| Summary: | This thesis demonstrates the relationship between depression symptomatology and white matter microstructure. Chapter 1 provides a systematic literature overview on white matter microstructure alterations of the reward system in depression. Findings suggest that localization and extent of white matter microstructure alterations in depression is highly dependent on the state (depression vs. remission) and the clinical subtype. Using a novel tractography algorithm, Chapter 2 provides a comprehensive instruction on how to delineate the two different branches of the MFB (supero-lateral medial forebrain bundle (slMFB) and infero-medial medial forebrain bundle (imMFB)), the main pathway of the reward system. An association between fractional anisotropy (FA), a diffusion tensor imaging (DTI)-based measure that is supposed to reflect white matter microstructure and hedonic tone, the capacity to derive pleasure from rewarding experiences is identified across a group of remitted depressed (RD) and never depressed (ND) young women. Chapter 3 uses a longitudinal design to investigate white matter microstructural changes of different pathways of the reward system from depression to remission. A distinct pattern of changes that depends on both the tract and the age is identified. Chapter 4 investigates the structural correlates of physical activity (PA). PA is reduced in depression and its benefit for depression symptomatology is well known. Using an MRI-sequence that has been shown to be specific to myelination we identify a positive correlation between PA and myelination of the right parahippocampal cingulum (PHC). This thesis contributes to the identification of structure-function associations related to the reward system in both patients with major depressive disorder (MDD) and healthy controls (HC). Results call for a careful stratification of clinically meaningful homogeneous subgroups if investigating participants with depression. Further the benefit of novel imaging methods for reconstruction of specific pathways and for a neurobiologically meaningful interpretation of the data is clearly shown.
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