of interest obtained from fMRI studies (e.g., Dosenbach et al. 2007; Fair et al. 2009). While ultimately any region definition technique should be subject to anatomical constraints, current anatomical parcellation schemes underestimate the number of functional areas in the brain. For example, the cytoarchitec-tonic parcellation of human orbital and medial prefrontal cortex in Ongur et al. (2003) finds many more distinctions than are defined by the AAL parcellation (Tzourio-Mazoyer et al. 2002). Networks formed using regions of interest thought to reflect functional areas or systems should presumably represent the underlying functional network structure more faithfully, but our knowledge of this architecture is currently incomplete. Node definition is the critical underpinning of network properties and organization (i.e., Zalesky et al. 2010; Smith et al. in press), and a better understanding of the organization of functional areas and systems is a clear and pressing challenge in neuroimaging (Power et al. 2010b).
