In the realm of medical research, which more commonly concerns itself with the diseased condition, a landmark study of healthy brain development has uncovered a number of surprises. Among them is the finding that, whereas childhood is characterized by improvement on tasks of cognitive and motor function, this progress levels off at around age 11 or 12, just prior to adolescence.

These results, announced today in the Journal of the International Neuropsychological Society, are the product of a National Institutes of Health (NIH) study, begun in 1999, that included 385 healthy children aged six to 18 years old.

Just before the teenage years, "the rate of growth for many skills kind of slows down," says Deborah Waber, an associate professor of psychiatry at Harvard University Medical School's Children's Hospital Boston and the lead author of a paper that reports the results of the behavioral component of the NIH Magnetic Resonance Imaging (MRI) Study of Normal Brain Development. (A separate analysis based on the brain imaging results is forthcoming.) Waber adds that when children are in elementary school, they may be learning "basic building blocks" of cognition and that after about 11 years of age, "children take these building blocks and use them."

Waber and her colleagues at hospitals in five other cities screened thousands of children in order to get a sample that was representative of the diverse demographics of the U.S. Study participants could not have behavioral or cognitive abnormalities or any risk of developing a mental or neurological disorder. Children who had been exposed to alcohol and cigarettes while in the womb were also excluded.

Subjects who qualified were then subjected to a battery of cognitive, behavioral, spatial and motor assessments. These included standard IQ tests as well as memory tasks, in which participants were asked to recite sequences of random digits or words they had learned. Tests of coordination required kids to place pegs in a peg-board under time constraints. Each subject performed the entire gauntlet of examinations three times in a four-year period so that their individual development could be tracked.

Intriguingly, the research also revealed that there were only a few cases where gender predicted ability. Boys performed, on average, better on perceptual analysis, whereas girls earned higher marks on tasks involving physical dexterity and processing speed. Girls were also slightly better at verbal tasks involving learning and reciting groups of nouns—however, beyond the age of 11, boys caught up to them.

"The biggest surprise for me was that the differences between children from different income groups were relatively modest," says Waber, compared with the findings of previous studies. She notes that the disparity between this and other research may have been this study's screening process, which excluded a higher percentage of low-income children than middle- and high-income children. She believes this could be the result of poor families' inadequate access to health care.

Although the difference was slight, high-income children outperformed their less wealthy peers on both IQ tests and an exam designed to replicate achievement in various academic subjects. Lower income kids performed at a lower level than others in reading comprehension and mathematical calculations but were competitive in basic cognition, memory and reading skills, indicating that poverty may affect development at the level where different abilities must be combined, such as verbal skills and memory, in the case of reading comprehension.

Waber says the research group will have a better idea of the physiological bases behind some of the behaviors they measured in the current paper when its results are combined with those from the imaging studies. In addition, she notes, the scientists also measured some hormonal data, which could further elucidate some of their findings. Eventually, she explains, the results of all these analyses will be a central database that other researchers can use as a baseline of "normal" subjects in studies of brain development in children, such as investigations into the effects of autism, drug use or prenatal exposure to illicit substances.

"A neurologist who notices something unusual in a child's MRI could use the database to help determine if the anomaly is within the normal range or variation, or if it is a cause for concern," NIH Director Elias Zerhouni said in a statement to the press. "A researcher studying an environmental toxin or genetic disease that affects brain development could use the database to help determine where and when development has strayed from its normal course."