Scientists have discovered that the brain undergoes surprisingly dramatic anatomical changes between the ages of 3 and 15, a finding that may not amaze parents of mercurial children but shatters some traditional assumptions about neural development.

During key periods, a research team reports in today's issue of the journal Nature, the amount of gray matter in some areas can nearly double within as little as a year, followed by a correspondingly drastic loss of tissue as unneeded cells are purged and the brain continues to organize itself.

"It's remarkable," said neurologist Arthur Toga of the University of California at Los Angeles' School of Medicine. "Even though the overall size of the brain is relatively mature, we're still seeing changes" in the form of "very local and discrete patterns."

As recently as a decade ago, it was widely assumed that such major growth spurts and subsequent cutbacks took place in the womb or very early in childhood, and that the overall structure of the brain changed very little, if at all, after the age of 5 or 6.

At that point, the brain usually has reached about 95 percent of the average adult volume, having increased fourfold in size since birth. Of course, extensive internal re-wiring takes place during childhood. The gross structure, however, was thought to be generally fixed. That led educators and child development experts to focus on the first few years of life as crucial for proper brain development.

"Basically, the theory said that the amount of gray matter went downhill from about age 3," said Jay N. Giedd, a child psychiatrist with the National Institute of Mental Health in Bethesda who participated in both the new study and a similar one last fall. "The idea was that it's a zero-sum game. The frame and structure of the house was complete, and the furniture was just being rearranged."

The new brain scan results, and two similar studies of slightly older age groups published last fall, are the first brain-imaging studies to show that the process continues.

"We now know that there are these sorts of critical periods," said UCLA neuroscientist Elizabeth R. Sowell, a coauthor of one of the earlier papers. "But I don't think anybody has yet figured out a way to make this clinically relevant. . . . Still, we're all hoping that perhaps the experts in education or psychology will see these things that we're showing them and find ways to make those connections."

Neuroscientists had long known that a two-stage process of growth and attrition is typical of brain development from the fetal period through early childhood.

First, the brain overproduces gray matter--bulk neurons that are not yet permanently "wired" into neural circuits. These cells then begin to arrange themselves into patterns depending on which connections are reinforced by mental or physical activity. Thereafter, the least-used cells and pathways die out in a phenomenon called "pruning" as white matter (chiefly fibers interconnecting nerve cells) forms to firm up the most robust connections.

In the new research, Toga, Giedd and colleagues from UCLA and McGill University in Canada conducted repeated three-dimensional brain scans of several normal children over intervals as short as two weeks and as long as four years.

The group concentrated on size and shape changes in a complex nerve fiber network called the corpus callosum, which connects the two hemispheres and is a reliable indicator of the level of activity in different parts of the brain.

The results indicate that from ages 3 to 6, the most rapid growth takes place in frontal-lobe areas involved in planning and organizing new actions, and in maintaining attention to tasks.

By contrast, during the period from 6 to puberty, the scientists found, the gray-matter spike shifts to the temporal and parietal lobes that play a major role in language skills and spatial relations. The growth rate then falls off fast, which may explain why, as a rule, the ability to learn languages declines sharply after the age of 12.

As children age, the growth moves in a sort of wave from the front of the brain to the rear, the team found. "We were quite surprised," Giedd said, "to see this unexpected increase in gray matter in the front part of the brain right before puberty," which occurs around age 11 in girls and 12 in boys.

Last fall, researchers reported in the journal Nature Neuroscience that they had found an unexpected increase in gray matter at the onset of adolescence, followed by a substantial loss in the frontal lobes from the mid-teens through the mid-twenties.

The frontal lobe of the brain is essential for inhibiting impulses, regulating emotion and planning and organizing behavior--all of which can be critical issues for teenagers and their parents.

Giedd believes the growing evidence of brain-structure mutability should be welcome news to teenagers.

"In the womb and during the first 18 months of life," when the brain undergoes its most drastic changes, "an infant doesn't have much say about the way things turn out. But during the teenage years, "a person has a lot to say" about the way his brain develops, Giedd said.

In that critical interval, he said, the rule for brain structures appears to be "use it or lose it. What we think then happens is that if a person is doing sports or academics or music, then those are the abilities that are going to be hard-wired" as the circuits mature. "The teenage years are a kind of critical time to optimize the brain."

Teenagers who recognize that tend to "feel empowered," Giedd said, especially if they "realize that the stakes are pretty high."

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