The Teenage Brain

Jay Giedd has been using mri since 1991 to understand how the brain develops from childhood through adolescence and into early adulthood. He has scanned the brains of about 1,800 children, teenagers, and young adults every two years and interviewed them about their lives and feelings. As it turns out, Dr. Spock was not entirely wrong: by the time a child reaches the age of six, the brain is 90 to 95 percent of its adult size. But massive changes continue to take place for at least another fifteen years. They involve not just the familiar “grey matter,” but a substance known as “white matter,” the nerve tissue through which brain cells communicate — literally the medium that delivers the messages. White matter develops continuously from birth onward, with a slight increase during puberty. In contrast, grey matter — the part of the brain responsible for processing information, or the “thinking” part — develops quickly during childhood and slows in adolescence, with the frontal and temporal lobes the last to mature.

And this is the crux: the frontal lobe, or more precisely the prefrontal cortex, is the home of the so-called “executive functions” : planning, organization, judgment, impulse control, and reasoning. The part that should be telling the sixteen-year-old not to dive off the thirty- foot cliff into unknown water. The seat of civilization.

What Giedd has witnessed via mri is a constant push and pull in the grey matter. Certain forces cause a process known as arborization, during which grey matter gets bushier and grows new dendrites. Balancing that is a regressive pull, a competition for survival of sorts, in which some branches of the grey matter thrive while others are sacrificed. Both processes are continuous; as some new pathways grow, others are being pruned back. The quantity of grey matter peaks in girls around the age of eleven and in boys around thirteen, after which the amount of white matter increases. As grey matter decreases, there is also an increase in myelination, a process during which neurons, or nerve fibres, are insulated to enhance their performance.

In the end, though, the amount of grey matter isn’t really the issue. “It’s much more related to quality than quantity,” explains Giedd. “This pruning process is normal and natural and healthy in terms of optimizing the brain for different environments. Our brains are built to be very adaptable during the teen years” — just the time when children start to figure out how to make it in the world. “The brain is incredibly plastic, which allows us to make it at the North Pole or the equator, to use a computer versus hunting with a stick. The teen brain is able to make changes depending on the demands of the environment.” (This might explain a thirteen- year-old’s ability to easily master new technology while parents struggle with the TV remote.)

What determines the fate of a cell is whether it has made a meaningful connection with other cells. This is a real use-it-or-lose-it process. As some scientists have noted, if an adolescent forgoes reading in favour of lying around on the couch playing video games, those unused synapses will be pruned. Nobel Prize-winning scientist Gerald Edelman has called this “neural Darwinism” — the survival of the fittest synapses. So scientists know that different activities — playing sports, speaking a second language, drinking, smoking, and so on — influence how the adolescent’s brain will ultimately be wired, though they aren’t clear what the implications are: Is the pianist going to do better in life than the crossword- puzzle fiend? Will the jock have a leg up, brain-wise, on the geek? “Can you actually see changes in the brain of someone doing music? The answer to that is yes,” says Giedd. “But is that a good thing particularly? Is it just that our brains will become specialized in whatever we spend our time doing or is there a more general benefit?”

A father compliments his thirteenyear- old daughter on her new dress, only to have her swivel around, glare at him, and hiss, “What’s that supposed to mean?” Nervous parents can rarely tell when an adolescent is going to fly off the handle. Why do they often have such hair-trigger responses? Two different mri studies indicate that teenagers do not process emotion the same way adults do. In fact, one study shows that the adolescent brain actually reads emotion through a different area of the brain. Dr. Deborah Yurgelun-Todd, director of neuropsychology and cognitive neuroimaging at McLean Hospital in Belmont, Massachusetts, has scanned both adults and teenagers as they were shown images of faces that are clearly expressing fear. All the adults correctly identified the emotion; many of the teens got it wrong (about half labelled the expression one of “shock,” “sadness,” or “confusion”). Yurgelun-Todd found that during the scan of the adults, both the limbic area of the brain — the area especially connected to emotions — and the prefrontal cortex lit up. When teens were seeing the same pictures, the limbic area was bright but there was almost no activity in the prefrontal cortex. They were having an emotional response essentially unmediated by judgment and reasoning.

In another brain-imaging study, Daniel Pine, a researcher at the National Institute of Mental Health, tried to determine how the brain was able to stay focused on a task while the subject was being exposed to faces that were registering strong emotion. The result: activity in the frontal cortex of the adults was steadier, indicating they were better able to stay on task than teenagers. The emotional faces seemed to activate key areas in the brains of both age groups but only the adults were able to mute that activity so they could stay focused. Teenagers are more at the mercy of their feelings.

There is another fascinating phenomenon that plays havoc with the family of a teen: the adolescent sleep pattern. Suddenly, the kid who always woke you up at sunrise, when you were desperate to sleep, turns thirteen or fourteen and can neither be dragged from bed in the morning nor forced into it at night. Making matters worse, this change invariably occurs as the sleep needs of the middle-aged parents are flipping around the other way. It may seem like just another case of teenage passive aggression, but it’s just biology; the circadian rhythm of the brain has changed and teenagers simply don’t want to — or can’t — go to bed before 12 or 1 a.m.

Why this happens has been the focus of some interest. Researchers at Brown University and Bradley Hospital in Providence, Rhode Island, measured the amount of melatonin, the hormone that helps regulate the sleep-wake cycle, in teenagers’ saliva over the course of the day. They discovered that the levels of the hormone increased later in the day and decreased later in the morning in teenagers than in adults and children. A separate study indicated that the biological trigger for sleep — called the sleep pressure rate — slowed down during adolescence.

So if teenagers appear to be cycling through the day at a different pace from the rest of the world, it’s because they are. In fact, because they are waking up when the world dictates — rather than when their bodies tell them to — teenagers are chronically sleep-deprived, which can have consequences ranging from superficial to severe. For starters, as Carlyle Smith, a psychology professor at Trent University in Peterborough, Ontario, who has studied how the adolescent brain processes information during sleep, notes, “They’re just sleepy.” They go to school tired, unfocused, and — because nobody likes to eat breakfast when they’d rather be sleeping — typically unfed. And as many teachers can attest, teenagers are also generally less able to absorb information in the morning. But by later in the afternoon, as the rest of the world is struggling not to nod off at their desks, teenagers begin to fire on all cylinders. “[As an adult], your temperature is at its high point shortly after lunch,” explains Smith, “and then it starts its way down and drops all night until 3 or 4 a.m., when it starts to go up again. Theirs doesn’t reach its height until later in the day.” As a result, teenagers are just starting to focus and become more verbally adept as the rest of the world is crashing. By midnight, while the rest of the family is doing its best to fall asleep, teenagers are wide awake and instant-messaging away.