Most early studies using either neuroimaging or electrophysiology were concerned with identifying individual brain regions or cells that were modulated by a particular stimulus or task. From the electrophysiology work of Hubel and Wiesel (1962) to cognitive activation paradigms in human neuroimaging (Posner & Raichle, 1994) this approach has been very successful. However, no brain region operates in isolation. Instead, brain regions are integrated in complex, distributed neural networks, and studying the interactions between regions is proving to be just as important as understanding the response properties of individual regions. The interaction between brain regions has been termed “functional connectivity” and can refer to any examination of inter-regional correlations in neuronal variability (Friston et al., 1993; Horwitz, 2003).
