The purpose of this study is to find out if children with attention-deficit, hyperactivity disorder (ADHD) have a difference in how their brain cells "fire" or react. The investigators also want to find if brain cell "firing" can tell us how severe of symptoms a child has from ADHD. Finally, the investigators want to see if giving an ADHD medication called atomoxetine can make the ADHD symptoms in a child better and if the improvement shows a change in brain "firing".