WASHINGTON - Can you get smarter than a fifth grader? Of course, but new research suggests some of the brain's basic building blocks for learning are nearly at adult levels by age 11 or 12.

It is the first finding from a study of how children's brains grow. The most interesting results are yet to come.

About 500 super-healthy people, from newborns to teenagers, recruited from super-healthy families, are having periodic MRI scans of their brains as they grow up. They also get a battery of age-appropriate tests of such abilities as IQ, language skills, and memory.

The project, funded by the National Institutes of Health, is tricky work.

Move during an MRI, and the image blurs. Because scientists cannot sedate healthy children, they are having to get crafty to keep their subjects still. Tired toddlers are put in the scanners at naptime; mom squeezes in for a cuddle, and earplugs help block the machines' noisy banging. Six-year-olds wear earphones and watch favorite videos beamed into the scanner.

The MRI images measure how different parts of the brain grow and reorganize throughout childhood.

Overlap them with the children's shifting behavioral and intellectual abilities at each age, and scientists expect to produce a long-sought map of normal brain development in children representative of the diverse American population.

Today, scientists are publishing a sneak peek at some surprising early results.

Performance on a variety of cognitive tasks - working memory, vocabulary, spatial recognition, reasoning, calculation - rapidly improves between age 6 and 10, but then levels off.

"We don't honestly know why," said Deborah Waber of Children's Hospital Boston, who led the analysis published in the Journal of the International Neuropsychological Society.

This is a snapshot of 6- to 18-year-olds' abilities during their first study visit. Results may change after researchers observe each child's progress with age and compare their MRI scans, she said.

The adolescent brain is still growing. Indeed, the region responsible for things such as impulse control and moral judgment is the last to mature, sometime in the early 20s, said Jordan Grafman of the National Institute of Neurological Disorders and Stroke.

The study did not evaluate those kinds of skills. "It's an incomplete picture," he said.

But the age finding does make sense, suggesting a foundation necessary for higher learning is in place by puberty, said John Gilmore, brain-development specialist at the University of North Carolina School of Medicine. He was not involved with the project.

Scientists already knew that before age 12, the brain was racing to wire itself, making more connections between nerve cells that in turn enlarge vital regions. This is a time of rapid learning, the reason it is easier to learn a foreign language as a young child than as a teenager or adult, Gilmore said.

After puberty, the process slows, and the brain "prunes" itself, focusing less on installing new wiring than on programming and refining what is already there.