Theoretically, different neuropsychiatric disorders could result from a variety of different network pathologies. Consider a simplified network comprised of clusters of nodes with dense local connectivity and a few long-range connections (Figure 2A), consistent with the small-world topographies identified in human brains. One set of brain pathologies could result from direct elimination of node(s), with resulting network dysfunction (Figure 2B). Ischemic stroke represents a classical example of a neuropsychiatric disease with such a mechanism. Alternatively, the functional network could be disrupted by elimination of connections between different nodes (Figure 2C), as may occur in diseases in which the primary pathology is in the white matter connections between brain regions, such as multiple sclerosis. A third possibility is that the strength of the connections between nodes is altered in a manner that results in relative hypo- or hyperactivity within a specific subnetwork (Figure 2D). Epilepsy may be a paradigmatic example of a disease resulting from such a process (Bettus et al., 2008), while recent work suggests that such alterations in the strength of connectivity between different brain regions are also critical
