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  Sleep, obviously, offers muscles and other parts of the body a chance to rest. But not the brain: measurements of its electrical activity reveal that it’s hardly dormant. Still, until 1953, when two researchers who were studying children’s sleep patterns described REM sleep for the first time, scientists assumed that the brain was inactive during sleep. But exactly why sleep is so important, regardless of your age, and what sleep means to the brain are questions scientists cannot answer yet. Perhaps sleep helps consolidate learning (more on that later in “Sleep and Learning: Caffeine in Your Beer,” on page 43).

  TEENAGERS: WHAT WERE YOU THINKING!

  In a child’s first year, the brain triples in size, until it’s almost three quarters of the size of an adult’s. The brain achieves its full growth at about age 17. The number of neurons doesn’t increase, but the number of synapses do as children imitate, learn, remember, and add to their experiences. By adulthood, the brain has 100 trillion synapses. But before adulthood comes adolescence, when the brain is flooded with hormones. Neurologists only recently have confirmed what every parent knows: The teenage brain is indeed different. In the teenage brain, the prefrontal cortex—the center of reasoning and impulse control—is still forming. In some people, that maturation may not occur until they are 25 years old. That’s why so many teenagers have trouble understanding the consequences of their impulsive, destructive behavior. That’s why, as every parent can tell you, teenagers are impulsive, emotionally erratic, and liable to make poor decisions. The answer is simple: The section of the brain that can foresee the future, the part that can predict the consequences of actions, is not fully developed.

  Every parent also knows that teenagers have a terrible time getting up in time for school and on weekends, preferring to sleep into the afternoon. Neuroscientists now know that teens aren’t being lazy—they really do need more sleep. Some schools have even pushed the start of the school day later to take into account the adolescent need for sleep.

  SEEING INSIDE THE BRAIN

  We’ve been able to learn more about the adolescent brain because today, we don’t have to drill into the brain to find out about it, as early peoples did. We have new technology that allows us to see what the brain looks like and what it’s doing. And that’s an excellent advance because the brain is delicate, and once a neuron is destroyed, it’s gone forever—though the brain often is able to compensate for the loss. Neurosurgeons even have a pithy saying: “You’re never the same once air hits your brain.”

  In 1917, László Benedek, a Hungarian neurologist, took the first pictures of the living brain with what was then the brand-new technology of X-rays. German physicist Wilhelm Roentgen won the Nobel Prize for discovering X-rays, in 1895, making it possible to see structures inside the body without surgery. By 1936, Benedek had come up with the idea of making three-dimensional X-ray pictures of the brain. But X-rays can only show large features of the brain, such as tumors; they cannot show the different layers inside the brain. Nor can they capture the brain’s activity and changes, which last only a split second.

  In the 1970s, Benedek’s idea had been developed into the computerized axial tomography (CAT) scanner—the CAT scan, which uses X-rays to show details of the brain’s soft tissue. (Now it’s called a computed tomography, or CT, scan.) With a burst of radiation, a CT scan takes a two-dimensional “slice” of the brain, and by putting two-dimensional X-rays together with the aid of a computer, researchers can obtain a three-dimensional image, as Benedek envisioned. Images of the brain from a number of X-rays are digitized, and then reconstructed so that researchers have a cross-sectional view of any part of the brain. CT scans can detect brain damage and can measure brain activity by monitoring blood flow as the patient performs a task. But just like Benedek’s early photos, CT scans have limits beyond revealing the brain’s structure.

  By the time X-rays were discovered, scientists were aware that living brains produce electrical activity. But not until the late 1920s did an Austrian psychiatrist, Hans Berger, actually record this activity for the first time. Berger made an electroencephalograph, another noninvasive way of studying the brain. Electrons fastened to a patient’s scalp pick up the electrical signals produced by the brain and send them to galvanometers, instruments that detect and measure small electrical currents. Like seismometers, which measure earthquakes, galvanometers used to be hooked up to pens that moved over a roll of graph paper to record the characteristic patterns of the current from the patient’s brain. Today, the patterns appear on a computer monitor, but electroencephalograms (EEGs) still allow scientists to monitor split-second brain activity and changes. EEGs can tell physicians whether you’re awake, asleep, or anesthetized—because your brain patterns look different in each of these states.

  During the 2005 debate over Terri Schiavo, the 41-year-old woman in Florida who had spent 15 years in a post-coma, or vegetative, state after a stroke, some physicians who had examined her reported that “her EEG is flat.” That meant that her brain was not producing any electricity at all. Since it was no longer functioning, its structure was deteriorating and filling with fluid so that it resembled the inside of a grapefruit. So EEGs can tell us a lot about the state of the brain, but they can’t tell us what regions of the brain do what.

  To look deeply into the brain, we now have positron emission tomography (PET) scans, which neuroscientists use frequently. To take a PET scan, a neurologist injects into a patient’s bloodstream a tiny amount of a radioactive substance, attached to glucose molecules that brain activity absorbs as fuel. In brain tissue, the glucose molecules give off gamma rays, recorded by sensors and then analyzed by computers to picture just where in the brain more glucose-molecule fuel is being used and where less is required. The result is a color-coded map of the brain, where red or yellow usually shows the more active areas that are using more fuel and blue indicates the less active areas that are consuming less fuel. The PET scan of a patient with Alzheimer’s disease, for example, is often mostly blue. Right now, PET scans can measure what is happening at 30-second intervals in a tiny portion of the brain. That, of course, is still not fast enough to keep up with brain activity.

  In 1977, there was a major breakthrough in brain imaging, the invention of functional magnetic resonance imaging, or fMRI. When you undergo magnetic resonance imaging (MRI), you lie on your back on a movable bed that slides into a giant circular magnet. MRI, like the other kinds of brain scans, isn’t at all painful—just uncomfortably noisy once the machine is turned on and begins generating a strong magnetic field. What happens is that the molecules in your body, including your brain, begin to behave like tiny magnets. The MRI machine’s magnetic field realigns the hydrogen atoms in your body so that instead of spinning in different directions, they all spin along the same axis, along the length of your body. Now the protons in the hydrogen atoms are facing either up toward your head or down toward your feet. The hydrogen atoms’ opposite directions means that most of them cancel out each other’s electrical charge. But a few remain, and when the machine sends a beam of radio waves to your brain or to the part of your brain that’s being scanned, the pulse makes them resonate and give off radio signals of their own. When the machine shuts off the pulse, the hydrogen atoms return to their normal alignment and release energy, giving off a signal. The machine’s sensors detect these signals and feed them into a computer, which generates an image of the different types of tissue in the brain.

  Magnetic resonance images are good at detecting changes in the brain, which occur every time you learn something new. MRI has been used to help education researchers develop new teaching methods that have proven to help elementary-school children—most of them are boys—with dyslexia and other reading difficulties. Magnetic resonance images showed researchers that parts of the brains of struggling readers were different from those of successful readers. In one study at the University of Washington, researchers took magnetic resonance images while struggling readers pronounced certain words while lying
in the machine. Then the boys were trained in a special curriculum designed to help them read more easily. At the end of the program, the boys underwent MRI again while they read words aloud. The images showed that their brains now looked more like those of normal readers—proof positive that the innovative reading training that they had been given worked. At Yale University, another research team is taking magnetic resonance images of dyslexic readers throughout their lives, for a long-term study of how the brain changes.

  Magnetic resonance images are often very colorful and beautiful, as well as clear and detailed. MRI is faster than PET, but it still can’t keep up with the extremely rapid changes inside the brain and give us the best possible picture of the brain at work.

  Now a new imaging technology in limited use can record brain activity by the millisecond. Magnetoencephalography, or MEG, is still extremely expensive, so there are only a few of the new machines that the imaging requires in existence. Because your brain—like your entire body—works by electricity, it produces a magnetic field. MEG works by detecting the very faint magnetic fields generated by the tiny electric currents from your neurons that are recorded on EEGs. When you have a MEG scan, you sit under a big, very heavy machine that positions magnetic detection coils bathed in helium over your head. The helium chills the coils to supersensitive, superconducting temperatures. Your brain’s magnetic field induces a current in the coils that in turn induces a magnetic field in an instrument called a superconducting quantum interference device, or SQUID. The magnetic field can be translated into computer-processed images that provide the most accurate monitoring and timing of brain-cell activity. Of all imaging technologies, MEG provides the best information, so let’s hope that it will become cheaper and more available in the future.

  Besides imaging, we’re learning a lot about the brain from developments in genetics. The Human Genome Project has linked certain genes with normal brain function, such as learning and memory, and with some mental disorders as well. By adolescence, the effects of genes become apparent—including genetically related dysfunctions such as depression or schizophrenia. We know that mental illnesses like these can begin in adolescence, and we can see the changes they cause in the brain in imaging. Pharmaceutical companies have developed new drugs that change brain chemistry in positive ways and help relieve the suffering of people with depression or schizophrenia. But some doctors and philosophers worry that if we know how to explain the brain, we can heal it but also may manipulate it. When the new types of antidepressants that affect the brain’s level of serotonin first became popular in the 1990s, some wondered if people would have drugs but not personalities.

  The debate over whether to give teenagers drugs that affect brain chemistry illustrates the pitfalls of treating brains that are still developing. For example, teenagers who take antidepressants may be more prone to violent acts, such as suicide or murder. The Native American teenager who killed 10 people at his high school in Red Lake, Minnesota, in 2005 was taking Prozac. While no direct connection between his ingestion of Prozac and his rampage exists, it does bring up disturbing questions about kids and psychotropic drugs. Very little is known about the effects of adult drugs on teens. But that’s another story….

  At the Neuropsychiatric Institute at the University of California, Los Angeles, researchers have developed their own advanced version of electroencephalography, called cordance or quantitative electroencephalography (QEEG), to link brain function and medication side effects. In studying teenagers who might be susceptible to antidepressants’ side effects, the researchers used QEEG to find changes in brain activity in the prefrontal region. They think that when these changes show up before a teenager begins taking an antidepressant, the patient could be vulnerable to side effects. When we are adults, our brains are no longer so soaked with hormones. That can mean that we may need a little help feeling like teenagers.

  THE CUTTING EDGE

  The National Science Foundation has made neuroscience research a top priority for the twenty-first century, and there have been plenty of new studies because now researchers can use noninvasive technology such as PET scanning and fMRI to look inside the brains of living people and see how they change. Among the surprises they’ve found is that your brain looks different depending on geography. If you live in Washington State, for example, your brain scan will look different than that of your cousin who lives in Florida. And if you yourself move from Washington to Florida, your scan will look different.

  We’ve learned that the hippocampus, a part of the brain, plays a crucial role in the formation of new memories about personal experiences and in navigation. If you have Alzheimer’s disease, your hippocampus will be one of the first parts of your brain to deteriorate. That’s why patients with Alzheimer’s disease usually don’t recognize close family members and usually easily lose their way.

  But that doesn’t have to happen to you. Researchers have found out that neurons in the hippocampus do regenerate, deep into old age. They used to think that people relentlessly shed neurons as they aged. But it turns out that there’s a constant cycle of neurogenesis, and that’s why the brain is plastic, able to change and adapt and even heal sometimes after a serious injury such as a bullet wound or stroke. Your brain cells go through a constant cycle of growth and replacement that continues from birth to death. Some parts of the brain may take over the function of other damaged parts.

  SEX ON THE BRAIN

  One more piece of recent news about the brain: “Sex matters.” That’s what the National Academy of Sciences said in 2001 in a major report on how sex differences affect human health. The academy’s adage applies to the workings of the brain as well. There are significant differences between male and female brains. We still don’t know why men forget their loved ones’ birthdays or won’t ask for directions on road trips, while women readily remember important dates and ask for directions. But we do know for sure that women’s brains are smaller. In January 2005, the president of Harvard University sparked major protests, especially from women scientists and engineers, by publicly speculating that differences in men’s and women’s brains might account for the fact that far fewer women than men do well in science and engineering. Besides size, there do seem to be quite a few differences in the structure, chemistry, and function of male brains versus female brains, but no one has uncovered any evidence that these differences mean that women are intellectually inferior.

  Some of those differences are there in your brain from birth, thanks to the sex hormones that bathe a fetus’s brain in the womb. Researchers used to think that your sex affected only your hypothalamus, a small structure at the base of your brain that regulates hormone production, and basic mammalian behaviors such as mating, eating, and drinking. But today scientists know that your sex seems to affect your brain in many ways and therefore how you think, remember, and behave.

  Information enters your brain in the form of sensory experiences—what you see, hear, taste, smell, and touch. Inside, brain cells called neurons form dendritic trees (multiarmed branching arrays of nerves), transmit this information to important parts of your brain, and receive signals from other neurons in return. Some parts of the brain are bigger, depending on whether you’re a man or a woman—bigger in volume, that is, relative to the overall volume of your brain, which weighs about three pounds. For example, parts of the frontal cortex (which is involved in cognition), parts of the limbic cortex (which is involved in emotional responses), and the amygdala (a small almond-shaped structure that responds to emotion-triggering information), are all bigger in women. Some investigators even have found sex-based differences at the level of cells. In women, for example, neurons are particularly dense in parts of the temporal lobe cortex that process language and comprehension. Could this explain why women generally do better on language tests such as the verbal portion of the SATs or why they choose English as their college major more often than men do?

  Other differences in brain structure between men and women a
lso may explain why anxiety disorders are more common in girls than they are in boys; why men tend to find their way around by estimating distance, whereas women are more likely to use landmarks; and why men and women seem to learn and remember differently.

  CHAPTER TWO

  MEMORY: SENIOR JEOPARDY!, ANYONE?

  If you’ve played the TV game show Jeopardy!, you’ve probably noticed that your response time has been slowing down over the years. That’s because the brain changes with age. (That’s why I believe that we should have a “senior” version of the game, where folks older than 50 get up to 10 minutes to answer each question, because they just can’t seem to pull the answer—“I know it! Don’t tell me…”—out of their memory banks.)

  Forgetting things isn’t all bad. (I’m not rationalizing just because I’ve always had a rotten memory. I’ve even forgotten names of people I’m interviewing—while I’m interviewing them!) In daily life, there are lots of things you don’t need to remember—such as phone numbers you’ll call only once or directions you’ll use only one time. You wouldn’t want to remember everything. That would be like saving all your junk mail.

  Those dendritic trees make up different memory systems in your brain. When you experience something, the information enters your brain through different sensory pathways. The information fuses at some point into an integrated perception. When you remember something, your brain is stimulated to activate a certain pattern of neurons. Each time, that pattern is just a little bit different. Your memory doesn’t work like a videotape or a DVD, where exactly the same experience is replicated each time you play it. Instead, your memory is constantly changing and evolving as you accumulate experiences and as your neurons grow and change.