WASHINGTON—Scientists are gaining new insights into remarkable changes in teenagers' brains that may help explain why the teen years are so hard on young people—and on their parents.
From ages 11 to 14, a young person loses a substantial fraction of the connections between cells in the part of the brain that enable him or her to think clearly and make good decisions. This loss is a vital part of growing up. It clears out, or "prunes," unneeded wiring to make way for more efficient information-processing in adults.
"Ineffective or weak connections are pruned in much the same way a gardener would prune a tree or bush, giving the plant the desired shape," said Alison Gopnik, a professor of child development at the University of California-Berkeley.
The pruning process "appears to follow the principle of use-it-or-lose-it," said Jay Giedd, a child development expert at the National Institute of Mental Health, in Bethesda, Md. "Neural connections or synapses that get exercised are retained, while those that don't are lost."
Like teenage pimples and body hair, changes inside the head can be upsetting.
"It certainly seems possible that normal adolescents who are experiencing these brain changes can react emotionally," said Ian Campbell, a neuroscientist at the University of California-Davis Sleep Research Laboratory.
"Teens may process emotions differently than adults," said Giedd, who calls the teenage brain "a work in progress."
Girls typically start pruning their brain cells about a year before boys do, but the loss ends up the same, Campbell said.
To figure out why teenagers are often moody, uncooperative and irresponsible, scientists make images of their brains. Their tools include electroencephalograms, which record brain waves, and functional magnetic resonance imaging, which measures activity in various regions of the brain.
"In the past decade, brain changes in adolescence have become the subject of intensive research," Campbell said.
For instance, he and a colleague at the University of California-Davis, psychiatrist Irwin Feinberg, attached EEG recorders to the skulls of two groups of children—one of 9- to 11-year-olds, the other of 12- to 14-year-olds—while they slept. The devices showed that the brain waves were 25 percent weaker in the older children than in the younger ones, the scientists reported in the December issue of the American Journal of Physiology.
These waves are produced by electrical vibrations in brain cells, or neurons. The more neurons vibrate in concert, the stronger the wave.
Campbell compared the effect to "crowd noise within a stadium. When all the members of the crowd yell together, the noise is very loud." Similarly, in the brain, he said, "the intensity is strongly affected by the number of neurons oscillating in unison."
Synaptic pruning is a good thing. It brings about "an improvement in speed in information-processing and a greater ability to build the long neuronal chains required for complex problem-solving," Campbell said. "There are situations in which less is more."
However, the loss of synapses makes it much harder for an adult to learn a new language without a foreign accent or to achieve first-class athletic or musical skills.
According to Gopnik's book, "The Scientist in the Crib," each neuron in the cerebral cortex, the front part of the brain, where higher-level thinking is centered, has about 2,500 connections, known as synapses, at birth. By the age of 2 or 3, the number has soared to about 15,000 synapses per neuron. Eventually, however, as a result of synaptic pruning, the average adult brain has only half as many connections. Alzheimer's patients have even fewer.
Other crucial changes occur in the teenage brain parallel with pruning. According to Giedd, "a major rearrangement of brain structure and function takes place during early adolescence."
Regions that specialize in language, for example, grow rapidly until about age 13 and then stop. The frontal lobes of the brain, which are responsible for high-level reasoning and decision-making, aren't fully mature until adulthood, around the early 20s, according to Deborah Yurgelun-Todd, a neuroscientist at Harvard's Brain Imaging Center in Belmont, Mass.
"Adolescents are more prone to react with gut instinct when they process emotions," Yurgelun-Todd said. "But as they mature into early adulthood, they are more able to temper their gut reactions with reasoned responses."
This is why some researchers, such as Abigail Baird, a psychologist at Vassar College in Poughkeepsie, N.Y., contend that teenage criminals shouldn't be subject to the death penalty.
"These studies have shown remarkable changes that occur in the brain during the teen years, Giedd said. "They also demonstrate what every parent can confirm: The teenage brain is a very complicated and dynamic area, one that is not easily understood."
For more information online, go to www.nimh.nih.gov/Publicat/teenbrain.cfm
(c) 2006, McClatchy-Tribune Information Services.
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