That conclusion was based on a brain-imaging study conducted at the U.S. Department of Energy's Brookhaven National Laboratory in collaboration with Mount Sinai School of Medicine in New York.
Researchers also found lower levels of the "dopamine transporter" proteins in certain brain regions of ADHD patients compared with controls. The study also found that for any given level of dopamine transporters in the brain, ADHD patients experienced much higher levels of inattention compared with control subjects.
"These results suggest that dopamine transporter levels alone cannot account for the severity of symptoms of inattention in ADHD," said Nora Volkow, director of the National Institute on Drug Abuse and the lead author of the study.
The research is to be published in an upcoming issue of the journal Neuroimage and is now available online.
Notable deaths of 2014 [PHOTOS]