Provided by the National Institute of Mental Health
New imaging studies are revealing—for the first
time—patterns of brain development that extend into the teenage years. Although
scientists don't know yet what accounts for the observed changes, they may
parallel a pruning process that occurs early in life that appears to follow the
principle of "use-it-or-lose-it:" neural connections, or synapses, that get
exercised are retained, while those that don't are lost. At least, this is what
studies of animals' developing visual systems suggest. While it's known that
both genes and environment play major roles in shaping early brain development,
science still has much to learn about the relative influence of experience
versus genes on the later maturation of the brain. Animal studies support a role
for experience in late development, but no animal species undergoes anything
comparable to humans' protracted childhood and adolescence. Nor is it yet clear
whether experience actually creates new neurons and synapses, or merely
establishes transitory functional changes. Nonetheless, it's tempting to
interpret the new findings as empowering teens to protect and nurture their
brain as a work in progress.
The newfound appreciation of the dynamic nature of the teen brain is emerging
from MRI (magnetic resonance imaging) studies that scan a child's brain every
two years, as he or she grows up. Individual brains differ enough that only
broad generalizations can be made from comparisons of different individuals at
different ages. But following the same brains as they mature allows scientists a
much finer-grained view into developmental changes. In the first such
longitudinal study of 145 children and adolescents, reported in l999, NIMH's Dr.
Judith Rapoport and colleagues were surprised to discover a second wave of
overproduction of gray matter, the thinking part of the brain—neurons and their
branch-like extensions—just prior to puberty.1 Possibly related to the influence
of surging sex hormones, this thickening peaks at around age 11 in girls, 12 in
boys, after which the gray matter actually thins some.
Prior to this study, research had shown that the brain overproduced gray matter
for a brief period in early development—in the womb and for about the first 18
months of life—and then underwent just one bout of pruning. Researchers are now
confronted with structural changes that occur much later in adolescence. The
teen's gray matter waxes and wanes in different functional brain areas at
different times in development. For example, the gray matter growth spurt just
prior to puberty predominates in the frontal lobe, the seat of "executive
functions"—planning, impulse control and reasoning. In teens affected by a rare,
childhood onset form of schizophrenia that impairs these functions, the MRI
scans revealed four times as much gray matter loss in the frontal lobe as
normally occurs.2 Unlike gray matter, the brain's white matter—wire-like fibers
that establish neurons' long-distance connections between brain regions—thickens
progressively from birth in humans. A layer of insulation called myelin
progressively envelops these nerve fibers, making them more efficient, just like
insulation on electric wires improves their conductivity.
Advancements in MRI image analysis are providing new insights into how the brain
develops. UCLA's Dr. Arthur Toga and colleagues turned the NIMH team's MRI scan
data into 4-D time-lapse animations of children's brains morphing as they grow
up—the 4th dimension being rate-of-change.3 Researchers report a wave of white
matter growth that begins at the front of the brain in early childhood, moves
rearward, and then subsides after puberty. Striking growth spurts can be seen
from ages 6 to 13 in areas connecting brain regions specialized for language and
understanding spatial relations, the temporal and parietal lobes. This growth
drops off sharply after age 12, coinciding with the end of a critical period for
learning languages.
While this work suggests a wave of brain white matter development that flows
from front to back, animal, functional brain imaging and postmortem studies have
suggested that gray matter maturation flows in the opposite direction, with the
frontal lobes not fully maturing until young adulthood. To confirm this in
living humans, the UCLA researchers compared MRI scans of young adults, 23-30,
with those of teens, 12-16.4 They looked for signs of myelin, which would imply
more mature, efficient connections, within gray matter. As expected, areas of
the frontal lobe showed the largest differences between young adults and teens.
This increased myelination in the adult frontal cortex likely relates to the
maturation of cognitive processing and other "executive" functions. Parietal and
temporal areas mediating spatial, sensory, auditory and language functions
appeared largely mature in the teen brain. The observed late maturation of the
frontal lobe conspicuously coincides with the typical age-of-onset of
schizophrenia—late teens, early twenties—which, as noted earlier, is
characterized by impaired "executive" functioning.
Another series of MRI studies is shedding light on how teens may process
emotions differently than adults. Using functional MRI (fMRI), a team led by Dr.
Deborah Yurgelun-Todd at Harvard's McLean Hospital scanned subjects' brain
activity while they identified emotions on pictures of faces displayed on a
computer screen.5 Young teens, who characteristically perform poorly on the
task, activated the amygdala, a brain center that mediates fear and other "gut"
reactions, more than the frontal lobe. As teens grow older, their brain activity
during this task tends to shift to the frontal lobe, leading to more reasoned
perceptions and improved performance. Similarly, the researchers saw a shift in
activation from the temporal lobe to the frontal lobe during a language skills
task, as teens got older. These functional changes paralleled structural changes
in temporal lobe white matter.
While these studies have shown remarkable changes that occur in the brain during
the teen years, they also demonstrate what every parent can confirm: the teenage
brain is a very complicated and dynamic arena, one that is not easily
understood.
--------------------------------------------------------------------------------
For More Information
Please visit the following links for information about organizations that focus
on children and adolescents and the human brain.
--------------------------------------------------------------------------------
All material in this fact sheet is in the public domain and may be copied or
reproduced without permission from the Institute. Citation of the source is
appreciated.
NIH Publication No. 01-4929
--------------------------------------------------------------------------------
References
1 Giedd JN, Blumenthal J, Jeffries NO, et al. Brain development during childhood
and adolescence: a longitudinal MRI study. Nature Neuroscience, 1999; 2(10):
861-3.
2 Rapoport JL, Giedd JN, Blumenthal J, et al. Progressive cortical change during
adolescence in childhood-onset schizophrenia. A longitudinal magnetic resonance
imaging study. Archives of General Psychiatry, 1999; 56(7): 649-54.
3 Thompson PM, Giedd JN, Woods RP, et al. Growth patterns in the developing
brain detected by using continuum mechanical tensor maps. Nature, 2000;
404(6774): 190-3.
4 Sowell ER, Thompson PM, Holmes CJ, et al. In vivo evidence for post-adolescent
brain maturation in frontal and striatal regions. Nature Neuroscience, 1999;
2(10): 859-61.
5 Baird AA, Gruber SA, Fein DA, et al. Functional magnetic resonance imaging of
facial affect recognition in children and adolescents. Journal of the American
Academy of Child and Adolescent Psychiatry, 1999; 38(2): 195-9.
Back to Mental Health Articles