The earliest rs-fcMRI studies (Biswal et al. 1995; Greicius et al. 2003; Fox et al. 2005) used seed correlation maps to define relationships between a single brain region (the seed) and the rest of the brain. In a typical seed-based analysis, researchers first delineate a particular region of interest, generally either a functionally defined region from a task-based fMRI study or an anatomically defined region. The rs-fcMRI timecourses of all voxels within the defined region are extracted and averaged together. This average timecourse is correlated with the rs-fcMRI timecourses of all other voxels in the brain to create a “seed map”. This map reveals the spatial locations of other brain regions whose timecourse correlates highly with the seed's. An example of the “map” produced by such correlations can be found in Fig. 1b. Seed map analysis yields a peculiar form of a network, in which only relationships to a single seed are defined, and relationships between non-seed regions are left undefined.
