The Associated Press

Children's brains change dramatically in key areas well into puberty, researchers report in a new study that contradicts some long-standing assumptions about brain development.

The anatomical changes - described as "fine-tuning" - surprised scientists in the United States and Canada who conducted the study published Thursday in the journal Nature.

The changes occurred between ages 3 and 15, in some cases years after the brain has reached its full size. Scientists had believed that neural development slowed after the first few years of life, and the brain was essentially organized by the time a child enters first grade.

In fact, they said, even in the mid-teens the amount of gray matter in some very active areas can double and neurons can interconnect rapidly while unneeded cells in other areas are flushed out.

The research continues two earlier studies of brain development published last fall. Researchers said they have not determined how the findings might be practically incorporated into new approaches to education or child development, but the study suggests how critically intertwined the stages of brain growth might be to a child's intellectual and emotional development.

"The teenage years are a kind of critical time to optimize the brain," one of the study's co-authors, child psychiatrist Jay Giedd of the National Institutes of Health, told the Washington Post. "If a person is doing sports or academics or music, those are the abilities that will be hardwired."

In the study, researchers at UCLA, the NIH and McGill University in Montreal scanned the normal brains of boys and girls ages 3 to 15. Some of the children participated as long as four years.

They saw a wave of growth in the fiber system that relays information between the brain hemispheres and is a good indicator of brain activity. The scans also showed new connections being made in some areas, while other areas shrunk.

In children of 3 to 6 years, the team saw burgeoning in the frontal networks that regulate the planning of new actions.

"In the very youngest children, there really is this furious growth going on in the frontal circuits of the brain," UCLA neurologist Paul Thompson, who helped develop the brain mapping technique, told the Los Angeles Times. "You see this extraordinary wave of peak growth that proceeds from the front of the brain to the back."

In teenagers up to 15, the researchers observed peak growth rates in areas in the middle and back of the brain associated with associative thinking and language.

The finding reinforces the wisdom of learning new languages early in life. By high school, the task may become biologically more difficult.

"The ability to learn a new language declines rapidly after age 12," said researchers reported. "Peak growth rates in linguistic regions, as well as their attenuation around puberty, may reflect the conclusion of the critical period for learning languages."

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