Network studies are inherently studies of the relationships between things, but several classical neuroanatomical and neurophysiological techniques for assaying relationships between brain regions were (and remain) of limited use in living humans. Structural studies using dissection or tracers faced obvious ethical limitations, and methods such as EEG that could demonstrate correlations in activity between brain regions had coarse spatial resolution and relatively superficial access to the brain. The introduction of PET and functional MRI (fMRI) in the 1980s and 90s, respectively, enabled reasonably precise, noninvasive measures of activity throughout the brain. This level of investigation localized function quite well, but did not explicitly measure relationships between brain regions. Innovative investigators were, however, able to harness these methods to develop the concepts of effective and functional connectivity during task performance (see (Friston, 2005; Horwitz, 2003) for discussions of both measures). Functional connectivity is defined as the temporal coherence, or statistical dependence, between measurements of activity in different neurons or neural ensembles. Effective connectivity often refers to changes in weighted relationships between regions as a consequence of condition or state. Measures of
