on touch, and was described by Aristotle more than 2,000 years ago. Happily, you can pull this one off at home. First, find a pencil, and lay it in front of you. Then, cross your middle and index fingers (they’re right next to each other, so that’s easy). Now, without looking at the pencil, lay both your fingers on it. You’ll have the distinctly odd impression that there are two pencils. (That’s why you avert your eyes. If you look at the pencil, you give your brain a chance to correct itself.)

Your Shifty Eyes

Some of the most captivating optical illusions are those that involve imaginary motion. Like the pattern of dots shown below, they appear to undulate hypnotically.

Figure 4-3. 

This illusion packs in two tricks. First, it uses contrasting colors that are perceived by different cells in the eye (without which the effect is much more subdued). Second, it varies the shading of different dots, placing the shadows above, below, and to the side of the various dots. (This trick is duplicated in hundreds of optical illusions.) However, neither of these details explains how a static image can fool your brain into seeing nauseating motion.

To really understand this illusion, you need to realize that your eye has a dirty secret—it’s only able to see fine detail in a small fragment of its visual field. The pinpoint-sized part of your eye that sees sharply is called the fovea. If you look at a person an arm’s length away, the fovea gives you a sharply detailed region that’s about the size of a dime.

Figure 4-4. 

Your brain uses a crafty trick called saccades to compensate for this weakness. Saccades are quick, automatic eye movements. They’re keenly important for reading books like this one, and they’re equally indispensable for taking in the full detail of a visual scene. On average, your eye performs two or three saccades each second, ricocheting about your visual field without you even realizing it, each time capturing the fine detail of another tiny region. Inside your brain, these separate dime-sized pictures are pasted together to create a single, seamless whole.


If you’re severely drunk, your saccades slow down, and you start to see the world the way your eye really perceives it—a patch of sharpness surrounded by a blurry field.

With this in mind, the drifting dots you saw earlier are easier to understand. As your eyes jump from one circle to the next, trying to stitch together the complete picture, your brain is confused by the alternate shading. After each saccade, the previously viewed dots aren’t quite where your brain expects them to be, and so it assumes that they’ve shifted ever-so-slightly to the side. This creates the impression of motion.

You can will away this illusion to a certain extent by focusing intently on a small section of the pattern, and refusing to move your eyes. In this case, the center stops moving, but the sides continue to swell and heave like an unsettled sea.


Saccades don’t just compensate for the blurriness of your vision outside the fovea. They also compensate for the unequal distribution of color sensitivity in your eye, and they mask your blind spot (which exists where the bundle of optic nerves exits your eye on the way to the brain).

One of the most famous illusions to take advantage of our shifty-eyed nature is the rotating snakes illusion, created by Akiyoshi Kitaoka, and shown in many alternate incarnations at www.psy.ritsumei.ac.jp/~akitaoka/rotsnakee.html. The effect is strongest out of your peripheral vision.

Figure 4-5. 

Saccades are one of the many ways your eyes can deceive you. During a saccade, your brain compensates for the sudden movement by temporarily shutting down your visual input. This ensures that you don’t see a dizzying blur streaking across your field of view. However, it also means you can miss sudden events that happen during a saccade (just as you can miss something important when you blink). Not only do the eyes lie, they also omit.

Keeping Focused

As you’ve seen, your brain keeps your gaze on the move, shifting your eyes to take in a full scene and moving your head to fix on important-seeming objects. This automatic movement creates a sticky problem. It means that it’s much more difficult to focus on something that doesn’t use the dynamics of sound, flashes of light, and bursts of movement to catch and hold your attention (for example, Gorillas in the Mist the book, rather than King Kong the movie).

This difference is particularly prominent in many business environments, where distractions abound and everything you’re expected to do is monumentally boring. In this situation, focusing on a task like data entry is an epic battle between the paranoid parts of your brain, which are constantly on alert and waiting for the cues that indicate danger, and the conscious parts, which just want to get the job done so you can head off to the pub. So what can you do to win the war and keep your attention where you want it?

First, recognize what you can’t change. Studies show that it’s all but impossible for the brain to tune out distractions by sheer willpower. In other words, if people are given a task and told to ignore something unrelated, they can’t. For example, experiments show that if you work on a computer monitor that has a background with a slowly moving starfield, the part of your brain that processes motion remains continuously active. Or, if you’re shown pictures of famous faces while working on word problems, the face-recognition region of your brain lights up like a Christmas tree. The same is true when unimportant sound intrudes on your senses—whether it’s a ringing telephone or a foul-mouthed coworker, it all gets processed. This is annoyingly inconvenient, but it makes sense. In our deep evolutionary past, tuning out a sound as loud as a hip-hop cellphone ringtone was likely to get you eaten.

With this in mind, here are a few good tips to keep your brain on task:

  • Don’t try to fight distractions; eliminate them. Unplug your phone, turn off your radio, and close the door to your workroom. If you insist on doing your taxes in front of the television, you’re asking for an audit. It’s a skewed battle because the television has the help of your superior colliculus to reel you in.

  • Make boring tasks just a little bit harder. Studies show that the brain will start to tune out some superfluous information when it’s wrestling with a challenge. (In the previously described studies, that means the parts of the brain that would ordinarily process the starfield’s motion or the famous faces become less active when you’re struggling with a tough task.) Obviously, this advice only lends itself to certain chores. For example, if you have to type a long list of names into a computer, you may be able to better keep your focus by racing against a clock, challenging yourself to enter names in larger batches, or playing a risqué rhyming game with each person’s middle name.

  • Resist the distractions you can control. Although the automatic processing of your brain gives us all a certain degree of distractibility, studies suggest that roughly half of the distractions that derail us from tedious tasks are self-generated. Examples include snacking endlessly and hunting down rare action figures on eBay. In the corporate world, some businesses have found that a mandatory email-free day once a week boosts productivity, sometimes dramatically. Another email wrangling option: limit the number of times per day you check (in the morning, at lunch, and an hour before heading out, for example).

  • Don’t worry about background noise. You should be able to tune out continual soft chatter, humming fans, and keyboard typing through a process known as adaptation, which is described later in this chapter (Ignoring Things). Essentially, the brain adjusts to a continuous stimulus, recognizing that it probably doesn’t indicate an immediate threat.

Distortions and Mismeasurements

Many of the most familiar optical illusions are distortions. They take advantage of the brain’s assumptions to skew the way you perceive contours, lengths, colors, and shading.

For example, the long diagonal lines in the following picture (which run from the top-left to bottom-right) are perfectly parallel. However, the pattern of cross marks in the line fools your brain into thinking they lean toward one another.

Figure 4-6. 

Here, your brain is confused by angles that aren’t quite what it expects. It’s as if your brain expects the hatch marks to cross each line at a right angle. You can almost feel your brain mentally twisting the lines to make them fit its expectation.

The following image shows a more ambitious pattern that easily blinds the brain. The image shows a series of concentric circles, but the brain is locked into a different interpretation, and insists on seeing a spiral. (Trace your finger around one of the circles if you don’t believe it’s concentric.)

Figure 4-7. 

The remarkable part of both these illusions isn’t that your brain is fooled—after all, its mistaken logic is reasonable and (more importantly) it’s blindingly fast. The amazing part is that even if you carefully measure the angle of the slanted lines or trace out the circles, thereby proving the illusion, you still can’t convince your brain that it’s made a mistake. In fact, no amount of pleading can convince your brain to alter its wonky interpretation. Your brain may take a lot of rules into account when it decides how to view a scene, but it has no interest in your slow-thinking deductive logic.


To put it another way, you aren’t in control of what you perceive. So expect flaws in your vision and be prepared to be fooled by magicians, UFO sightings, and apparent paranormal phenomena. Seeing may be believing, but only if you don’t mind being royally snookered.

Faulty Comparisons

Along with distortions of shape, your brain can also mislead you when sizing up the length, size, and color of an object. And when the brain’s assumptions fail, the effects tell us quite a bit about the brain’s book of visual rules, tricks, and shortcuts.

For example, the following illusion shows two curved shapes. The bottom shape appears to be larger, but it’s actually identical to the top shape.

Figure 4-8. 

This illusion works because in its haste, your brain makes a few simplifications. It notices the way the left edges of both shapes line up, and takes that into account, discounting the fact that the lined-up edge is gently slanted. When your brain then turns its gaze to the right side, it correctly notices that the bottom shape kicks out a bit further. Thus, the brain concludes that the bottom shape is bigger, missing the fact that the left edge of the bottom shape actually sits a bit further to the right than the edge of the top shape. (If the shapes were truly lined up, the top-left corner of each shape would be positioned on the same vertical line.)

A similar faulty rule is on display in the orange circles of the next optical illusion.

Figure 4-9. 

Here, your brain makes two correct observations: the orange circle on the left is small compared to its blue neighbors, and its counterpart on the right is large compared to its neighbors. However, once the brain settles on this intriguing fact, it becomes blind to the fact that both orange circles are the same size. Instead, the proportionally larger one (on the right) seems larger than the one on the left.

Color Confusion

Shapes and sizes aren’t the only thing that can confuse your brain. Your brain can easily make similar mistakes when comparing brightness or colors. In the carefully set up illusion below, two cubes are shown with different lighting. In the center of the front face of the cube is a square that appears to be yellow in one figure and blue in the other. However, the color is actually identical in both—it’s the dull shade of gray that’s shown in the bar below. (Virtually the only way to convince yourself of this illusion is to use bits of paper to block out the rest of the picture, so that you see only the squares in question.)

Figure 4-10. 

In this illusion, the brain isn’t exactly wrong—it’s simply compensating for what it believes is a difference in lighting. It concludes that a square that appears gray under blue light is probably yellow, and a square that appears gray under a yellow light is probably blue. In other words, your brain’s perception has a built-in routine for evaluating lighting conditions. This is the reason you can see quite normally at home in the evening, even though the artificial lights you’re using cast a yellow-red shade of light that’s dramatically different than the blue-tinted radiance of the sun at noon.


Cameras can’t adjust themselves to compensate for the color of light. This is one of the reasons why it’s much easier for your eyes to interpret a scene than for your camera to take a great picture. Your brain can smooth out the oddities and inconsistencies of lighting conditions, but film (or the electronic sensor in your digital camera) isn’t as forgiving.

A similar effect is at work in the legendary same-color illusion. Here, two squares that are filled with exactly the same shade of gray (A and B) appear to be dramatically different. Once again, it’s almost impossible to accept this illusion unless you cover up almost everything else in the picture except the two squares in question.

Figure 4-11. 

The remarkable part of this illusion is that the brain picks up on a range of clues to make an emphatic conclusion—everything from the 3-D shape of the cylinder that casts the shadow to the pattern of the checkerboard, which darkens significantly but imperceptibly around square B. (The latter part is the most significant factor in the illusion. The brain is deeply attached to the idea of a regular checkerboard pattern, and prefers to see that over anything else.)

The 3-D World

So far, you’ve seen how the brain has built-in assumptions that help it interpret shapes, sizes, and colors (and sometimes lead to quirky mistakes). The brain also has a bag of tricks that it uses to convert the 2-D image that’s projected on your eye to a realistic understanding of the 3-D world in front of you.

Consider the classic example of two lines, shown below. Even though a ruler will tell you that the lines are the same length, the brain stubbornly insists that the top one is shorter.

Figure 4-12. 

One explanation for this illusion is that the brain is biased towards picking out the cues of 3-D objects. Lines that angle inward are typically seen in objects that are nearby (like the table in the picture below). Lines that angle outward are more common in distant objects (like the back corners of the room). Here’s an example that illustrates by comparing two lines that have the same length, but are placed in two different spots in a 3-D scene.

Figure 4-13. 

In the two-lines illusion, your brain is well aware of the fact that both lines are really and truthfully the same length. However, your brain also believes that the bottom-most line is farther away. If two objects look the same in your eye, but one is farther away, there’s only one possible conclusion—namely, the object that’s farther away is bigger. Thus, the brain “corrects” the length of the second line to take the imagined distance into account.

At first, it seems odd that the brain is so willing to skew the size of things based on their perceived distance. However, on second thought it makes a lot of sense. If the brain didn’t perform this automatic adjustment, your father would appear to shrink to midget size as soon as he began walking away from you.

The brain has several other tricks for translating the 2-D picture in your eye into a 3-D model. It assumes that objects close to the horizon are farther away, and it compares unknown objects against nearby known objects to infer distance.


A similar effect underpins the horizon moon illusion. In this scenario, the moon appears to be much larger than usual when it’s low in the sky. This is because the brain sees the moon in relation to distant objects and the horizon. But when the moon is high in the sky, the brain has no such frame of reference, and so the moon appears tiny and insignificant.

Another 3-D cue is shading. When the brain takes in a scene, it expects to find a sun-like light source radiating from above, and it uses patterns of shading to infer contours and shapes. Humans co-opt these automatic assumptions with artful applications of makeup. To an unbiased observer (say, a computer or an alien being from another planet), makeup would seem like little more than face paint. But for the easily influenced human brain, makeup is processed like shadows, and suggests a more sharply defined face.

Lastly, the brain uses one physical detail to see in three dimensions: the slightly different vantage point that’s provided by each of your two eyes. You probably already knew this, but it’s a less important factor than you probably thought. The separation of your eyes helps your brain accurately judge depth for very close objects, but it’s useless for far off ones. As you can readily test, if you cover one eye and wander around the house you might have trouble doing some precision tasks (like tying a knot or chopping tomatoes), but you won’t have any difficulty interpreting the shapes around you as 3-D objects.

Seeing Things

One of the hardest challenges for the brain’s visual systems is picking out shapes. It’s an extraordinarily difficult task. Shapes can not only be moved, rotated, resized, distorted, and obscured, but they can also exist in an endless number of variations.

The brain deals with this problem using a toolkit of assumptions. And the brain does a good job—it can easily beat computerized shape-spotters when scanning pictures, faces, and moving scenes. However, the brain’s eagerness to find shapes also leads it to find shapes where there aren’t any, as with the white triangle at the forefront of the following picture.

Figure 4-14. 

When confronted with this picture, your brain doesn’t need to conjure up a white triangle. There’s a reasonable alternate explanation—that the image contains three pacman-like circles with wedges cut out of them, and the wedges are lined up with the gaps between the blue triangles inside. However, a just-so arrangement like this would be unlikely in the natural world, so your brain quickly dismisses that possibility. In essence, your brain picks up on a few clues and performs a rapid analysis to determine the most likely explanation. However, you don’t merely think about that most likely explanation, you also see it.

If you rotate the pacmen around, the illusion disappears, and the image reverts to a collection of harmless shapes.

Figure 4-15. 


This hints at one of the key limitations of vision. Our brains are tuned to see what’s mostly likely in the ordinary, natural world. However, we haven’t caught up with the way that manmade products can deliberately hijack these assumptions. In other words, our natural-born visual senses set us up to be the perfect dupes in a world filled with manmade objects.

The brain’s obsessive pattern matching isn’t limited to shapes. It happens with faces (which we see in unlikely places like house fronts and :) punctuation) and speech sounds (for example, if parts of a word are beeped out in a recording, we “hear” the full word based on what makes sense in the context of a sentence).

The brain is also primed to identify letters and spot words. Can you read this sentence?

Figure 4-16. 

If you said “I LIKE IUMRING TQ GQNGIUSIQNS,” you have a perverse sense of humor. However, you’re also entirely correct.

Making Something out of Nothing

The brain doesn’t just organize seemingly unrelated input into patterns; it also has a nasty habit of imagining something into existence. The dubious Rorschach inkblot test is a good example. Take a look at the card shown below:

Figure 4-17. 

What do you see—a masked face, two bears exchanging a high five, or a meaningless splatter of red and black ink?

When looking at an inkblot, most of us are aware of the imaginative power we’re investing to transform ambiguous input into a meaningful picture. However, there are many cases where your brain performs the same operation without you realizing the creative leap that it’s taking.

Have you ever thought you heard a telephone ringing or a person calling your name while running a noisy appliance like a vacuum? This effect is spurred by the brain’s pattern-matching systems, which run wild when hunting through a din of sound. A great illustration of this phenomenon is an experiment that asked volunteers to determine when Bing Crosby’s White Christmas began to play in the background of a noisy recording. The devious trick was that there was no White Christmas—only thirty seconds of white noise. But primed with the expectation of hearing the familiar tune, about a third of the participants reported that they heard it. Incidentally, some people seem to be more susceptible to illusions that draw on imagination like this one. It’s thought that they have more creative, fantasy-based minds, which are perfect for free-wheeling brainstorming sessions but not as good at skeptical inquiry.


The authors of the book Mind Hacks (O’Reilly, 2004) describe the White Christmas illusion, and have provided a noisy recording that you can use to test your friends at www.mindhacks.com/book/48/whitechristmas.mp3.

The white noise study was very small and far from conclusive. Other studies have argued that people who believe in ESP are more likely to find meaningful patterns in random arrangements of dots. In other words:

[Noisy Input with Little Obvious Information] + [Your Brain] = [Things That Go Bump in the Night]

The brain’s over-eager pattern-matching system may be a plausible explanation for UFO sightings, ghosts, and other late-night creepies.

Ignoring Things

Your brain has another skill that’s just as important as finding patterns in the chaos. Not only can your brain imagine new objects into existence, it can also block out the things it wants to ignore.

As you learned earlier, your brain is hard-wired to focus attention on threatening sights and sounds. In order to better separate these potentially dangerous cues, the brain filters out repetitive, unchanging stimuli like a whirring air conditioner or the rocking motion of a boat at sea.


There are many different neurological processes supporting this “tune-out” behavior. At the lowest level, constantly stimulated neurons temporarily stop firing. (For this reason, your eyes jitter imperceptibly back and forth even when you hold your gaze steady. If they didn’t, the same neurons would always be stimulated by the sight in front of you. They’d get tired out, stop firing, and everything would fade out into blackness until you looked somewhere else.) The brain also has higher-level processes that adapt to constant stimuli and direct attention away from things that aren’t changing in favor of those that are.

Most of the time, your brain’s tune-out feature is exactly what you want. After all, who wants to be bothered thinking about the sound of air rushing by your ears, the feeling of weightiness as you sit in your couch, or the tactile sensation of clothes rubbing against your skin? Instead, your brain notices each one of these phenomenon briefly when they first appear, and then quickly adapts to ignore them. However, sometimes this effect can lead to some interesting illusions.

You’re no doubt keenly aware of the way the brain adapts itself to different levels of brightness. (If not, try walking from a darkened room into a bright summer day without getting run over.) However, the following version is more fun:

Figure 4-18. 
  1. Stand in a doorway, with your arms down at your sides.

  2. Place the back of both hands against the door frame on either side.

  3. Push up with as much strength as you can muster. Keep this up for a couple of minutes.

  4. Now relax and walk away from the door.

For the next few minutes, you’ll have the sensation that your arms are drifting up, weightless—in much the same way that your brain might adjust to a stronger gravitational field on another planet. After only a couple of minutes in the doorway, your brain becomes accustomed to the fact that it needs to exert more effort to keep your arms up than at your sides.

As this experiment shows, the brain’s tendency to ignore things is really a remarkable ability to adapt itself to its current environment. There are dozens of do-it-to-yourself experiments that show similar adaptations at work. For example, if you scatter your living room furniture haphazardly, you’ll spend the first few hours bumping into sofas, the next few hours steering yourself effortlessly (and subconsciously) through the chaos, and the following weeks wondering why everyone looks at you so oddly when you have them over for tea. A similar automatic adjustment and eventual ignoring happens with smells, but even more quickly. If you want to know if the scent from last night’s curry cook-off is still around, you’ll need to step outside your house and then come back in, because your nose tunes out even the strongest smells after just a few sniffs. And if you want to answer the age-old question “Do I smell OK?” you’ll need the help of a friend, because your brain is perpetually filtering out the familiar odor of your own body.

Lastly, your brain also adjusts itself automatically to pleasure, making sure you don’t get too much of it no matter how many triple-chocolate sundaes you down in a single sitting. Chapter 6 has more about this frustrating fact.

Other Perception-Distorting Assumptions

You’ve no doubt seen illusions that use ambiguous pictures, which can be interpreted in different ways. One legendary example is the two faces and vase, shown below.

Figure 4-19. 

The interesting thing about this sort of illusion is the fact that when you see it for the first time, you’re likely to settle on just one way of seeing it. You’ll remain oblivious to the alternative possibility until a smug friend points it out.

This sort of automatic interpretation is obvious with contours and shapes, but it also applies to more complex meanings that we assign at a higher level. In fact, these sorts of snap judgments often color what we see, even though they aren’t specifically related to vision.

For example, consider the following figure, which was the subject of a cross-cultural study. If you were asked to express this scene in a couple of sentences, how would you describe it? Think out your answer before continuing.

Figure 4-20. 

Most Westerners describe this scene pretty plainly. There’s a group of people gathered in discussion (possibly a family), there appears to be a window on the left above one of the women, and the shading of the floor and corner of the wall make it clear that everyone is gathered indoors. But these obvious “facts” aren’t quite as obvious to people with different experiences and hence different assumptions engraved in their brains.

When researchers showed this picture to East Africans, nearly all of them said the woman on the left was balancing a box on her head. And the corner of the room in the back was interpreted as a tree, under which the family is sitting. Now if you look at the figure again, you’ll probably agree that this interpretation makes just as much sense as your own. As a Westerner who has spent much of your life indoors, your brain is used to interpreting scenes using the boxlike shapes and angular cues of modern architecture (like windows and the corners of walls). Rural East Africans have a different store of experience to bring to bear on new scenes. All this shows that a surprising amount of higher-level reasoning can leak into processes like hearing and seeing, and color the results without you even realizing it.

Dizzy Yourself Silly with Optical Illusions on the Web

You’ve now finished your tour through the oddities of human perception. But the fun doesn’t need to end here.

Once upon a time, the only reliable place for average people to see optical illusions was in a somewhat baroque object known as a book. (If you’re reading a non-electronic version of this text, you know what I’m talking about.) Books were wildly popular for many years, before becoming replaced by electronic pixels. Today, books are mostly remembered by some of the over-16 crowd as an odd early version of the Internet.

No matter what you think about the march of progress and the colonization of Earth by computers, the Internet has been good for optical illusions. There are dozens of Web sites that show optical illusions in all their glory. Here are some of the best:

  • Akiyoshi Illusions. The legendary creator of the rotating snakes illusion (shown on Your Shifty Eyes) also provides pages of hand-crafted illusions. See www.ritsumei.ac.jp/~akitaoka/index-e.html.

  • Purves Labs. This top-notch lab studies optical illusions and offers a see-it-yourself section that includes some of the most remarkable color and brightness illusions ever created. See www.purveslab.net.

  • Michael Bach’s Illusions. Many of the illusions on this Web site are outfitted with multimedia extras, such as little movies that move parts of the optical illusion around so you can verify what you really want to know—that two lines are the same length, two shapes are the same color, and so on. See www.michaelbach.de/ot.

  • Wikipedia. The free-for-all online encyclopedia describes a selection of optical illusions, including many of the examples you’ve seen in this chapter. See http://en.wikipedia.org/wiki/Optical_illusion.

  • Mighty Optical Illusions. Although it’s a bit noisy—try to use your brain’s ignoring power on the Google ads distributed about the top, side, and bottom—this Web site has a solid illusion catalog, and ranges farther afield to get some interesting examples from real life. See www.moillusions.com.

Incidentally, the human race takes advantage of the idiosyncrasies of its visual hardware. Most of us spend hours transforming collections of flickering dots on a screen into an impression of real people. This optical illusion, known as television, is surely one of the most impressive visual ruses ever documented.

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