Behavioural and electrophysiological evidence for subtypes of ADHD : a developmental perspective

Abstract

The suggestion that subtypes of Attention-Deficit/Hyperactivity disorder (ADHD) may represent distinct disorders has been made by numerous previous researchers. However, the specific impairments associated with each putative subtype have not been identified. The current project tested the general hypothesis that any proposed ADHD subtypes would have characteristic behavioural and/or electrophysiological impairments. Participants were children (5-12 years old), adolescents (12-18 years old), and adults (18 years and over) classified as either unmedicated ADHD-I (ADHD-Inattentive type) or unmedicated ADHD-CH (formed by combining ADHD-Combined type and ADHD-hyperactive/impulsive type participants), or matched controls. Study 1 used a traditional line bisection task to assess orienting of visuospatial attention, thought to be a function of the dorsolateral prefrontal and posterior parietal regions. Bisection bias differed between ADHD groups for children and adolescents, but not for adults. Study 2 tested orienting of visuospatial attention using a computerized line bisection task that required less motor action compared to the traditional task, and thus possibly less involvement from the dorsolateral prefrontal cortex. Bisection bias differed between ADHD groups for children, but not for adolescents or adults. The Stroop Colour-Word task during concurrent EEG was used in Study 3 (note: adult participants only). This task produces cognitive interference, the resolution of which is a function of the anterior cingulate cortex. No group differences were found. Callosal functioning was tested electrophysiologically in Study 4 using the Poffenberger task, again using adult groups only. Differences in directional interhemispheric transfer speeds were found between the ADHD groups. Overall the results indicate that participants with ADHD-I have normal visuospatial attention, and normal functioning of the anterior cingulate cortex, but that adults have impairment in interhemispheric callosal transfer, particularly from the right-to-left hemisphere. This may indicate impairment of the right hemisphere, specifically in the cells of origin of callosal fibres. In contrast, children with ADHD-CH appear to have impairment of visuospatial attention, indicating impairment of the posterior parietal region, while adults with ADHD-CH showed unexpected enhanced callosal connectivity. Finally, these results strongly suggest that ADHD subtypes represent distinct groups that are characterised by unique cognitive and electrophysiological markers. This work has significant implications on the assessment and treatment of ADHD.