On this first week of school, children with ADHD face extra challenges. 

They have a hard time paying attention and focusing, especially when they're not interested in what the teacher is teaching. 

Research has indicated that the classic symptoms of impulsivity and inattention could be caused by a disruption in the transmission of dopamine - a chemical in the brain that helps cells to communicate.

Dr. Nora Volkow, of the National Institute on Drug Abuse, and her colleagues, theorizes that faulty dopamine transmission is actually to blame for the underlying struggle many patients have completing tasks when there is no immediate reward on the horizon.

"Dopamine is considered a neurotransmitter that is crucial for our ability to perceive rewards and to be motivated in our behavior," Volkow says.

Featured this week in the Journal of the American Medical Association, the study compared what's called the dopamine reward pathway in the brains of 53 adults who had ADHD with 44 adults who did not, using images taken at Brookhaven National Laboratory in New York.

"There was a lower concentration of dopamine markers in the brain of individuals with ADHD, specifically in the areas of the brain that are involved with reward and motivation," says Volkow.

Researchers say the findings help explain why ADHD patients have such a hard time focusing on tasks they don't find interesting.

The study reinforces the concept that creating ways to make school and work tasks seem more rewarding to patients with ADHD can help them succeed.