Copyright

This book is designed to give information on various medical conditions, treatments, and procedures for your personal knowledge and to help you be a more informed consumer of medical and health services. It is not intended to be complete or exhaustive, nor is it a substitute for the advice of your doctor. You should seek medical care promptly for any specific medical condition or problem you may have. All efforts have been made to ensure the accuracy of the information contained in this book as of the date published. The authors and the publisher expressly disclaim responsibility for any adverse effects arising from the use or application of the information contained herein.

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© Frances E. Jensen with Amy Ellis Nutt 2015

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Source ISBN: 9780007448319

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This book is dedicated to my two sons, Andrew and Will. Watching them grow into young men as they emerged through their teen years has been the joy of my life, and shepherding them through this time was probably the most important job of my life. Together we went on a journey, and as much as I taught them, they taught me. The product is this book, and I hope that it informs not only those people helping to raise adolescents, but also the teenagers themselves.

When I was a boy of fourteen, my father was so ignorant I could hardly stand to have the old man around. But when I got to be twenty-one, I was astonished by how much he’d learned in seven years.

—MARK TWAIN

I would that there were no age between sixteen and three-and-twenty, or that youth would sleep out the rest, for there is nothing in the between but getting wenches with child, wronging the ancientry, stealing, fighting …

—THE WINTER’S TALE, WILLIAM SHAKESPEARE

Contents

Cover

Title Page

Copyright

Dedication

Epigraph

List of Illustrations

Introduction: Being Teen

1 Entering the Teen Years

2 Building a Brain

3 Under the Microscope

4 Learning: A Job for the Teen Brain

5 Sleep

6 Taking Risks

7 Tobacco

8 Alcohol

9 Pot

10 Hard-Core Drugs

11 Stress

12 Mental Illness

13 The Digital Invasion of the Teenage Brain

14 Gender Matters

15 Sports and Concussions

16 Crime and Punishment

17 Beyond Adolescence: It’s Not Over Yet

Postscript: Final Thoughts

Glossary

Notes

Selected Bibliography

Resources

List of Seacrhable Terms

Acknowledgements

About the Authors

Also by Amy Ellis Nutt

About the Publisher

Illustrations

FIG. 1
The Basics of Brain Structure (Brain images courtesy of and with permission from John Detre, MD, and Paul Yushkevich, PhD, University of Pennsylvania).

FIG. 2
The “Homunculus” (Artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania. Brain image courtesy of and with permission from John Detre, MD, and Paul Yushkevich, PhD, University of Pennsylvania).

FIG. 3
The Lobes of the Brain (Created by the author, artwork adapted by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania. Brain image courtesy of and with permission from John Detre, MD, and Paul Yushkevich, PhD, University of Pennsylvania).

FIG. 4
Maturing Brain: The Brain “Connects” from Back to Front (A, C: Reprinted from N. Gogtay et al., “Dynamic Mapping of Human Cortical Development During Childhood Through Early Adulthood,” Proceedings of the National Academy of Sciences 101, no. 21 [May 25, 2004], 8174–79, copyright 2004 National Academy of Sciences, U.S.A. B: Brain image courtesy of and with permission from John Detre, MD, and Paul Yushkevich, PhD, University of Pennsylvania).

FIG. 5
Multitasking Is Still Not Perfect in the Teen Brain (With kind permission from Springer Science and Business Media and the author: M. Naveh-Benjamin et al., “Concurrent Task Effects on Memory Encoding and Retrieval: Further Support for an Asymmetry,” Memory & Cognition 34, no. 1 [2006], 96, fig. 3A, © 2006).

FIG. 6
Anatomy of Neuron, Axon, Neurotransmitter, Synapse, Dendrite, and Myelin (Created by the author, artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania).

FIG. 7A
Inhibitory Cells Can Stop Signaling (Created by the author, artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania).

FIG. 7B
Excitatory and Inhibitory Synapses (Created by the author, artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania).

FIG. 8
The Young Brain Has More Excitatory Synapses Than Inhibitory Synapses (Courtesy of and created by the author).

FIG. 9
Gender Differences in Rate of Cortical Gray Matter Growth (Reprinted with permission from Macmillan Publishers Ltd./Nature Neuroscience: J. N. Giedd et al., “Brain Development in Children and Adolescents: A Longitudinal Study,” Nature Neuroscience 2, no. 10 [1999], 861–63, © 1999).

FIG. 10
Long-Term Potentiation (LTP) Is a Widely Used Model of the “Practice Effect” of Learning and Memory (Created by the author, artwork adapted by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania. Brain image courtesy of and with permission from John Detre, MD, and Paul Yushkevich, PhD, University of Pennsylvania).

FIG. 11
New Receptors Are Added to Synapses During Learning and Memory and Long-Term Potentiation (Created by the author, artwork adapted by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania).

FIG. 12
Gray Matter and White Matter Develop Differently Throughout Life (Courtesy of and with permission from Arthur Toga, Institute of Neuroimaging and Informatics, Keck School of Medicine, University of Southern California).

FIG. 13
Adolescents’ Synaptic Plasticity Is “Way Better” Than Adults’ (Reprinted from N. L. Schramm et al., “LTP in the Mouse Nucleus Accumbens Is Developmentally Regulated,” Synapse 45, no. 4 [Sept. 15, 2002], 213–19, copyright © 2002 Wiley-Liss, Inc.).

FIG. 14
The Developmental Control of the Circadian System (Reprinted from M. H. Hagenauer and T. M. Lee., “The Neuroendocrine Control of the Circadian System: Adolescent Chronotype,” Frontiers in Neuroendocrinology 33, no. 3 [Aug. 2012], 211–29, © 2012, with permission from Elsevier and the author. Additional artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania).

FIG. 15
Ventral Tegmental Area (VTA) Dopamine Neurons from Young Mice Are Able to Fire More Action Potentials Than Those from Adult Mice When Stimulated (Reprinted from A. N. Placzek et al., “Age Dependent Nicotinic Influences over Dopamine Neuron Synaptic Plasticity,” Biochemical Pharmacology 78, no. 7 [Oct. 1, 2009], 686–92, © 2009, with permission from Elsevier. Additional artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania).

FIG. 16
Rates of Alcohol, Cigarette, and Illicit Drug Use from the National Institutes of Health (Courtesy National Institute of Drug Abuse, a component of the National Institutes of Health, U.S. Department of Health and Human Services. Adapted by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania. Available at http://www.drugabuse.gov/sites/default/files/nida_mtf2012_infographic_1_1000px_3.jpg).

FIG. 17
Shared Synaptic Biology of Learning and Addiction (Artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania).

FIG. 18
The Adolescent Brain Responds to Nicotine More Robustly Than the Adult Brain (Reprinted from T. L. Schochet et al., “Differential Expression of Arc mRNA and Other Plasticity-Related Genes Induced by Nicotine in Adolescent Rat Forebrain,” Neuroscience 135, no. 1 [2005], 285–97, © 2005, with permission from Elsevier. Additional artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania).

FIG. 19
Alcohol Decreases LTP (A: Reprinted from T. A. Zhang et al., “Synergistic Effects of the Peptide Fragment D-NAPVSIPQ on Ethanol Inhibition of Synaptic Plasticity and NMDA Receptors in Rat Hippocampus,” Neuroscience 134, no. 2 [2005], 583–93, © 2005, with permission from Elsevier. B: Created by the author, artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania).

FIG. 20
Alcohol Affects LTP in Adolescents More Than in Adults (Reprinted from G. K. Pyapali et al., “Age and Dose-Dependent Effects of Ethanol on the Induction of Hippocampal Long-Term Potentiation,” Alcohol 19, no. 2 [Oct. 1999], 107–11, © 1999, with permission from Elsevier. Additional artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania).

FIG. 21
Increases in Marijuana and Substance Abuse in Teens in the Last Decade (A: Adapted from National Monitoring the Future Study 1997–2011 by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania. Available at http://files.eric.ed.gov/fulltext/ED529133.pdf. B: Courtesy of Substance Abuse and Mental Health Services Administration, Department of Health and Human Services, National Treatment Episode Data Set 2007, adapted by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania. Available at http://www.samhsa.gov/data/DASIS/TEDS2k7AWeb/TEDS2k7AWeb.pdf).

FIG. 22
Effects of Cannabinoids on Learning. (A: Reprinted by permission from Macmillan Publishers Ltd.: N. Stella et al., “A Second Endogenous Cannabinoid That Modulates Long-Term Potentiation,” Nature 388 [Aug. 21, 1997], 773–78, © 1997. B: Artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania).

FIG. 23
Long-Term Effects of Marijuana on Gray and White Matter in the Teen Brain. (A: Reprinted with permission from M. H. Meier et al., “Persistent Cannabis Users Show Neuropsychological Decline from Childhood to Midlife,” Proceedings of the National Academy of Sciences 109, no. 40 [Oct. 2, 2012], E2657–64, copyright 2012 National Academy of Sciences, U.S.A. B: Reprinted with permission from D. Arnone et al., “Corpus Callosum Damage in Heavy Marijuana Use: Preliminary Evidence from Diffusion Tensor Tractography and Tract-Based Spatial Statistics,” NeuroImage 41, no. 3 [July 1, 2008], 1067–74, © 2008, with permission from Elsevier).

FIG. 24
Enhanced Effects of Cocaine on the Behavior of Adolescent Rats. (Reprinted from A.L. Wheeler et al., “Adolescent Cocaine Exposure Causes Enduring Macroscale Changes in Mouse Brain Structure,” Journal of Neuroscience 33, no. 5 [Jan. 30, 2013], 1797–1803a, with permission from Society for Neuroscience. Additional artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania).

FIG. 25
Stress Can Decrease Learning and LTP. (A, C: Artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania. B: Created by the author, artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania. D: Reprinted with permission from M. R. Foy et al., “Behavioral Stress Impairs Long-Term Potentiation in Rodent Hippocampus,” Behavioral and Neural Biology 48, no. 1 [July 1987], 138–49, © 1987, with permission from Elsevier. Additional artwork by Mary A. Leonard, Biomedical Art and Design, University of Pennsylvania).

FIG. 26
Enhanced Response to Fearful Stimuli in Adolescents Compared with Children and Adults. (Reprinted from B. J. Casey et al., “Transitional and Translational Studies of Risk for Anxiety,” Depression and Anxiety 28, no. 1 [Jan. 2011], 18–28, © 2011 Wiley-Liss Inc.).

FIG. 27
Prior Juvenile Diagnoses in Adults with Mental Disorders. (Reprinted from J. Kim-Cohen et al., “Prior Juvenile Diagnoses in Adults with Mental Disorder: Developmental Follow-Back of a Prospective-Longitudinal Cohort,” Archives of General Psychiatry 60, no. 7 [July 2003], 709–17, copyright © 2003 American Medical Association. All rights reserved).

FIG. 28
Gender Differences in Brain Connections (Reprinted with permission from M. Ingalhalikar et al., “Sex Differences in the Structural Connectome of the Human Brain,” Proceedings of the National Academy of Sciences 111, no. 2 [Jan. 14, 2014], 823–28, copyright 2014 National Academy of Sciences, U.S.A.).

FIG. 29
How Accountable Should Society Hold Adolescents? (Justice John Paul Stevens, United States Supreme Court; Steve Drizin, Northwestern University in Chicago; www.fairsentencingofyouth.org).

FIG. 30
A Recap of Brain Development and the Critical Stage of Adolescence (Courtesy of the author).

Introduction Being Teen

What was he thinking?

My beautiful, auburn-haired son had just returned home from a friend’s house with his hair dyed jet-black. Despite my inward panic, I said nothing.

“I want to get red streaks in it,” he told me nonchalantly.

I was gob-smacked. Is this really my child!? I’d begun to ask the question often during my fifteen-year-old son Andrew’s sophomore year at a private high school in Massachusetts, all the while trying to be empathetic. I was a divorced working mother of two teenage sons, putting in long hours as a clinician and professor at Boston Children’s Hospital and Harvard Medical School. So if I sometimes felt guilty about the time I spent away from my boys, I also was determined to be the best mother I could be. After all, I was a faculty member in a pediatric neurology department and actively researching brain development. Kids’ brains were my business.

But my sweet-natured firstborn son had suddenly become unfamiliar, unpredictable, and bent on being different. He had just transferred from a very conventional middle school that went through ninth grade, where jackets and ties were the norm, to a very progressive high school. Upon arriving, he took full advantage of the new environment, and part of that was to dress in what you might describe as an “alternative” style. Let’s face it, his best friend had spiky blue hair. Need I say more?

I took a deep breath and tried to calm myself. Getting mad at him, I knew, wouldn’t do either of us any good and probably would only alienate him further. At least he felt comfortable enough to tell me about something he wanted to do before he actually did it. This was an opportunity, I realized, and I quickly seized it.

Instead of damaging your hair with some cheap, over-the-counter dye, what if I take you to my hair guy for the red streaks? I asked him. Since I also was going to pay for it, Andrew happily agreed. My hair stylist, who was a sort of punk rocker himself, got totally into the task. He did a great job, actually—so good that Andrew’s girlfriend at the time was inspired to color her hair in exactly the same black-and-red motif. She attempted this herself, and needless to say had different results.

Thinking back to those days, I realize so much of what I thought I knew about my son during this turbulent time of his life seemed turned on its head. (Was that a compost pile in the middle of his bedroom, or laundry?) Andrew seemed trapped somewhere between childhood and adulthood, still in the grip of confusing emotions and impulsive behavior, but physically and intellectually more man than boy. He was experimenting with his identity, and the most basic element of his identity was his appearance. As his mother and a neurologist, I thought I knew everything there was to know about what was going on inside my teenager’s head. Clearly I did not. I certainly didn’t know what was going on outside his head either! So as a mother and a scientist, I decided I needed to—I had to—find out.

Professionally, I was primarily studying the brains of babies at that time and running a research lab largely devoted to epilepsy and brain development. I was also doing translational neuroscience, which means, simply, trying to create new treatments for brain disorders. Suddenly, however, I had a new scientific experiment and project: my sons. My younger son, Will, was just two years Andrew’s junior. What would I be in for when Will reached the same age as his older brother? There was so much I didn’t get. I had watched Andrew, almost overnight, morph into a different being, yet I knew, deep inside, he was still the same wonderful, kind, bright kid he’d always been. So what happened? To figure it out I decided to delve into the world of research on this somewhat foreign species in my household called the teenager, and use that knowledge to help me and my sons navigate their way more smoothly into adulthood.

The teen brain has been a relatively neglected area of study until only the past decade. Most research dollars in neurology and neuropsychology are spent on infant and child development—from learning disabilities to early enrichment therapy—or, at the other end of the spectrum, on diseases of the elderly brain, especially Alzheimer’s. Up until just a few years ago, the neuroscience of the adolescent brain was underfunded, underresearched, and obviously not well understood. Scientists believed—incorrectly, as it turned out—that brain growth was pretty much complete by the time a child started kindergarten; this is why for the past two decades parents of infants and toddlers, trying to get a jump on their children’s education, have inundated their kids with learning tools and accessories like Baby Einstein DVDs and Baby Mozart Discovery Kits. But the adolescent brain? Most people thought it was pretty much like an adult’s, only with fewer miles on it.

The problem with this assumption is that it was wrong. Very wrong. There are other misconceptions and myths about the teenage brain and teenage behavior that are now so ingrained they are accepted societal beliefs: teens are impulsive and emotional because of surging hormones; teens are rebellious and oppositional because they want to be difficult and different; and if teenagers occasionally drink too much alcohol without their parents’ consent, well, their brains are resilient, so they’ll certainly rebound without suffering any permanent effects. Another assumption is that the die is cast at puberty: whatever your IQ or apparent talents may be (a math or science type versus a language arts type), you stay that way for the rest of your life.

Again, all wrong. The teen brain is at a very special point in development. As this book will reveal, I learned that there are unique vulnerabilities of this age window, but there is also the ability to harness exceptional strengths that fade as we enter into adulthood.

The more I studied the emerging scientific literature on adolescents, the more I understood how mistaken it was to look at the teenage brain through the prism of adult neurobiology. Functioning, wiring, capacity—all are different in adolescents, I learned. I was also aware that this new science of the teenage brain wasn’t reaching most parents, or at least wasn’t reaching parents who don’t have a background in neuroscience as I did. And this was just the audience who needed to know about this new science of the adolescent brain: parents and guardians and educators who are just as perplexed, frustrated, and maddened by the teenagers in their care as I was.

When my younger son, Will, was sixteen, he passed his driver’s test. He’d rarely given me cause to worry, but that changed early one morning. A few weeks after getting his license, he had started to drive himself to school in our 1994 Dodge Intrepid—a big, old, safe car. All seemed to go well. As usual, Will left around 7:30, as school started at 7:55. Off he went. Just as I was walking out the door to go to my job, at about 7:45, I got a call from Will: “Mom, I’m okay, but the car is totaled.” Well, first, I was thankful he had the presence of mind to lead off with telling me he was okay, but I had visions of his car wrapped around a tree. I said, “I’m on my way,” and jumped in my car. As I was approaching the school entrance, I saw the flashing lights of the police cars. What had he done? Well, simply put, he had decided that he could squeeze a left turn into the school entrance in the path of rapidly moving traffic going in the opposite direction. This might have worked if there had been another mother like me driving in the opposite direction who would have shaken her head and slammed on her brakes. But in Will’s case that morning, it was a twenty-three-year-old guy, a construction worker in a Ford F-150 on his way to work. He was no more in the mood to give the right-of-way than Will had been to wait to cross the road. So—the accident happened. It was good to know that 1994 airbags still worked in 2006.

There was Will standing by his completely trashed car at the very entrance of his school, looking sheepish as basically the entire school drove by him as students and staff arrived for the day. What a lesson for Will. I recognized that immediately—and was so thankful that he and the other driver had emerged unscathed from this battle of wills as to who had the right-of-way.

What was he thinking? I asked myself, almost reflexively.

Then: Oh, no, here we go again.

This time, however, I quickly calmed myself. I knew a lot more now. I knew Will’s brain, like Andrew’s, like every other teenager’s, was a work in progress. He clearly was no longer a child, and yet his brain was still developing, changing, even growing. I hadn’t recognized that until Andrew made me sit up and take stock of what I knew about the pediatric brain, that it’s not so much what is happening inside the head of an adolescent as what is not.

The teenage brain is a wondrous organ, capable of titanic stimulation and stunning feats of learning, as you will learn in this book. Granville Stanley Hall, the founder of the child study movement, wrote in 1904 about the exuberance of adolescence:

These years are the best decade of life. No age is so responsive to all the best and wisest adult endeavor. In no psychic soil, too, does seed, bad as well as good, strike such deep root, grow so rankly or bear fruit so quickly or so surely.

Hall said optimistically of adolescence that it was “the birthday of the imagination,” but he also knew this age of exhilaration has dangers, including impulsivity, risk-taking, mood swings, lack of insight, and poor judgment. What he couldn’t possibly have anticipated back then is the breathtaking range of dangers teens would be exposed to today through social media and the Internet. How many times have I heard from friends, colleagues, even strangers who have reached out to me after hearing me speak, about the crazy things their teenage kids or their friends just did? The daughter who “stole” her father’s motorcycle and crashed it into a curb. The kids who went “planking”—lying facedown, like a board, on any and every surface (including balcony railings), and then taking photos of one another doing it. Or worse: “vodka eyeballing,” pouring liquor directly into the eye to get an immediate high, or, scared about passing a drug test for a weekend job, ingesting watered-down bleach, thinking it would “clean” their urine of the pot they had smoked the night before.

Children’s brains continue to be molded by their environment, physiologically, well past their midtwenties. So in addition to being a time of great promise, adolescence is also a time of unique hazards. Every day, as I will show you, scientists are uncovering ways in which the adolescent brain works and responds to the world differently from the brain of either a child or an adult. And the way that the adolescent brain responds to the world has a lot to do with the impulsive, irrational, and wrongheaded decisions teens seem to make so frequently.

Part of the problem in truly understanding our teenagers lies with us, the adults. Too often we send them mixed messages. We assume that when our kid begins to physically look like an adult—she develops breasts; he has facial hair—then our teenager should act like, and be treated as, an adult with all the adult responsibilities we assign to our own peers. Teenagers can join the military and go to war, marry without the consent of their parents, and in some places hold political office. In recent years, at least seven teens have been elected mayors of small towns in New York, Pennsylvania, Iowa, Michigan, and Oregon. Certainly the law often treats teens as adults, especially when those teens are accused of violent crimes and then tried in adult criminal courts. But in myriad ways we also treat our teens like children, or at least like less than fully competent adults.

How do we make sense of our own conflicting messages? Can we make sense?

For the past few years I’ve given talks all over the country—to parents, teens, doctors, researchers, and psychotherapists—explaining the risks and rewards that pertain to the new science of the adolescent brain. This book was prompted by the tremendous, even overwhelming, number of responses I have received from parents and educators (and sometimes even teens) who heard me speak. All of them wanted to share their own stories, ask questions, and try to understand how to help their kids—and, in the process, themselves—navigate this thrilling but perplexing stage of life.

The truth of the matter is, I learned from my own sons that adolescents are not, in fact, an alien species, but just a misunderstood one. Yes, they are different, but there are important physiological and neurological reasons for those differences. In this book I will explain how the teen brain offers major advantages on the one hand but unperceived and often unacknowledged vulnerabilities on the other. I am hoping you will use this as a handbook, a kind of user’s manual or survival guide to the care and feeding of the teenage brain. Ultimately, I want to do more than help adults better understand their teenagers. I want to offer practical advice so that parents can help their teenagers, too. Adolescents aren’t the only ones who must navigate this exciting but treacherous period of life. Parents, guardians, and educators must, too. I have—twice. It is humbling, exhilarating, confusing, all at the same time. As parents, we brace ourselves for what will be quite a roller-coaster ride, but in the vast majority of cases the ride slows down, evens out, and gives one a lot of stories to tell afterward!

Nearly a decade ago, when it became clear to me that being a parent of teenagers was nothing like taking care of overgrown children, I said, Okay, let’s work on it together. I stayed in my sons’ faces. I remember one time, when Andrew was still a sophomore in high school, the inevitable point arrived when exams were just around the corner and he was still paying more attention to sports and parties than books and homework. Because I’m a scientist, I know learning is cumulative—everything new is based on something you just learned, so you have to hang in there, you have to stay on top of it. So I got a pad of paper and I went through each chapter of Andrew’s textbooks, and on one side of the paper I picked out a problem for him to solve and on the other side, folded, was the answer. All he needed was a model, a template, a structure. It was a turning point for him and me. He realized he actually had to do the work—sit down and do it—in order to learn. He also realized working on his bed, with everything spread out around him, wasn’t helping. He needed more structure, so he sat himself at his desk, with a pencil sharpener and a piece of paper in front of him, and he learned how to impose order on himself. He needed the external cues. I could plan and he couldn’t at that point. Having a structured environment helped him learn, and eventually he got really good at it, sitting in his chair at his desk for hours. I know because I’d check in on him. I also knew this was a good example of place-dependent learning. Scientists have shown that the best way to remember what you’ve learned is to return to the place where you learned it. For Andrew, that was his desk in his bedroom. As I will explain later, teenagers are “jacked up” on learning—their brains are primed for knowledge—so where and how they learn is important, and setting up a place where homework is done is something any parent can help teens do. And because homework is one of the main things kids do at home, you can stay involved with your teenagers even if you don’t happen to have an MD or PhD in the subject or subjects they’ve neglected for months. You can offer to proofread assignments, spell-check their essays, or simply make sure they are sitting in a comfortable desk chair. While you might not have the right hair guy to get red streaks, the point is that you can at least spring for a home hair dye when they want to transform themselves on the outside. Let them experiment with these more harmless things rather than have them rebel and get into much more serious trouble. Try not to focus on winning the battles when you should be winning the war—the endgame is to help get them through the necessary experimentation that they instinctively need without any longterm adverse effects. The teen years are a great time to test where a kid’s strengths are, and to even out weaknesses that need attention.

What you don’t want to do is ridicule, or be judgmental or disapproving or dismissive. Instead, you have to get inside your kid’s head. Kids all have something they’re struggling with that you can try to help. They can be all over the place: forgetting to bring their books home, crumpling important notes in the bottom of their backpacks, misconstruing homework assignments. Sometimes—or most of the time—they are just not organized, not paying attention to the details of what’s going on around them, and so expecting them to figure out how to do their homework can actually be expecting too much. Your teenagers won’t always accept your advice, but you can’t give it unless you’re there, unless you’re trying to understand how they’re learning. Know that they are just as puzzled by their unpredictable behavior and the uneven tool kit they call their brain. They just aren’t at a point where they will tell you this. Pride and image are big for teens, and they are not able to look into themselves and be self-critical.

That’s what this book is all about—knowing where their limits are and what you can do to support them. So that you won’t get angry or confused at your teens or simply throw your hands up in surrender, I want to help you understand what makes them so infuriating. Much of what is in this book will surprise you—surprise you because you probably thought teenagers’ recalcitrant behavior was something they could, or at least should, be able to control; that their insensitivity or anger or distracted attitude was entirely conscious; and that their refusal to hear what you suggest or request or demand they do was entirely willful. Again, none of these things are true.

The journey I will take you on in this book will actually shock you at times, but by the end of the journey I promise you will gain insight into what makes your teens tick because you’ll have a much better understanding of how their brains work. I make an effort in this book to reveal, wherever possible, the real data from real science journal articles. There is much data out there that has not been “translated” for the public. Even more important, the teen generation is one that holds information in great esteem. So when you talk to teens, you owe it to them to have actual data. I inserted as many figures into this book as I could where the actual science is shown, and I point out where it applies to our knowledge of the strengths and weaknesses of being a teen. There are lots of myths about teenagers out there that need to be debunked: this book is an attempt to chip away at those myths and explore the new science that is available to inform us.

For this book to be truly effective, however, you must remember a simple rule: First, count to ten. It became a kind of mantra for me when I was raising my sons. But it means more than just taking a deep breath. Let me explain. In leadership courses I’ve taken for my professional career, one theme that is always emphasized is the Boy Scouts’ motto, “Be prepared.” I learned in these seminars that the average time an American businessperson spends preparing for a meeting is about two minutes. We probably spend more time just scheduling those meetings than actually thinking about what we’re going to say or do in them. I don’t mean the big presentations. I mean the one-on-one encounters, which we too often step into cavalierly without taking much time for reflection beforehand. When I heard this statistic, initially it shocked me, but then I thought about my own professional world, where I am the head of a large university neurology department and have my own lab with many graduate and postgraduate students, and I realized, Yep, that’s pretty much what does happen. Not a lot of time is devoted to planning or “rehearsing” for all those one-on-one encounters with colleagues and staff, and yet it’s these more personal, more direct interactions that often play a pivotal role in the success of an organization. Similarly, the impression you give others in these encounters can affect the direction your career takes; this is why it’s so important to plan ahead, at least for more than just a few minutes, and think about how the other person will react during one of these meetings. In your mind, go through what you want to say, step by step, and imagine the range of responses. Now imagine that the other person is your teenage son or daughter. Being prepared for both positive and negative responses will help guide you as you consider your options about what to say or do next. If you appear hotheaded or mentally disorganized, you lose credibility, whether it’s with a colleague, an employee, or your teenager.

For parents or teachers, or anyone who has a caring relationship with a teenager, reading this book will arm you with facts—and with fortitude. Changing the behavior of your teen is partly up to you, so you have to come up with a plan of action and a style of action that fits your household and your kids, as well as your needs and wants. Remember, you are the adult, and if your child is under eighteen, you also are legally responsible for that “child.” Certainly the courts will hold you accountable for your child and, by extension, for the environment you provide for him or her. So take the lead, take control, and try to think for your teenage sons and daughters until their own brains are ready to take over the job. The most important part of the human brain—the place where actions are weighed, situations judged, and decisions made—is right behind the forehead, in the frontal lobes. This is the last part of the brain to develop, and that is why you need to be your teens’ frontal lobes until their brains are fully wired and hooked up and ready to go on their own.

But the most important advice I want to give you is to stay involved. As the mother of two sons I adore, I couldn’t physically maneuver them into doing what I wanted them to do when they were teenagers, not in the way I could when they were small children. Eventually they were simply too big to just pick up and put down where I wanted them to be. We lose physical control as children leave childhood. Our best tool as they enter and move through their adolescent years is our ability to advise and explain, and also to be good role models. If there’s anything I’ve learned with my boys, it’s that no matter how distracted or disorganized they seemed to be, no matter how many assignments they forgot to bring home from school, they were watching me, taking the measure of their mom as well as all the other adults around them. I will talk much more about this later in the book, but just so you know, it all turned out okay in my life and the lives of my sons. Here’s the bottom line on my two “former teenagers”: Andrew graduated from Wesleyan University with a combined MA-BA degree in quantum physics in May 2011 and is now in a joint MD-PhD program. Will graduated from Harvard in 2013 and landed a business-consulting job in New York City. So, yes, you can survive your teenagers’ adolescence. And so can they. And you will all have a lot of stories to tell after it’s all over.

1
Entering the Teen Years

In July 2010 I received an e-mail from the frustrated mother of a nineteen-year-old who had just finished his freshman year of college. The mother had heard me give a talk to parents and teachers in Concord, Massachusetts, about the teenage brain, and her e-mail expressed a wide range of emotions, from sadness to confusion to anger, about the boy, whose behavior had suddenly become downright “weird.”

“My son gets angry easily,” she wrote. “He puts a wall around him so he would not talk. He stays up all night and sleeps all day. He stops doing things he used to enjoy…. He was once charming, intelligent, outgoing. These days, good mood is rare. I thought I did all that hard work to raise him, to send him to a very good college, and it all ended up like this.”

The woman ended her e-mail with a simple question: “How do I help him?”

Letters and e-mails and calls like these are what prompted me to write this book. Nine months after that mother asked how she could help her son, I received a similar e-mail, this time from the mother of an eighteen-year-old girl. Her daughter, who had once seemed so levelheaded, she wrote, had let her grades slip in high school. She became defiant, ran away from home, and was hospitalized for depression. “This year has been difficult for us,” the mother wrote. “Sometimes it seems as if she has been replaced by an alien. It is because of the behavior and the things that she says. She is a completely different person.”

I knew how these women felt. At one time, I felt helpless, too. Because I was newly divorced as my older son, Andrew, entered adolescence, I was painfully aware that my children’s future, as well as their present, was largely up to me. There was no pulling my hair out and saying, “Go talk to your father about it!” When you’re a single parent, the buck stops with you. As parents, we want to open a few doors for our kids—that’s all, really. To gently nudge them in the right direction. During their childhood, everything seems to go pretty much by plan. Our kids learn what’s appropriate and what isn’t, when to go to bed and when to get up in the morning, what not to touch, where not to go. They learn the importance of school, of being polite to their elders, and when they are physically hurt or emotionally wounded, they come to us seeking solace.

So what happens when they reach fourteen, fifteen, or sixteen years old? How is it that the cute, even-tempered, happy, and well-behaved child you’ve known for more than a decade is suddenly someone you don’t know at all?

These are a few of things I say to parents right off the bat: The sense of whiplash you are feeling is not unusual. Your children are changing, and also trying to figure themselves out; their brains and bodies are undergoing extensive reorganization; and their apparent recklessness, rudeness, and cluelessness are not totally their fault! Almost all of this is neurologically, psychologically, and physiologically explainable. As a parent or educator, you need to remind yourself of this daily, often hourly!

Adolescence is a minefield, for sure. It is also a relatively recent “discovery.” The idea of adolescence as a general period of human development has been around for aeons, but as a discrete period between childhood and adulthood it can be traced back only as far as the middle of the twentieth century. In fact the word “teenager,” as a way of describing this distinct stage between the ages of thirteen and nineteen, first appeared in print, and only in passing, in a magazine article in April 1941.

Mostly for economic reasons, children were considered miniadults well into the nineteenth century. They were needed to sow the fields, milk the cows, and split the firewood. By the time of the American Revolution half the population of the new colonies was under the age of sixteen. If a girl was still single at eighteen, she was considered virtually unmarriageable. Well into the early twentieth century, children over the age of ten, and often children much younger, were capable of most kinds of work, either on the farm or later in city factories—even if they needed boxes to stand on. By 1900, with the Industrial Revolution in full swing, more than two million children were employed in the United States.

Two things in the decades spanning the middle of the twentieth century—the Great Depression and the rise of high schools—not only changed attitudes about the meaning of childhood but also helped to usher in the era of the teenager. With the onset of the Depression after the stock market crash of 1929, child laborers were the first to lose their jobs. The only other place for them was school, which is why by the end of the 1930s, and for the first time in the history of American education, most fourteen- to seventeen-year-olds were enrolled in high school. Even today, according to a 2003 survey by the National Opinion Research Center, Americans regard finishing high school as the number one hallmark of adulthood. (In much of the United Kingdom a teenager is treated as an adult even if he or she does not finish high school, and in England, Scotland, and Wales it is legal not only to leave school at age sixteen but to leave home and live independently as well.) In the 1940s and ’50s, American youth, most of whom were not responsible for the economic survival of their families, certainly did not seem like adults—at least not until they graduated from high school. They generally lived at home and were dependent on their parents, and as more and more children found themselves going to school beyond the eighth grade, they became a kind of class unto themselves. They looked different from adults, dressed differently, had different interests, even a different vocabulary. In short, they were a new culture. As one anonymous writer said at the time, “Young people became teenagers because we had nothing better for them to do.”

One man foresaw it all more than one hundred years ago. The American psychologist Granville Stanley Hall never used the word “teenager” in his groundbreaking 1904 book about youth culture, but it was clear from the title of his fourteen-hundred-page tome—Adolescence: Its Psychology and Its Relations to Physiology, Anthropology, Sociology, Sex, Crime, Religion and Education—that he regarded the time between childhood and adulthood as a discrete developmental stage. To Hall, who was the first American to earn a PhD in psychology, from Harvard University, and the first president of the American Psychological Association, adolescence was a peculiar time of life, a distinct and separate stage qualitatively different from either childhood or adulthood. Adulthood, he said, was akin to the fully evolved man of reason; childhood a time of savagery; and adolescence a period of wild exuberance, which he described as primitive, or “neo-atavistic,” and therefore only slightly more controlled than the absolute anarchy of childhood.

Hall’s suggestion to parents and educators: Adolescents shouldn’t be coddled but rather should be corralled, then indoctrinated with the ideals of public service, discipline, altruism, patriotism, and respect for authority. If Hall was somewhat provincial about how to treat adolescent storm and stress, he was nonetheless a pioneer in suggesting a biological connection between adolescence and puberty and even used language that presaged neuroscientists’ later understanding of the malleability of the brain, or “plasticity.” “Character and personality are taking form, but everything is plastic,” he wrote, referring to pliability, not the man-made product. “Self-feeling and ambition are increased, and every trait and faculty is liable to exaggeration and excess.”

Self-feeling, ambition, exaggeration, and excess—they all helped define “teenager” for the American public in the middle of the twentieth century. The teenager as a kind of cultural phenomenon took off in the post-World War II era—from teenyboppers and bobbysoxers to James Dean in Rebel Without a Cause and Holden Caulfield in The Catcher in the Rye. But while the age of adolescence became more defined and accepted, the demarcation between childhood and adulthood remained—and remains—slippery. As a society, we still carry the vestiges of our centuries-old confusion about when a person should be considered an adult. In most of the United States a person must be between fifteen and seventeen to drive; eighteen to vote, buy cigarettes, and join the military; twenty-one to drink alcohol; and twenty-five to rent a car. The minimum age to be a member of the House of Representatives is twenty-five; to be president of the United States, thirty-five; and the minimum age to be a governor ranges among states from no age restriction at all (six states) to a minimum age of thirty-one (Oklahoma). There is generally no minimum age requirement to testify in most courts, enter into a contract or sue, request emancipation from one’s parents, or seek alcohol or drug treatment. But you must be eighteen to determine your own medical care or write a legally binding will, and in at least thirty-five states those eighteen or younger must have some type of parental involvement before undergoing an abortion. What a lot of mixed messages we give these teenagers, who are not at a stage to weed through the logic (if there is any) behind how society holds them accountable. Very confusing.

So what does being a teenager mean? Man-child, woman-child, quasi-adult? The question is about much more than semantics, philosophy, or even psychology because the repercussions are both serious and practical for parents, educators, and doctors, as well as the criminal justice system, not to mention teens themselves.

Hall, for one, believed adolescence began with the initiation of puberty, and this is why he is considered the founder of the scientific study of adolescence. Although he had no empirical evidence for the connection, Hall knew that understanding the mental, emotional, and physical changes that happen in a child’s transition into adulthood could come only from an understanding of the biological mechanics of puberty.

One of the chief areas of focus in the study of puberty has long been “hormones,” but hormones have gotten a bad rap with parents and educators, who tend to blame them for everything that goes wrong with teenagers. I always thought the expression “raging hormones” made it seem as though these kids had taken a wicked potion or cocktail that made them act with wild disregard for anyone and anything. But we are truly blaming the messenger when we cite hormones as the culprit. Think about it: When your three-year-old has a temper tantrum, do you blame it on raging hormones? Of course not. We know, simply, that three-year-olds haven’t yet figured out how to control themselves.

In some ways, that’s true of teenagers as well. And when it comes to hormones, the most important thing to remember is that the teenage brain is “seeing” these hormones for the first time. Because of that, the brain hasn’t yet figured out how to modulate the body’s response to this new influx of chemicals. It’s a bit like taking that first (and hopefully last!) drag on a cigarette. When you inhale, your face flushes; you feel light-headed and maybe even a bit sick to your stomach.

Scientists now know that the main sex hormones—testosterone, estrogen, and progesterone—trigger physical changes in adolescents such as a deepening of the voice and the growth of facial hair in boys and the development of breasts and the beginning of menstruation in girls. These sex hormones are present in both sexes throughout childhood. With the onset of puberty, however, the concentrations of these chemicals change dramatically. In girls, estrogen and progesterone will fluctuate with the menstrual cycle. Because both hormones are linked to chemicals in the brain that control mood, a happy, laughing fourteen-year-old can have an emotional meltdown in the time it takes her to close her bedroom door. With boys, testosterone finds particularly friendly receptors in the amygdala, the structure in the brain that controls the fight-or-flight response—that is, aggression or fear. Before leaving adolescence behind, a boy can have thirty times as much testosterone in his body as he had before puberty began.

Sex hormones are particularly active in the limbic system, which is the emotional center of the brain. That explains in part why adolescents not only are emotionally volatile but may even seek out emotionally charged experiences—everything from a book that makes her sob to a roller coaster that makes him scream. This double whammy—a jacked-up, stimulus-seeking brain not yet fully capable of making mature decisions—hits teens pretty hard, and the consequences to them, and their families, can sometimes be catastrophic.

While scientists have long known how hormones work, only in the past five years have they been able to figure out why they work the way they do. Because sex hormones are present at birth, they essentially hibernate for more than a decade. What, then, triggers them to begin puberty? A few years ago, researchers discovered that puberty is initiated by what appears to be a game of hormonal dominoes, which begins with a gene producing a single protein, named kisspeptin, in the hypothalamus, the part of the brain that regulates metabolism. When that protein connects with, or “kisses,” receptors on another gene, it eventually triggers the pituitary gland to release its storage of hormones. Those surges of testosterone, estrogen, and progesterone in turn activate the testes and ovaries.

After sex hormones were discovered, for the rest of the twentieth century they became the dominant theory of, and favorite explanation for, adolescent behavior. The problem with this theory is that teenagers don’t have higher hormone levels than young adults—they just react differently to hormones. For instance, adolescence is a time of increased response to stress, which may in part be why anxiety disorders, including panic disorder, typically arise during puberty. Teens simply don’t have the same tolerance for stress that we see in adults. Teens are much more likely to exhibit stress-induced illnesses and physical problems, such as colds, headaches, and upset stomachs. There is also an epidemic of symptoms ranging from nail biting to eating disorders that are commonplace in today’s teens. We have a tsunami of input coming at teens from home, school, peers, and, last but not least, the media and Internet that is unprecedented in the history of mankind. Why are adults less susceptible to the effect of all this stimulation? In 2007, researchers at the State University of New York (SUNY) Downstate Medical Center reported that the hormone tetrahydropregnanolone (THP), usually released in response to stress to modulate anxiety, has a reverse effect in adolescents, raising anxiety instead of tamping it down. In an adult, this stress hormone acts like a tranquilizer in the brain and produces a calming effect about a half hour after the anxiety-producing event. In adolescent mice, THP is ineffective in inhibiting anxiety. So anxiety begets anxiety even more so in teens. There is real biology behind that.

In order to truly understand why teenagers are moody, impulsive, and bored; why they act out, talk back, and don’t pay attention; why drugs and alcohol are so dangerous for them; and why they make poor decisions about drinking, driving, sex—you name it—we have to look at their brain circuits for answers. The elevated secretion of sex hormones is the biological marker of puberty, the physiological transformation of a child into a sexually mature human being, though not yet a true “adult.”

While hormones can explain some of what is going on, there is much more at play in the teenage brain, where new connections between brain areas are being built and many chemicals, especially neurotransmitters, the brain’s “messengers,” are in flux. This is why adolescence is a time of true wonder. Because of the flexibility and growth of the brain, adolescents have a window of opportunity with an increased capacity for remarkable accomplishments. But flexibility, growth, and exuberance are a double-edged sword because an “open” and excitable brain also can be adversely affected by stress, drugs, chemical substances, and any number of changes in the environment. And because of an adolescent’s often overactive brain, those influences can result in problems dramatically more serious than they are for adults.

2
Building a Brain

The human body is amazing, the way it neatly tucks all these complex organs into this finite space and connects them into one smoothly functioning system. Even the average human brain is said by many scientists to be the most complex object in the universe. A baby brain is not just a small adult brain, and brain growth, unlike the growth of most other organs in the body, is not simply a process of getting larger. The brain changes as it grows, going through special stages that take advantage of the childhood years and the protection of the family, then, toward the end of the teen years, the surge toward independence. Childhood and teen brains are “impressionable,” and for good reason, too. Just as baby chicks can imprint on the mother hen, human children and teens can “imprint” on experiences they have, and these can influence what they choose to do as adults.

Such was the case with me. I “imprinted” on neuroscience and medicine pretty early on. My experiences cultivated in me a curiosity that I found irresistible, sustaining me from my high school years through medical school and graduate research, and to this very day. I grew up the oldest of three children in a comfortable family home in Connecticut, just forty minutes from Manhattan. I happened to live in Greenwich, which even back then was the home of actors, authors, musicians, politicians, bankers, and the independently wealthy. The actress Glenn Close was born there, President George H. W. Bush grew up there, and the great bandleader Tommy Dorsey died there.

My parents were from England; they had immigrated after World War II, and my dad came over after medical school in London to do his urological surgery residency at Columbia. To them, Greenwich seemed a great place to settle within commuting distance of New York City. It was a matter of convenience, and they were pretty oblivious to the celebrity status of the town. Perhaps because of my father, I was open-minded about learning math and science. For me a major “imprinting” moment that propelled me in the direction of medicine was a ninth-grade biology class at Greenwich Academy. The best part to me, memorable in fact, was when we each got a fetal pig to dissect. While many of my classmates slumped in their seats at the proposition of slicing up these small mammals, some rushing to the girls’ washrooms with waves of nausea, a few of us jumped into the task at hand. It was one of those defining moments. The scientists had separated from those destined to be the writers, lawyers, and businesspeople of the future.

Injected with latex, the pigs’ veins and arteries visibly popped out with their colorful hues of blue and red. I’m a very visual person; I also like thinking in three dimensions. That visual-spatial ability comes in handy with neurology and neuroscience. The brain is a three-dimensional structure with connections between brain areas going in every direction. It helps to be able to mentally map these connections when one is trying to determine where a stroke or brain injury is located in a patient presenting with a combination of neurological problems—definitely a plus for a neurologist. Actually, that’s how the minds of most neurologists and neuroscientists work. We’re a breed that tends to love to look for patterns in things. I’ve never met a jigsaw puzzle, in fact, that I didn’t like. My attraction to neuroscience in high school and college began at a time before CT scans and MRIs, when a doctor had to imagine where the problem was inside the brain of a patient by picturing the organ three-dimensionally. I’m good at that. I like being a neurological detective, and as far as I’m concerned, neuroscience and neurology turned out to be the perfect profession for me to make use of those visuospatial skills.

If the human brain is very much a puzzle, then the teenage brain is a puzzle awaiting completion. Being able to see where those brain pieces fit is part of my job as a neurologist, and I decided to apply this to a better understanding of the teen brain. That’s also why I’m writing this book: to help you understand not only what the teen brain is but also what it is not, and what it is still in the process of becoming. Among all the organs of the human body, the brain is the most incomplete structure at birth, just about 40 percent the size it will be in adulthood. Size is not the only thing that changes; all the internal wiring changes during development. Brain growth, it turns out, takes a lot of time.

And yet the brain of an adolescent is nothing short of a paradox. It has an overabundance of gray matter (the neurons that form the basic building blocks of the brain) and an undersupply of white matter (the connective wiring that helps information flow efficiently from one part of the brain to the other)—which is why the teenage brain is almost like a brand-new Ferrari: it’s primed and pumped, but it hasn’t been road tested yet. In other words, it’s all revved up but doesn’t quite know where to go. This paradox has led to a kind of cultural mixed message. We assume when someone looks like an adult that he or she must be one mentally as well. Adolescent boys shave and teenage girls can get pregnant, and yet neurologically neither one has a brain ready for prime time: the adult world.

The brain was essentially built by nature from the ground up: from the cellar to the attic, from back to front. Remarkably, the brain also wires itself starting in the back with the structures that mediate our interaction with the environment and regulate our sensory processes—vision, hearing, balance, touch, and sense of space. These mediating brain structures include the cerebellum, which aids balance and coordination; the thalamus, which is the relay station for sensory signals; and the hypothalamus, a central command center for the maintenance of body functions, including hunger, thirst, sex, and aggression.

I have to admit that the brain is not very exciting to look at. Sitting atop the spinal cord, it is light gray in color (hence the term “gray matter”) and has a consistency somewhere between overcooked pasta and Jell-O. At three pounds, this wet, wrinkled tissue is about the size of two fists held next to each other and weighs no more than a large acorn squash. The “gray matter” houses most of the principal brain cells, called neurons: these are the cells responsible for thought, perception, motion, and control of bodily functions. These cells also need to connect to one another, as well as to the spinal cord, for the brain to control our bodies, behavior, thoughts, and emotions. Neurons send most of their connections to other neurons through the “white matter” in the brain. The commonly used brain imaging tool, magnetic resonance imaging, or MRI, shows the distinction between gray and white matter beautifully. On the outside surface, the brain has a rippled structure. The valleys or creases are referred to as sulci and the hills are referred to as gyri. Figure 1 shows an image from a brain’s MRI scan, like those done on patients. There are two sides to the brain, each called a hemisphere. (When an MRI image shows a cut across the middle in one direction or the other [slice angles A and B], it is easier to see the two sides.) The most superficial layer of the brain is called the cortex and it is made up of the gray matter closest to the surface, with the white matter located beneath it. The gray matter is where most of the brain cells (neurons) are located. The neurons connect directly to those close by, but in order to connect to neurons in other parts of the brain, in the other hemisphere, or in the spinal cord to activate muscles and nerves in our face or body, the neurons send processes down through the white matter. The white matter is called “white” because in real life and also in the MRI scans its color is light, owing to the fact that the neuron processes running through here are coated with a fatty insulator-like substance called myelin, which truly is white in color.

As I said before, sheer size—or even weight, for that matter—doesn’t mean everything. A whale brain weighs about twenty-two pounds; an elephant brain about eleven. If intellect were determined by the ratio of brain weight to body weight, we’d be losers. Dwarf monkeys have one gram of brain matter for every twenty-seven grams of body matter, and yet the ratio for humans is one gram of brain weight to forty-four grams of body weight. So we actually have less brain per gram of body weight than some of our primate cousins. It is the complexity of the way neurons are hooked up to one another that matters. Another example of how little the weight of the brain has to do with its functioning, at least in terms of intelligence, is that the human female brain is physically smaller in size than the male brain but IQ ranges are the same for the two sexes. At only 2.71 pounds, the brain of Albert Einstein, indisputably one of the greatest thinkers of the twentieth century, was slightly underweight. But recent studies also show that Einstein had more connections per gram of brain matter than the average person.

FIGURE 1. The Basics of Brain Structure: A magnetic resonance imaging (MRI) scan of a brain. The horizontal and vertical cross sections (slice angles A and B) show the cortex (gray matter) on the surface and the white matter underneath.

The size of the human brain does have a lot to do with the size of the human skull. Basically, the brain has to fit inside the skull. As a neurologist, you have to measure the size of children’s heads as they grow up. I have to admit there were occasions when I did this with my own sons—just like noting changes in their height—to make sure they were on track and in the normal range for skull size. When they were older, they thought I was nuts, of course, but when they were babies and toddlers, I just couldn’t resist coming at them with a tape measure I’d take from my sewing kit, then trying to get them to stop squiggling free so I could take just one more measurement. The truth is, skull size doesn’t tell us a lot. It’s a gross measurement, and the skull can be large or small for a variety of reasons. There are disorders in which the head is too big and disorders in which the head is too small. The most important characteristic of the skull is that it limits the size of the brain. Eight of the twenty-two bones in the human skull are cranial, and their chief job is to protect the brain. At birth, these cranial bones are only loosely held together with connective tissue so that the head can compress a bit as the baby moves through the birth canal. The skull bones are loosely attached and have spaces between them: one of these is the “soft spot” all babies have at birth, which closes during the first year of life as the bones fuse together. Most growth in head size occurs from birth to seven years, with the largest increase in cranial size occurring during the first year of life because of massive early brain development.

So with a fixed skull size, human evolution did its best to jam as much brain matter inside as possible. Homo erectus, from whom the modern human species evolved, appeared about two million years ago. Its brain size was only about 800 to 900 cubic centimeters, as opposed to the approximately 1,500 cubic centimeters of today’s Homo sapiens. With modern human brains nearly double the size of these ancestors’, the skull had to grow as well and, in turn, the female pelvis had to widen to accommodate the larger head. Evolution accomplished all of this within just two million years. Perhaps that’s why the brain’s design, while extraordinarily ingenious, also gives a bit of the impression that it was updated on the fly. How else to explain the cramped conditions? Like too many clothes crammed in too small a closet, the evolution-sculpted brain looks like a ribbon repeatedly folded and pressed together. These folds, with their ridges (gyri) and valleys (sulci), as seen in Figure 1, give the human brain an irregular surface appearance, the result of all that tight packing inside the skull. Not surprisingly, humans have the most complex brain folding structure of all species. As you move down the phylogenic scale to simpler mammals, the folds begin to disappear. Cats and dogs have some, but not nearly as many as humans do, and rats and mice have virtually none. The smoother the surface, the simpler the brain.

While the brain looks fairly symmetrical from the outside, inside there are important side-to-side differences. No one is really sure why, but the right side of your brain controls the left side of your body and vice versa; this means that the right cortex governs the movements of your left eye, left arm, and left leg and the left cortex governs the movements of your right eye, right arm, and right leg. For vision, the input from the left side of the visual field goes through the right thalamus to the right occipital cortex, and information from the right visual field goes to the left. In general, visual and spatial perception is thought to be more on the right side of the brain.

The image of the body, in fact, can actually be “mapped” onto the surface of the brain, and this map has been termed the “homunculus” (Latin for “little man”). In the motor and sensory cortex, the different areas of the body get more or less real estate depending upon their functional importance. The face, lips, tongue, and fingertips get the largest amount of space, as the sensation and control necessary for these areas have to be more accurate than for other areas such as the middle of the back.

An early-twentieth-century Canadian neuroscientist, Wilder Penfield, was the first to describe the cortical map, or homunculus, which he did after doing surgery to remove parts of the brain that caused epileptic seizures. He would stimulate areas of the surface to determine which parts would be safe to remove. Stimulating one area would cause a limb or facial part, for instance, to twitch, and having done this on many patients he was able to create a standard map.

FIGURE 2. The “Homunculus”: A “map” of the brain illustrating the regions that control the different body parts.

The amount of brain area devoted to a given body part varies depending on how complicated its function is. For instance, the area given to hands and fingers, lips and mouth, is about ten times larger than that for the whole surface of the back. (But then, what do you do with your back anyway—except bend it?) This way all the brain regions for the same part of the body end up in close proximity to one another.

My undergraduate thesis at Smith College in Northampton, Massachusetts, examined several of those areas of the brain given over to individual body parts and whether overstimulation of one of the body’s limbs might result in more brain area devoted to that part. This was actually an early experiment in brain plasticity, to see if the brain changed in response to outward stimulation. Many impressive studies that have been done since the late 1970s back up the whole concept of imprinting. Some of the most famous work, which inspired me to do my little undergraduate thesis, was done by a pair of Harvard scientists named David Hubel and Torsten Wiesel. The term that started to be used was “plasticity,” meaning that the brain could be changed by experience—it was moldable, like plastic. Hubel and Wiesel showed that if baby kittens were reared with a patch on one eye during the equivalent of their childhood years (they looked sort of like pirate kittens!), for the rest of their lives they were unable to see out of the eye that had been patched. The scientists also saw that the brain area devoted to the patched eye had been partially taken over by the open eye’s connections. They did another set of experiments where the kittens were raised in visual environments with vertical lines and found that their brains would respond only to vertical lines when they were adults. The point is that the types of cues and stimuli that are present during brain development really change the way the brain works later in life. So my experiment in college showed basically the same thing, not for vision, but rather for touch.

I actually had some fun showing off this cool imprinting effect in our everyday lives. Our beloved cat had died at the ripe old age of nineteen, and we all missed her so. Of course, it didn’t take long before Andrew, Will, and I were at the local animal shelter, looking at kittens to bring home. We fell in love with the runt of a litter and brought home the most petite and needy little tabby kitten you could ever imagine. The boys came up with a name: Jill. Jill was always on our laps; she was a very people-friendly cat. I remembered the experiments on brain plasticity and said to Andrew and Will, when we hold her, let’s massage her paws and see if she becomes a more coordinated cat. So anytime we had her in our laps, we would massage her paws with our hands, spreading them out, touching the little “fingers” that cats’ paws contain. Sure enough, Jill started to use her paws much more than any other cat we had ever had (and I have had back-to-back cats since the age of eight). She used her paws for things most cats don’t. She was very “paw-centric,” going around the house batting small objects off tables and taking obvious pleasure in watching them hit the ground. This was a source of consternation as not all the things she knocked off were unbreakable. She also often used her left paw to eat, gingerly reaching into the cat food can with her paw and scooping up food to bring to her mouth. Watching her, we started to notice that she almost always used her left paw to do these things. We had a left-pawed cat! Then suddenly we realized that when we picked her up to massage her paws, she was facing us and because we are all right-handed, we were always stimulating her left paw much more than her right! Home neuronal plasticity demonstration project accomplished. I know if we could have looked into her brain, we’d have seen that she had more brain space given over to her paws, and especially her left paw, than the average cat. This same phenomenon of reallocating brain space based on experience during life happens in people, too. We call this part of life the critical period, when “nurture,” that is, the environment, can modify “nature.” But more on that later.

So what I have just told you is that brain areas for vision and body parts are compartmentalized in different places, but that they can shrink or grow relative to one another during development based on how much the senses are used. Structurally, the human brain is divided into four lobes: frontal (top front), parietal (top back), temporal (sides), and occipital (back). The brain sits on the brainstem, which connects to the spinal cord. In the rear of the brain, the cerebellum regulates motor patterning and coordination, and the occipital lobes house the visual cortex. The parietal lobes house association areas as well as the motor and sensory cortices (which include the homunculus in Figure 2). The temporal lobes include areas involved in the regulation of emotions and sexuality. Language is also located here, more specifically in the dominant hemisphere (the left temporal lobe for right-handed people and 85 percent of left-handed people, and the right temporal lobe for that small group of truly strong lefties). The frontal lobes sit most anteriorly and this area is concerned with executive function, judgment, insight, and impulse control. Importantly, as the brain matures from back to front in the teen years the frontal lobes are the least mature and the least connected compared with the other lobes.

FIGURE 3. The Lobes of the Brain: A. The brain matures from the back to the front. B. The cortex of the brain can be divided into several main areas based on function.

The brain is divided into specialized regions for each of the senses. The area for hearing, or the auditory cortex, is in the temporal lobes; the visual cortex is in the occipital lobes; and the parietal lobes house movement and feeling in the motor and sensory cortices, respectively. Other parts of the brain have nothing to do with the senses, and the best example of this is the frontal lobes, which make up more than 40 percent of the human brain’s total volume—more than in any other animal species. The frontal lobes are the seat of our ability to generate insight, judgment, abstraction, and planning. They are the source of self-awareness and our ability to assess dangers and risk, so we use this area of the brain to choose a course of action wisely.

Hence, the frontal lobes are often said to house the “executive” function of the human brain. A chimpanzee’s frontal lobes come closest to the human’s in terms of size, but still make up only around 17 percent of its total brain volume. A dog’s frontal lobes make up just 7 percent of its brain. For other species, different brain structures are more important. Compared with humans, monkeys and chimpanzees have a much larger cerebellum, where control of physical coordination is honed. A dolphin’s auditory cortex is more advanced than a human’s, with a hearing range at least seven times that of a young adult. A dog has a billion olfactory cells in its brain compared with our measly twelve million. And the shark has special cells in its brain that help it detect electrical fields—not to navigate but to pick up electrical signals given off by the scantest of muscle movements in other fish as they try to hide from this deadly predator.

We humans don’t have a lot else going for us other than our wile and wit. Our competitive edge is our ingenuity, brains over brawn. This edge happens to take the longest time to develop, as the connectivity to and from the frontal lobes is the most complex and is the last to fully mature. This “executive function” thus develops slowly: we certainly are not born with it!

So in what order are these brain regions all connected to one another during childhood and adolescence? This could never have been learned before the advent of modern brain imaging. New forms of brain scans, called magnetic resonance imaging (MRI), not only can give us accurate pictures of the brain inside the skull but also can show us connections between different regions. Even better, a new kind of MRI, called the functional MRI, abbreviated fMRI, can actually show us what brain areas turn one another on. So we can actually see if areas that “fire” together are “wired” together. In the last decade, the National Institutes of Health conducted a major study to examine how brain regions activate one another over the first twenty-one years of life.

What they found was remarkable: the connectivity of the brain slowly moves from the back of the brain to the front. The very last places to “connect” are the frontal lobes (Figure 4). In fact, the teen brain is only about 80 percent of the way to maturity. That 20 percent gap, where the wiring is thinnest, is crucial and goes a long way toward explaining why teenagers behave in such puzzling ways—their mood swings, irritability, impulsiveness, and explosiveness; their inability to focus, to follow through, and to connect with adults; and their temptations to use drugs and alcohol and to engage in other risky behavior. When we think of ourselves as civilized, intelligent adults, we really have the frontal and prefrontal parts of the cortex to thank.

Because teens are not quite firing on all cylinders when it comes to the frontal lobes, we shouldn’t be surprised by the daily stories we hear and read about tragic mistakes and accidents involving adolescents. The process is not really done by the end of the teen years—and as a result the college years are still a vulnerable period. Recently a friend of mine told me about his son’s college classmate, Dan, an all-around great kid who’d rarely caused his parents to worry. He was popular, had been a star ice hockey player in high school, and was a finance major in college. Over the summer my friend’s son got a phone call from Dan’s mother. Dan had drowned the night before, she told him. He’d been out with friends, drinking, and sometime between three and four in the morning, on their way home, the group—there were eight of them—decided they wanted to cool off, so they stopped at the local tennis club. The club was closed, of course, but the locked gate didn’t stop them. All eight scaled the fence and jumped into the pool. It was only after they’d gotten home that someone said, “Where’s Dan?” Racing back to the club, they found their friend facedown in the water. The medical examiner listed the cause of death as accidental drowning due to “acute alcohol intoxication.” One of the news reports I read made me shake my head: “Police are asking kids and adults to think twice about potential dangers before taking any risks that could turn deadly.”

FIGURE 4. Maturing Brain: The Brain “Connects” from Back to Front: A. A functional MRI (fMRI) scan can map connectivity in the brain. Darker areas indicate greater connectivity. B. Myelination of white matter tracks cortex maturation from back to front; this is why the frontal lobes are the last to be connected. C. Serial connectivity scans reveal that frontal lobe connectivity is delayed until age twenty or older.

“Think twice.”

How many times have we all said this to our teenage sons and daughters? Too many times. Still, as soon as I heard about Dan, I called my boys to tell them the story. You have to remember this, I told them. This is what happens. Drinking and swimming don’t go together. Neither does the decision to suddenly scale a fence in the middle of the night, or jump into a pool with seven friends who are also intoxicated.

How parents deal with these tragic stories and talk about them with their own kids is critical. It shouldn’t be, “Oh, wow, I’m so glad that wasn’t my child.” Or, “My teenager would never have done that.” Because you don’t know. Instead, you have to be proactive. You have to stuff their minds with real stories, real consequences, and then you have to do it again—over dinner, after soccer practice, before music lessons, and, yes, even when they complain they’ve heard it all before. You have to remind them: These things can happen anytime, and there are many different situations that can get them into trouble and that can end badly.

One of the reasons that repetition is so important lies in your teenager’s brain development. One of the frontal lobes’ executive functions includes something called prospective memory, which is the ability to hold in your mind the intention to perform a certain action at a future time—for instance, remembering to return a phone call when you get home from work. Researchers have found not only that prospective memory is very much associated with the frontal lobes but also that it continues to develop and become more efficient specifically between the ages of six and ten, and then again in the twenties. Between the ages of ten and fourteen, however, studies reveal no significant improvement. It’s as if that part of the brain—the ability to remember to do something—is simply not keeping up with the rest of a teenager’s growth and development.

The parietal lobes, located just behind the frontal lobes, contain association areas and are crucial to being able to switch between tasks, something that also matures late in the adolescent brain. Switching between tasks is nearly a constant need in today’s world of information overload, especially when you consider the fact that multitasking—doing two cognitively complex things at the same time—is actually a myth. Chewing gum and doing virtually anything else is not multitasking because chewing gum involves no real cognitive focus. Both talking on a cell phone and driving, however, do involve cognitive focus. Because there are limits to how many things the human brain can focus on at any one time, when someone is engaged in multiple cognitively significant activities, like talking and driving, the brain must constantly switch back and forth between the two tasks. And when it does, neither of those tasks is being accomplished particularly well.

The parietal lobes help the frontal lobes to focus, but there are limits. The human brain is so good at this juggling that it seems as though we are doing two tasks at the same time, but really we’re not. Scientists at the Swedish medical university Karolinska Institutet measured those limits in 2009 when they used fMRI images of people multitasking to model what happens in the brain when we try to do more than one thing at a time. They found that a person’s working memory is capable of retaining only between two and seven different images at any one time; this means that focusing on more than one complex task is virtually impossible. Focusing chiefly happens in the parietal lobes, which dampen extraneous activity to allow the brain to concentrate on one thing and then another.

The problem of having immature parietal lobes was illustrated in a segment on Good Morning America in May 2008 by the ABC TV correspondent David Kerley and his teenage daughter Devan. Using a course set up by Allstate Insurance, and with her father in the passenger seat, Devan, who had been driving for a year, was instructed about speed, braking, and turning and allowed to take a practice run through the course. Then she was given a series of three “distractions” to handle while navigating the course’s twists and turns. First, she was handed a BlackBerry and told to read the text on the screen while driving. She hit several cones. Next, three of her friends were put in the backseat and a lively conversation ensued. Devan hit more cones. Finally, Devan was handed a package of cookies and a bottle of water, and just passing the cookies around and holding the bottle of water caused her to run over several more cones. Multitasking is not only a myth but a dangerous one, especially when it comes to the teenage brain.

“Multitasking” has become a household word. The research in Sweden suggests that there are limits. Teenagers and young adults pride themselves on their ability to multitask. Have today’s teens and young adults imprinted on a multitasking world? Maybe. In studying how young adults these days handle distractions, researchers at the University of Minnesota have shown that the ability to successfully switch attention among multiple tasks is still developing through the teenage years. So it may not come as a surprise to learn that of the nearly six thousand adolescents who die every year in automobile accidents, 87 percent die because of distracted driving.

The question of whether today’s teens and young adults have a special skill set for learning while distracted was more formally tested in 2006 by researchers at the University of Missouri. They took twenty-eight undergraduates, including kids in their late teens, and asked them to memorize lists of words and then recall these words at a later time. To test whether distraction affected their ability to memorize, the researchers asked the students to perform a concurrent task—placing a series of letters in order based on their color by pressing the keys on a computer keyboard. This task was given under two conditions: when the students were memorizing the lists of words and when the students were recalling those lists for the researchers. The Missouri scientists discovered that simultaneous tasks affected both encoding (memorizing) and retrieving (recalling). When the keyboard task was given while the students were trying to recall the previously memorized words (which is akin to taking a test or exam), there was a 9 to 26 percent decline in their ability to memorize the words. The decline was even more if the concurrent distracting task occurred while they were memorizing, in which case their performance decreased by a whopping 46 to 59 percent.

These results certainly have implications for the teen bedroom during a homework night! I not-so-fondly remember walking in on my sons during evening homework time to find them with the television on, headphones attached to iPods, all the while messaging someone on the lower corner of their computer screens and texting someone else on their iPhones. It wasn’t a problem, they protested, when I suggested they concentrate on their homework, assuring me their course reviews for the next day’s exams were totally unaffected by the thirty-two other things they were doing at the same time. I didn’t buy it. So I buttressed my argument with the Missouri data. I put Figure 5 in this book in case you want to use it to make the same point to your teen.

FIGURE 5. Multitasking Is Still Not Perfect in the Teen Brain: College students were tested under three conditions: No Distraction (full attention), Distracted Attention (DA) when memorizing (DA at encoding), and Distracted Attention when recalling (DA at retrieval). Students performed poorly when multitasking during recall, and even worse when they multitasked while memorizing.

Attention is only one way we can assess how the brain is working. There’s a lot more under the hood of the brain than just the four lobes, so returning to Figure 3 let’s start at the back, where we find the brainstem at the very bottom of the brain, attached to the spinal cord. The brainstem controls many of our most critical biological functions, like breathing, heart rate, blood pressure, and bladder and bowel movements. The brainstem is on “automatic”—you are not even aware of what it does, and you normally don’t voluntarily control what it does. The brainstem and spinal cord are connected to the higher parts of the brain through way station areas, like the thalamus, which sits right under the cortex. Information from all the senses flows through the thalamus to the cortex. Right below the cortex are structures called the basal ganglia, which play a big role in making coordinated and patterned movements. The basal ganglia are directly affected by Parkinson’s disease and account for the trembling and the appearance of being frozen, or unable to move, which are the hallmark symptoms of Parkinson’s patients.

As we move closer to the cortex, we encounter structures that together make up what is called the limbic system. The limbic system gets involved in memories and also emotions. A part of the brain we will talk about a lot in this book is the hippocampus. The hippocampus is a little seahorse-shaped structure underneath the temporal lobe. In fact the name “hippocampus” comes from the Latin word for “horse” because of the shape. The hippocampus is truly the brain’s “workhorse” for memory processing—it is used for encoding and retrieving memories.

So what do we know about our memory workhorse? It has the highest density of excitatory synapses in the brain. It is a virtual beehive of activity, and turns on with every experience. As we will explain later, the hippocampus in the adolescent brain is relatively “supercharged” compared with an adult’s.

The connection of the hippocampus to memory was recognized some six decades ago through the unforeseen consequences of one patient’s radical brain surgery. This surgery was performed in 1953 on a twenty-seven-year-old Connecticut man who, until his death several years ago, was known only by his initials, H.M. He underwent an experimental operation in an attempt to cure him of frequent and severe epileptic seizures. So incapacitating was H.M.’s epilepsy that he was unable to hold down even a factory job. When the Yale neurosurgeon William Beecher Scoville removed most of H.M.’s medial temporal lobe, which was causing his seizures, the operation appeared to be a success. By cutting away brain tissue in the area of the seizures, Scoville dramatically reduced their frequency and severity. In the process, though, he also removed a large portion of H.M.’s hippocampus. (That the hippocampus is critical for memory formation was unknown at the time; the case of H.M. shed much light on the subject.) What became clear when H.M. awoke was that while his seizures were by and large gone, so, too, was his ability to turn short-term memories into long-term memories. Essentially, H.M. could remember his past—everything before the time of the operation—but for the rest of his life he had no short-term memory and could not remember what happened to him, what he said or did or thought or felt or whom he met, in the decades following the surgery. H.M.’s loss, as often happens in the history of science, was neuroscience’s gain. For the first time researchers could point to a specific brain region (the temporal lobe) and brain structure (the hippocampus) as the seat of human memory.

Next door to the hippocampus, in another part of the limbic system under the temporal lobe, is another key brain structure, the amygdala, which is involved in sexual and emotional behavior. It is very susceptible to hormones, such as sex hormones and adrenaline. It is sort of the seat of anger, and when stimulated in animal experiments, it has been shown to produce rage-like behavior. The limbic system can be thought of as a kind of crossroads of the brain, where emotions and experiences are integrated.

A slightly unbridled and overexuberant immature amygdala is thought to contribute to adolescent explosiveness; this explains in part the hysteria that greets parents when they say no to whatever it is their adolescent thinks is a perfectly reasonable request. Cross that immature amygdala with a teen’s loosely connected frontal lobe, and you have a recipe for potential disaster. For example, the sixteen-year-old patient of a colleague of mine was so incensed when his parents said driving was a “privilege” (for which he did not yet qualify), and not a “right,” that he stole the car keys and drove away from the house. He didn’t get very far, though. He forgot the garage door was closed and plowed right through it. One of my colleagues also told me that, because he himself had three grown daughters, rather than sons, he had few “terrible teen tales” to tell. Then he reconsidered: “Oh, yes, there was the weekend we were away and the ‘couple of friends’ became a party that got out of hand, including the raid on our wine cellar, a minor fender bender with our stolen liquor in the trunk, and maybe a navel ring (which I never knew about until years later after it disappeared). But all’s well that ends well.”

3
Under the Microscope

If you pick out any random region of brain and look at it under a microscope, you’ll find it jam-packed with cells. In fact, there is almost no space between the billions of cells in the brain. Evolution made sure of that, putting to use every cubic micron wisely. A cell is the body’s smallest unitary building block, and each has its own command center, called a nucleus, a large oval body near the center of the cell. There are more than two hundred different types of cells making up every organ, tissue, muscle, etc. A unique cell type in the brain is the neuron. This is a cell we will talk about frequently in this book. Thoughts, feelings, movements, and moods are nothing more than neurons communicating by sending electrical messages to one another.

I remember my first time looking at brain cells under a microscope. In the mid- to late 1970s the only way to study changes in neurons, for instance the changes that occur during learning, was by looking through a microscope at individual cells over a given period of time. Today, we have amazing tools—brain imaging scans and specialized microscopes—that allow us to look into the brain and see cells and synapses change in real time. If you are learning something right now, as you read this, your neurons will change in about fifteen minutes, creating more synapses and receptors. Changes start within milliseconds of learning something new, and can take place over a period of minutes and hours. When I look at brain cells under a microscope, I think of the billions of neurons that are interconnected and how we’re still trying to figure out the wiring. What we know now is that no two human brains are wired exactly the same, and experience shapes us all differently. It’s the final frontier, our own internal frontier, and we’re just now beginning to see all the patterns.

There are 100 billion neurons in the human brain and you could place about 30,000 of them on the head of a pin, but placed end to end the neurons in just one person’s cortex would stretch for 100,000 miles—enough to circle the globe four times. At birth, we have more neurons than at any other time in our life. In fact, our brains are at their densest before birth, between the third and sixth months of gestation. Dramatic pruning of much of that gray matter occurs in the last trimester and first year of life. Still, by the time a baby is born, he or she has a brain brimming with neurons. Why? An infant’s overabundance of neural cells is needed to respond to the barrage of stimuli that comes with entering into the world. In response to all those new sights, sounds, smells, and sensations, neurons branch out in the baby’s brain, creating a thick forest of neural connections. So why aren’t all babies tiny Mozarts and Einsteins? Because when we are born, only a very small percentage of that overflow of neurons is wired together. The information is going in, being absorbed by the neurons, but it doesn’t know where to go next. Like someone plunked down in the middle of a strange and bustling metropolis, the infant brain is surrounded with possibilities and yet has no map, no compass, to navigate this strange new world. “All infants are born in a state of psychedelic splendor similar to an acid trip” is how Daniel Levitin, a neuroscientist at McGill University in Montreal, Canada, colorfully describes it.

A neuron responds to a stimulus with a burst of activity, called an action potential, which is actually an electrical signal that passes, in relay fashion, from the point of contact with the stimulus down the receiving limb of the neuron, called the dendrite, through the cell.

When we see the color red, smell a rose, move a muscle, or remember someone’s name, action potentials are happening.

FIGURE 6. Anatomy of Neuron, Axon, Neurotransmitter, Synapse, Dendrite, and Myelin: Signals between cells flow in one direction, from an axon to a dendritic spine, through a synapse. Axons with myelin coating transmit signals faster than those without. At the synapse, a transfer of neurotransmitter molecules binds to the synaptic receptor on the spine.

If you think of each neuron’s cell body as a point in a relay, there must be an incoming and an outgoing signal. Once the outgoing signal reaches the axonal bouton, or end point, it sets off a reaction causing the bouton to release packages of chemical messengers, called neurotransmitters. The point of contact between two neurons is called a synapse, and is actually a space no more than two-millionths of an inch wide. The synapse is truly where the action takes place in the brain. The signal heads down the neuron through the axon to the synapse and is then released as a chemical message. Like liquid keys, these neurotransmitters cross the synapse and lock onto the neuron on the other side, and in this way carry information from one cell to another. Once opened, the receptor causes a chain reaction of signals going down the receiving cell, triggering a pulse, or an action potential, which travels from a dendrite and through the cell body and out the axon toward another cell.

In order for neurons to survive, they need helper cells called glia. There are several types of glia: astrocytes, microglia, and oligodendrocytes. To put it simply, the astrocytes defend the neuron by helping to nourish it and cleaning up the unwanted chemicals around it. This helps keep the brain’s neurons at optimal functioning level. The microglia are tiny cells that move around the neuron and really activate when there is an infection or inflammation—they move through brain tissue to the site of action to fight these injuries, like an army-in-waiting. But because the brain is efficiently designed, microglia also have an everyday purpose, a kind of housekeeping duty, so that even when they are not activated, they are still helping maintain the health and well-being of the synapses. Oligodendrocytes are the cells that make the myelin that goes around the axon of neurons. These cells are tightly packed in the white matter, wrapping whitish-colored myelin around axons to insulate them, much like rubber around an electrical cord, allowing faster speeds of signal transmission down the axon.

While you are born with the vast majority of your neurons, most of the synapses in the cortex are not fully formed. In lower areas, like the brainstem, synapses are indeed almost fully mature. In the cortex, however, synapses are produced after birth in a burst of activity, which I mentioned earlier, known as the critical period. During this stage of development, a baby’s brain creates an astonishing two million synapses every second, allowing the infant to reach mental milestones like color vision, grasping, facial recognition, and parental attachment. It’s as if an infant’s brain is sending out billions of antennae, scanning the world for information. For each synapse to survive, it must find another neuron to send information to; this is why the number of synapses in a baby’s brain peaks in childhood. The gray matter—the brain tissue responsible for processing information—continues to thicken throughout childhood as the brain’s cells form extra connections, those limb-like dendrites. Known as arborization, this thickening is like a tree growing extra branches and roots. Stimulation, experiences, repeated sensations—all contribute to the creation of these new neural pathways. In adolescence, this “overgrowth” is responsible for a teen’s heightened capacity to learn new things quickly—everything from operating the new TV remote to speaking Mandarin Chinese. The profusion of gray matter, though, can also cause a kind of cognitive dissonance in which the brain has trouble picking out the right signals from all the “noise.” As a result, by late adolescence the brain has begun to prune away excess synapses and streamline connections.

Synapses come in two flavors: ones that excite, or turn on, the next neuron, and ones that inhibit, or turn off, the next neuron.

FIGURE 7A. Inhibitory Cells Can Stop Signaling: Inhibitory cells release inhibitory neurotransmitters onto spines, which will stop a signal in a neuron and turn the cell “off.”

FIGURE 7B. Excitatory and Inhibitory Synapses: Excitatory axons release excitatory neurotransmitters, such as glutamate, which bind to excitatory receptors and turn the neuron “on.” Inhibitory axons release inhibitory neurotransmitters, like GABA, which bind to inhibitory receptors and turn the neuron “off.”

Whether or not the synapse is excitatory or inhibitory depends upon the type of neurotransmitter the axon puts out and also on the custom-made receptor, or lock, which is the part of the synapse poised to “receive” the neurotransmitter. If you imagine the neurotransmitter as a simple geometric shape, say a square or a circle, the specific receptor for that “flavor” of neurotransmitter will have the complementary shape in order to make a perfect fit. Just as “you can’t put a square peg in a round hole,” these neurotransmitter “keys” will fit into only the perfect receptor “locks.” This helps the synapse not confuse messages. In addition to the near-perfect pairing of neurotransmitters to receptors, another way the signal is kept clean is that the astrocyte helper cells immediately clean up any leftover neurotransmitter hanging around after it gets released. This happens in milliseconds, as the timing of these signals between brain cells has to be rapid, sharp as a burst.

Once the neurotransmitter has bound and locked itself into the receptor on the receiving neuron, this pairing sets off a chain reaction. Inside the dendritic side of the synapse, there are lots of proteins that rush to work when the synapse gets excited or inhibited. The signal needs to get down the dendrite to the cell body of the neuron, where it sends a positive charge for an excitatory signal or a negative charge for an inhibitory signal. Depending on which charge is sent, the receiving neuron will get a message to either stop or start functioning. If the message is positive, the receiving neuron will send the information down its own axon and across another synaptic cleft, and so on. A neuron can have up to ten thousand synapses and can send a thousand electrical impulses every second. In one-tenth of the time it takes to blink your eyes, a single neuron can simultaneously send a signal to hundreds of thousands of other neurons.

Some of the most common excitatory neurotransmitters are epinephrine, norepinephrine, and glutamate. Inhibitory neurotransmitters, like gamma-aminobutyric acid (GABA) and serotonin, act as antianxiety nutrients, calming the body and telling it to slow down. A lack of serotonin can result in aggression and depression. Dopamine is a special neurotransmitter because it is both excitatory and inhibitory. It is also, along with epinephrine and several others, a hormone. When it acts on the adrenal glands, it is acting hormonally; when it acts in the brain, it is a neurotransmitter. As a brain chemical messenger, dopamine helps motivate, drive, and focus the brain because it is integral to the brain’s reward circuitry. It’s the “I gotta have it” neurochemical that not only reinforces goal-directed activity but also can, in certain circumstances, lead to addiction. The more dopamine that is released in the brain, the more the reward circuits are activated, and the more those circuits are activated, the bigger the craving. It doesn’t matter if the craving is at the dinner table or the card table, in the boardroom or the bedroom. For instance, scientists know that high-calorie foods produce more dopamine in the brain. Why? Because higher calories increase our chance for survival. When we crave ice cream or gambling or sex, we may not actually be craving sweets, money, or orgasms. We’re craving dopamine.

Inhibiting a neural response is just as important as activating one when it comes to “executive” brain function. Examples of things that bind to inhibitory synapses are sedatives such as barbiturates, alcohol, and antihistamines. Synapses will be critical in our discussion of the adolescent brain because both the number and the type of synapses in our brains change as we age. They also change in relation to the amount of stimulation our brains experience. One topic that will come up later is the effect of illegal and illicit drugs and alcohol on these synapses, which we will cover in the chapter on addiction.

A popular instrument used by researchers to test inhibition is the Go/No-Go task in which subjects are told to press a button (the “Go” response) when a certain letter or picture appears, and not to press it (the “No-Go” response) when the letter X appears. Several studies have shown that children and adolescents generally have the same accuracy, but the reaction times, the speed at which a subject successfully inhibits a response, dramatically decrease with age in subjects age eight to twenty. In other words, it takes longer for adolescents to figure out when not to do something.

Signals move from one area of the brain to another along fiber tracks, and some of these tracks travel down through the core regions of the brain in order to send signals to and from the spinal cord. Brains are intricately interconnected by these fibers, and research using special brain scans is rapidly evolving to look at these connections. Because axons are designed to have a rapid pulse of electricity run through them to the connection point at the synapse, they act like electrical wires conducting an electrical signal. And just as an electrical wire needs insulation in order for the electricity not to dissipate along its length, so do the axons. Since we don’t have rubber in our brains, our axons are coated with a fatty substance called myelin. (See Figure 6.) The brain requires myelin in order to function normally, to get a signal from one region of the brain to another and also down to the spinal cord. As we said before, myelin is made by oligodendrocytes, and has a white hue due to its fatty content: hence the term “white matter.” By essentially “greasing” the “wires,” myelin allows signals to travel down axons faster, increasing the speed of a neural transmission as much as a hundredfold. Myelin also aids the speed of transmission by helping to cut down the synapses’ recovery time between neural firings, thereby allowing a thirtyfold increase in the frequency with which neurons transmit information. The combination of increased speed and decreased recovery time has been estimated by researchers as roughly equivalent to a three-thousand-fold increase in computer bandwidth. (Myelin also is the target of attack in the disease multiple sclerosis, or MS. Patients with MS have areas of inflammation in their white matter that come and go, and this is why they can lose functions like walking, sometimes only temporarily until the inflammation passes.)

At birth, a baby’s cortex contains little myelin; this explains why the electrical transmissions are so sluggish and an infant’s reaction times so slow. However, the baby’s brainstem is almost as fully myelinated as an adult’s, so it can control automatic functions like breathing, heartbeat, and gastrointestinal function necessary to stay alive. Connections to and from many other areas of the brain occur after birth, beginning with the motor and sensory areas at the bottom and back of the brain. As these areas become wired with myelin, infants are better able to process basic information from their senses—their eyes, ears, mouth, skin, and nose. Within the first year, the neural tracts that support brain regions involved in vision and other primary senses, as well as those involved in gross motor activity, are completed. This is, in part, why it takes about a year for a baby to become coordinated enough to walk. Much of the brain becomes insulated by age two, and high-level areas involved in language and fine motor coordination follow over the next few years when children are particularly primed to learn to talk and improve their fine motor skills. The more complex areas of the brain, especially the frontal lobes, take much, much longer and are not finished until a person is well into his or her twenties.

All of this learning is dependent on excitation, the driving force in our brains. Excitatory signals between neurons build brain connections and are required for brain development. Excitation can come from outside or inside your brain, but regardless, if a particular pathway of cells and their synapses are activated repeatedly, the synapses between them strengthen. Thus, cells that “fire” together “wire” together.

In the developing brain, especially in early childhood, as groups and pathways of neurons and their synapses get activated, the process of excitation “turns on” the molecular machinery in the cell. This actually results in the building of more synapses, a process we term synaptogenesis (birth of synapses). Synapses are increased in infancy through adolescence, peaking in early childhood. Because synaptogenesis is so dependent upon brain cells being activated by one another, a child’s brain has more excitatory than inhibitory neurotransmitters and synapses compared with an adult’s brain, where there is more balance between the two.

Excitation is a key element of learning. The period in early life in which excitation is so prominent is also called the “critical period,” when learning and memory are more robust than in later life. This allows the brain to be very sensitive to excitation and grow. Unfortunately, the abundant excitation in the developing brain carries a price: the risk for overexcitation. This explains why diseases that are a result of overexcitation, like epilepsy, are more common in childhood than adulthood. Seizures are the main symptom in epilepsy, and they are caused by too many brain cells turning on together without enough inhibition to balance them.

FIGURE 8. The Young Brain Has More Excitatory Synapses Than Inhibitory Synapses: The number of synapses increases from infancy through adolescence, peaking in early childhood.

Arborization, or the branching out of neurons, peaks in the first few years of life but continues, as we’ve seen, into adolescence. Gray matter density peaks in girls at age eleven and in boys at age fourteen, and waxes and wanes throughout adolescence.

White matter, or myelin, however, has only one trajectory in adolescence: up. Jay Giedd and colleagues at the National Institute of Mental Health scanned the brains of nearly one thousand healthy children, ages three to eighteen, and discovered this pattern of wiring. As we saw in Figure 4, researchers at the University of California, Los Angeles, built on those findings and compared the scans of young adults, ages twenty-three to thirty, with those of teenagers, ages twelve to sixteen. They found that myelin continues to be produced well past adolescence and even into a person’s thirties, making the communication between brain areas ever more efficient.

FIGURE 9. Gender Differences in Rate of Cortical Gray Matter Growth: Like the body, the male brain is on average larger than the female brain. Rates of growth in male and female brains also are different. In females, the growth rate of two areas important for cognitive maturity—the frontal lobes and the parietal lobes—peaks in the early teen years, but in males the peak does not occur until the late teens.

Without those insulated connections, a signal from one area of the brain, say fear and stress coming from the amygdala, has trouble linking up with another part of the brain, for instance the frontal cortex’s sense of judgment. For adolescents whose brains are still being wired, this means they sometimes find themselves in dangerous situations, not knowing what they should do next. This was confirmed scientifically in a 2010 study conducted by the British Red Cross into how teenagers react to emergencies involving a friend drinking too much alcohol. More than 10 percent of all children and young teens between the ages of eleven and sixteen have had to cope at one time or another with a friend who was sick, injured, or unconscious owing to excessive alcohol consumption. Half of those had to deal with a friend who passed out. More broadly, the survey found that nine out of ten adolescents have had to deal with some kind of crisis involving another person during their teenage years—a head injury, choking, an asthma attack, an epileptic seizure, etc. Forty-four percent of the teens surveyed admitted to panicking in that emergency situation, and nearly half (46 percent) acknowledged they didn’t know how to respond to the crisis at all.

Dan Gordon, a fifteen-year-old boy from Hampshire, England, who was interviewed by the Guardian for a story about the study, spoke about a house party he attended at which there was widespread underage drinking. After one girl passed out on the floor, facedown, she began to vomit, and the others in the room, all teenagers, panicked. Thinking only that they needed to prevent her from choking, they stood her up and, with effort, walked her outside for fresh air and waited for her to wake up. Dan admitted to the reporter that neither he nor anyone else at the party had thought to call for an ambulance. In other words, the teenagers’ amygdalae had signaled danger, but their frontal lobes didn’t respond. Instead, the teens acted in the moment.

My son Andrew witnessed something similar during college. He was visiting his then-girlfriend at a college in Boston. The girlfriend’s roommate also had an out-of-town visitor, a shy freshman girl from the South who quickly became intoxicated at a party in another student’s room. When Andrew and his girlfriend returned to her dorm, they found the young girl passed out, and just as in Dan Gordon’s story, they all panicked. Instead of calling 911 or campus security, or driving her to an emergency room, they found a couple of friends to help, and then drove all the way out to our house, about ten miles away.

“We didn’t want to call campus security,” Andrew’s girlfriend explained, as I observed the young girl, whom they had helped into the house and who was now almost unresponsive. “She’s a freshman. If we brought her to the health center, me and my roommate could get in trouble.”

Andrew and his former girlfriend were both twenty-one at the time, but the visiting student was just eighteen.

“What about taking her to the hospital?” I said.

“We didn’t know how drunk she was,” the other friend said. “She was talking when we put her in the car, and now she’s completely out of it.”

None of them in fact knew the girl—they had met her briefly for the first time earlier that day, when she had arrived to visit the roommate. She had her wallet and an ID from her South Carolina college with her, but no other information. The roommate who had invited her to Boston was nowhere to be found. Already drowsy, she was rapidly becoming more sedated, and then she vomited on the floor. At that point, I insisted they get her to a local community hospital just a mile from our house. It took three of them to half-carry her back to the car. About fifteen minutes later, I got a call from Andrew’s girlfriend, who said the hospital was going to admit the girl for observation. The poor thing spent an unhappy night in the hospital, and the college crew picked her up the next afternoon. On their way back to Boston, they stopped by my house to gather things they had left there the night before. The young freshman looked pale and very tired, but otherwise was fine. Apparently her blood alcohol level had peaked at 0.34, which was more than four times the legal driving limit, and life threatening. Had she not been taken to the hospital, where her stomach was pumped and charcoal administered to prevent her body from absorbing any more alcohol, I shudder to think of what might have happened. As I had a captive audience, I sat them all down in the kitchen, turned on my laptop, and showed them a chart about blood alcohol levels and the effects on coordination and consciousness. I pointed out that 0.4, which was only a little more than her blood alcohol reached at its height, can be lethal. Turns out she had done about seventeen Jell-O shots that evening—to the best of her memory. There was no point in asking the usual question—“What were you thinking?”—but I felt this was a good teaching moment to show them all how close she had come to a very different end the night before.

The young girl recovered and hopefully learned her lesson, but obviously the consequences of poor decision-making can be, and often are, disastrous for teens. Bennett Barber was sixteen years old on New Year’s Eve 2008 when he left an unsupervised party at a friend’s house in Marblehead, Massachusetts, and began to walk home. It was around 11:30 p.m., snow was falling, and the wind was gusting up to thirty miles per hour. Dressed in jeans and sneakers, Bennett was drunk and disoriented, and although his home was only a half mile away, he became lost. With the temperature plunging into single digits, Bennett eventually collapsed, face-first, into a snowbank. At three o’clock in the morning his mother notified the police, and a search party was sent out into the subfreezing night. Hours later, a firefighter discovered a beer bottle in the snow and followed a blurry set of footprints. When he found Bennett, the boy was semiconscious and suffering from hypothermia. He was also missing a sneaker and a sock. The high school hockey player was taken by ambulance to Massachusetts General Hospital, where his core temperature was only 88 degrees and his right foot appeared to be frozen solid. Isolated in a special chamber to raise his body temperature, he was eventually transferred to a burn center for treatment of his frostbite.

Bennett later told his father why it had taken so long for authorities to rescue him. He was trying to elude them, he said. The police report filled in the details:

He remembers seeing all the lights, but told his father that he hid every time someone with a light went by, because he did not want to get in trouble for drinking.

The teenage girl who hosted the spontaneous party when her parents went out for the night initially told the police that Bennett was drunk when he arrived and that she had walked him part of the way home. Not until 5:00 a.m. did she admit the truth, that there had been more than a dozen people at the house, many of them drinking alcohol, all underage, and that she tried to clear them out around 11:30 before her parents returned home. Two girls said they were going to walk Bennett up the street, “but when they went outside with him and he was too drunk,” they took him back inside and left him alone while they helped their friend clean up. That was the last time they saw Bennett.

Teen consumption of alcohol was only half the problem. The other half was the poor decision-making on the part of Bennett and his friends at the party, the lying that led to a delay before the police found Bennett, and even his panic at the thought of being caught by the police. All the teenagers involved exhibited a stunning lack of insight.

What scientists tell us is that insight depends on the ability to look outside oneself, and because that skill arises in the frontal and prefrontal lobes, it takes time to develop. The dynamic changes taking place in the brain are part of what make the adolescent years an age of exuberance. But a malleable, still-maturing teenage brain can be a scary proposition. Anything can happen—much of it not good. Teenagers may look like adults, they may even think like adults in many ways, and their ability to learn is staggering, but knowing what teenagers are unable to do—what their cognitive, emotional, and behavioral limitations are—is critically important.

4
Learning A Job for the Teen Brain

What did I do wrong?”

Often that’s the second question I get from parents of teenagers. The first question is usually rhetorical:

“How could my [son/daughter] [fill in the blank]?”

Most of the parents who come up to me after a talk, or e-mail me or stop me at the grocery store, are exhausted or exasperated or both, and all of them could fill in the blank in that question with a whole host of perplexing actions, from “Why would my teenage daughter sneak out of the house in the middle of the night to be with her boyfriend after they just spent the whole weekend together?” to “How could my son raid the liquor cabinet of his friend’s parents—and then leave the empty bottles behind to boot?!”

A neighbor of mine with a sixteen-year-old was flummoxed when she caught her son smoking pot in his room when he was supposed to be studying. That was bad enough, she told me, but what astonished her even more was the fact that he had the window wide open (it was the middle of winter, mind you) in order to air out his room—and the wind was blowing the smoke back into the room, under the door and down the stairs, where it wafted toward my horrified friend in the kitchen!

“How could he be that stupid?” she asked me.

Parents quickly blame themselves for a teen’s poor behavior, even though they’re not exactly sure how or why they’re to blame. With biological parents, the guilt may come from passing on flawed DNA; and with biological and nonbiological parents or guardians, the guilt comes from questioning how they raised the child. In either case, you, the parent, are to blame, right? Yes, the two scenarios are different, but no, it’s not because of the genes or anything you did or didn’t do or because the teenager was somehow struck on the head and woke up as an alien species from the planet Adolescent.

Teenagers are different because of their brains and specifically because of two unusual aspects of their brains at this stage of their development. Their brains are both more powerful and more vulnerable than at virtually any other time in their lives. Even as they are learning things faster, their brains are eliminating gray matter and shedding neurons. How both of these facts can be true is because of something called neural plasticity.

Even as a teenager I used to wonder about the brain. Did it make a difference where a person grew up? How he or she grew up? Was the brain at all like the rest of the body—capable of changing depending on what went into it or what it was exposed to? I enjoyed turning these questions over in my head, and when I got to college they turned up again, only this time I began to have inklings of some of the answers.

During one summer while I was still in high school I volunteered at the Greenwich chapter of the Association for Retarded Citizens (ARC), now known simply as the Arc, which aids people with intellectual and developmental disabilities. Some of those who regularly attended the Greenwich ARC were born with Down syndrome, and though they had varying abilities, most were self-sufficient. They were able to swim and to participate in the theater program; some even learned to read and write. Because of Greenwich’s affluence, not only was the local ARC always well funded, but many of the kids came from very privileged backgrounds as well. To this day I remember being astonished when a limousine dropped off a tot for his day of activities with us. These children were really in an unusually enriched situation, and the effects of this gifted environment showed. Despite their handicaps and rather serious diagnoses, they were active and curious and engaged, and many were approaching milestones for reading and arithmetic close to those expected for normal kids their age. I knew that not only were they getting a great day at the ARC, but when they went home, they were often given physical therapy and tutoring there, too.

While at Smith, I had an opportunity to see what life was like for the mentally and developmentally disabled who did not have the same advantages as the children at the Greenwich ARC. I volunteered several hours a week at the Belchertown State School, a seventy-year-old state institution for the cognitively handicapped, located just a few miles from Smith. Belchertown’s residents ranged from children to the very elderly, many of whom had spent most of their lives at the institution. Before it closed in 1992, Belchertown housed as many as 1,500 people, ages one to eighty-eight, living in thirteen dormitories. The hospital was understaffed, even after a local newspaper exposed overcrowding and maltreatment in the 1960s. When I volunteered in 1975, I primarily spent time in the children’s dormitory. It was not a pleasant place. The rooms smelled of disinfectant, toys were few and far between, and many of the kids hadn’t been bathed in quite some time. Like the children at Greenwich’s ARC, some were more disabled than others, but even those who were more functioning seemed to lag far behind their peers at ARC. They sat in corners and rocked and had difficulty speaking, and their eyes appeared vacant.

This was a time at the height of the nature-versus-nurture debate, and my psychology and biology professors at Smith were keen on discussing how much a person’s makeup, from personality to intelligence to likes and dislikes, is dependent on genes (nature) and how much on the influence of environment (nurture). There was clearly little nurturing going on at Belchertown, while at ARC there were always activities, directed therapies, teaching, and, most of all, stimulation.

At some point I realized the children at Belchertown who had the same disabilities and the same hurdles to overcome were far worse off than the kids at ARC in Greenwich, and at least from my limited viewpoint, environment seemed to be the overwhelming determining factor. It was pure and simple: the brains of the ARC children were being stimulated and encouraged, and the brains of the Belchertown children were not.

Like fingerprints, no two brains are identical. Everything we do, think, say, and feel influences the development of our most precious organ, and those developments trigger ever more changes until the thread of action and reaction is too complex to unwind or undo. Our brains, in essence, are self-built. They not only serve the particular needs and functions of the particular individual, but also are shaped—landscaped if you will—by the individual’s particular experiences. In neuroscience, we refer to the human brain’s unique ability to mold itself as plasticity. Thinking, planning, learning, acting—all influence the brain’s physical structure and functional organization, according to the theory of neuroplasticity.

As far back as Socrates, some believed the brain could be “trained,” or changed, much as a gymnast trains his or her body to balance on a high beam. In 1942 the British physiologist and Nobel Prize winner Charles Sherrington wrote that the human brain was like “an enchanted loom, where millions of flashing shuttles weave a dissolving pattern, always a meaningful pattern, though never an abiding one.” In essence, the human brain, said Sherrington, was always in a state of flux.

Five years after Sherrington, Donald Hebb, an American neuropsychologist, was struck by a kind of accidental inspiration that led to the first quasi-experimental test of the theory of brain plasticity. When the forty-three-year-old researcher took rat pups home from his lab at Canada’s McGill University and gave them to his children as pets, he allowed the rodents to roam freely around the house. Hebb’s inspiration was to compare the brains of these free-roaming pet rats with those of rats kept in cages in his lab. After several weeks he put both groups of rats through a kind of intelligence test involving a maze. The pet rats, which had free access to explore the environment of Hebb’s home and unfettered interaction with one another as well as with Hebb and his family, performed significantly better on the maze test than the lab rats confined to small cages.

By the late 1990s researchers had confirmed a range of changes associated with experience and stimulation, including brain size, gray matter volume, neuron size, dendritic branching, and the number of synapses per neuron. The more stimulation and experience, they concluded, the larger the neurons, the bushier the dendrites, the higher the number of synapses, and the thicker the gray matter.

During my senior year at Smith College in 1977–78, I wrote my first professional journal article under the tutelage of Nico Spinelli, a professor in both the psychology department and the computer and information science department at the University of Massachusetts Amherst. He was doing pioneering experiments in the plasticity of the visual cortex. Previous research had looked at the brains of mammals raised in a deprived environment. Spinelli wanted to see if plasticity was still at work in a “normal” environment. So we took kittens raised with their mothers in a standard animal facility and gave them what’s called avoidance training. In these experiments, a “safe” and an “unsafe” stimulus were associated with two different visual stimuli: vertical lines and horizontal lines. As the kittens learned to associate the safe stimulus with either the horizontal or the vertical lines, the number of neurons in those parts of the visual cortex expanded. The results, which were published in the journal Science, confirmed “that early learning produces plastic changes in the structure of the developing brain,” or, to put it more simply, young brains are shaped by experience.

Of course, adult brains can be shaped by experience as well. Researchers in neural plasticity have found that even in the last decades of life, adult brains can be remodeled, just not as easily or as constantly as during childhood and adolescence. Whereas kids’ brains will respond and change in response to virtually any stimulation, so-called adult plasticity occurs only in specific behavioral contexts. For instance, cab drivers in London (a notoriously difficult city to navigate) have been found by scientists to have an enlarged hippocampus particularly in the area responsible for spatial memory. Violinists and cellists, who must use their hands fluidly and rapidly, have been shown to have an enhanced motor cortex. And in an unusual experiment conducted several years ago, Patricia McKinley of McGill University was able to show that learning the tango, which involves both complex movement and a fine sense of balance, improved the ability of senior citizens, ages sixty-eight to ninety-one, to switch between two different cognitive tasks. “Plasticity,” then, is just another way of saying “learning.”

In the first few years of childhood there is a critical period of plasticity in which learning comes quickly and easily. Evolution experts believe this is the brain’s way of helping us adapt early to the specific environment in which we are raised. The concept is the same as that of imprinting, whereby a baby duckling develops a keen and powerful preference to follow the mother duck over any other. When I was five years old, I saw this in action, although I obviously didn’t know it at the time. It was Easter, and my baby brother had just been born. Perhaps because of that, friends of my parents gave me my own “baby”—a baby chick, that is, much to my parents’ consternation. I loved that fuzzy little animal and was absolutely fascinated that it would follow me around the house, through the swinging door between the kitchen and the dining room, even out of the house and around the yard. Because I was with the chick almost from its birth, it had determined I was its mother. Years later I would read the children’s book Are You My Mother? by P. D. Eastman to my sons. Basically, the book is really all about imprinting. A young hatchling leaves its nest too early while its mother is out foraging for food, and goes on a journey, asking every animal and object it meets—a kitten, a hen, a dog, a cow, a car, even an enormous power shovel—the question of the title. Luckily the power shovel lifts the young bird up and deposits it back in its nest beside its real mother.

Five-year-old me, of course, was the only mother my baby chick had. Unfortunately, the end of the relationship was sudden and brutal. About a week after Easter, after I’d just gotten home from kindergarten, my baby chick was once again following me all over the house, but this time, as I skipped between the kitchen and the dining room, the little hatchling failed to make it through the swinging door and was squished. I cried for days.

Thirteen years later, as a freshman at Smith, I created my own chick-imprinting experiment for a class in advanced biology. In order to imprint them to sound, I exposed my baby chicks to a specific sound or tone every day over a week. At the end of this training period, the chicks were placed on a kind of runway and were then exposed to two sounds, one of them being the familiar tone I’d played for them for seven straight days. Every one of the chicks toddled toward the familiar tone: they had imprinted to sound. I remember this so well because my mother was visiting me at the time of the experiment and she helped me type the results!

But how does learning actually happen? Young brains and old brains work much the same way, by receiving information from the senses—hearing, seeing, tasting, touching, smelling. Sensory information is transmitted by synapses through a network of neurons and is stored, temporarily, in short-term memory. This short-term memory region is highly volatile and is constantly receiving input from the nearly continuous information our senses encounter every minute of our waking life. After information is processed in the short-term memory region, it is compared with existing memories, and if the information matches, it is discarded as redundant. (Brain space is too limited and too precious to allow duplicates to take up neural real estate.) If, on the other hand, the information is new, then it is farmed out to one of several locations in the brain that store long-term memories. Although nearly instantaneous, the transmission of sensory information is not perfect. In the same way that the otherwise seamless signal coming from your TV is occasionally interrupted, briefly distorting the picture, so, too, does degradation occur as information races up and down the axons of your brain’s neurons. This explains why our memories are never perfect, but have holes or discontinuities, which we occasionally fill in, albeit unconsciously, with false information.

The brain is programmed to pay special attention to the acquisition of novel information, which is what learning really is. The more activity or excitation between a specific set of neurons, the stronger the synapse. Thus, brain growth is a result of activity. In fact, the young brain has more excitatory synapses than inhibitory synapses.

The more a piece of information is repeated or relearned, the stronger the neurons become, and the connection becomes like a well-worn path through the woods. “Frequency” and “recency” are the key words here—the more frequently and the more recently we learn something and then recall it or use it again, the more entrenched the knowledge, whether it’s remembering the route between home and work or how to add a contact to your smartphone’s directory. In both cases, the mental machinery of learning is dependent on the synapse, that minuscule space where packets of information are passed from one neuron to another by electrical or chemical messengers. For these neural connections to be made, both sides of a synapse need to be “on,” that is, in a state of excitation. When an excitatory input exceeds a certain level, the receiving neuron fires and begins the molecular process, called long-term potentiation, by which synapses and neuronal connections are strengthened. The process of long-term potentiation, or LTP, is a complex cascade of events involving molecules, proteins, and enzymes that starts and ends at the synapse.

FIGURE 10. Long-Term Potentiation (LTP) Is a Widely Used Model of the “Practice Effect” of Learning and Memory: A. The hippocampus is located inside the temporal lobe. B. Brain cell activity recorded in hippocampal slices from rodents shows changes in cell signals after a burst of stimulation. C. LTP experiments commonly record repeated small responses to stimuli until a burst is given (akin to the “practice effect”), after which point responses from the neuron to the original stimulus become much larger, as if “memorized” or “practiced.”

The process of LTP begins with the main excitatory neurotransmitter, glutamate, being released at the axon terminal of one neuron across the synapse to the receptor on the dendrite of the receiving neuron. Glutamate is directly involved in building stronger synapses. How does it do this? Glutamate acts as a catalyst and sets off a chain reaction that eventually builds a bigger and stronger synapse, or connection in a brain pathway. When glutamate “unlocks” the receptor, it triggers calcium ions to zip around the synapse. The calcium, in turn, activates many molecules and enzymes and interacts with certain proteins to change their shape and behavior, which in turn can change the structure of synapse and neuron to make them more or less active. Calcium can alter existing proteins very rapidly, within seconds to hours, and it can also activate genes to make new proteins, a process that can take hours to days. The end result is a synapse that is bigger and stronger and that can cause a bigger response in the activated cell. In experiments, this increased response can be measured electrically as a bigger signal. Compared with the response before the “training” and the consequent building of a stronger synapse, the response in the cell after this strengthening, or potentiation, is much larger, and these measurements are the typical ones used in LTP experiments. In fact, if you are learning any of this at all, you are building new synapses as you read. Only minutes after you learn a new thing, your synapses start to grow bigger. In a few hours they are virtually cemented into a stronger form.

John Eccles, who would go on to win a Nobel Prize for his early work in the study of synapses, was perplexed by how much stimulation was needed to produce a synaptic change. “Perhaps the most unsatisfactory feature of the attempt to explain the phenomena of learning,” he wrote, “is that long periods of excess use or disuse are required in order to produce detectable synaptic change.” What Eccles failed to realize is that the repetitions he observed so frustratingly—those “long periods of excess use”—were the brain at work, learning and acquiring knowledge. After repeated stimulation, a brain cell will respond much more strongly to a stimulus than it initially did. Hence, the brain circuit “learns.” And the more ingrained the knowledge, the easier it is to recall and use. As when skiers race through a slalom course, the quickest route down becomes worn by use. Ruts develop. By the time the last competitors race through the gates, the route is so deeply entrenched in the snow that they can’t ski out of it, nor do they need or want to. The deeply imprinted line, in fact, guides them down without their having to search for it.

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