http://chronicle.com/weekly/v53/i19/19a01401.htm
| From the issue dated January 12,
2007 |
Who's Minding the Teenage Brain?
Scientists find clues to why adolescents seek out and find trouble
By RICHARD MONASTERSKY
Pittsburgh
In the time it takes you to eat dinner tonight, two adolescents somewhere in
the United States will contract HIV. Over the next month, nearly half of all
high-school students will sneak a drink of alcohol. And sometime over the course
of 2007, one in 12 high schoolers will try to kill themselves.
It is one of the great paradoxes of modern existence: Humans grow far
stronger and healthier during their second decade of life, but their chances of
dying rise rapidly at the same time. In the vast majority of cases, it all comes
down to a bad decision. A 17-year-old honor student gets behind the wheel while
drunk. A high-school freshman tries methamphetamine — and gets hooked. A
pack of fans at a football game picks a fight with their rivals. A depressed
girl, alone in her room, chooses to check out.
Any parent of a teenager — or anybody who hasn't blocked out memories of
those turbulent years — knows that adolescents often have difficulty
navigating through their world. Now scientists are starting to find out why.
Peering into the minds of maturing youngsters, researchers are discovering what
happens within the developing brain that can steer adolescents into risky
territory.
"What we want to do is classify what is particular about adolescence that
makes it vulnerable," says Beatriz Luna, an associate professor of psychiatry
and psychology at the University of Pittsburgh. "That will start giving us a
clue as to why this is the time where you see the breakdowns, when you see
schizophrenia appear, and depression, and bipolar disorder and risk-taking
behavior, and substance abuse. Why then?"
Along with several other researchers, Ms. Luna has switched in recent years
to studying the adolescent brain — a subject of inquiry that didn't exist a
generation ago. In 1990 only two papers were published on this topic, while the
number in 2005 topped 150.
"It's really incredible that there isn't a long history of doing this work,"
says Linda P. Spear, a professor of psychology at the State University of New
York at Binghamton, who studies adolescence in rats. "It just blows my mind
because it's so critical."
Through new experimental techniques that illuminate the thoughts of
youngsters and through research on animals, investigators have found that the
brains of teenagers are changing far more than was previously thought. And the
way their minds ripen seems specifically to lead them into danger. Neural
systems that respond to thrills, novelty, and rewards develop well before the
regulatory systems that rein in questionable actions. The teenage brain, in
essence, is a turbocharged car with a set of brakes still under
construction.
Out of Control
In a darkened room at the University of Pittsburgh Medical Center, an
11-year-old girl is letting researchers spy on her thoughts. The sixth grader,
Erika, wears a net of electrodes on her hair that pick up the neuronal chatter
inside her skull. She peers at a computer screen as a line of five horizontal
arrows pops into view. The central arrow, which points toward the left, is
flanked by arrows pointing to the right. Erika's job is to ignore the
distracting outside arrows and press a button indicating the orientation of the
central one.
The task requires quick responses, and Erika's brain is buzzing with
activity, which appears as wavy lines on another computer screen in the room.
Those squiggles reveal that children Erika's age — who are in the early
stages of adolescence — have trouble dealing with the conflicting
information on the screen.
When adults perform the same task, their brains have built-in mechanisms for
spotting incongruent situations and for catching errors. If they press the wrong
key in their haste, their anterior cingulate cortex, toward the front of the
skull, emits an electrical signal within about 80 milliseconds — faster
than the blink of an eye — even before the person grows aware of having
made a mistake. At a subconscious level, this area of the adult brain is
monitoring its actions and quickly noting when something goes awry. That fast
circuitry tells the rest of the brain, "Stop. There's a problem. Pay more
attention."
But an 11-year-old brain has not yet developed such a sophisticated warning
system. Mistakes pass without triggering that error signal, says Cecile D.
Ladouceur, an assistant professor of psychiatry at Pitt, who is studying the
electrical readings. This preawareness detection system doesn't begin to work
until mid- to late adolescence, according to work that she and other researchers
have conducted.
Although it is still early in her studies, Ms. Ladouceur is trying to relate
the results to the real-world risks that teens take. With adults, the fast
monitoring system of the anterior cingulate cortex may help inhibit dangerous
activity, serving as a defense mechanism that younger people have yet to
develop.
"So when adolescents are together and you see, for example, boys
skateboarding and they decide to skateboard down this steep street — well,
they had the idea and the next thing you know they're doing it," she says. "To
what extent is the monitoring system not inhibiting that behavior?"
In another experiment at Pitt, Ms. Luna's team tests Erika's skill at
controlling her actions. She rests her chin on a plastic stirrup, just like
during an eye exam, and stares into the monitor while a yellow dot jumps around
the screen. Although her instinct is to follow the dot, Erika must look in the
opposite direction.
Adolescents can perform this task just as accurately as older individuals,
but the action looks quite different inside the skull. In adults, a distributed
network of brain areas helps fight against the urge that pulls their eyes toward
the dot. However, the brains of adolescents don't recruit from as many regions,
Ms. Luna and her colleagues have found. Instead, teenagers tax just a few parts
of the brain, particularly the prefrontal cortex just behind the forehead, a
region associated with planning and controlling actions.
"Adolescents seem to be pushing the prefrontal cortex more than adults, and
that's what you see in the adult brain when it's doing something difficult,"
says Ms. Luna. While it looks like adults and adolescents can perform this task
equally well, she says, the brain scans show "it's a lot harder for the
adolescents. And when you do something hard, you're vulnerable to error. That's
where the idea of a vulnerable brain system comes."
Growing Through Loss
Until relatively recently, scientists thought that the neural systems of the
brain matured early in life, well before puberty. After all, the brain reaches
90 percent of its adult size by age 6, and some types of faculties — the
rudiments of language, for example — can only be learned in a window of
time in early childhood.
During the 1990s, however, researchers started to study the brains of
children and adolescents using newer techniques, including
magnetic-resonance-imaging machines (MRI), which are considered safer than
previous types of scanning devices. The MRI studies and other experiments found
that the developmental story, which was supposed to end in childhood, actually
had an extended epilogue.
Part of that additional chapter involves the synapses — the connections
between nerve cells that provide the brain with its tremendous computational
capacity. Humans have an estimated 100 billion nerve cells, or neurons, but each
one connects with 1,000 and 10,000 other neurons, which compounds the
possibilities enormously. Out of all those connections, the synapses that don't
get used wither away while repeated activity reinforces the active ones. In some
areas of the brain, 50 percent of the synapses disappear.
The pruning of synapses occurs at different times throughout the brain. The
visual cortex matures by age 7, but parts of the prefrontal cortex, which is
involved in cognition, do not finish their fine-tuning until after
adolescence.
As the pruning progresses, another process adds a fatty insulation called
myelin to the long shafts of some neurons. The myelin layer speeds up how
quickly electrical messages can move along the nerve cells, in some cases by as
much as 100 times. The prefrontal areas of the brain are some of the last to
develop these faster links.
For Ms. Luna, the changes she sees in behavior and mental activity match well
with how the brain physically changes in adolescence. As teens grow, their
prefrontal areas can communicate better with distant brain regions, which
creates a more distributed and sophisticated network for processing information.
In brain scans of people performing the eye-tracking task, Ms. Luna has found
that adults employ a much more specialized set of brain regions to distribute
the workload, when compared with adolescents.
"In theory," she says, "you can imagine that very quick neural transmission
would allow something like the prefrontal cortex, which is really the conductor
of the brain, to go out to different places and say, 'Hey, parietal cortex, or
cerebellum, or temporal cortex. Come and help out so I don't have to do all the
work and we'll be a real efficient machine.'"
Addicted to Novelty
As it grows, the adolescent brain seems hard-wired to seek out exciting and
potentially dangerous situations. While the control centers linked to the
prefrontal cortex take their time to mature, the pleasure-seeking systems of
other regions get a kick start in puberty and go into overdrive, according to
recent studies.
B.J. Casey, director of the Sackler Institute for Developmental Psychobiology
at the Weill Medical College of Cornell University, has been exploring the way
people of different ages respond to rewards and risks by scanning them as they
play games. In a recent study, Ms. Casey and Adriana Galvan, a former doctoral
student, had subjects perform a task on a computer in which they could win $25.
If subjects answered a question correctly, either a single coin or a big pile
would appear on the screen.
In scans of all ages, the coins set off a deeply buried area of the brain
called the nucleus accumbens, a region that responds to various types of
rewards. The adolescent accumbens seemed particularly primed for big payoffs.
The region lit up especially brightly when teenagers saw the pile of money, and
it showed an anemic response to the single coin, when compared with adults, the
Cornell group reported in the Journal of Neuroscience last summer.
Other researchers have found similar activity in this particular brain
region. A team at the National Institutes of Mental Health in 2005 saw that the
accumbens responded more in adolescents than in adults when they received a
reward.
Ms. Casey says it makes sense that the adolescent brain is primed to seek
rewards and take risks. "From an evolutionary perspective," she says, "the only
way you're going to leave your comfortable village and go out and mate somewhere
else that isn't going to be as safe as to be a risk taker."
To understand why the accumbens might be acting up more in adolescence,
researchers are looking into how younger brains deal with dopamine, a key
neurotransmitter molecule produced by some neurons to send signals to other
nerve cells. From studies on animals, researchers know that the dopamine system
is intimately connected with the way mammals respond to rewards.
"A lot of things we do naturally, like sex and food, will cause dopamine to
squirt into the nucleus accumbens," says R. Andrew Chambers, an assistant
professor of psychiatry at the Indiana University School of Medicine, who uses
rats as a model to study substance abuse in human adolescents.
Pleasure is not the only sensation that gets the dopamine system going. In
rats, novelty has a powerful effect on the adolescent brain. "When an animal is
put in a place where it's never been, the dopamine system operates about as
robustly as when it gets a hit of cocaine," says Dr. Chambers.
Evolution may have tuned the mammalian reward system to seek out new
experiences, whether it be trying an unknown food or exploring a foreign region,
he says. "It is so powerful for mammals because it's so critical for survival,"
he says. "This is what's going to tell you what the resources and dangers are
around you."
Dr. Chambers suggests that the dopamine system of adolescents might react so
strongly because the prefrontal cortex is still developing. Neuroscientists call
this trait plasticity, and it confers benefits as well as risks. It helps
explain the creativity of adolescence and early adulthood, before the brain
becomes set in its ways. But it also makes adolescents more prone to addiction,
says Dr. Chambers.
"The conditions in the brain, the circuits that mediate motivation and
addiction, are so plastic and so influenced by addictive drugs that the
motivational disorder can take hold more aggressively during that period," he
says.
At least that's the story for rodents. At present, researchers have gained
most of their knowledge about the dopamine system from work on animals, because
they need to use invasive techniques to monitor this molecule. But some
investigators are starting to study the genetics underlying dopamine activity in
humans.
That is why when Erika first arrives at the Pittsburgh lab, an assistant
scrapes the inside of the girl's cheek to collect some cells. By analyzing
Erika's DNA, the research team will check what variations of genes she carries
for regulating the strength of the dopamine signal. Ahmad R. Hariri, an
assistant professor of psychiatry and director of the developmental-imaging
genetics program at Pitt, has yet to examine the data his group is collecting
for adolescents, but he has started to find some intriguing links in adults. "We
have some exciting preliminary findings showing that the genetic variations in
dopamine-related genes do in fact predict the variability in the reward
circuitry," he says.
As they gather more data, Mr. Hariri will try to extend the research to
adolescents to see if the pattern persists between particular genetic variations
and an exaggerated reward system. In the long run, such findings could suggest
ways to better prevent or treat problems such as addiction, he says.
With Friends Like These ...
When teenagers get into trouble, they like to have company — a tendency
that shows up in crime statistics. "Adolescents are more likely to commit crimes
in groups whereas adults are more likely to commit crimes by themselves," says
Laurence Steinberg, a professor of psychology at Temple University.
Mr. Steinberg has started to look at this phenomenon, testing the commonly
held assumption that the presence of peers makes adolescents take more risks. In
one study, he and Margo Gardner, a former graduate student, used a computer game
in which players tried to earn points by driving a car across a screen. As they
progressed, a yellow light popped up and it eventually turned red, at the same
time as a brick wall appeared in the car's path. When the yellow light appeared,
players had to decide whether to stop immediately or risk forging ahead, in the
hope of gaining more points.
The Temple team had adolescents, young adults, and adults play the game alone
and with peers. Adolescents were far more likely to take risks in a group, while
adults drove more conservatively when surrounded by others. College-age adults
also drove more aggressively in company, though they took fewer risks than
adolescents did.
"This is consistent with data from driving studies indicating that
adolescents have more accidents when there are other teenage passengers in the
car," says Mr. Steinberg. He is looking into extending their research by testing
the influence of peers while subjects undergo a brain scan.
Gregory S. Berns, an associate professor of psychiatry and behavioral
sciences at Emory University, is also scanning teenagers to see how they respond
to social forces. In a previous experiment on adults, his group found that peers
can have a strong effect on the brain, actually coloring the way we see the
world.
Dr. Berns and his team scanned subjects while they mentally rotated objects
and compared them, a task they could do accurately on their own. The trick was
that the experimenters hired actors to perform the same tasks, and their
answers — sometimes purposefully wrong — were shown to the subjects as
they solved the problems. In many cases, when the actors answered incorrectly,
the subjects followed suit.
According to the scanning data, the subjects were not just altering their
answers to go along with the group. "The group's responses were changing
activity in the subjects' visual and perception regions, which suggests that
other people can change the perception of what you can see," says Dr. Berns. In
current work, he is testing teenagers to see whether they are more susceptible
to social interference.
Although these kinds of studies are just starting up, some researchers
hypothesize that the brain has a social-emotional processing system that
operates somewhat independently from the more cold, cognitive network. While the
latter doesn't mature until adulthood, the social-emotional system gets revved
up by hormonal changes in puberty, suggests Mr. Steinberg.
To see that system in action, simply look at a group of teens laughing, he
says. "They laugh louder than adults do, and the same thing happens when they
get angry or upset," he says. "It leads to people getting more aroused, and
aroused more easily."
Compounding Risks
In recent decades, changes in American society have been stacking the deck
against adolescents in this vulnerable stage of life. Busy parents are providing
less supervision during kids' preteenage and teenage years, so they must fend
for themselves much more than they did in the past, says Mr. Steinberg.
Teenagers are also running up significant sleep deficits, which can affect
their ability to moderate their behavior. Research has shown that people's daily
rhythm changes as they enter adolescence, causing them to stay up later and
require more sleep overall. But school schedules force teenagers to rise earlier
than their bodies are naturally prepared to do. Distractions like the Internet
and increasing homework loads add to the problem by keeping high schoolers up
even later.
And for reasons not well understood, children are entering puberty
significantly earlier than in the past, so they spend longer going through the
risky years of adolescence. By age 8, for example, 47 percent of
African-American girls and 15 percent of European American girls have started to
develop breasts, or pubic hair, or both.
The new scientific findings are leading scientists — and society at
large — to rethink how to treat teenagers. The Supreme Court, for example,
quoted Mr. Steinberg's work when it ruled in 2005 that states could not execute
people for crimes they committed as juveniles.
Because the brains of adolescents are not ready to fully regulate their
behavior, "adolescents need more supervision," says Mr. Steinberg. "We need to
build that into the way in which the laws and other kinds of social policies
regulate their behavior."
Ronald E. Dahl, a professor of psychiatry at Pittsburgh, has reached a
similar conclusion from his own research on adolescents. He calls for adults to
provide kids with more "scaffolding and monitoring, so that risks are relatively
less, but as [show responsibility and develop skills, you gradually give them
more freedom."
That kind of support — in the form of supervised after-school programs
or restricted driving licenses — is critical because it allows the
adolescent brain to acquire its social and emotional fluency, he suggests. When
that learning process breaks down, either because of genetic susceptibilities or
problems in their lives, teenagers can develop depression, anxiety, or the other
types of mood disorders that make their appearance during this stage of life.
Stunted growth in this domain can also send people veering toward addiction, he
says.
He and his team are trying to peer into the hypothesized window for social
and emotional learning by testing how the brains of adolescents with mood
disorders react differently from those of unafflicted teens. As a participant in
the study, Erika represents a normal kid just climbing onto the roller coaster
of puberty. Inside and out, she is starting to show the telltale signs that her
brain and body are entering adolescence. Like most of her peers, her attention
is shifting away from family and toward friends. "She's starting to spend more
time on the phone," says her mother.
The day of testing wears on, and Erika grows bored with the procedures. But
while waiting to get her brain scanned, she lights up as conversation turns to
the MySpace social-networking Web site, where she has posted a page in violation
of its age restrictions. Some adults in the room, clueless about the MySpace
phenomenon, ask what Erika does on the site. The 11-year-old answers in a vague
way that leaves the experimenters just as uninformed as before. Her broad face
curls into a grin when she explains the site's appeal.
"It's awesome," she says. "It's just
awesome." http://chronicle.com
Section: Research &
Publishing
Volume 53, Issue 19, Page A14
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