The primary motor cortex reached peak thickness at age 7.4 in children with ADHD, about five months earlier than in normal children, researchers found. Shaw said it was possible that the early maturation of the primary motor cortex contributed to the fidgety behavior characteristic of ADHD.
Dr. F. Xavier Castellanos of New York University said the research helps explain why children with ADHD often choose younger playmates, and it should reassure parents who are worried about their children fitting in.
"They may be 11 but their brain is 8. They can't act their chronological age," he said. "This lets parents know that having younger playmates is OK and to be expected," said Castellanos, a former National Institute of Mental Health researcher involved in the early stages of the study.
The study, which focused on one aspect of brain development, did not explain why some people continue to experience ADHD symptoms as adults.
Dr. Bradley S. Peterson of Columbia University, who was not connected to the study, said that although the brains of children with ADHD reached the appropriate thickness, there was no way of knowing from the study whether individual cells were normal.
"Billions of cells make up brain tissue, and we cannot measure all the cells and all the connections between the cells," he said. "Subtle deficits could easily remain."
In addition, he said, the study did not examine the process of cortical thinning that takes place in late adolescence -- a second developmental milestone in which unneeded connections are pruned to shape the adult brain.
Government researchers plan to continue tracking some study participants through adulthood, Shaw said.
"We have not captured this later transition," he said. "It is possible some people never quite get there and that is what accounts for the persistent" ADHD.
denise.gellene@latimes.com
