http://www.walrusmagazine.com/articles/science-the-teenage-brain/

 
Thursday, October 26, 2006
 


The Teenage Brain
Why adolescents sleep in, take risks, and wont listen to reason

by Nora Underwood

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In his speech at the launch of the 1997 I Am Your Child campaign, director and
actor Rob Reiner stated that "by the age of ten, your brain is cooked." And
until recently, most child experts, including Dr. Spock, would have agreed. They
considered the first few years of a child's life to be the most important - and
the experiences a child had during those years to play a crucial role in
defining the kind of person he or she would ultimately become. That
understanding also helped create a whole generation of obsessively child-focused
parents, who, with the best of intentions, have tried to cram a lifetime of
"educating" into a few short years, subjecting their unwitting fetuses to a diet
of Eine kleine Nachtmusik and their pre-verbal toddlers to basic arithmetic and
multiple viewings of Baby Einstein dvds. (A wise elementary-school principal
once noted, "I very much doubt Einstein was doing any of this when he was
young." ) 

Somewhere along the line, or so many of us believed, the window of opportunity
would close. The foundations of the adult-to-be would be laid, and the worst
damage would be done. The majority of brain development does, in fact, take
place in the early years, when billions of synaptic circuits that will last the
child's lifetime are forming. But growth and change don't end there. Important
developmental changes, scientists are discovering, are still taking place in a
big way through the adolescent years - and into the mid-twenties. Perhaps this
helps to explain the growing phenomenon of adult children who linger on under
the parental roof; their growing may not be over, despite their arrival at
"adulthood." 

In recent years, researchers have finally been able to get real insight into the
workings of the brain thanks to magnetic resonance imaging (mri), using the
technology to map blood flow to the areas of the brain that are activated by
exposure to various stimuli. By scanning the same group of adolescents over a
period of years or by comparing the brain responses of teenagers to those of
adults, researchers are putting together a portrait of adolescence that confirms
what many parents have always suspected: adolescents might as well be a whole
different species. They are, as one neuroscientist puts it, a
"work-in-progress." 

Over the past decade, scientists have started to grasp exactly how distinctive
the adolescent brain is and how crucial the years between ten and twenty- five
are in terms of its development. And their discoveries have implications not
only for parents, educators, and the medical community but also for
policymakers. "I wouldn't disagree with Rob Reiner that the first three years
are important," says Jay Giedd, chief of brain imaging in the child-psychiatry
branch of the National Institute of Mental Health in Bethesda, Maryland. "I
would just say that so are the next three and the next three and the next three,
up to twenty-five and perhaps even beyond." 

This news may not come as a surprise to the mother who still lies awake at 3
a.m., waiting for her basement-dwelling, twenty-two-year-old post-grad son to
come home. What science suggests is that "adulthood" as we have defined it
doesn't necessarily signal the end of childhood development - or of parental
worries. 

If the media is to be believed, the stereotypical teen is a selfish, volatile,
rude, rebellious hormone-head, capable of little more than taking outrageous
risks, ingesting too many harmful substances (legal and otherwise), committing
crimes, crashing parties, trashing houses, and generally being a layabout. Of
course, this is a gross misrepresentation: many teenagers pass through
adolescence smoothly and happily, without becoming parents themselves, dropping
out of school, or acquiring a criminal record instead of a degree. Still,
there's a stubborn tendency in the culture to ascribe every negative teen moment
to "hormones." Recent brain research, however, relieves hormones of much of the
blame for this period of "storm and stress," as psychologist G. Stanley Hall,
father of adolescent research, called it. 

The full extent to which hormones actually influence adolescent behaviour
remains unknown. So is what role they play in brain development. Hormones are
certainly responsible for the most obvious hallmarks of puberty; at some
mysterious point in a child's life, a protein called kisspeptin causes the
hypothalamus - an area in the brain that orchestrates certain autonomic
nervous-system functions - to secrete the gonadotropin-releasing hormone, which
sets the pubertal changes in motion. Ultimately, estrogen and testosterone are
responsible for the physical transformations - breast and genital development,
body-hair growth, deepening of the voice, and so on - but by no means all the
behavioural changes of adolescence. Hormones may have nothing to do with the
fact that your daughter can't bear your singing voice, for instance; it's a safe
bet, however, that a teenager's fixation on sex and social standing is pretty
much hormone related. 

But puberty does have an impact on how they think. For instance, as Giedd points
out, boys fairly predictably base their decisions on the question "Will this
lead to sex? " Giedd adds: "They may not say it in that way or it may not be
that blatant, but if you just sort of go with that model it works pretty well."
When girls make decisions, he adds, they are more likely to keep the social
group, and their place in it, in mind. But Giedd feels that puberty's influence
doesn't extend much outside that realm. "Your ability to do a logic problem or
to do geometry or to do other things seems to be more [related to] age itself."
Researchers have also found that the onslaught of testosterone in both male and
female adolescents at puberty literally swells the amygdala - the brain centre
associated with the emotions. Perhaps we can blame the amygdala for the slammed
doors and sudden tears that overcome previously sunny children when they hit
adolescence. 

So hormones are not the only players in the changes that characterize
adolescence. And while it is difficult to tease out the varying roles played by
chromosomes, hormones, and other factors in teen behaviour, the insights that
mri reveals are nothing short of astounding. 

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
instantmessaging away. 

What is the fallout from a world that runs against the adolescent clock? There
are four non-rem stages of sleep, and stages three and four, the deepest, which
occur during the first third to first half of the night, are particularly useful
to adolescents, who still have those frontal lobes to myelinate and lots of
overall growing left to do (growth hormone is released during deep sleep). But
because teenagers are so often deprived of rem sleep, which occurs during the
last part of the night, their memories can suffer; they lose out on the stage of
sleep that sees the information they've absorbed throughout the day replayed and
consolidated. "Kids should be getting over nine hours of sleep," says Smith.
"Most are getting one to two hours less than they should. They're missing quite
a chunk of rem sleep and that's important for understanding new things. If you
don't get much rem sleep, you're not going to learn as fast as people who do." 

In one study, Smith set his subjects, who ranged in age from eighteen to
twenty-two, to learning a logic task and then deprived them of the last half of
the night of sleep. A week later, after the participants had recovered, the
researchers tested them again. All had forgotten between 20 and 30 percent of
what they'd learned. Once in a while, this kind of sleep loss is no problem.
People can catch up. But when sleep deprivation becomes chronic, the
consequences are compounded. "You're forgetting 20 percent, but 20 percent every
day," says Smith. "And that goes on for months and months and months. That's an
inefficient system." 

Chronic sleep deprivation also increases the risk of developing depression
(though, paradoxically, if someone is already depressed, sleep deprivation tends
to help them feel better). This is a particularly serious issue for adolescents,
as certain mental-health disorders tend to manifest themselves during these
years. "There's so much confusion over this," Smith admits, "but one of the
worries is if you just keep on with the sleep deprivation, eventually [that
person] will become depressed. And we're seeing a lot more depressed kids around
now." 

But it's not easy to fight nature; perhaps the best parents can do is to
encourage a slowdown of activity at a reasonable time in the evening, keep
technology out of the bedroom and caffeine out of the fridge, and let their kids
catch up on weekends. 

Most adults know what they're up against because they remember their own
night-owl days. They may have dabbled in rule-breaking, underage drinking, and
general wildness as teenagers and now they shudder at the thought of their own
children doing the same or worse. They were lucky, but will their kids inherit
their luck? (The bad news for former hellraisers: some research suggests a
person's tendency to take risks is partly genetic.) 

In fact, there's some indication that cultivating unhealthy habits through this
whole tumultuous period of development can have serious long-term effects. Those
who start smoking during adolescence, for example, will likely have a much
harder time quitting later in life than those who take up smoking in their
twenties; the addiction, according to researchers at Duke University in Durham,
North Carolina, appears to get hard-wired during the teen years. 

Evidence from some studies also suggests that alcohol is more likely to damage
memory and learning ability in the hippocampus of the evolving adolescent brain.
At the same time, adolescent rats - whose brains are rela- tively similar to
those of adolescent humans - suffer less from some of alcohol's other effects,
including sedation. That sounds like a good thing, but if it is indeed true for
adolescents (and for obvious ethical reasons researchers don't put adolescents
through alcohol- related trials), it means they can drink more, and for longer
periods - and therefore run a greater risk of long-term damage. Repeated alcohol
use during these years may also lead to lasting memory and learning impairment -
not to mention the fact that young binge drinkers are more likely to set
themselves up with a lifetime alcohol- abuse problem. 

This is one area where brain-research findings have affected how Giedd, the
father of four, behaves as a parent. "In terms of substance abuse and alcohol,
I'm a lot less hip now," he says. "I wouldn't have the mentality of, 'Oh it's
better to have them do it at home.' [Adolescence is] a very vulnerable time in
brain development to be exposed to these other substances." Giedd is surprised
by how many parents say that their kids are going to drink and take drugs
anyway, so they might as well do it at home, in a safe environment.
"Biologically, it's a time when the cement is setting. If people cannot do these
things until the age of nineteen, the odds of them not having trouble as adults
go up enormously." 

But experimenting, taking risks, and searching for good times are, it would
seem, all part of the adolescent picture. As difficult as it is for parents to
grasp, adolescents don't always make poor choices just to get their goats, or
because they're suddenly gripped by temporary insanity. This sort of behaviour
appears to be a predictable part of the identity-formation process, which begins
in the early years but dramatically accelerates during adolescence. That's when
children begin playing different roles, trying on different hats, figuring out
if they're gay, straight, or bisexual, whether they're a geek, a jock, or cool.
At the same time, their frontal lobes aren't fully developed, which means that
the appetite for experimentation doesn't necessarily go along with the capacity
to make sound judgments or to see into the not-so-distant future. In other
words, by their very nature, teenagers are not especially focused on, or
equipped to assess, the consequences of their actions. 

A 2004 mri study suggested that adolescent brains are less active than those of
adults in regions that motivate reward-based behaviour. James Bjork, a
neuroscientist at the National Institute on Alcohol Abuse and Alcoholism, and
his colleagues conducted a brain scan on twelve adolescents between the ages of
twelve and seventeen and a dozen adults aged twenty-two to twenty- eight. During
the scan, the participants responded to targets on a screen by pressing a
button; the object was to win (or avoid losing) varying amounts of money. The
researchers found that areas of the brain associated with seeking gain lit up in
both age groups. But in the adolescents, there was less activity. Adults, says
Bjork, may have developed circuitry that enables them to motivate themselves to
earn relatively modest rewards - the satisfaction felt after volunteering at
church, say, or walking through a ravine. Adolescents, on the other hand, "may
need activities that either have a very high thrill payoff or reduced effort
requirement or a combination of the two." Examples, he adds, would be "sitting
on the couch playing violent video games or sitting on the couch and pounding
alcohol." 

Even if, in quiet conversation, teenagers understand the risks of certain
actions - drinking and driving, sex without protection, jumping off cliffs -
when the moment of truth actually arrives, reason can be shot to hell. In the
heat of the moment, the limbic area of the brain lights up like a pinball
machine while the prefrontal cortex, the good angel that tamps down intense
feeling and helps us navigate through emotional situations, is essentially
asleep. In addition, experts have found that teenagers have a higher level of
dopamine, a neurotransmitter connected to pleasure, movement, and sexual desire,
which may increase the need for extra stimulation through risk-taking. 

Some teenagers slide through adolescence unscathed. But there's no doubt that
adolescents in the throes of hormone surges and brain development are extremely
vulnerable - to making poor choices, to mental-health problems, to death and
injury. A quick look at the statistics paints a troubling picture. According to
Statistics Canada, adolescents between fourteen and nineteen are more likely to
commit property crimes and violent offences than any other age group; 25 percent
of teenagers reported binge drinking at least once a month in 2000-2001, a rate
second only to the twenty- to thirty-four-year-olds. During that same period,
the pregnancy rate for girls between fifteen and nineteen was thirty-six out of
1,000. Most discouraging is the suicide rate for teenagers: currently about
eighteen for every 100,000, with the highest rate occurring among teenaged boys
(although girls are hospitalized for attempted suicide at a far greater rate
than boys). 

In fact, the three leading causes of death for teenagers in North America are
accidents, suicide, and homicide. Unsurprisingly, the majority of accidents
involve motor vehicles; in 2004, in the United States, about 20 percent of
accidents that resulted in fatalities were due to a driver who had a high
blood-alcohol level. According to the Insurance Institute for Highway Safety,
injuries suffered by teenagers in car crashes have become a pressing public-
health problem. Sixty-two percent of teenage passenger deaths in 2004 occurred
when another teenager was driving. And teenage drivers are more likely to be at
fault in crashes. 

All of which is not going to make parents sleep any better - if indeed they can
get to sleep in the first place. 

Teenage speeding, irresponsibility, and status-seeking are not the only
explanations for the statistics (though teenagers have been shown to take
greater risks behind the wheel when their friends are with them). In fact, they
also appear to be at a disadvantage because they have not refined the ability to
multitask - driving while drinking a beverage, listening to music, talking on a
cellphone, or even chatting with a passenger. One sensible response to this,
according to many scientists and policy-makers, is graduated licensing, which is
already in place everywhere in Canada except Nunavut. In 1996, many American
states started to introduce some aspects of graduated licensing, and according
to a 2003 report in the Journal of Safety Research, they have seen a decrease in
crash rates. 

So if adolescents are a work-in-progress in terms of judgment, should they be
held accountable for their crimes in the same way adults are? Recent adolescent
brain-development research was used in arguments against the juvenile death
penalty in the United States. If adolescents aren't yet fully capable of
controlling their emotional responses or understanding the consequences of their
actions, groups like the International Justice Project said, then they should
not be punishable by death. In March 2005, when the US Supreme Court finally
abolished the juvenile death penalty, there were seventythree people on death
rows across the United States for crimes they had committed before the age of
eighteen. Many brain researchers believe that science should be part of the
debate. But, Giedd adds, "it becomes a very slippery slope: the same data that
might support abolishing the juvenile death penalty could be used to take away
teenagers' ability to make their own reproductive-rights decisions." 

Despite these new findings, has brain science told us anything we don't already
know? Bjork's answer: "As Jay Giedd says, a lot of what we're finding out in
brain research is the neuroanatomical, neurometabolic correlate of what grandma
always told you." Indeed, brain mapping has provided proof of a neurological and
biological basis for what sometimes ails the stillforming adult (and the adults
who love and live with them). 

Of great urgency for Giedd and others now is why certain disorders - anxiety and
eating disorders, substance abuse, schizophrenia - develop during adolescence,
but not autism, adhd, Alzheimer's, and others. "Many of the things that plague
adults really do hap- pen during the teen years," says Giedd, "so identifying
them early, treating them early, when the brain is more plastic, would seem to
make more sense in terms of really having a lifelong impact." Parents are wise
not to assume that misery and anxiety are just part of the teenage rite of
passage; it may be that serious unhappiness in adolescence is an early-warning
sign of adult disorders. 

Another task for scientists is to determine which things in a teenager's
environment and experience will, for better or worse, influence brain
development. "So many things have already been put forward - music, education in
general, learning a second language, bacteria, viruses, video games, diet,
sleep, exercise," says Giedd, "and all of them are probably true to some
extent." 

But what the general culture has to offer to teenagers is only one part of the
equation. The brain has always been built for learning by example and experience
- which experiences lead to pain, which lead to good outcomes. And for Giedd,
that facility is what will give adolescents the best chance to grow up well -
the ability to learn from the people around them. "It's the little things, the
day-to-day things that we say in the car or when we're solving problems, how we
handle relationships, emotions, our work ethic," he says. "They will believe
much more what we do than what we tell them." 

In fact, if there is anything parents can take away from all the scientific
research into adolescent brain development, it's that their influence, patience,
understanding, and guidance are very necessary - even when the teenager or young
adult shrinks away from affection, grunts, slams doors, blasts music, rolls
eyes, breaks house rules, and seems incapable of following simple instructions.
Developing brains often can't handle organizational problems; they have more
trouble making social, political, and moral judgments; they have to be reminded
of potential consequences and carefully directed toward risks that aren't quite
so, well, risky. Developing adults need appropriate amounts of independence,
freedom, and responsibility. 

"I would say with a clear conscience that the teen brain is different than the
adult brain," says Giedd. "Just as I would feel comfortable saying men are
taller than women." We ignore those differences at our peril, he adds. Teenagers
may drive the family car, move away from home, go to college, and spend their
early twenties wrestling with life decisions, all of which are a normal part of
growing up. But as Giedd says, just because adolescents have left childhood
behind, "parents shouldn't say, 'My work is done.'" 
- Published November 2006

Nora Underwood is a senior editor at The Walrus


C 2006 The Walrus Magazine

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