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<p>This thesis presents research investigating structural neural correlates of major depressive disorder (MDD). Although there are clear clinical differences between early- and late-onset MDD, they are still subject to the same diagnostic criteria and treatment strategy. Whether these differences translate into differences in cortical structure was examined in this study. By directly comparing early-onset (EOD) and late-onset (LOD) patients, we test whether age-of-onset results in changes in the extent or spatial pattern of cortical thinning.</p> <p>Chapter 1 provides a general background on the cerebral cortex, followed with a focus on cortical thickness. Chapter 2 presents a comprehensive review of the clinical and neurobiological literature on major depressive disorder as it pertains to age-of-onset. Three working hypotheses regarding the differences between early- and late-onset depression are presented and discussed.</p> <p>The results presented in this thesis show that there are both differences and similarities in cortical thickness between patients with EOD and LOD, with differences reflecting spatial extent, region-specificity, and magnitude of thickness differences. We confirmed the hypothesis of greater thinning in the dorsal lateral prefrontal cortex in depressed patients compared to healthy controls. We also correlated cortical thickness with clinical variables, which resulted in the finding of a positive correlation in the posterior cingulate cortex with illness severity.</p> <p>Few studies have used age-of-onset as a factor, which may account for some of the heterogeneity and inconsistent results seen in studies of MDD. We found that depression onset in early life is associated with greater disturbances in cortical thickness than LOD, possibly reflecting atypical development. These results provide novel insights into vulnerability and how development of depression is differentially affected by age.</p> / Master of Science (MSc)