Teenage Brain: A work in progress
> 1/9/2006 10:35:26 AM

New imaging studies are revealing—for the first time—patterns ofbrain development that extend into the teenage years. Althoughscientists don't know yet what accounts for the observed changes, theymay parallel a pruning process that occurs early in life that appearsto follow the principle of "use-it-or-lose-it:" neural connections, orsynapses, that get exercised are retained, while those that don't arelost. At least, this is what studies of animals' developing visualsystems suggest. While it's known that both genes and environment playmajor roles in shaping early brain development, science still has muchto learn about the relative influence of experience versus genes on thelater maturation of the brain. Animal studies support a role forexperience in late development, but no animal species undergoesanything comparable to humans' protracted childhood and adolescence.Nor is it yet clear whether experience actually creates new neurons andsynapses, or merely establishes transitory functional changes.Nonetheless, it's tempting to interpret the new findings as empoweringteens to protect and nurture their brain as a work in progress.

The newfound appreciation of the dynamic nature of the teen brain isemerging from MRI (magnetic resonance imaging) studies that scan achild's brain every two years, as he or she grows up. Individual brainsdiffer enough that only broad generalizations can be made fromcomparisons of different individuals at different ages. But followingthe same brains as they mature allows scientists a much finer-grainedview into developmental changes. In the first such longitudinal studyof 145 children and adolescents, reported in l999, NIMH's Dr. JudithRapoport and colleagues were surprised to discover a second wave ofoverproduction of gray matter, the thinking part of the brain—neuronsand their branch-like extensions—just prior to puberty.1Possibly related to the influence of surging sex hormones, thisthickening peaks at around age 11 in girls, 12 in boys, after which thegray matter actually thins some.

Prior to this study, research had shown that the brain overproducedgray matter for a brief period in early development—in the womb and forabout the first 18 months of life—and then underwent just one bout ofpruning. Researchers are now confronted with structural changes thatoccur much later in adolescence. The teen's gray matter waxes and wanesin different functional brain areas at different times in development.For example, the gray matter growth spurt just prior to pubertypredominates in the frontal lobe, the seat of "executivefunctions"—planning, impulse control and reasoning. In teens affectedby a rare, childhood onset form of schizophrenia that impairs thesefunctions, the MRI scans revealed four times as much gray matter lossin 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 progressivelyfrom birth in humans. A layer of insulation called myelin progressivelyenvelops these nerve fibers, making them more efficient, just likeinsulation on electric wires improves their conductivity.

Advancements in MRI image analysis are providing new insights intohow the brain develops. UCLA's Dr. Arthur Toga and colleagues turnedthe NIMH team's MRI scan data into 4-D time-lapse animations ofchildren's brains morphing as they grow up—the 4th dimension beingrate-of-change.3 Researchers report a wave of white mattergrowth that begins at the front of the brain in early childhood, movesrearward, and then subsides after puberty. Striking growth spurts canbe seen from ages 6 to 13 in areas connecting brain regions specializedfor language and understanding spatial relations, the temporal andparietal lobes. This growth drops off sharply after age 12, coincidingwith the end of a critical period for learning languages.

While this work suggests a wave of brain white matter developmentthat flows from front to back, animal, functional brain imaging andpostmortem studies have suggested that gray matter maturation flows inthe opposite direction, with the frontal lobes not fully maturing untilyoung adulthood. To confirm this in living humans, the UCLA researcherscompared MRI scans of young adults, 23-30, with those of teens, 12-16.4They looked for signs of myelin, which would imply more mature,efficient connections, within gray matter. As expected, areas of thefrontal lobe showed the largest differences between young adults andteens. This increased myelination in the adult frontal cortex likelyrelates 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 teenbrain. The observed late maturation of the frontal lobe conspicuouslycoincides 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 mayprocess emotions differently than adults. Using functional MRI (fMRI),a team led by Dr. Deborah Yurgelun-Todd at Harvard's McLean Hospitalscanned subjects' brain activity while they identified emotions onpictures of faces displayed on a computer screen.5 Youngteens, who characteristically perform poorly on the task, activated theamygdala, a brain center that mediates fear and other "gut" reactions,more than the frontal lobe. As teens grow older, their brain activityduring this task tends to shift to the frontal lobe, leading to morereasoned perceptions and improved performance. Similarly, theresearchers saw a shift in activation from the temporal lobe to thefrontal lobe during a language skills task, as teens got older. Thesefunctional changes paralleled structural changes in temporal lobe whitematter.

While these studies have shown remarkable changes that occur in thebrain during the teen years, they also demonstrate what every parentcan confirm: the teenage brain is a very complicated and dynamic arena,one that is not easily understood.


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-onsetschizophrenia. A longitudinal magnetic resonance imaging study. Archives of General Psychiatry, 1999; 56(7): 649-54.

3 Thompson PM, Giedd JN, Woods RP, et al. Growthpatterns in the developing brain detected by using continuum mechanicaltensor maps. Nature, 2000; 404(6774): 190-3.

4 Sowell ER, Thompson PM, Holmes CJ, et al. Invivo evidence for post-adolescent brain maturation in frontal andstriatal regions. Nature Neuroscience, 1999; 2(10): 859-61.

5 Baird AA, Gruber SA, Fein DA, et al. Functionalmagnetic resonance imaging of facial affect recognition in children andadolescents. Journal of the American Academy of Child and Adolescent Psychiatry, 1999; 38(2): 195-9.

From Teenage Brain: A work in progress, National Institute for Mental Health (NIMH)

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