On Human Brain Networks in Health and Disease

The brain is a complex system whose function relies on a diverse set of connections or interactions between brain regions. Using the mathematical framework of complex networks, these interaction patterns can be parsimoniously represented as brain graphs: each brain area is represented as a network node and each connection is represented as a network edge. These methods have been used to demonstrate that human brain networks display properties such as a small-world architecture that may directly facilitate cognitive processes. Moreover, mounting evidence suggests that these properties are altered in disease states, potentially providing important biomarkers for psychiatric and neurological disorders and informing our understanding of the mechanisms of altered cognitive function. Here, the basic concepts in network science are reviewed, and the properties of healthy and diseased brain networks discussed. Relationships between network diagnostics and alterations in behavioural or cognitive variables associated with Alzheimer's disease, schizophrenia and epilepsy are highlighted. Key Concepts The brain is a complex system that can be represented by a graph. In a graph, nodes represent brain regions and edges represent the links or connections between those areas, forming a complex network. Brain networks display properties such as a small-world architecture or a hierarchical modular organisation that may directly facilitate cognitive processes. These properties are altered in disease states, potentially providing important biomarkers for psychiatric and neurological disorders. Keywords: brain network; graph theory; Alzheimer's disease; schizophrenia; epilepsy