

The Teen Brain: Still Under Construction
Introduction
One of the ways that scientists have searched for the causes of
mental illness is by studying the development of the brain from birth to
adulthood. Powerful new technologies have enabled them to track the
growth of the brain and to investigate the connections between brain
function, development, and behavior.
The research has turned up
some surprises, among them the discovery of striking changes taking
place during the teen years. These findings have altered long-held
assumptions about the timing of brain maturation. In key ways, the brain
doesn’t look like that of an adult until the early 20s.
An
understanding of how the brain of an adolescent is changing may help
explain a puzzling contradiction of adolescence: young people at this
age are close to a lifelong peak of physical health, strength, and
mental capacity, and yet, for some, this can be a hazardous age.
Mortality rates jump between early and late adolescence. Rates of death
by injury between ages 15 to 19 are about six times that of the rate
between ages 10 and 14. Crime rates are highest among young males and
rates of alcohol abuse are high relative to other ages. Even though most
adolescents come through this transitional age well, it’s important to
understand the risk factors for behavior that can have serious
consequences. Genes, childhood experience, and the environment in which a
young person reaches adolescence all shape behavior. Adding to this
complex picture, research is revealing how all these factors act in the
context of a brain that is changing, with its own impact on behavior.
The
more we learn, the better we may be able to understand the abilities
and vulnerabilities of teens, and the significance of this stage for
life-long mental health.
The fact that so much change is taking
place beneath the surface may be something for parents to keep in mind
during the ups and downs of adolescence.
The "Visible" Brain
A clue to the degree of change taking place in the teen brain came
from studies in which scientists did brain scans of children as they
grew from early childhood through age 20. The scans revealed
unexpectedly late changes in the volume of gray matter, which forms the
thin, folding outer layer or cortex of the brain. The cortex is where
the processes of thought and memory are based. Over the course of
childhood, the volume of gray matter in the cortex increases and then
declines. A decline in volume is normal at this age and is in fact a
necessary part of maturation.
The assumption for many years had
been that the volume of gray matter was highest in very early childhood,
and gradually fell as a child grew. The more recent scans, however,
revealed that the high point of the volume of gray matter occurs during
early adolescence.
While the details behind the changes in volume
on scans are not completely clear, the results push the timeline of
brain maturation into adolescence and young adulthood. In terms of the
volume of gray matter seen in brain images, the brain does not begin to
resemble that of an adult until the early 20s.
The scans also
suggest that different parts of the cortex mature at different rates.
Areas involved in more basic functions mature first: those involved, for
example, in the processing of information from the senses, and in
controlling movement. The parts of the brain responsible for more
"top-down" control, controlling impulses, and planning ahead—the
hallmarks of adult behavior—are among the last to mature.
What's Gray Matter?
The details of what is behind the increase and decline in gray matter
are still not completely clear. Gray matter is made up of the cell
bodies of neurons, the nerve fibers that project from them, and support
cells. One of the features of the brain's growth in early life is that
there is an early blooming of synapses—the connections between brain
cells or neurons—followed by pruning as the brain matures. Synapses are
the relays over which neurons communicate with each other and are the
basis of the working circuitry of the brain. Already more numerous than
an adult's at birth, synapses multiply rapidly in the first months of
life. A 2-year-old has about half again as many synapses as an adult.
(For an idea of the complexity of the brain: a cube of brain matter, 1
millimeter on each side, can contain between 35 and 70 million neurons
and an estimated 500 billion synapses.)
Scientists believe that
the loss of synapses as a child matures is part of the process by which
the brain becomes more efficient. Although genes play a role in the
decline in synapses, animal research has shown that experience also
shapes the decline. Synapses "exercised" by experience survive and are
strengthened, while others are pruned away. Scientists are working to
determine to what extent the changes in gray matter on brain scans
during the teen years reflect growth and pruning of synapses.
A Spectrum of Change
Research using many different approaches is showing that more than gray matter is changing:
- Connections
between different parts of the brain increase throughout childhood and
well into adulthood. As the brain develops, the fibers connecting nerve
cells are wrapped in a protein that greatly increases the speed with
which they can transmit impulses from cell to cell. The resulting
increase in connectivity—a little like providing a growing city with a
fast, integrated communication system—shapes how well different parts of
the brain work in tandem. Research is finding that the extent of
connectivity is related to growth in intellectual capacities such as
memory and reading ability.
- Several lines of evidence suggest
that the brain circuitry involved in emotional responses is changing
during the teen years. Functional brain imaging studies, for example,
suggest that the responses of teens to emotionally loaded images and
situations are heightened relative to younger children and adults. The
brain changes underlying these patterns involve brain centers and
signaling molecules that are part of the reward system with which the
brain motivates behavior. These age-related changes shape how much
different parts of the brain are activated in response to experience,
and in terms of behavior, the urgency and intensity of emotional
reactions.
- Enormous hormonal changes take place during
adolescence. Reproductive hormones shape not only sex-related growth and
behavior, but overall social behavior. Hormone systems involved in the
brain's response to stress are also changing during the teens. As with
reproductive hormones, stress hormones can have complex effects on the
brain, and as a result, behavior.
- In terms of sheer intellectual
power, the brain of an adolescent is a match for an adult's. The
capacity of a person to learn will never be greater than during
adolescence. At the same time, behavioral tests, sometimes combined with
functional brain imaging, suggest differences in how adolescents and
adults carry out mental tasks. Adolescents and adults seem to engage
different parts of the brain to different extents during tests requiring
calculation and impulse control, or in reaction to emotional content.
- Research
suggests that adolescence brings with it brain-based changes in the
regulation of sleep that may contribute to teens' tendency to stay up
late at night. Along with the obvious effects of sleep deprivation, such
as fatigue and difficulty maintaining attention, inadequate sleep is a
powerful contributor to irritability and depression. Studies of children
and adolescents have found that sleep deprivation can increase
impulsive behavior; some researchers report finding that it is a factor
in delinquency. Adequate sleep is central to physical and emotional
health.
The Changing Brain and Behavior in Teens
One interpretation of all these findings is that in teens, the parts
of the brain involved in emotional responses are fully online, or even
more active than in adults, while the parts of the brain involved in
keeping emotional, impulsive responses in check are still reaching
maturity. Such a changing balance might provide clues to a youthful
appetite for novelty, and a tendency to act on impulse—without regard
for risk.
While much is being learned about the teen brain, it is
not yet possible to know to what extent a particular behavior or ability
is the result of a feature of brain structure—or a change in brain
structure. Changes in the brain take place in the context of many other
factors, among them, inborn traits, personal history, family, friends,
community, and culture.
Teens and the Brain: More Questions for Research
Scientists continue to investigate the development of the brain and
the relationship between the changes taking place, behavior, and health.
The following questions are among the important ones that are targets
of research:
- How do experience and environment interact with
genetic preprogramming to shape the maturing brain, and as a result,
future abilities and behavior? In other words, to what extent does what a
teen does and learns shape his or her brain over the rest of a
lifetime?
- In what ways do features unique to the teen brain play
a role in the high rates of illicit substance use and alcohol abuse in
the late teen to young adult years? Does the adolescent capacity for
learning make this a stage of particular vulnerability to addiction?
- Why is it so often the case that, for many mental disorders, symptoms first emerge during adolescence and young adulthood?
This
last question has been the central reason to study brain development
from infancy to adulthood. Scientists increasingly view mental illnesses
as developmental disorders that have their roots in the processes
involved in how the brain matures. By studying how the circuitry of the
brain develops, scientists hope to identify when and for what reasons
development goes off track. Brain imaging studies have revealed
distinctive variations in growth patterns of brain tissue in youth who
show signs of conditions affecting mental health. Ongoing research is
providing information on how genetic factors increase or reduce
vulnerability to mental illness; and how experiences during infancy,
childhood, and adolescence can increase the risk of mental illness or
protect against it.
The Adolescent and Adult Brain
It is not surprising that the behavior of adolescents would be a
study in change, since the brain itself is changing in such striking
ways. Scientists emphasize that the fact that the teen brain is in
transition doesn't mean it is somehow not up to par. It is different
from both a child's and an adult's in ways that may equip youth to make
the transition from dependence to independence. The capacity for
learning at this age, an expanding social life, and a taste for
exploration and limit testing may all, to some extent, be reflections of
age-related biology.
Understanding the changes taking place in
the brain at this age presents an opportunity to intervene early in
mental illnesses that have their onset at this age. Research findings on
the brain may also serve to help adults understand the importance of
creating an environment in which teens can explore and experiment while
helping them avoid behavior that is destructive to themselves and
others.
Alcohol and the Teen Brain
Adults drink more frequently than teens, but when teens drink they
tend to drink larger quantities than adults. There is evidence to
suggest that the adolescent brain responds to alcohol differently than
the adult brain, perhaps helping to explain the elevated risk of binge
drinking in youth. Drinking in youth, and intense drinking are both risk
factors for later alcohol dependence. Findings on the developing brain
should help clarify the role of the changing brain in youthful drinking,
and the relationship between youth drinking and the risk of addiction
later in life.
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NIH Publication No. 11-4929
2011
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