Corpus callosum morphology and function in attention deficit hyperactivity disorder and the relationship between the corpus callosum and cognitive functioning in healthy adults.

Abstract

The corpus callosum (CC) is the largest fiber¹ tract in the brain and connects homological regions of the two cerebral hemispheres. Research with split-brain patients, whose CC has been surgically severed, and neurologically intact groups has shown that the CC is important for sustained and divided attention. Due to its role in attention, the CC is of interest to clinical conditions in which attention is affected, such as attention deficit hyperactivity disorder (ADHD). Although the size of the CC has been examined in children and adolescents with ADHD, the results have been inconsistent. Therefore, the first of three studies in this thesis synthesized the current research in a metaanalysis, which analyzed the data from 13 studies that examined CC area in children and adolescents with ADHD, when compared to healthy controls. This study found that the splenium, the most posterior region of the CC, was smaller in ADHD and the rostral body, an anterior region, was smaller in boys with ADHD compared with controls. Thus, there is evidence for differences in area in both the anterior and posterior regions of the CC in ADHD. It was not known whether these differences persist into adulthood, however, because CC size had not been examined in adults with ADHD. Therefore, the second study examined CC area and structural integrity in young adults with ADHD compared with healthy controls using magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI), respectively. The difference in the size of the splenium was not present in this adult sample, although the genu (an anterior region of the CC) was smaller and two midposterior regions were larger in adults with ADHD when compared with controls. In addition, a reduction in the integrity of the genu and greater integrity in the splenium was found in ADHD. The relationship between CC morphology and measures of attention and IQ was also examined in young adults with ADHD and controls in order to assess the functional significance of differences in the CC. The integrity of the splenium was correlated with performance on the Stroop task, which requires attentional control. Hence, this study indicated that the morphology of the CC is atypical in young adults with ADHD and that these differences in the CC may impact on cognitive functioning. Interestingly, an estimate of performance IQ was negatively correlated with CC area in controls. This result conflicts with previous research on the relationship between IQ and the CC in healthy adults although the literature has yielded inconsistent findings. The third study, therefore, examined the relationship between IQ and both CC area and integrity in more detail in a larger sample of young adults. A negative correlation was found between the area of posterior regions of the CC and an estimate of performance IQ, while an estimate of verbal IQ was associated with decreased structural integrity in the genu. This study supports the hypothesis that differences in CC size and or integrity may have cognitive consequences. In summary, this thesis confirms the view that the development of the CC is atypical in children and young adults with ADHD. In addition, differences in CC integrity were associated with cognitive functioning in young adults with ADHD. Finally, the morphology of the CC is related to cognitive performance in healthy adults. ¹ American spelling is used throughout the thesis in order to be consistent with the published papers which have been published or prepared using American spelling.