A brief overview of research into brain development during adolescence.
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.
Hormones aren't the only reason adolescents act crazy. Their gray matter differs from children's and adults'.
You probably recognize the species: it's known for making stupid decisions... barely able to plan beyond the next minute... clueless when it comes to reading parents' facial expressions... exhibits poor self-control... seems to think with its hormones more than its brain... all thumbs when juggling several tasks.
Such is Homo teenageris.
But while the adolescent mind has been well documented, the reason kids passing through and just beyond puberty seem to be members of a different species has been a puzzle, though raging hormones and simple rebellion are handy scapegoats. It turns out there's a good reason adolescent brains seem different: they are. Contrary to the notion that the brain has fully matured by the age of 8 or 12, with the truly crucial wiring complete as early as 3, it turns out that the brain is an ongoing construction site. "Maturation does not stop at age 10, but continues into the teen years and even the 20s," says Jay Giedd of the National Institute of Mental Health. "What is most surprising is that you get a second wave of overproduction of gray matter, something that was thought to happen only in the first 18 months of life."
The brain reaches about 95 percent of its adult volume by the age of 5 or so. To get beyond such crude measures, in the early 1990s Giedd and colleagues began snapping images of the brains of healthy kids with magnetic resonance imaging (MRI) every two years. The first surprise came last May, with the discovery that the corpus callosum, the cable of nerves that connects the right half of the brain to the left, "continues growing into your 20s," says Giedd. Although the effect of an immature corpus callosum is not crystal clear, let us simply note that this structure has been implicated in intelligence, consciousness and self-awareness.
Until now, studies of the brains of children and adolescents have shown that their gray matter decreases with age. The rule seems to be "use it or lose it": connections among neurons that are not used wither away, a process called pruning. But neuroscientists led by Elizabeth Sowell of UCLA's Lab of Neuro Imaging found that the story is not so simple. They used MRI to compare the brains of 12- to 16-year-olds to those of twentysomethings. What they found will surprise no one who has a teen or is a teen or just remembers being a teen: the frontal lobes, responsible for such "executive" functions as self-control, judgment, emotional regulation, organization and planning, undergo the greatest change between puberty and young adulthood. They grow measurably between 10 and 12 (with girls' growth spurt generally coming a little earlier than boys'), then shrink into the 20s as extraneous branchings are pruned back into efficient, well-organized circuitry. Giedd's team, using MRI to scan the brains of 145 healthy 4- to 21-year-olds, also "found that the grey matter in the frontal lobes increased through age 11 or 12," as they reported in the journal Nature Neuroscience. "Then there is a noticeable decline. It looks like there is a second wave of creation of gray matter at puberty, probably related to new connections and branches, followed by pruning." Neuronal connections that underlie cognitive and other abilities stick around if they're used, but wither if they're not.
Toddlers are pretty much at the mercy of their parents when it comes to the kind and amount of environmental stimulation they get, and thus which connections remain. Teenagers, however, create their own world. "Teens thus have the power to determine their own brain development, to determine which connections survive and which don't," says Giedd. "Whether they do art, or music, or sports, or videogames, the brain is figuring out what it needs to survive and adapting accordingly."
Things get even more interesting once neuroscientists look beyond the frontal lobes. When the UCLA team scanned the brains of 19 normal children and adolescents, ages 7 and 16, they found that the parietal lobes (which integrate information from far-flung neighborhoods of the brain, such as auditory, tactile and visual signals) are still maturing through the midteens. The long nerve fibers called white matter are probably still being sheathed in myelin, a fatty substance that lets nerves transmit signals faster and more efficiently. As a result, circuits that make sense of disparate information are works in progress through age 16 or so. The parietal lobes reach their gray-matter peak at 10 (girls) or 12 (boys), and are then pruned. But the seats of language, as well as emotional control, called the temporal lobes, do not reach their gray-matter maximum until age 16, Giedd finds. Only then do they undergo pruning. If teens are hardly models of emotional maturity, at least they have a good excuse.
Studies at McLean Hospital outside Boston find that many teenagers are unable to read emotions in people's faces. Brain regions that light up with activity when adults read "fear" in faces are nearly dark in these teens. Their brains' emotional centers light up. But the thinking regions stay dark, as if they are unable to integrate visual, emotional and cognitive information. No wonder looking daggers at a teen hardly gets a rise out of him.
Might hormones be responsible for the changes the brain undergoes during and after adolescence? Research into this question is only in its infancy. But in one suggestive finding, Giedd reports that in girls the hippocampus, which responds to estrogen, grows faster than in boys. The hippocampus forms memories. In boys, the amygdala, which responds to androgen, grows faster than in girls. The amygdala is in charge of emotions like fear and anger.
Together, the experiments suggest that the teen brain reprises one of its most momentous acts of infancy, the overproduction and then pruning of neuronal branches. "The brain," says Sowell, "undergoes dynamic changes much later than we originally thought." Maturity is not simply a matter of slipping software (learning) into existing equipment. Instead, the hardware changes. Those changes partly reflect signals from the world outside, and seem to be a peculiarly human adaptation. Think of it as nature's way of giving us a second chance.
The brain, say textbooks, is mostly finished growing and connecting by the age of 12 or so. But new studies using MRI show that puberty brings a neuronal growth spurt, followed by the loss of circuits that are not used. Might the pattern of maturation even explain tongue studs?
ARTICLE #3 Adolescent Brain Development A collaboration of Cornell University, University of Rochester, and the NYS Center for School Safety
http://www.actforyouth.net/documents/may02factsheetadolbraindev.pdf Research now supports what parents have long suspected—that the teenager’s brain isdifferent than the adult brain. Recent research by scientists at the National Institute ofMental Health (NIMH) using magnetic resonance imaging (MRI) has found that the teenbrain is not a finished product, but is a work in progress. Until recently most scientists
believed that the major "wiring" of the brain was completed by as early as three years ofage and that the brain was fully mature by the age of 10 or 12. New findings show thatthe greatest changes to the parts of the brain that are responsible for functions such as self-control, judgment, emotions, and organization occur between puberty and adulthood.
This may help to explain certain teenage behavior that adults can find mystifying, such as poor decision-making, recklessness, and emotional outbursts.
The brain is still developing during the teen years Dr. Jay Giedd of the NIMH has reported that brain “maturation does not stop at age 10, but continues into the teen years and even into the 20’s. What is most surprising is that you get a second wave of overproduction of gray matter, something that was thought to happen only in the first 18 months of life (Begley, 2000).” Following the overproduction of gray matter, the brain undergoes a process called “pruning” where connections among neurons in the brain that are not
used wither away, while those that are used stay—the “use it or lose it” principle. It is thought that this pruning process makes the brain more efficient by strengthening the connections that are used most often, and eliminating the clutter of those that are not used at all.
What does this mean for teens? According to Dr. Giedd, this is exciting news for teens. “…unlike infants whose brain activity is completely determined by their parents and environment, the teens may actually be able to control how their own brains are wired and
sculpted.” Kids who “exercise” their brains by learning to order their thoughts, understand abstract concepts, and control their impulses are laying the neural foundations that will serve them for the rest of their lives. "This argues for doing a lot of things as a teenager," says Dr. Giedd. "You are hard-wiring your brain in adolescence. Do you want
to hard-wire it for sports and playing music and doing mathematics–or for lying on the couch in front of the television?"
Alcohol use and the developing teen brain Recent research suggests that alcohol use affects adolescents and adults differently, which makes sense given what we now know about the changes going on in the teen brain. While more research needs to be done in this area, Duke University scientists say “the available research suggests that adolescents are more vulnerable than adults to the affects of alcohol on learning and memory (White, 2001). Not only do they react differently to the initial affects of alcohol, studies
suggest that teens who repeatedly use alcohol can suffer long-term effects. Preliminary studies using rats have shown that those with repeated alcohol exposure during adolescence are more sensitive to alcohol-induced impairments later in life (White, 2001).
Research on humans by Brown, et al. (2000) has shown the first concrete evidence that heavy, on-going alcohol use by adolescents can impair brain functioning. Brown’s research on 15 and 16 year olds showed cognitive impairments in teen alcohol abusers, compared with non-abusing peers, even weeks after they stop drinking. This suggests
that abuse of alcohol by teens may have long-term negative effects on the make up of their brains.
Teens and understanding emotions Teens also differ from adults in their ability to read and understand emotions in the faces of others. Recent research shows that teens and adults actually use different regions of the brain in responding to certain tasks. In a study conducted at Boston’s McLean Hospital, psychologist Deborah Yurgelun-Todd and colleagues showed pictures of people wearing fearful expressions to teenagers between the ages of 11 and 17 while the teens had their brains scanned using functional magnetic resonance imaging (fMRI). She found that compared to adults the teens' frontal lobes (the seat of goal-oriented rational thinking) are less active and their amygdala (a structure in the temporal lobe that is involved in discriminating fear and other emotions) is more active. The teens often misread facial expressions, with those under the age of 14 more often
seeing sadness or anger or confusion instead of fear. Older teenagers answered correctly
more often and exhibited a progressive shift of activity from the amygdala to the frontal lobes. The results suggest that "in teens, the judgment, insight and reasoning power of the frontal cortex is not being brought to bear on the task as it is in adults. Teens just process information differently from adults” (Yurgelun-Todd, 2002).
Implications It is important to note that experts caution careful interpretation of this new information about adolescent brain development, as it is still very early in the analysis and understanding of what it all means. Yet it is also true that these findings add new dimensions to issues facing young people, as well as their parents and teachers,
and they pose a challenge to policy makers (NIH, 2000). If the choices adolescents make about using drugs and alcohol and engaging in or avoiding challenging learning tasks have long-term and irreversible consequences for the development of their brains, then discouraging harmful choices and encouraging healthy ones is all the more urgent. This
new research may also provide a compelling explanation for why adolescents often fail to heed adults' warnings about such choices; they may simply not be able to understand and accept arguments that seem logical and decisive to adults. It is also possible that teens are misperceiving or misunderstanding the emotions of adults, leading to miscommunication
both in terms of what the teen thinks the adult is feeling and in terms of the teen’s response. Perhaps most importantly, teenagers are empowered with opportunities to develop their brains through the activities in which they choose to participate.
Brain Regions and functions Frontal lobe—self-control, judgment, emotional regulation; restructured in teen years
Corpus callosum—intelligence, consciousness and self-awareness; reaches full maturity in 20’s
Parietal lobes—integrate auditory, visual, and tactile signals; immature until age 16