Event-related FMRI of inhibitory control in the predominantly inattentive and combined subtypes of ADHD.

BACKGROUND AND PURPOSE To examine the neurophysiological basis for the pronounced differences in hyperactivity and impulsiveness that distinguish the predominantly inattentive type of attention-deficit/hyperactivity disorder (ADHD-PI) from the combined type of the disorder (ADHD-C). METHODS Event-related brain responses to a go/no-go test of inhibitory control were measured with functional magnetic resonance imaging (fMRI) in 11 children with ADHD-C and 9 children with ADHD-PI, aged 7 to 13 years, who were matched for age, sex, and intelligence. RESULTS There were no significant group differences in task performance. Children with ADHD-C and ADHD-PI activated overlapping regions of right inferior frontal gyrus, right superior temporal lobe, and left inferior parietal lobe during inhibitory control. However, the magnitude of the activation in the temporal and parietal regions, as well as in the bilateral middle frontal gyrus, was greater in children with ADHD-PI than those with ADHD-C. Conversely, children with ADHD-C activated bilateral medial occipital lobe to a greater extent than children with ADHD-PI. CONCLUSIONS The results provide preliminary evidence that phenotypic differences between the ADHD-C and ADHD-PI subtypes are associated with differential activation of regions that have previously been implicated in the pathophysiology of ADHD and are thought to mediate executive and attentional processes.