Amphetamine and its effect on the brain's incentive mechanisms were studied in this report from Stanford. Using healthy volunteers, research studied the effect of amphetamine on brain activation of reward centers. There was some elevated mood seen during the testing, but the most notable finding was that amphetamine decreased the amount of ventral striatal activation during anticipation of gain, but it boosted activation during loss anticipation.
It's clear that further lines of investigation involving amphetamine, reward, and mood, will be important as the numbers of U.S. children and now adults are diagnosed with ADHD, but some of the results are also cautionary. Because fMRI measures blood flow oxygenation, it raises the possibility of negative (and unwanted) effects on normal brain reward systems. Also, the mood properties of stimulants need to be considered as possible confounding variables when evaluating whether students or others have improved on stimulant trials.
Amphetamine, Mood, and Reward
CDC: More Kids with ADHD