Your Brain: The Missing Manual

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For most of this book, you'll focus on what your brain does, and pay less attention to its plumbing. It's not that the brain lacks interesting hardware. But you can easily spend a lifetime studying your brain's biological workings without having the faintest idea why your company laid you off, your spouse ran off with another lover, and your dreams are filled with gorillas in tuxedos serving you shrimp cocktails.

To get practical information that can help with life's day-to-day challenges, you need to concentrate on your brain's software—in other words, the thoughts, emotions, and higher-level processes that are endlessly at work in your squishy gray matter. In this book, you'll explore these phenomena closely. But, before you get started, there are a few underlying details to get out of the way. You need a crash-course in brain basics.

In this chapter, you'll take a quick tour to see what your brain looks like and how it's structured. You'll take a close look at neurons—the tiny wires that convey electrical signals in your brain—and find out how your brain plugs into the rest of your body. Along the way, you'll dispel a few myths about the brain, peer into its evolutionary history, and learn a few of the secrets of mental health.

It's time to meet your brain.

Lurking in the space between your ears is a very soft, reddish, jelly-like organ. (If you were expecting your brain to be firm and deep grey, like a wrinkled walnut, you are no doubt thinking of a preserved brain. The living brain is much squishier, and it's covered in deep red arteries.)

Figure :

The average human brain weighs in at about three pounds. By comparison, an elephant's brain tips the scale at 11 pounds while a cat's brain—brace yourself, cat lovers—is a mere ounce. Bigger animals tend to have bigger brains, and some scientists suggest that a high brain-to-body weight ratio distinguishes the smart species from the dullards. In other words, the larger the brain is as a percentage of body weight, the smarter the creature. This calculation puts a few of our favorite animals at the top of the list (like dolphins and chimpanzees), but it needs a bit of fudgery to deal with really small animals (like birds and mice), which would otherwise appear to be raging geniuses.

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It's time to meet your brain.

Lurking in the space between your ears is a very soft, reddish, jelly-like organ. (If you were expecting your brain to be firm and deep grey, like a wrinkled walnut, you are no doubt thinking of a preserved brain. The living brain is much squishier, and it's covered in deep red arteries.)

Figure :

The average human brain weighs in at about three pounds. By comparison, an elephant's brain tips the scale at 11 pounds while a cat's brain—brace yourself, cat lovers—is a mere ounce. Bigger animals tend to have bigger brains, and some scientists suggest that a high brain-to-body weight ratio distinguishes the smart species from the dullards. In other words, the larger the brain is as a percentage of body weight, the smarter the creature. This calculation puts a few of our favorite animals at the top of the list (like dolphins and chimpanzees), but it needs a bit of fudgery to deal with really small animals (like birds and mice), which would otherwise appear to be raging geniuses.

You can check out the brain weight of your favorite animal at http://faculty.washington.edu/chudler/facts.html.

Of course, size isn't everything. Although all mammals have some strikingly similar brain hardware (and, to a lesser extent, so do all creatures that have any sort of brain), there are key anatomical differences. To really understand your brain, you need to dig deeper.

Figure :

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Much as archaeologists examining an ancient site often find the ruins of multiple cities, each built on top of the previous one, neuroscientists peering into the brain find newer biological hardware built over the old stuff. In this section, you'll get the chance to peel back the layers.

The human brain is, like all the products of evolution, a work-in-progress. Although we won't see the human brain change in our lifetimes, millions of years of evolution have left their fingerprints all over it. Here's what's been happening:

The human brain has grown, becoming physically larger. In fact, there's a strong case that humans suffer far more pain giving birth than almost any other animal because of our comparatively huge heads, which we need to carry around our outsized brains.
Existing brain hardware has been adapted for different uses. The human brain is remarkably flexible. In deaf children, it can assign brain parts normally used for hearing to other tasks, like understanding sign language. In blind children, the brain can recruit the speech processing regions to interpret the tactile sensation of Braille letters. Over millions of years, similar but more profound shifts can occur. For example, many researchers believe that human speech hijacked some serious brain space in our early ancestors, and crowded out other skills.
New features have been bolted on top of old ones. It's much easier for evolution to change what's already there than create a whole new brain from scratch. That means there's some deep, dark animal ancestry in your brain. If evolution were a building contractor, you'd find it leaving a few frightening things in the basement.

In the following section, you'll slice open your brain (metaphorically speaking) and get a closer look.

No one knows why big-brained humans won the evolutionary arms race. Although it's tempting to conclude that smarter humans could build better tools (and therefore catch more nutritious animals), the brain has a significant evolutionary disadvantage—it's a hugely expensive energy hog. One of the more likely explanations for our success is that bigger human brains helped us attract mates and negotiate sticky group dynamics. In other words, we're all the descendants of a few sexy nerds.

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So far, you've looked at the brain's shape, structure, and history. But you haven't yet seen it in action.

You probably already know that the brain is an electrical appliance more complex than any circuit board. But the brain also communicates with chemicals, using tiny compounds to transmit information, control mood, and interact with the rest of the body. Once you understand a few facts about your brain's wiring system, you'll have an easier time tackling some of the more sophisticated topics in this book.

Your brain holds hundreds of billions of nerve cells. These cells come in two flavors: neurons (which get all the attention) and glial cells (which play an essential but often-overlooked supporting role).

Neurons carry electrical signals through your brain, and through the rest of your body. Estimates range, but the most widely cited calculations suggest that you have 100 billion neurons. (If you need an ego boost, compare that with the 300,000 neurons in the brain of the humble fruit fly.) Amazingly, there are at least 10 times as many glial cells, which provide nourishment, protection, waste disposal, speed enhancement (see ), and other support services for the spotlight-hogging neurons.

Here's a look at a single neuron:

Figure :

Up close and personal, a neuron looks like some form of futuristic vegetation. It receives messages through tree-like branches called dendrites. It then sends an electrical signal down a long tube-like structure called the axon. Add up the cumulative effect of several billion of these electrical impulses and you get a symphony, a treatise on law, or an episode of Buffy the Vampire Slayer.

This picture of the neuron isn't proportionally accurate. In a real brain, the body of the cell (the top-left section of the picture) would be much smaller, while the dendrites, axons, and axon terminals (the branches at the end of the axon) would snake out far, far longer.

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You've now completed your first tour of the brain. Although you don't yet know all the reasons for the peculiar behavior of the planet's dominant species, you now have some of the tools that you can use to start asking the right questions. This makes it a good time to take a step back and change focus from low-level biology to more general guidelines. In this final section, you'll consider how you can keep your mental machine running in tip-top shape through the decades.

First, it's important to realize that the solution isn't to grow a bigger brain. After birth, it's rare for new neurons to appear in the brain. In fact, the story of the brain's development (which is told in ) is largely the story of neurons and synapses dying off in waves as your body lumbers into old age. But don't panic yet. There's good reason to think that the loss of a few million neurons over the years is no big deal. In fact, it just might be part of the brain's natural housekeeping.

Rather than count the number of neurons in your head, it's more important to take note of the connections between them. As you've already learned, neurons are constantly being rewired. In healthy brains, the ratio of synapses to neurons grows as the number of neurons declines. In other words, leaner brains can become more efficient to compensate for their loss of neurons.

So what can you do to keep your brain in its best working form? There may be no way to dodge bad genes, bad luck, injury, and disease, but studies of brain aging consistently identify a few characteristics in old-aged but nimble-brained people. Here are a few practical guidelines if you hope to become a quick-witted fast-talking 90-year-old cribbage shark:

You are what you do. The brain is constantly rewiring the connections between your neurons, strengthening the ones you use and weakening the ones that you don't. In other words, when you spend a day munching Cheetos, watching

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Several times a day, the average human puts whatever he or she is doing on hold and trundles off in search of some food.

At this precise moment, a small-scale drama unfolds in the brain. The deepest levels of the brain notice the shortage of food and trigger the physical feelings of hunger. The higher levels fire up food cravings, strategize about where to get the next food fix, and attempt to rationalize how a triple cheeseburger makes for a responsible breakfast. Here, the human brain shows its impressive abilities once again. In even the most well-adjusted person, it can convert a brightly-colored box of Oreos into a subtle interplay of desire, pleasure, guilt, and regret.

It may well be that food guilt is the most reliable way to separate humans from other animals. Although other species have muscled their way into our territory in various other skill areas—demonstrating clear evidence of tool making, social bonding, and the contemplation of past and future—they aren't known to feel guilty after polishing off half a bag of ill-gotten dog food.

Clearly, the brain is deeply involved in the story of why (and what) we eat. In this chapter, you'll start by teasing apart the puzzle of food. For example, what does the brain do with all the calories it consumes? And how can you optimize its performance by eating the right foods? The answers aren't earth-shattering, but it all adds up to a good review if you don't have mom around to nag you about the virtues of a proper breakfast.

Next, you'll consider a subtly different side of the same issue—the human appetite. From a neurological standpoint, your desire for food is ruled by a cocktail of neurotransmitters and hormones that scientists have yet to puzzle out in its entirety. By exploring the biological basis for appetite, you'll gain insight into why many of us eat all wrong—and whether there's any hope to deny your brain the fast food, chocolate éclairs, and deep-fried Twinkies it craves.

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Your brain is an energy hog. Although it accounts for a fraction of your body weight (typically, about two percent), it devours an astounding 20 percent of the energy you use. And your brain's hunger is insatiable, whether you're asleep, awake, or focused on the very worst reality television. If your brain is deprived of energy for as little as 10 minutes, it suffers permanent damage. No other human organ is nearly as temperamental.

Before getting to the details of exactly how the brain gets its fuel, it's worth asking a preliminary question—namely, what the heck is the brain doing that it needs so much juice? Right now, your brain is using its calories in the following ways:

Performing the normal housekeeping of all living cells, such as cleaning up debris, transporting nutrients, repairing cells, and so on.
Building neurotransmitters (the chemicals that transmit messages from one neuron to another) and distributing them throughout your brain.
Rewiring your brain circuitry with the new information you're learning.
Firing electrical signals in your neurons, and keeping your brain's electrical system up and ready.

Out of all these tasks, the last one consumes the most energy. Neurons can easily fire an electrical signal hundreds of times a second, and a single neuron can talk to thousands of other neurons—each of which may also fire their own electrical signals to pass the message along. All this adds up to a lot of blinking lights in the big switchboard that we call the brain.

Incidentally, your brain's energy use is roughly 20 watts—enough to power a very dim bulb.

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Glucose—simple sugar—is the raw fuel that powers your brain. Unlike the muscles in your body, your brain can't tap the energy reserves in your body fat. (So thinking hard might tire you out, but it won't slim you down.)

Figure : Your Friend: The Glucose Molecule

Studies consistently find that very low glucose levels weaken the brain's ability to concentrate, remember, and pay attention. Some anthropologists even believe that our early ancestors kicked their brains into high gear when they discovered starchy tubers, a rich source of carbohydrates that can be readily broken down into sugar. (Potatoes, turnips, cassava and many other root vegetables fall into this category.) Although there's no concrete proof, the idea certainly gives French fry fans some serious food for thought.

Under normal conditions, the brain always gets the trickle of sugar it needs to stay functioning. However, certain drugs and diseases can bring on hypoglycemia, a condition in which even the brain's bare minimum sugar requirements can't be met. (For example, hypoglycemia is a possible side-effect of the blood-lowering medication taken by diabetes patients.) If this happens to you and your brain is deprived of sugar, you're likely to experience weakness, confusion, dizziness, and ultimately unconsciousness.

In other words, nothing craves glucose like a working brain.

Now that you know your brain loves sugar, you might try to overclock it with a steady diet of chocolate, fudge icing, and gummy bears. Not so fast. The problem is that unlike the muscles in your body, your brain can only store the tiniest amount of glucose. Instead, it depends on your body to feed it a constant sugar supply through your blood. And simple, sugary foods don't stick around in your bloodstream for very long.

To understand the problem, consider what happens when you eat a quintuple-chocolate frosted donut:

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So far, you've seen how your brain runs on glucose—and how to establish a steady supply. But although glucose is your brain's fuel, it's not the only ingredient your brain needs to stay shipshape.

Here are some other diet essentials for a balanced brain:

Protein. Proteins are broken down into amino acids—incredibly versatile building blocks that the body uses to create a variety of compounds, including key neurotransmitters involved in attention and memory. This may be the reason that protein-rich meals appear to increase alertness (or this may just be a consequence of the fact that protein slows down the absorption of glucose, stabilizing blood sugar levels). Either way, it's a good idea to eat small amounts of low-fat protein at breakfast and lunch. Popular choices include yogurt, peanut butter, or a boiled egg. More exotic but equally nutritious choices include roasted crickets and steamed mealworms.
Fat. Fat gets a bad rap, but it's actually responsible for a lot of essential functions in the body, and the brain is no different. In fact, your neurons are in large part built out of the stuff. Their membranes are composed of fatty acids, and their long axons are often wrapped in fatty insulation (which increases the speed that the electrical signal travels from one end to the other). However, not all fats are equal. Many studies suggest that the omega 3 fats found in many fish are serious brain boosters. Diets rich in omega 3 fats are linked to healthier brains that have more resilient memories and a diminished risk of depression and degenerative diseases like Alzheimer's.
Although the exact benefits of omega 3 fats are still being debated, there's good reason to support the popular legend that seafood is brain food. Other omega 3 all-stars include avocados and olive oil.

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You've now learned what the brain does with your dinner. However, you haven't considered how it gets what it wants—in other words, what neurological process underpins the hunger pangs that can drive you out of bed for a midnight snack or cripple your resolve when strolling past the vending machine.

In truth, the full appetite story is still shrouded in mystery. This isn't because the human appetite is a particularly strange phenomenon, but because there are many overlapping influences that come into play. At any given moment, your desire to eat (or ignore) food is shaped by the time of day, the current fullness of your stomach, your emotional state, and the amount of fat, sugar, and protein that's circulating in your body.

Although even the sharpest brain scientist can't discern the appetite's exact equation, we do know the brain center that evaluates these factors and triggers your hunger. It's the hypothalamus, the ancient control center that sits at the top of the brain stem. (You first met the hypothalamus in , where you learned how it controls the pituitary gland, the brain's 24-hour pharmacy shop.) In studies with unfortunate rats, scientists discovered that damage to one section of the hypothalamus causes rats to lose their appetite and willingly starve. Damage to another section causes rats to eat insatiably and balloon up to three times their normal size.

Figure :

The appetite-controlling system of the hypothalamus is surprisingly complex. The hypothalamus includes neurons that react to the distension of the stomach, and others that respond to the levels of sugar and fat in the blood stream. It also pays attention to two more recently discovered hormones: ghrelin and leptin.

They may sound like two nasty hobbits, but these two hormones play a key role in shaping your appetite.

Ghrelin. This hormone is produced by the lining of your stomach. Its presence rises before meals, and falls after you eat. Ghrelin appears to act on the hypothalamus to stimulate appetite. In studies, a quick shot of ghrelin gave participants a voracious appetite worthy of an all-you-can-eat Chinese buffet.

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Sleep is one of the quirkiest brain behaviors. If it wasn't such a fundamental part of your life, you'd find the whole idea more than a bit outlandish. Think about it: For nearly a third of the day, your brain paralyzes your body. It then slips into a state of supposed rest that has bursts of electrical activity as energetic as when you're awake. And to top things off, the sleeping brain reels with hallucinations that rival those induced by the most potent controlled substances.

Scientists who study sleeping brains have unearthed all kinds of fascinating things. But they still can't agree on why we do it. In fact, they still can't completely agree that we actually need to do it. And the story gets even stranger when neuroscience shifts its attention to the surreal world of dreams.

In this chapter, you'll take a long, sober look at the sleeping brain. First up: a consideration of possible reasons your brain craves sleep (including a look at why it entertains itself with wild, convoluted flights of fancy while you're out cold). As you size up the science of sleep, you'll also dip into its many practical uses—for example, how sleep bolsters learning, how to harness the creativity of your dreams, and how to get a good nap.

Most humans are well adjusted to the basic schedule of modern life—sleeping through breakfast, dozing off after lunch, and watching late night television when they should be deep asleep. Against this backdrop, it's amazing to realize that every human has a built-in timepiece that, if properly calibrated, can get you to bed at night and up in the morning with flawless punctuality.

This time-keeping device is embedded in a region of the brain called the suprachiasmatic nucleus (SCN). This small bundle of neurons is a part of the hypothalamus, which—as you've discovered in previous chapters—is a deep, ancient structure in the core of the brain that performs key tasks, like regulating the release of hormones and controlling appetite.

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Most humans are well adjusted to the basic schedule of modern life—sleeping through breakfast, dozing off after lunch, and watching late night television when they should be deep asleep. Against this backdrop, it's amazing to realize that every human has a built-in timepiece that, if properly calibrated, can get you to bed at night and up in the morning with flawless punctuality.

This time-keeping device is embedded in a region of the brain called the suprachiasmatic nucleus (SCN). This small bundle of neurons is a part of the hypothalamus, which—as you've discovered in previous chapters—is a deep, ancient structure in the core of the brain that performs key tasks, like regulating the release of hormones and controlling appetite.

Figure :

Scientists have discovered how the SCN works by putting good-natured people in dark caves for long amounts of time. Not only is this an entertaining way for brain researchers to while away a weekend, it also turns out to be surprisingly informative. Confining people in caves tells us how humans manage their time when they have no external cues to indicate whether it's morning, midnight, or midday.

Famous time-isolation studies have used actual caves, an underground glacier, a bomb shelter, and less impressive-sounding research laboratories.

During cave studies, volunteers are free to sleep whenever they like. However, they gravitate to a 24- to 25-hour cycle that closely resembles what we think of as a normal human day. As this cycle, called the circadian rhythm, draws to its close, the participants get ready to sleep. As the cycle starts again, they pass through their deepest sleep, and then rise to start a new day. The cave studies show that you don't need the rising and setting of the sun to know when to get out of bed. Instead, the SCN keeps an internal clock running all the time.

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Now that you know about the sleep-regulating pacemaker implanted in your brain, you may be curious about why it exists. In other words, why is sleep so important that there's a module in your head dedicated to nagging you about it?

As you've learned, there's a highly accurate clock lodged in your brain. This timepiece could do a splendid job running your life, getting you out of bed without an alarm clock and sending you to bed before sluggishness sets in—if you weren't scrambling its time-keeping with the bad habits of modern living (and possibly artificial lights).

Fortunately, now that you understand how the SCN works, you can improve the situation. Here are some good tips:

Use light to shape your sleep. Having trouble sleeping on time? The easiest way to adjust your biological clock is with light. So turn it on bright to wake up in the morning and dim it to slow down in the evening. And if you really need a solid night's sleep, try to make do with no artificial lights at all (or shift to weaker sources of illumination, like candles, oil lamps, or a roaring fireplace). If all else fails, relocate to a darkened cave for the night. (If you've ever had your light constrained by the sun—for example, when camping or in the midst of a power outage—you've probably noticed how uncomplicated sleep becomes.)
Work late, but not all night. If you're planning to work late, set 4:00 a.m. as your absolute limit. At that point, your body is ready to shut itself down for a couple of hours at least. Those who stay up past this point will have poor coordination, slow reflexes, and are at an increased risk for accidents.
Use the sun to bridge time zones. If you're traveling across time zones, give the sun a chance to reset your biological clock. For example, a walk in the bright sun is a perfect way to adjust your clock on arrival. You can begin shifting your clock a day or two ahead of your trip. To do so, hunt down a jet lag calculator on the Web, or just keep two good rules of thumb in mind. Before traveling

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To continue your exploration into the brain and its sleep habits, you need to take a closer look at exactly what your brain does while you're snoozing.

The sleeping brain goes through a cycle that typically lasts about 90 minutes, and repeats that cycle about four times each night. The different stages of the cycle are characterized by dramatically different forms of brain activity. Researchers can spot these stages by hooking a sleeper up to an EEG machine, which records the brain's electrical activity.

Figure :

Here's a quick rundown of the sleep stages your brain travels through every night:

Stage 1. This is a drowsy semi-conscious stage. Breathing slows and you may experience hypnagogic imagery—visual and auditory hallucinations (for example, flashes of light and sounds of crashing surf) that have no overarching narrative.
Stage 2. This is light sleep. Brain activity slows, but is punctuated by brief spikes of activity called sleep spindles, which last one or two seconds. Half of all the hours you spend asleep are spent in this stage.
Stage 3. This is a transitionary period of ever-deepening sleep.
Stage 4. This is the deepest stage of sleep. Heartbeat and blood pressure have slowed, and the brain shows a slow, steady form of activity known as delta waves. This is also the stage of sleep when sleepwalking and sleeptalking strike. If you're woken up while in stage 4 sleep, you'll feel groggy and confused.

The best time to wake up is at the beginning of a sleep cycle, while you're still in stage 1 or stage 2 sleep. If you're getting the recommended 8 hours of sleep, you'll find it easy to wake up between sleep cycles. At this point, sleep is at its lightest, and minor stimulus—a birdsong, a sunrise, a bulging bladder—can nudge you into full wakefulness. By comparison, if you aren't allowing yourself enough time to sleep and you're using an alarm clock to start the day, you may find yourself shocked out of stage 3 or stage 4 sleep. In that case, you're apt to feel like you've fallen under a cement truck.

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REM sleep is named after the rapid eye movements that sleepers experience. Unlike other stages of sleep, REM sleep is easy to identify. When you're experiencing REM sleep, your eyes dart back and forth under your eyelids. However, the rest of your body is essentially paralyzed, which acts as a safeguard to prevent you from acting out particularly violent dreams.

REM sleep is closely identified with the phenomena of dreaming. If someone wakes you out of REM sleep, you're sure to report a vivid dream. However, other sleep states also produce dreams. Usually, these are fuzzier, more sedate dreams, and often they're little more than general feelings and soft-focus visions. But occasionally, vivid dreams are reported in non-REM sleep, most commonly at the end of a long sleep indulgence (say, a Sunday morning).

Current science suggests that our biological drive to rest just might have less to do with the tender ministrations of sleep, and more to do with the freewheeling chaos of dreams. Here are some tantalizing reasons to think REM sleep is a critical part of every brain's night:

When deprived of REM sleep (for example, by being repeatedly woken up in the middle of a sleep cycle), the brain fights back, plunging itself into REM sleep more quickly.
If you don't get your normal amount of REM sleep in a night, you're brain alters its sleep cycle the next night, spending more time in REM sleep to compensate.
Adults spend about 20 percent of their sleeping hours in REM sleep. Newborns spend about 50 percent of their time in REM sleep, and fetuses are thought to stay in a nearly-perpetual state of REM sleep. In later years, REM sleep declines to a more modest 15 percent of sleep time. This correlation between periods of heavy brain development and long intervals of REM sleep hints that REM sleep might be playing an important role we haven't quite pinned down.

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The most obvious hallmark of REM sleep is irrational dreaming with vivid, hallucinatory detail. However, it's quite possible that REM sleep isn't designed to create dreams. Instead, dreams might simply be a side effect of the lower-level brain processes that are going on during REM sleep (possibly memory consolidation and emotional regulation). As your brain is flooded with a chaotic series of images and memories, the reasoning centers of your brain do what they're trained to do—they struggle to make sense of the disorganized mass of information by weaving it into a barely logical story.

Even if dreams aren't anything more than noise in the higher regions of your brain while the older, more primitive levels do their housekeeping, they can still be mind-bendingly fascinating. Dreams can also be useful, by providing insight into your emotions or giving you a burst of creative thinking.

Although you undoubtedly remember a few of your most memorable dreams, you probably don't have as good an idea about the overall pattern of your dreaming, and how your dreams compare to the nightly visions of other people. Large dream studies shed some light into these questions by comparing the dream journals of hundreds of volunteers, sometimes over long periods of time. Here are some of their discoveries:

There's not much sex. Sure, it happens, but not nearly as often as a lusty Freudian psychiatrist might have you believe. (That said, sex plays a commanding role in daytime fantasies.)
Dreams incorporate the ordinary. Most dreamworld objects, people, and themes come straight out of personal experience. Often, it's recent experience, and it's not necessarily important, unusual, or emotionally charged. (For example, I once had a dream about putting away my socks after a day where I did, indeed, put my socks away. Neuroscientists would conclude that this dreary yawn of a dream isn't an indication of a developmentally delayed brain, but a perfectly sensible example of ordinary dreaming.)

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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.

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.

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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'

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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 :

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 :

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.

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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 :

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 :

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.

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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 :

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 :

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.

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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 :

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 :

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).

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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 :

Stand in a doorway, with your arms down at your sides.

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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 :

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 :

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.

Wh

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