The Dana Guide to Brain Health

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Today, we know beyond any doubt that the brain plays a crucial role in our everyday health, and, indeed, makes us who we are. But the Internet, television, and newspapers abound in contradictory information about it, and, as a result, the insights we wish for and the medical choices we need to make are rarely as clear as they need to be -- especially when a family member's well-being is at stake. For decades The Dana Foundation, a philanthropic organization founded in 1950 dedicated to science, health, and ...
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Overview

Today, we know beyond any doubt that the brain plays a crucial role in our everyday health, and, indeed, makes us who we are. But the Internet, television, and newspapers abound in contradictory information about it, and, as a result, the insights we wish for and the medical choices we need to make are rarely as clear as they need to be -- especially when a family member's well-being is at stake. For decades The Dana Foundation, a philanthropic organization founded in 1950 dedicated to science, health, and education, has focused its medical initiatives on brain research. The culmination of that relentless commitment is now all here in this complete health guide to the brain. The editors -- Floyd E. Bloom, M.D.; M. Flint Beal, M.D.; and David J. Kupfer, M.D. -- are three of the world's leading medical experts in neuroscience, neurology, and psychiatry. Together with more than one hundred of America's most distinguished scientists and medical professionals, they have created an essential, easy-to-understand, and practical reference guide to the brain and how it works.

The Dana Guide to Brain Health is a resource that today's health consumer can trust, or what Foundation chairman William Safire calls "a kind of basic 'bible' of the brain." The first truly accessible all-in-one source of its kind, this illustrated home reference features the latest facts and medical discoveries, combined with clear, up-to-date information on seventy-two psychiatric and neurological disorders, their diagnoses, and their treatments. Filled with informative diagrams, tables, sidebars, graphs, charts, photographs, and drawings, along with a section listing every major consumer advocacy/information organization related to brain disorders, The Dana Guide to Brain Health sets a new standard. The Dana Guide to Brain Health is simply the most authoritative, comprehensive, and clearly written guide to the bodily organ that is the key to our everyday health. No home should be without it.

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Editorial Reviews

Boston Globe
An irreproachably comprehnsive look at the state of current neuroscience on how the brain works when it is normal and how it works when things go wrong... The most accessible gateway to neuroscience imaginable.
Psychology Today
If your interest in the brain is more practical than theoretical, this encyclopedic family guide will tell you everything you need to know and then some.
Publishers Weekly
Edited by three leading physicians from the Dana Foundation, a private foundation specializing in science and health, this volume is a comprehensive reference guide to the brain. Covering an array of topics, the book includes detailed information that goes beyond encyclopedia-type entries about medical testing, childhood development, autism and cerebral palsy and treatments for these and other diseases. The writing is clear and accessible. Some of the topics such as the healthy brain and childhood development will interest many readers, but most will probably use the book as a home medical reference, looking up individual ailments, researching illnesses that run in the family or finding helpful strategies for coping with brain difficulties. For the various illnesses, the authors define symptoms, steps to diagnosis and treatment options. Useful approaches on living with an illness are also included: for example, epilepsy sufferers are urged to have their bathroom doors open out instead of inward so the door can still open even if someone has fallen against it. Patients newly diagnosed with brain-related ailments will find this an invaluable resource. (Jan.) Copyright 2002 Cahners Business Information.
Library Journal
This book calls itself the first authoritative home reference about the brain. While that claim can be disputed, there is no doubt that the three editors (all leading brain experts) and the more than 80 contributors (all physicians from major American medical institutions) are eminently qualified. Divided into four broad sections ("Understanding Your Brain," "Your Brain Through Life," "The Healthy Brain," and "Conditions of the Brain and Nervous System"), the chapters cover such diverse topics as brain development, brain health, and 72 major neurological and emotional conditions, including emotional and control disorders, infectious and autoimmune disorders, disorders of movement and muscle, and disorders of the senses. Crammed into sidebars is a wealth of additional information on such topics as common sleep disorders, prenatal tests and the brain, treatments for multiple sclerosis, and the implications of the Huntington's Disease gene. A plethora of clear black-and-white line drawings illustrate a variety of subjects, ranging from disk herniation to the facial nerves affected in Bell's palsy to the development of the fetal brain to the brain's various emotional centers. Along with helpful cross references, the book includes a glossary, an up-to-date if unannotated bibliography, a list of resource groups with full contact information, and appendixes listing the drugs used to treat brain and nervous system diseases and disorders. Although the book offers a wealth of information, it is broad, not deep; for instance, chapters on pediatric and adult brain tumors do not mention proton beam therapy as a possible treatment modality. And, surprisingly, considering the qualifications of the writers, the entries are not signed. Still, given the relatively modest price, amount of information covered, and target audience of educated lay readers, this guide is recommended, with slight reservations as noted, for all large public and consumer health libraries. Patrons who would prefer a more condensed overview can try John J. Ratey's User's Guide to the Brain: Perception, Attention and the Four Theaters of the Brain and Mark Dubin's How the Brain Works. (Index not seen.) [The book takes its name from the Dana Foundation, a private philanthropic organization focusing on science, health, and education, whose publishing division directed this project.-Ed.]-Martha E. Stone, Massachusetts General Hosp. Lib., Boston Copyright 2002 Cahners Business Information.
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Product Details

  • ISBN-13: 9780743203975
  • Publisher: Free Press
  • Publication date: 1/6/2003
  • Pages: 768
  • Product dimensions: 7.38 (w) x 9.25 (h) x 1.75 (d)

Read an Excerpt

Chapter One: How to Think About the Brain

Since a human brain weighs on average some three pounds, it is easy to hold one in your hands. This simple fact somehow makes it even harder to imagine how such a small mass of tissue can be the source of all that we think of as human. Yet that is what the brain is, and how that can possibly be is one of the most fundamental questions in brain science. What is the link between the anatomy of a brain and the workings of a human mind? The big challenge is that there are no obvious moving parts within the brain -- it does not operate mechanically as our hearts and lungs do. If we simply look at the brain, our only remote clue about how it works is that it seems to be made up of different parts, easily discernible to the naked eye (see illustrations on page 4). In addition to the cerebral hemispheres (resembling a pair of large walnuts pressed together), the smaller structure that sits behind them (the cerebellum) is visible, as is the stalk that connects to the spinal cord (the brain stem). But there are many more regions than these three.

One easy way to think about the brain would be to view each of these different regions as having a clear function. Every part would be a sort of independent minibrain, controlling one aspect of our mental and behavioral repertoire: movement, emotion, ethics, balance, mathematical thinking, and so on. Simple and attractive though this idea is, it quickly runs into problems. After all, such a scenario would merely be miniaturizing the problem, not solving it; we would still have to figure out how each of those minibrains operates. And, as neuroscientists have learned through extensive observations and experiments, the brain just doesn't work that neatly.

Let's start with a straightforward way of trying to match up the brain's physical structures with specific functions. We know that within the animal kingdom, each species has a very different range of abilities and behavior patterns. If the brains of different animals diverge in form, that would give us significant clues about what structures are important for what kinds of functions. For instance, no animal has a language function anywhere near as sophisticated as ours. If there is a particular structure for language, it should be especially well developed in human brains, and small or nonexistent in the brains of other species.

However, the brains of very different creatures, such as a reptile, a bird, and a mammal, differ mainly in size. In all cases we can make out the same big features: the hemispheres, the brain stem, and the cerebellum. So whatever makes one species so different from another -- and above all makes the human species so different even from other primates -- is not some new, clearly conspicuous structure in their brains that no other animal has.

If, however, we look at various animals' brains for a difference not in quality but in quantity, then one clue about the physical basis of mental differences becomes apparent. The biggest discrepancy appears in the surface of the outer layer of the hemispheres. This layer is called the cortex, after the Latin for "bark," because it wraps around the brain the way its namesake wraps around a tree. In a rat or rabbit, for example, the surface of the cortex is completely smooth. In a cat it has clear convolutions. By the time we look at monkeys and apes, and eventually humans, the cortex takes on an ever more wrinkled appearance. Why?

Imagine trying to hold a sheet of paper in one fist. The more you crumple the paper, the more the sheet will fit inside your fingers. In a way, this is what has happened to the cortex within the skull. As species have become more sophisticated, the surface of their cortices has increased faster than the limited confines of their heads. The only way to develop more "working surface" in the cortex was to fold and wrinkle it. We can see this same evolutionary trend in the development of an individual human. The brain of the six-month-old fetus has a completely smooth cortex. But in the final three months of pregnancy, the baby's neurons proliferate at an astonishing 250,000 a minute. The cortex expands enormously so that by birth it has become as walnutlike as we know it. (For more on the brain's prenatal development, see chapter 5.)

Mapping the Regions -- The Top-Down Approach

We can call this method of thinking about the brain -- looking at its physical regions and their traits -- the top-down approach. The surface area of the cortex and the degree to which it is wrinkled seem to hold a clue about how a species' brain relates to its mental abilities. Small wonder, then, that the cortex has fascinated many brain researchers. But how might it accommodate the uniqueness of our human traits?

The top-down approach has given us some valuable insights into how the cortex is organized and how it plays a part in brain function. We know, for example, that despite the way its surface looks the same everywhere, different parts of the cortex participate in different processes. Certain areas, along with many deep brain structures below the cortex, seem to relate directly to the processing of each of the senses: vision, hearing, smell, and so on. As an example, let's take one thin strip of cortex that straddles the brain a little like a hair band. This region is called the somatosensory (that is, body-sensing) cortex. (See pp. 138-39 for more about it.) The cells in this strip collect signals from other brain structures, which in turn are activated by impulses buzzed up the spinal cord that report on touch, pain, or temperature felt in certain parts of the body. Clearly, this strip of cortex must contain some sort of representation of the body. How else would you know that a pain was in your toe as opposed to your hand?

So far, so good. The most logical way of thinking about your body being "mapped" in the brain would be in direct relation to size. A large part of the body like the back would have a large allocation of brain territory, and a small area like the fingertips or the tongue would be represented by a correspondingly meager area of cortex. But here is a simple experiment you can do at home to prove that this "obvious" scenario is wrong. All you need are a pair of sharp pencils, or unbent paper clips, and a willing friend.

Ask the friend to close his or her eyes and turn away. Hold the pencils so their points are close together -- three-eighths inch or so. Gently touch both pencil points to your friend's skin in different parts of the body, and ask if you are applying one or both points. (You can also try touching just one point at a time to see if your friend feels a clear difference between one and two points.) When you touch both pencils to your friend's back, he or she will almost always report feeling a single point. Now position the points much closer, only one-sixteenth inch apart, and apply them to your friend's fingertip or (with permission) tongue. Surprisingly, this time your friend will be able to feel two distinct points. Even though the fingertips and tongue represent only small fractions of our bodies, they are extremely sensitive to physical detail. Despite their small size, the fingers and the tongue have the lion's share of territory in the relevant strip of brain. That's because our brains are organized according to the functional needs of our bodies rather than simple physical size. Our fingers and tongue have to be more sensitive to touch than our back -- so they have more brain territory allocated to them.

Thus we can start to see that the structures of our brains are in tune with our daily lives. But testing such primitive processes as touch doesn't help with the question of how our cortex works differently from those of other species. So let's go back to what is arguably the monopoly of us humans, language.

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Table of Contents

Using Our Heads: A Foreword xiii
Introduction: Welcome to Your Brain xvii
How to Read This Book xxi
Contributors xxiii
Your Brain: A Primer xxxii
Part I Understanding Your Brain
1. How to Think About the Brain 3
2. How We Know: Learning the Secrets of the Brain 14
3. Basic Brain Care: Protecting Your Mental Capital 31
4. The Brain-Body Loop 41
Part II Your Brain Through Life
5. Prenatal Development 61
6. Brain Development in Childhood 83
7. The Adolescent Brain 102
8. The Brain in Adult Life and Normal Aging 116
Part III The Healthy Brain
9. The Body Manager 135
B1 The Major Senses: Sight, Hearing, Taste, Smell, and Touch 136
B2 Body Regulation 142
B3 Basic Drives: Eating, Sleeping, and Sex 150
B4 Movement, Balance, and Coordination 158
B5 Pain Perception 165
B6 Consciousness 171
10. Emotions and Social Function 179
B7 Emotions 180
B8 Inhibition and Control 185
B9 Temperament 190
B10 Attention and Motivation 196
11. Learning, Thinking, and Remembering 200
B11 Decision Making and Planning 201
B12 Intelligence 210
B13 Learning and Memory 217
B14 Speech, Language, and Reading 226
B15 Visualization and Navigation 232
B16 Creativity, Talents, and Skills 236
Part IV Conditions of the Brain and Nervous System
12. Conditions That Appear in Childhood 247
C1 Dyslexia 248
C2 Attention Deficit/Hyperactivity Disorder 254
C3 Mental Retardation 259
C4 Cerebral Palsy 265
C5 Autism 270
C6 Metabolic Diseases 278
C7 Neurofibromatosis 284
C8 Hydrocephalus 288
C9 Spina Bifida 293
C10 Tumors of Childhood 298
13. Disorders of the Senses and Body Function 306
C11 Sleep Disorders 307
C12 Narcolepsy 315
C13 Epilepsy and Seizures 319
C14 Dizziness and Vertigo 327
C15 Seeing Problems 332
C16 Hearing Problems 341
C17 Smelling and Tasting Problems 347
C18 Autonomic Disorders 352
C19 Chronic Fatigue Syndrome 357
14. Emotional and Control Disorders 362
C20 Depression 363
C21 Anxiety and Panic 371
C22 Social Phobia (Social Anxiety Disorder) 376
C23 Obsessive-Compulsive Disorder 379
C24 Bipolar Disorder 385
C25 Schizophrenia 390
C26 Borderline Personality Disorder 394
C27 Eating Disorders 399
C28 Post-Traumatic Stress Disorder 405
C29 Substance Abuse and Addiction 409
C30 Alcoholism 419
C31 Violence and Aggression 426
C32 Suicidal Feelings 429
15. Infectious and Autoimmune Disorders 434
C33 Multiple Sclerosis 435
C34 Shingles/Herpes Zoster 444
C35 Neurological Complications of AIDS 448
C36 Lyme Disease 453
C37 Meningitis 458
C38 Viral Encephalitis 462
C39 Creutzfeldt-Jakob Disease 468
C40 Systemic Lupus Erythematosus 471
16. Disorders of Movement and Muscles 476
C41 Parkinson's Disease 477
C42 Parkinsonism Plus 484
C43 Tremors 490
C44 Dystonia, Spasms, and Cramps 494
C45 Tourette's Syndrome and Tics 497
C46 Ataxia 501
C47 Huntington's Disease 505
C48 Peripheral Neuropathy 511
C49 Guillain-Barre Syndrome 516
C50 Bell's Palsy 520
C51 Myopathies 523
C52 Myasthenia Gravis 530
C53 Amyotrophic Lateral Sclerosis 535
17. Pain 539
C54 Headache 540
C55 Migraines 548
C56 Back Pain and Disk Disease 553
C57 Chronic Pain 559
C58 Trigeminal Neuralgia 565
18. Nervous System Injuries 569
C59 Ischemic Stroke 570
C60 Hemorrhagic Stroke 576
C61 Brain Trauma, Concussion, and Coma 581
C62 Spinal Cord Injury 591
C63 Paraneoplastic Syndromes 600
C64 Brain Tumors 604
C65 Nutritional Disorders 610
C66 Chemicals and the Nervous System 617
19. Disorders of Thinking and Remembering 624
C67 Alzheimer's Disease 625
C68 Amnesias 635
C69 Dementia 639
C70 Trouble with Speech and Language 643
C71 Apraxias 648
C72 Agnosias 651
Glossary 655
Appendix A Drugs Used to Treat the Brain and Nervous System 661
Appendix B Suggested Reading 669
Appendix C Resource Groups 673
The Dana Alliance for Brain Initiatives 687
The European Dana Alliance for the Brain 695
About the Editors 701
Index 703
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First Chapter

Chapter One: How to Think About the Brain

Since a human brain weighs on average some three pounds, it is easy to hold one in your hands. This simple fact somehow makes it even harder to imagine how such a small mass of tissue can be the source of all that we think of as human. Yet that is what the brain is, and how that can possibly be is one of the most fundamental questions in brain science. What is the link between the anatomy of a brain and the workings of a human mind? The big challenge is that there are no obvious moving parts within the brain -- it does not operate mechanically as our hearts and lungs do. If we simply look at the brain, our only remote clue about how it works is that it seems to be made up of different parts, easily discernible to the naked eye (see illustrations on page 4). In addition to the cerebral hemispheres (resembling a pair of large walnuts pressed together), the smaller structure that sits behind them (the cerebellum) is visible, as is the stalk that connects to the spinal cord (the brain stem). But there are many more regions than these three.

One easy way to think about the brain would be to view each of these different regions as having a clear function. Every part would be a sort of independent minibrain, controlling one aspect of our mental and behavioral repertoire: movement, emotion, ethics, balance, mathematical thinking, and so on. Simple and attractive though this idea is, it quickly runs into problems. After all, such a scenario would merely be miniaturizing the problem, not solving it; we would still have to figure out how each of those minibrains operates. And, as neuroscientists have learned through extensive observations and experiments, the brain just doesn't work that neatly.

Let's start with a straightforward way of trying to match up the brain's physical structures with specific functions. We know that within the animal kingdom, each species has a very different range of abilities and behavior patterns. If the brains of different animals diverge in form, that would give us significant clues about what structures are important for what kinds of functions. For instance, no animal has a language function anywhere near as sophisticated as ours. If there is a particular structure for language, it should be especially well developed in human brains, and small or nonexistent in the brains of other species.

However, the brains of very different creatures, such as a reptile, a bird, and a mammal, differ mainly in size. In all cases we can make out the same big features: the hemispheres, the brain stem, and the cerebellum. So whatever makes one species so different from another -- and above all makes the human species so different even from other primates -- is not some new, clearly conspicuous structure in their brains that no other animal has.

If, however, we look at various animals' brains for a difference not in quality but in quantity, then one clue about the physical basis of mental differences becomes apparent. The biggest discrepancy appears in the surface of the outer layer of the hemispheres. This layer is called the cortex, after the Latin for "bark," because it wraps around the brain the way its namesake wraps around a tree. In a rat or rabbit, for example, the surface of the cortex is completely smooth. In a cat it has clear convolutions. By the time we look at monkeys and apes, and eventually humans, the cortex takes on an ever more wrinkled appearance. Why?

Imagine trying to hold a sheet of paper in one fist. The more you crumple the paper, the more the sheet will fit inside your fingers. In a way, this is what has happened to the cortex within the skull. As species have become more sophisticated, the surface of their cortices has increased faster than the limited confines of their heads. The only way to develop more "working surface" in the cortex was to fold and wrinkle it. We can see this same evolutionary trend in the development of an individual human. The brain of the six-month-old fetus has a completely smooth cortex. But in the final three months of pregnancy, the baby's neurons proliferate at an astonishing 250,000 a minute. The cortex expands enormously so that by birth it has become as walnutlike as we know it. (For more on the brain's prenatal development, see chapter 5.)


Mapping the Regions -- The Top-Down Approach

We can call this method of thinking about the brain -- looking at its physical regions and their traits -- the top-down approach. The surface area of the cortex and the degree to which it is wrinkled seem to hold a clue about how a species' brain relates to its mental abilities. Small wonder, then, that the cortex has fascinated many brain researchers. But how might it accommodate the uniqueness of our human traits?

The top-down approach has given us some valuable insights into how the cortex is organized and how it plays a part in brain function. We know, for example, that despite the way its surface looks the same everywhere, different parts of the cortex participate in different processes. Certain areas, along with many deep brain structures below the cortex, seem to relate directly to the processing of each of the senses: vision, hearing, smell, and so on. As an example, let's take one thin strip of cortex that straddles the brain a little like a hair band. This region is called the somatosensory (that is, body-sensing) cortex. (See pp. 138-39 for more about it.) The cells in this strip collect signals from other brain structures, which in turn are activated by impulses buzzed up the spinal cord that report on touch, pain, or temperature felt in certain parts of the body. Clearly, this strip of cortex must contain some sort of representation of the body. How else would you know that a pain was in your toe as opposed to your hand?

So far, so good. The most logical way of thinking about your body being "mapped" in the brain would be in direct relation to size. A large part of the body like the back would have a large allocation of brain territory, and a small area like the fingertips or the tongue would be represented by a correspondingly meager area of cortex. But here is a simple experiment you can do at home to prove that this "obvious" scenario is wrong. All you need are a pair of sharp pencils, or unbent paper clips, and a willing friend.

Ask the friend to close his or her eyes and turn away. Hold the pencils so their points are close together -- three-eighths inch or so. Gently touch both pencil points to your friend's skin in different parts of the body, and ask if you are applying one or both points. (You can also try touching just one point at a time to see if your friend feels a clear difference between one and two points.) When you touch both pencils to your friend's back, he or she will almost always report feeling a single point. Now position the points much closer, only one-sixteenth inch apart, and apply them to your friend's fingertip or (with permission) tongue. Surprisingly, this time your friend will be able to feel two distinct points. Even though the fingertips and tongue represent only small fractions of our bodies, they are extremely sensitive to physical detail. Despite their small size, the fingers and the tongue have the lion's share of territory in the relevant strip of brain. That's because our brains are organized according to the functional needs of our bodies rather than simple physical size. Our fingers and tongue have to be more sensitive to touch than our back -- so they have more brain territory allocated to them.

Thus we can start to see that the structures of our brains are in tune with our daily lives. But testing such primitive processes as touch doesn't help with the question of how our cortex works differently from those of other species. So let's go back to what is arguably the monopoly of us humans, language. Surely if we understood how our brains process language, we would have a route into understanding the physical basis of what makes us so special.

Paul Broca was a physician working in Paris during the mid-nineteenth century. He has earned his place in neurological history thanks to one of his patients, a Monsieur Leborgne. Everyone knew this unfortunate man by his nickname, "Tan," because that was all he could say. Leborgne had a severe speech problem, an aphasia (C70), which meant he could not articulate words. When Tan died, Broca examined his brain and discovered a clear hole in the side of its left hemisphere. Tan's aphasia was obviously related to the damage in this region, henceforth known as Broca's area. But does this mean that Broca had discovered the mind's center for speech? Far from it.

Within a decade, a German physician, Carl Wernicke, identified a second site, also on the left-hand side of the brain but clearly well behind Broca's area, where damage gave rise to a different type of speech problem. Wernicke's aphasia is also referred to as jargon aphasia because although a person with this problem can articulate words perfectly well, all that comes out of his or her mouth is a string of gibberish.

By the end of the twentieth century, scientists had come to realize that there are still more brain regions involved in speech. Imaging techniques have made it possible for us to see the brain at work in conscious humans without causing any pain or harm. Positron-emission tomography (PET) and functional magnetic resonance imaging (fMRI) exploit the facts that the brain is very greedy for oxygen or glucose and that the hardest-working brain regions are hungriest of all. (For more on these technologies, see chapter 2.) Studies have now revealed that, during such seemingly simple behaviors as using language, many brain regions are working together, rather like the instruments in an orchestra. Each region will be making a specialized contribution, but the whole is somehow more than the sum of its parts. What we don't know yet is how all the different brain regions involved in any one task, be it language or vision or memory, somehow come together.

But what might we learn about the particular contribution of one brain region? Let's look at an area toward the front of our brains, the prefrontal cortex. This area is twice the size it should be for a primate of our body weight. Could this contain the secret of our awesome mental abilities?

Again, as long ago as the mid-nineteenth century people realized that there was something special about the prefrontal cortex. This was demonstrated in 1848 in a most dramatic way by Phineas Gage, a railway worker in Vermont. One day, Gage was working to clear the path for a new railroad when the gunpowder he was using exploded prematurely. As a consequence, the bar with which he had been tamping down the explosive shot right through his prefrontal cortex. In effect, he had speared himself through the head. Surprisingly, Gage lost sight in one eye but otherwise appeared to be unaffected by this horrific accident. His movements and senses all were as before, and he actually went back to work. Only then did his workmates start to notice a difference. Gage was not badly affected in how he walked, pronounced words, ate, or did other normal human activities, but a far more subtle change had occurred. He had become very unpleasant and antisocial, cursing in inappropriate situations. So could the prefrontal cortex be the brain's center for character (or good character)?

In fact, in the decades since Phineas Gage had his accident, scientists have studied many other patients suffering damage to the prefrontal cortex. More recently, they have observed the region's activity in healthy people. The prefrontal cortex has now been implicated in a welter of seemingly disparate functions, ranging from "forward planning" to "working memory." In people suffering from clinical depression this area appears overactive, and in those with schizophrenia it can be underactive. There is clearly no single common, easily identifiable theme to these findings. So where does that leave us in our effort to localize functions within the brain, to match up different functions with different structures?

The emerging picture is certainly not a brain composed of autonomous minibrains. Rather, every function is divided among many brain regions, and every brain region participates in the many functions that make up the human behavioral repertoire. We can say that certain regions of the brain are more active than others when it comes to certain functions, but we can't say those functions are confined to particular areas. And we are still a long way from knowing how to assemble the different structures of the brain to make up a human mind.


Studying the Cells -- The Bottom-Up Approach

We can also think about the brain in the opposite way, or "bottom up." This alternative approach involves starting with the brain's most basic components and then figuring out how they connect with each other.

The most basic working unit of the brain is a special type of cell, the neuron. You have approximately 100 billion neurons -- as many trees as there are in the Amazon rain forest. But neurons are not the only cells in the brain: they are actually outnumbered ten to one by another type, glial cells. These cells maintain a healthy and nurturing microenvironment within our heads for the neurons to operate at their best.

So what do neurons actually do? Since the 1920s, neurologists have known that neurons generate minute electrical signals. Each neuron alive in your brain at this moment is producing a tiny voltage, a potential difference between the charge inside the cell and the charge outside. Under certain conditions, such as when a signal comes in from a neighboring cell, tiny channels open in the wall of the neuron so that there is a sudden, brief interchange of ions (atoms with an electrical charge, in particular sodium and potassium). This ion interchange causes a temporary shift in the neuron's charge -- an electrical blip called an action potential. Action potentials last for only about one thousandth of a second. Yet a neuron will typically fire off a hundred or so every second. This traffic in charges represents the "moving parts" of the brain, the actions that make it work. Ultimately all that we are -- all our memories, hopes, and feelings -- can be boiled down to the banal transfer of a few ions across the membrane wall of our brain cells.

Using the right sensors, we can read those electrical signals through the bone of the skull; the result is the valuable diagnostic tool called the electroencephalogram (EEG; for more information, see chapter 2). Over the last few decades, furthermore, technology has enabled neurophysiologists to record the activity of a single neuron. Therefore, we have a fairly good idea of what makes those little building blocks of our brains work.

What happens once a neuron has generated an action potential? This tiny blip, some eighty thousandths of a volt in amplitude, buzzes away at speeds up to 250 miles per hour along the biological equivalent of a wire: an axon. But unlike any household electrical circuit, the brain isn't wired so that all neurons form one single continuous network. Instead, in most cases, there is a gap between the axon of one neuron and the next neuron. This gap is called a synapse. It is as impossible for the action potential to cross a synapse as it is for a car to screech down a road and then float across a river. This might seem to be a cumbersome weakness in our wiring, but it is actually a powerful advantage.

The brain has an alternative way to send a signal across the fluid-filled gap. When the electrical impulse invades the end of the axon, it triggers the release of a chemical that can spread across the synapse and activate the target neuron.

This chemical, because it transmits a signal, is known as a transmitter (or a "neurotransmitter," if we want to make absolutely clear that it is at work in the brain). Once the transmitter hits the target cell, it enters into a kind of molecular handshake with a custom-made protein, a receptor, on the outside of that cell. This molecular handshake then causes the opening of the tiny channels into that neuron so that ions can cross over, once again generating the electrical signal. The brain therefore is not like a computer or any other electrical device, because it operates by means of a cascade of alternating electrical and chemical events.

Furthermore, there are many different transmitters in the brain, each with several different subtypes of receptors. So unlike a standard electronic circuit within a computer, which can only be on or off, the brain has a powerful spectrum of functions. Different chemicals will trigger different states within the brain. To appreciate just how important chemical signaling is to brain function, and hence to our mental abilities, let's take a look at drugs.

All drugs that modify moods and feelings, whether prescribed or proscribed, do so by changing the availability or the efficacy of different transmitters within the brain. For example, some 30 years ago scientists discovered that the drug morphine worked by imitating a naturally occurring neurotransmitter called enkephalin (literally, "in the head"). But that does not mean that it is natural or safe to take the most abused derivative of morphine, heroin. Enkephalin is released in minute amounts as and when it's needed in the brain; then, even more important, it is disposed of very rapidly. Not so with heroin. First, it is not released in a small quantity exactly where it is needed; a heroin user effectively marinates his or her whole brain, setting the drug free to act wherever there are appropriate receptors. Second, when heroin does encounter a receptor and enters into a molecular handshake, the drug can't be removed as readily as its natural counterpart. Because heroin is a different chemical, it will remain stubbornly in place. The result is like a handshake with an excessively strong grip. And just as the hand being gripped quickly starts to turn numb, the brain's special receptors become less sensitive. The heroin user needs increasing amounts of the drug to obtain the same effect, one sign of an addiction (C29).

The powerful effects of drugs on the brain surely demonstrate the importance of transmitters and, above all, of the connections over which they operate. Even the awesome number of neurons in our brains is dwarfed by the number of connections between them. There can be as many as 10,000 inputs to any one neuron. One estimate has it that counting each connection in your cortex alone, one a second around the clock, would take you 32 million years!


Making Connections -- The Dynamic Approach

Looking at the connections our brain cells forge is a sort of middle approach, halfway between studying large brain regions and examining single cells. It is this aspect of the brain that will most likely allow us to discover what it is about these squishy organs that makes humans such an intelligent species, and makes each one of us unique.

As you'll see in more detail in Part II of this book, we are born with pretty much all the neurons we will ever have. (In fact, many brain cells die off during childhood.) But the marvelous feature of being human is that many of the connections among those neurons are laid down after we are born. This forging of connections in the most basic and broadest sense underpins what we refer to as learning. We have highly adaptable brains that reflect and benefit from our experiences. In contrast, simpler organisms like bugs operate at the dictates of their genes, following preprogrammed instincts.

We call the adaptability of our human brains plasticity. Our brains reflect each new experience. As a consequence, we become individuals. Everyone undergoes different experiences, and everyone's brain develops differently. Of course genes play an important part in constructing the molecular machinery at work on each side of your synapses. But there are about 1 billion more connections in your brain than genes in your chromosomes; it is impossible for each connection to be programmed by a gene. Instead, the connections are shaped by your experiences.

The basis of this adaptability is the growth of connections between cells, strengthened and promoted by the activation of the relevant neurons. An axon coming from one neuron makes contact with the next neuron along the circuit by means of what are called dendrites on the receiving neuron. The more dendrites a neuron has, the more connections it will be able to make and the greater the circuitry underpinning a particular process or function. Just as a muscle grows with appropriate exercise, so selective circuits in the brain branch out and expand as they are worked. We can see this change even at the level of a single neuron. In one study with adult rats, half were housed in humane but isolated cages while the other half were housed collectively and exposed to interactive objects, such as ladders. The neurons from the group in the temporarily "enriched" environment showed more dendrites emanating from a single cell than those in the nonenriched group.

The more sophisticated a species, the longer it takes for an individual to grow to adulthood. We humans are the most sophisticated animals of all, so we take many years to develop. Our brains need that much time to collect and store the experiences that shape our minds. Childhood is usually a time of exploration, of making the mental connections that go along with the growing connections among our brain cells. That is why an individual's circumstances in youth help mold that person's personality, skills, and other qualities. (For more on brain development in childhood and adolescence, see chapters 6 and 7.)

But learning doesn't have to stop in childhood. The plasticity of our brains means that they can usually adapt to further challenges. A recent study showed that London taxi drivers, who have to memorize all the street names and routes of that huge city, have a larger part of the brain relating to memory than do other adults of a similar age. Another striking example of our brain's ability to learn entails not a lifetime at a profession but merely five days spent practicing a piano exercise -- this study showed that over such a short time the brain territory allocated to the fingers became enhanced. Even more amazing, mere mental practice has a similar effect on the brain.

But is the brain's power to learn limited as we get older? We have already briefly explored the physical basis of "blowing the mind" with drugs. Sadly, old age can bring the horror of "losing one's mind" because of degenerative diseases like Alzheimer's (C67). In this disorder, still not fully understood, the connections that a person's brain has so painstakingly accumulated throughout life gradually become dismantled: increasingly, everything around the person comes to "mean" less. Imaging techniques have now revealed that certain brain regions in Alzheimer's patients shrink far faster than in healthy individuals of a similar age. This finding has an encouraging implication: the symptoms of senile dementia that come with this disease are not a natural consequence of aging but are due to some special factor or factors that are as yet a matter of conjecture.

In fact, healthy older brains retain their plasticity. We can see this in the often remarkable recoveries of people who have had strokes (C59, C60). Parts of their brains suffered severe damage, having been deprived of blood and oxygen for significant periods. Nevertheless, many of these people are able to offset the damage and regain functions they initially lost. Their brains create new neural pathways, or start to use old ones, to bypass the damaged areas. Once again, the brain responds to experience by creating new connections and new functionality. (For more about the brains of older adults, see chapter 8.) In the near future the brain sciences will be shedding more light on these mechanisms of plasticity, as well as giving us insight into the specific losses that characterize Alzheimer's disease and other disorders.

Because of plasticity, as we go through life, our brains become increasingly personalized. Everything we encounter will be interpreted in the light of all that we have seen before. It is this personalization of the brain that gives rise to the mind. Viewed in this way, the mind is not some airy, whimsical alternative to the physical brain, but the aspect of it that makes each of us unique.

The Director of the Royal Institution of Great Britain, Dr. Susan Greenfield, has a memory that captures nicely the mystique of the physical human brain and its relationship to mind. Here is how she tells it:

"Once upon a time, over 25 years ago, I was undertaking a dissection of the human brain as part of a college class. Each pair of us students had our own plastic bucket, containing in preservative liquid the organ that had once defined a unique person. I stared down at this odd object in my fingers, resembling two compacted giant walnuts with a smaller walnut on the back. A macabre thought struck me: What if I weren't wearing protective gloves and got a piece of this brain stuff stuck under my fingernail? Would that be a thought or a memory, a habit or a feeling? Exactly what part of the individual would be nestling on top of my finger?"

Evocative, fundamental questions remain to tantalize. We are at an exciting time, when we no longer need to think about the brain as a collection of static anatomical regions, nor as a mere mass of generic cells and chemicals. We can now peek into the brain and see it shaping and reshaping every moment of our lives. It is truly the most dynamic and the most personal part of our bodies.

Copyright © 2003 by The Charles A. Dana Foundation

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