This is the VOA Special English Health Report.

A.D.H.D. is attention deficit hyperactivity disorder. Children with A.D.H.D. might have trouble paying attention. They might not seem to listen. They might forget things. They might not be able to stay seated or play quietly.

They might talk too much. And they might act and speak without thinking about the results of their behavior. These are among the signs listed on the Web site of the Centers for Disease Control and Prevention in the United States.

Barbara Eddy with daughters, Allegra, left, and Francesca. The mother from Pasadena, California, has A.D.H.D.

Researchers have been looking for the cause or causes of attention deficit hyperactivity disorder. Now, a brain-imaging study offers more evidence that could lead to new ways to treat it.

Researchers say they observed shortages in the brain's reward system in patients with A.D.H.D. The study found that levels of certain proteins were lower than normal.

Nora Volkow is director of the National Institute on Drug Abuse and the lead author of the study. She says the lack of attention and self-control that mark A.D.H.D. could be caused by problems in the flow of dopamine.

NORA VOLKOW: "Dopamine is considered a neurotransmitter that is crucial for our ability to perceive rewards and to be motivated in our behavior."

If people cannot sense a reward, she says, then they might have difficulty completing an activity.

Researchers compared the pathways that transmit dopamine in fifty-three adults with A.D.H.D. to those of forty-four adults without the disorder.

NORA VOLKOW: "There was a lower concentration of dopamine markers in the brain of individuals with A.D.H.D., specifically in the areas of the brain that are involved with reward and motivation."

Doctor Volkow says the dopamine levels were directly related to the severity of the patient's inattention.

The study used brain images taken at Brookhaven National Laboratory in New York State. Gene-Jack Wang is chairman of the Brookhaven medical department. He says the finding might also help explain why people with A.D.H.D. are more likely to abuse drugs or overeat. He says they might be trying to increase their dopamine levels to make up for the deficits in their reward system.

The Brookhaven Lab is part of the United States Department of Energy. The National Institutes of Health supported the research. The study appeared in the Journal of the American Medical Association.

And that's the VOA Special English Health Report, written by Caty Weaver with Carol Pearson. You can find transcripts and MP3s of our reports and share comments at voaspecialenglish.com. I'm Mario Ritter.