This paper combined a review of brain imaging studies with a new brain scan study. The review concluded that the brain differences found in children with ADHD, across several fronto-striatal and connected networks, seem to carry on into adulthood.
The new scan study looked at 11 adult men who had ADHD in childhood and still had symptoms, and who had never taken ADHD medication (average age 29). It compared them with a group of healthy men of similar age (14 were analyzed). During a task that needed sustained attention, the men with ADHD used some front and side brain regions less than the comparison group, and used regions toward the back of the brain more. The extra activity in the cerebellum was read as the brain trying to compensate. That cerebellar activity was tied to lower activity in a left frontal region (r=-.71 and r=-.81).
On a task with rewards, the men with ADHD responded faster to targets (p=.01). During the reward part, one frontal region was less active in the ADHD group, but this showed up only in the 6 men who also had childhood conduct disorder, not in the 5 who did not.
The authors note the sample was small and all male, and the two groups differed in estimated IQ (92 versus 108).
Source: Cubillo A, Halari R, Smith A, et al.. A review of fronto-striatal and fronto-cortical brain abnormalities in children and adults with Attention Deficit Hyperactivity Disorder (ADHD) and new evidence for dysfunction in adults with ADHD during motivation and attention. Cortex (2012). doi:10.1016/j.cortex.2011.04.007