Brain laterality in adults with attention deficit hyperactivity disorder

The study of ADHD has been mired in speculation about what might represent its core deficit. DSM-IV criteria delineates a set of behavioral indications. However, determining the underlying neural or cognitive substrates remains challenging. Barkley (1996) proposed that the core deficit was poor behavioral inhibition, and although this model has been supported, recent work has offered some challenges. Impairments of processing speed (e.g., color naming) and language operations have been discovered across multiple studies. Some researchers suggest that the fundamental deficit is one of early information processing. This dissertation was designed to investigate cognitive operations among adults with ADHD, with regard to lateralized information processing and later stage executive functions. We used two lateralized cognitive measures to assess processing of linguistic and emotional stimuli. Next, we performed an fMRI study to investigate executive functions by noting differences in brain activation during a simple (the forward digit span) and a more complex (the backward digit span) working memory task. We found that adults with ADHD over-emphasized a right hemisphere strategy during both laterality tasks. This was associated with an impairment for word processing and normal or superior ability to process stimuli typically processed using right hemisphere resources (i.e., nonword recognition in a lexical decision task, and negative emotional intonation in a dichotic listening task). During both laterality tasks there were indications that adult ADHD subjects had reduced interhemispheric communication. Furthermore, we discovered that under certain attention conditions group differences were eliminated, suggesting that processing differences in ADHD were attributable to management and use of cognitive resources rather than to capacity limitations per se. The fMRI study supported these results, showing that adult ADHD subjects over emphasized a right hemisphere strategy during the forward digit span. During the backward digit span, they failed to activated posterior parietal areas thought to be important for performing mental transformation operations required by this task, and instead, activated left hemisphere Wernicke's area. We concluded that deferring to a right hemisphere strategy impairs their ability to processes symbol-bound information and disrupts executive functions dependent upon the efficient use of left hemisphere language functions. Consequently, adult ADHD subjects appear to expend greater resources in processing the linguistic components of task stimuli during the backward digit span.