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.

FIGURE 11. New Receptors Are Added to Synapses During Learning and Memory and Long-Term Potentiation (LTP): Axonal signals that originally elicited a small response elicit a larger response in the neuron after LTP, owing to the development of a larger synapse over time.

The process of fine-tuning and turning off neuronal connections that may have been made in childhood but are no longer needed is called pruning, and it accelerates during mid- to late adolescence when unneeded synapses are removed. Scientists call this pruning phase a kind of “neural Darwinism,” in which only the “fittest,” that is, the most used neurons, survive. What accounts for gray matter loss so early in a person’s development when many cognitive and behavioral functions have yet to fully develop? Researchers have discovered in the past few years a direct correlation between an adolescent’s decrease in gray matter and the increase in white matter. Scientists know that gray matter continues to decrease through adulthood, especially after the age of sixty, but they also believe that gray matter loss in adolescence is a distinctly different process. In later life gray matter declines as a function of degenerative processes, that is, cell shrinkage and death, whereas in adolescence gray matter decline is a product of the brain’s plasticity. (“Use it or lose it.”)

Researchers at UCLA discovered not only that effective pruning increased brain efficiency but also that higher intelligence could be correlated with prolonged, accelerated neuronal growth in childhood followed by vigorous cortical pruning in adolescence. What is less, it turns out, is actually more, and this is why in the midst of all the chaos of the teenage years, adolescents are developing a leaner, more efficient adult mental “machine.” Adults have an advantage over children and teens in that their white matter is more extensive, meaning that they have more speedy connections between brain areas, such as the frontal lobes.

FIGURE 12. Gray Matter and White Matter Develop Differently Throughout Life: Children and teens have more gray matter and synapses than adults, because as we age our brains remove unnecessary connections for greater efficiency. However, in old age white matter also diminishes, contributing to age-related cognitive issues such as memory loss and abnormal conditions such as dementia.

The real news in all of this science is that the teen brain is more than capable of learning, and this fact should not be taken for granted! LTP is indeed more robust in teens. Adolescent animals similarly show faster learning curves than adults, and scientists wanted to know whether this was because they had better synaptic plasticity, given all the increase in excitatory synapses, which are required for LTP. Studies were performed on brain slices from rats to look at the LTP in an adolescent rat versus an adult rat. They found that the strength of LTP was “way better” in the adolescents. The “before” and “after” comparisons following the burst of stimulation showed that synapses in adolescent slices had about one and a half times as much increase and that it lasted a lot longer.

So what this means is that memories are easier to make and last longer when acquired in teen years compared with adult years. This is a fact that should not be ignored! This is the time to identify strengths and invest in emerging talents. It’s also the time when you can get the best results from remediation, special help, for learning and emotional issues. We’ve all long thought that the IQ with which you were “branded” in grade school after taking one of those aptitude tests was the final word on your intellectual destiny. Not true. There is solid data to show that your IQ can change during your teen years, more than anyone had ever expected. Between thirteen and seventeen years of age, one-third of people stay the same, one-third of people decrease their IQ, and a remarkable one-third of people actually significantly raise their IQ. The changes in increased IQ were also associated with changes in brain scans. When verbal IQ increased, so did gray matter in the center of the brain, responsible for articulating speech. When nonverbal IQ increased, so did gray matter in the area of the brain related to hand movements. The frustrating part of this study is that the researchers did not track what these people did during those formative years. We’d love to know the secret of how to make your IQ go up during your teen years. At least we are already learning the kinds of things that can make it go down, but we will come back to this later.

FIGURE 13. Adolescents’ Synaptic Plasticity Is “Way Better” Than Adults’: To test if teens have better learning abilities than adults because they have better LTP, researchers compared adolescent hippocampal brain slices with those from adults. The signal after the burst stimulation in the adolescent (B) was much higher, and lasted longer, than in the adult (A).

Researchers at the University of Colorado’s Institute for Behavioral Genetics recently discovered that compared with children with lower IQs, children with high IQs may have an extended learning period during which they are able to maintain a rapid pace of new knowledge acquisition. This extended learning period does not necessarily lead to higher IQ but could have potential long-term benefits. This kind of information needs to get out there: teens need to become aware that this is one of the golden ages for their brains!

That, of course, doesn’t really help you in dealing with the here and now of your dizzyingly confusing teenagers. It’s important to remember that even though their brains are learning at peak efficiency, much else is inefficient, including attention, self-discipline, task completion, and emotions. So the mantra “one thing at a time” is useful to repeat to yourself. Try not to overwhelm your teenagers with instructions. Remember, although they look as though they can multitask, in truth they’re not very good at it. Even just encouraging them to stop and think about what they need to do and when they need to do it will help increase blood flow to the areas of the brain involved in multitasking and slowly strengthen them. This goes for giving instructions and directions, too. Write them down for your teens in addition to giving them orally, and limit the instructions to one or two points, not three, four, or five. You can also help your teenagers better manage time and organize tasks by giving them calendars and suggesting they write down their daily schedules. By doing so on a regular basis, they train their own brains.

Perhaps most important of all, set limits—with everything. This is what their overexuberant brains can’t do for themselves. So be clear about the amount of time you will allow your teenager to socialize “virtually,” either on the Internet or through texting. Best-case scenario: limit the digital socializing to just one to two hours a day. And if your teenager fails to comply, take away the phone or the iPod, or limit computer use to homework. Also, insist on knowing the user names and passwords for all their accounts.

None of this means your kids are going to immediately go along with the program. In fact, it’s virtually a certainty that there will at least be occasional slip-ups, perhaps a lot of them. That’s why it’s up to you to keep tabs, to check on teenagers as they do their homework and spend time on the computer. The more on top of it you are, the fewer the temptations for your adolescents, and the fewer the temptations, the more their brains will learn how to do without the constant distractions.

In dealing with those unexpected emotional outbursts from your teenage sons and daughters, something as simple as counting to ten before responding can help you stay calm. Being angry, or treating the meltdown like a childish tantrum, is not advisable. Adolescents believe they’re adults, and though we know better, the more you treat them that way, the greater the chance they’ll actually try to act that way, too. Because I’m a doctor and a scientist, I could sit my kids down and tell them, Look, you don’t believe me when I say that you’re being irrational or impulsive or overly sensitive, but let me tell you why it’s your brain’s “fault.” By the time you finish reading this book, you’ll be able to do the same thing. And trust me, it works. I’ve seen it not only with my own sons but also in talking to teenagers after I’ve given presentations in high schools. They’re actually fascinated by the neuroscience and that there’s a logic, or rationale, behind what otherwise seem like inexplicable upheavals in their lives. You do run the risk, however, that these “brain explanations” become a kind of adolescent ammunition.

“My brain made me do it.” Your teenage son might be tempted to say when he decides to drive off with Dad’s car and not tell anyone until he comes homes after midnight.

“Well, no,” you have to say, “your brain is sometimes an explanation; it’s never an excuse.”

Your teenagers are knowledgeable and self-aware enough to know that they are not automatons, and this means they have the capacity to modify and the responsibility for modifying their own behavior. This is what you must remind them—and then remind them again and again and again. The brain science isn’t an excuse for crazy, stupid, illegal, or immoral behavior. It’s an explanation and a framework, about which they should be encouraged to read more. In the same way that I called my sons after I heard the news of Dan’s drowning, when you read or hear about something similar, call your teenagers, or sit down with them, and remind them why these things happen. You can’t expect them to automatically grasp innuendo and more subtle connections, so the better thing to do is err on the side of overkill and point out the obvious, explicitly. I did this so many times with my kids that they named me Captain Obvious!

There is a logical reason why plasticity is front-loaded in childhood and adolescence: survival depends on knowledge of one’s environment, so the young brain must be flexible and moldable depending on the type of environment in which the person is growing up. The growth of synapses makes teens sensation-seeking learning machines, but the fact that their brain signals can also easily slide off the tracks makes those growth spurts somewhat dangerous. Evolutionarily, being open to new ideas, having new things to learn, leads to useful experiences that are necessary for survival.

For adults, it’s our myelin that in part allows certain brain signals to speed their way to the frontal lobe, where we respond by putting the brakes on that impulse to skydive or drive at 120 miles an hour. So pruning happens just as myelin growth increases, giving adolescents a short window to experience the world and figure out what will make them happier, healthier, and, one can only hope, wiser. This delicate balance is why some teen behaviors can appear so confounding, like the teenage boy a colleague told me about who was stopped on a side street for driving at 113 miles an hour and given a speeding ticket. The boy was furious—not that he thought he shouldn’t have been ticketed, since he did admit he was driving way over the speed limit, but because the violation was for “reckless driving,” when in fact, he told his father, he’d thought about it and planned it all out well in advance. He knew exactly what he was going to do and where to do it, and even picked a straight road with little traffic and good weather to do it in. On the other hand, he then went out and drove 113 miles an hour!

There’s an additional, evolution-based reason for this apparent conflict of self-interest. Scientists at University College London recently asked fifty-nine young people, ages nine to twenty-six, to guess the odds that certain bad things might happen to them. The forty unfortunate events ranged from getting lice to getting seriously injured in an auto accident. After the subjects made their guesses, they were told the real odds of those bad things happening. Then they were asked to project the odds again. The majority of the subjects were good at remembering the actual risk if it wasn’t as bad as their original guess. But the adolescents were worse at recalling the risk when the risk was worse than their original guess. As it turns out, there are more areas of the brain that process positive information, whereas negative information is centered in the prefrontal cortex. In other words, adolescents have less ability to process negative information than adults do, and so they are less inclined not to do something risky, and less likely to learn from the ensuing mistake or misadventure, than adults are.

Once someone is out of adolescence, synaptic plasticity, and learning, require more effort. In the same way that a young adult eventually settles down into a life and a routine, so, too, does the brain. The middle-aged man who played electric guitar in a rock band in high school would have trouble picking up the instrument again at age forty-five. Twenty-five years ago, those guitar-playing neurons were in constant use, but by the time he reaches adulthood they’ve been dormant for so long they were essentially left behind, just like that electric guitar in the attic. Adults also have less glutamate and dopamine and fewer receptors available; therefore they are less cognitively flexible.

But don’t tell my dad that. He’s now in his early nineties and still active, and his favorite gadget is his iPad, which he never puts down. He’s always e-mailing me, sending me virtual clippings from medical articles he’s read, attaching them to an e-mail that just reads, “I thought you’d like to see this.” My father’s brain is engaged all the time, and the Internet has helped him stay stimulated and on top of current issues and events. If my dad had been born just twenty years earlier than he was, I’m not sure what he’d be doing now or what he’d be able to do. My mother, who is also in her nineties, prefers to play solitaire on the iPad. She says it’s easier than shuffling cards. Having grown up in England, she worked in British Intelligence during World War II, and her brain remains sharp. The good news about brain plasticity is that it may peak in childhood and adolescence but it never entirely stops—at least not until we do. The more you learn, the easier it is to learn the next thing.