Date of Award
Master of Arts
The purpose of this study was to explore the brain-behavior relationship of the frontostriatal circuit to executive functioning (EF) in children with Attention-Deficit/Hyperactivity Disorder (ADHD) and developmental dyslexia. It was expected that the volume and asymmetry of the caudate nucleus head and body would be related to ADHD; the relationship between caudate volume and asymmetry and dyslexia was exploratory. It also was expected that children with ADHD and children with dyslexia would be impaired on measures of EF compared to those without each disorder. Lastly, it was predicted that verbal and spatial working memory would mediate the relationship between the volumes of the left and right caudate nuclei, respectively, and performance on other EF measures. One hundred five children from the Southern Illinois region who successfully completed a full-day neuropsychological test battery and an 8-minute structural magnetic resonance imaging (MRI) scan were included in this study. Results indicated that children with ADHD had greater rightward asymmetry for the caudate head as hypothesized, but not leftward asymmetry for the caudate body, when compared to those without it; however, there were no differences in caudate asymmetry for those with dyslexia. An exploratory factor analysis of the data revealed three EF factors: EF abilities in the home, problem solving/perseveration, and working memory/fluency. The ADHD and the dyslexia groups were more impaired than those without each disorder on EF abilities in the home and working memory/fluency. Further analysis revealed that working memory was a significant covariate in the relationship between diagnosis and performance on EF measures for these groups and greatly reduced EF differences between groups when looking at dyslexia. Children with ADHD-Combined Type were not more impaired on a measure of inhibition when compared to those with ADHD-Predominantly Inattentive Type, but low power may have affected the ability to find a significant effect. The two subtypes were similar on all other EF measures. The diagnostic groups did not differ on a complex non-EF measure requiring attention (i.e., a verbal long-term memory task), which shows a dissociation between performance on complex tasks requiring attention with and without an EF component. The mediation models were not tested further since there was no significant relationship between left and right caudate volume and performance on EF measures. These results indicate that the caudate head volume is related to the pathophysiology of ADHD, suggesting that more research is needed using segmentation. In addition, results showed that deficits in EF go beyond working memory in ADHD given that ADHD is still related to executive dysfunction after controlling for working memory. In contrast, the difference between children with and without dyslexia was no longer significant on EF measures after controlling working memory, suggesting that working memory may be the main factor driving EF impairment in dyslexia. Further work on this topic is indicated. An exploratory analysis revealed that left caudate head volume approached significance when correlated with a verbal working memory measure; therefore, further research is needed in the area of brain-behavior relationships of the frontostriatal circuit and performance on EF measures, especially working memory.
This thesis is Open Access and may be downloaded by anyone.