Brain scans of children with attention deficit hyperactivity disorder show abnormalities in the fiber pathways along which brain signals pass, scientists said on Monday.

The finding indicates the disorder may be more than just a chemical imbalance, they added.

Using an imaging technique called diffusion tensor imaging, researchers found subtle anatomical differences in children diagnosed with ADHD that may affect communication between key areas of the brain - the frontal cortex, basal ganglia, brain stem and cerebellum.

“These areas are involved in the process that regulate attention, impulsive behavior, motor activity, and inhibition - the key symptoms in ADHD children,” said lead researcher Manzar Ashtari, an associate professor of radiology and psychiatry at North Shore-Long Island Jewish Health System in New Hyde Park, New York.

The research was presented at the annual meeting of the Radiological Society of North America.

In another study, Ashtari found the brain irregularities diminished in children who had been medicated with stimulant drugs for an average of 2-1/2 years.

“The findings ... indicate that the therapeutic effect of stimulants may involve a brain normalization process,” said co-researcher Sanjiv Kumra, a psychiatrist at Zucker Hillside Hospital in Glen Oaks, New York.

“Typically, ADHD is described as a chemical imbalance, but our research has shown that there may also be subtle anatomical differences in areas of the brain that are important in this disorder,” Kumra said.

Between 3 percent and 5 percent of American children are diagnosed with ADHD. Millions take stimulants to counteract the behavior, which can be disruptive both at home and in school and can lead to problems later in life.