Health Background


Growth and Development: The Teenage Brain

The Decade of the Brain (1990-2000), declared by then-President George Bush, focused scientific research on the human brain. One outcome was new imaging studies that revealed — for the first time — patterns of brain development that extended into the teenage years. Previously, research had shown that the brain overproduced gray matter (neurons and their branch-like extensions), the thinking part of the brain, for a brief period in a child's early development: in utero and for about the first 18 months of life. The brain then underwent a cutting back period, following a "use it or lose it" principle, i.e., neural connections or synapses that were exercised were retained; those that weren't exercised were lost.

In the first longitudinal study of the brains of 145 children and adolescents, published in 1999, scientists at the National Institute of Mental Health (NIMH) discovered a second wave of overproduction of gray matter, just prior to puberty. This thickening peaks at around age 11 in girls and 12 in boys. The researchers speculate that this change may be due to the influence of surging sex hormones.

Following the pre-puberty surge, the gray matter actually thins some, again following a "use it or lose it" principle. Further, the teen's gray matter waxes and wanes in different functional brain areas at different times in development. The gray matter growth spurt just prior to puberty is largely in the frontal lobe, the seat of "executive functions," such as planning, impulse control, and reasoning. In teens who developed early onset schizophrenia, the MRI scans revealed that they lost four times more gray matter in the frontal lobe as non-schizophrenic teens. The teen's gray matter waxes and wanes in different functional brain areas at different times in development.*

Unlike gray matter, the brain's white matter (wire-like fibers that establish neuron's long-distance connections between brain regions) thickens progressively from birth in humans. A wave of white matter growth begins at the front of the brain in early childhood, moves rearward, and then subsides after puberty. From age 6 to age 13, there are growth spurts 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, signifying the end of a critical period for learning languages.

In this new brain research, magnetic resonance imaging (MRI) of children's brains at two-year intervals over a period of years as they mature allows scientists to track developmental changes in the human brain. The MRI scan data is then transformed into 3-D time-lapse animations of children's brains morphing as they grow up.

In a subsequent study using the same techniques, scientists at NIMH and the University of California at Los Angeles (UCLA) showed that gray matter diminished in a back-to-front wave. This study followed 13 healthy children and teens every two years as they grew up, for ten years. The new study also found that the first areas to mature (e.g., extreme front and back of the brain) are those with the most basic functions, such as processing these senses and movement. Areas involved in spatial orientation and language (parietal lobes) follow. Areas with more advanced functions — integrating information from the senses, reasoning and other "executive" functions (prefrontal cortex) — mature last, during young adulthood.

*This information is from Teenage Brain: A Work in Progress A brief overview of research into brain development during adolescence, 2001

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