Figure 2. The brain as a network.

(A) The brain can be represented as a network where the nodes correspond to constitutive elements, or nodes (voxels from MRI, neurons, etc.), and these nodes form links based on some type of interaction between nodes. Complex networks also have a tendency to exhibit a modular topology, where links are concentrated within modules. The presence of chronic pain may alter the properties of these modules through changes in inter- or intra-modular interactions, through peripheral and central mechanisms (figure adapted from Bullmore et al. [23]) These may be viewed as subcomponents of the local modules depicted in Fig. 1. (B) Resting state fMRI show that several networks composed of modules (regions) can be identified from the fluctuating patterns of intrinsic activity seen in the human brain. Shown are four major resting state brain networks that might be relevant to pain processes (adapted from [8]). (C) Number of hits in PubMed for search of “Resting state AND fMRI”. The number of studies using resting state analyses exhibits an exponential growth over the last decade. Also shown are resting state studies performed in patients suffering from Alzheimer’s disease and schizophrenia, which are a magnitude higher than the number of studies conducted in chronic pain.