Attention deficit（不足）hyperactivity（活动过度）disorder is known by the letters A．D．H D．Children with A．D．H．D．might have trouble paying attention．They might not seem to listen and might forget things．They might not be able to stay seated or play quietly．They might talk too much and act and speak without thinking about the results of their behavior．These are among the signs named on the web site of America’s Centers for Disease Control and Prevention．
Doctors usually identify the disorder in children．But experts say the behaviors often last into adulthood．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 some proteins were lower than normal．Nora Volkow, director of the National Institute on Drug Abuse, says the lack of attention and self –control that mark A.B.H.D. could be caused by problem in the flow of dopamine（多巴胺）. She says people might have difficulty completing an activity if they cannot expect some kind of reward in return. Researchers studied the pathways on which dopamine travels and A.D.H.D. Doctor Volkow says the researchers then compared the pathways to those of 44 adults without the disorder. “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. And the dopamine levels were directly linked to the severity, of the patient’s inattention she says.”
The study used brain images taken at Brookhaven National Laboratory in new York State. Gene-Jack Wang is chairman of the Brookdhaven 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 attempting to increase their dopamine levels to make up for the deficits in their reward system.

Title	●The finding of
The    of Children with A.D.H.D.	●They are lacking in     . ●Perhaps they don’t seem to listen. ●Perhaps they are  . ●Perhaps they can’t sit for   or play quietly. ●They might talk too much and act and speak without      the results of their behavior
The       of A.D.H.D.	●patients with A.D.H.D are    of the brain’s reward. ●Levels of some proteins are lower than normal. ●Levels of dopamine are     as well.
Reasons of abusing Drugs	●They are trying to     their dopamine.