Evaluating the strength of functional connectivity during the resting state

Knowledge about the intrinsic functional architecture of the human brain has been greatly expanded by the study of the spontaneous fluctuations observed during the resting-state. Recently, there has been a surge of interest in resting state functional connectivity (RSFC) which has been used to identify large-scale brain networks including the default-mode network (DMN). When these results are thresholded for statistical significance, a considerable amount of gray matter remains uncharacterized. For both potential clinical and basic research purposes, we present a method for evaluating the strength of functional connectivity throughout the entire brain. The value of the method comes from its ability to portray the full picture of functional connection without any prior assumptions on possible membership in any of the major brain networks. After thresholding, a map of the strength of functional connectivity averaged over 30 healthy subjects exhibited striking similarity to the structural connectivity maps of Hagman et al.. Our comprehensive assessment demonstrates the predominance of the DMN, in agreement with Fransson´s voxel-wise frequency analysis. Comparison of lobar ROIs demonstrated that the functional connectivity in parietal areas is significantly higher than in frontal areas (p <; 10^-6). As a complementary approach to other methods such as ICA, future application of our comprehensive brain analysis includes mapping and characterizing changes in functional connection in both health and disease.