One theory about Attention Deficit Disorder is that it may stem from underactive brain activity in an area known as the pre-frontal cortex. That in turn may disrupt communications with other parts of the brain -- and undermine the sophisticated neural network that produces what we think of as "attention."
For reasons that aren't well understood, psychostimulant medications help both to rein in impulsive behavior and boost the "attention network" into higher gear.
This study at Stanford's Department of Psychology shows the brains of two boys before and after taking methylphendiate (MPH), or Ritalin, a drug commonly used to treat ADD.
The boy with ADD showed more activity in the affected striatal structures when taking the drug than when not. The healthy boy (control), in contrast, showed the reverse - less activity in those areas when taking the drug than when not.