Positron emission tomography (PET) scans were used to measure dopamine synaptic markers in adults with unmedicated ADHD and adults without ADHD. The study found that
A reduction in dopamine synaptic markers associated with symptoms of inattention was shown in the dopamine reward pathway of participants with ADHD.
(Volkow, et al., 2009).
This means that for people with ADHD, there may be a biological deficit in the way the brain gets motivated.
Volkow, N., et al. (2009). Evaluating Dopamine Reward Pathway in ADHD. JAMA, 302(10):1084-1091.
The abstract for the study can be found here: http://jama.ama-assn.org/cgi/content/abstract/302/10/1084