Macroscopic networks in the human brain: mapping connectivity in healthy and damaged brains

The human brain contains a network of interconnected neurons. Recent advances in functional and structural in-vivo magnetic resonance neuroimaging (MRI) techniques have provided opportunities to model the networks of the human brain on a macroscopic scale. This dissertation investigates the possibilities to map connectivity in the healthy and damaged human brain with macroscopic network models in four studies. The first study of this dissertation shows that structural connectivity patterns in the human brain may be an important identifier of brain function. Hence brain parcellations based on structural connectivity patterns are considered useful alternatives to anatomical brain atlases. The second study underlines the close relationship between brain structure and brain function by showing that the topographically distributed structural hubs of the neocortical network coincide with key areas from known neurocognitve networks. Furthermore this study shows that the hubs have different betweenness centrality and node degree values between males and females and that the network topology of the female neocortex has a higher small-worldness score. The third study provides a plausible description of the structural network between neocortical areas of the visual system. The results demonstrate graph theoretically that areas from the visual network are divided into two modules which coincide with the known ‘what’ and ‘where’ streams. The last study simulates the effect of lesions of stroke patients with visual field defects in the structural neocortical networks of a set of healthy controls by homotopically mapping their lesions onto them. The results show that estimated network damage of the occipital lobe was positively correlated with the measured visual field defect. In conclusion this dissertation shows in four studies that brain structure and brain function can be consistently related to one another, highlighting the importance and feasibility of considering both, structural and functional modalities in brain connectivity research. This work contributes to human brain research by describing how to map connectivity in healthy as well as damaged brains; furthermore it provides maps of the neocortical structural network on a macroscopic level which depict local connectivity patterns and network features.