Having established a method to measure functional relationships within the brain, one must decide what relationships to study. Hundreds of rs-fcMRI (and fMRI) studies state that they are investigating “networks”, but the meaning of the term “network” varies significantly (detailed below). Networks are studied in a wide variety of fields, and an entire branch of mathematics, called graph theory, is devoted to the study of networks. Networks, from both an intuitive and a more formal graph theoretical perspective, are collections of items (or nodes) that possess pairwise relationships (called edges). It becomes immediately obvious how entities such as the Internet or transportation systems are well-defined networks in this sense.
