The brain's default network and its adaptive role in internal mentation

Abstract

During the many idle moments that comprise daily life, the human brain increases its activity across a set of midline and lateral cortical brain regions known as the "default network." Despite the robustness with which the brain defaults to this pattern of activity, surprisingly little is known about the network's precise anatomical organization and adaptive functions. To provide insight into these questions, this article synthesizes recent literature from structural and functional imaging with a growing behavioral literature on mind wandering. Results characterize the default network as a set of interacting hubs and subsystems that play an important role in "internal mentation"-the introspective and adaptive mental activities in which humans spontaneously and deliberately engage in every day.

Fig 1

Prevalence of default network studies in the literature. A. Total number of studies relating to the default network separated by year published. Data was obtained from a PubMed search on the phrases “Default Mode” OR “Default Network” filtered to discard unrelated studies. Published dates reflect the date in which studies were printed rather the advanced E-Publication date. B. Studies are further classified based on their methodology and participant cohort to highlight the prevalence of resting-state functional connectivity techniques and the relevance of the default network to psychiatric and neurological disease. fMRI or other = Task-evoked fMRI and all other experimental techniques (i.e. event-related potentials, diffusion tensor imaging, etc.); rs-fcMRI = resting-state functional connectivity MRI; both = studies employing rs-fcMRI and at least one other neuroimaging technique.

Fig. 2

Cortical distribution of task-induced deactivations. Data from four previously-published meta-analyses of task-induced deactivations is re-plotted on inflated brain surfaces (Caret software, Van Essen 2005) for ease of visual comparison. Permission was obtained from the authors of these studies. A. Task-induced deactivations averaged over 9 distinct PET studies from Shulman and others (1997; see also Buckner and others 2005 2008), thresholded at a 0.5% PET count signal change. B. A conjunction image reflecting task-induced deactivations from 9 distinct PET studies from Mazoyer and others (2001, Figure 1; thresholded at p < 0.001 uncorrected). C. Data from Spreng and others (2009; Figure 1), which represents an activation likelihood estimate (ALE) meta-analysis of 16 studies, plotted using an FDR-corrected threshold of p < 0.05. D. Task-induced deactivations from the ALE meta-analysis of Laird and others (2009, Figure 1), plotted using an FDR-corrected threshold of p < 0.005. ¹Figure adapted with permission from John Wiley and Sons ©; see Buckner and others (2008; Figure 2).

Fig. 3

Converging approaches highlight the anatomical organization of the default network. A. This diagram, from Binder and others (2009, Figure 7B), outlines the major white matter connections between default network regions as revealed from anatomical tracing studies in macaques. B. Diffusion tractography from a single human participant highlights white matter tracts connecting the medial prefrontal, medial parietal, and medial temporal cortices within the default network. Figure from Greicius and others (2009; Figure 1B) C. Co-activation of default network regions (including the hippocampal formation) at rest as revealed by independent component analysis. Figure from Greicius and others (2004, Figure 2A). D. Using a seed-based temporal correlation rs-fcMRI approach, Fox and others (2005) revealed robust spontaneous BOLD correlations between default network regions. Figures were reproduced or adapted with permission from ¹Oxford University Press ©, ²© 2004, National Academy of Sciences, U.S.A., and ³Elsevier Limited © (adapted from Raichle and Snyder 2007, Figure 1C).

Fig. 4

The default network comprises hubs and subsystems. A. The network properties of eleven regions of interest within the default network were explored using graph analysis. Results revealed the aMPFC and PCC (yellow regions) qualified as hubs within the network. B. The rs-fcMRI clustering properties of all remaining regions were next explored with hierarchical clustering. Distinct clusters are color-coded to highlight the dMPFC subsystem (blue) and the medial temporal lobe subsystem (green). C. Similar clustering patterns emerged in task-evoked analyses when participants engaged in introspective tasks including reflecting on their present mental states and envisioning their future. Panels adapted with permission from Elsevier Limited © (see Andrews-Hanna and others 2010a, Figures 1B,D and 5B).

Fig. 5

Default network activation is linked to spontaneous thought. A combined fMRI / thought sampling study revealed activation of several default network regions prior to reports of mind wandering when participants were behaviorally probed during a sustained attention task. Interestingly, participants also activated frontoparietal control regions when experiencing spontaneous thoughts, a finding which is currently a matter of debate (McVay and Kane 2010; Smallwood 2010). Figure reproduced with permission from Christoff and others (2009, Figure 2).

Fig. 6

Awake resting states are associated with spontaneous internal mentation. Andrews-Hanna and others (2010b) administered retrospective thought sampling questionnaires to 139 participants after staring at a fixation crosshair between 2-4 consecutive resting runs in the MRI scanner. Participants reported spending nearly half of their time engaged in episodic past or future thought, with a preference towards thinking about the recent past and immediate future. Figure modified from Andrews-Hanna and others (2010b, Figure 8).

Fig. 7

Proposed functional-anatomic organization of the major default network components. A schematic drawing of the default network hubs (yellow) and subsystems (blue = dMPFC subsystem; green = MTL subsystem) is highlighted along with each components’ hypothesized functions and the tasks that frequently activate them. Arrows reflect approximate strength of connectivity between default network components. See text for references. Note regional anatomic boundaries are approximate.