, 2006 and Raichle et al., Selleck Ibrutinib 2001). Until the study of spontaneous BOLD activity, however, the association

of regions within a functional system was to some extent dependent upon sets of task paradigms. Task-based approaches left functional systems open to an interpretation that rather than being a fundamentally related group of brain regions within a brain-wide context, a functional system thus defined might be just a transient and task-specific association of brain regions. The subgraphs presented herein were derived in task-free data using methods with no prior information about node identity. There is substantial agreement between aspects of paradigm-driven functional system definition in neuroimaging, and paradigm-free subgraphs derived in task-free activity. Even if one were to object that the areal network included functional

assumptions via meta-analytic localizers, the modified voxelwise analysis, which returned very similar results, made no such assumptions. In a brain-wide context, several functional systems are distinguished from each other by spontaneous activity. This task-free definition of brain functional organization can inform perspectives on cognitive function. For example, dorsal selleck chemicals llc and lateral frontal cortex appears to be apportioned among a variety of distributed subgraphs, many of which correspond to functional systems with known characteristics (Figure 2). This organization does not appear consistent with accounts of cognition that posit rostro-caudal gradients or hierarchies across frontal cortex (Badre and D’Esposito, 2009 and O’Reilly, 2010). In a related manner, the finding no of similar graph properties (relatively dense internal relationships and relatively few external relationships) in visual, SSM, and default mode systems may inform the degree to which the default mode system is seen as a processing type of system versus a control type of system. Such a finding need not contradict the description of posterior members of the default mode

system as cortical hubs (Buckner et al., 2009), but it may alter the understanding of what it means to be a hub. Recent investigations into the structure of functional brain organization using a variety of methods (Erhardt et al., 2010 and Yeo et al., 2011) have found some similar (but not identical) sets of resting state networks as the subgraphs reported here. We consider convergence across methods to be a key indicator of the validity of findings. We find the graph theoretic framework to be especially useful, because it is capable of describing the overall graph (no such measures are presented in this article, but small-world measures are an example), portions of the system (e.g., subgraphs), or individual nodes of the system (e.g., local efficiency) within a common framework. Our findings have substantial implications for past and future graph-based analyses.