The existence of a default mode network (DMN) in the brain is well established. Converging evidence suggests a fundamental role for the default mode network (DMN) in the working of the brain, however its function remains controversial.
The DMN consists of a set of brain regions that show highly correlated brain activity during ‘rest’, and a reduced level of activation during most attentionally demanding tasks. Regions within the DMN, including the posterior cingulate cortex (PCC), the retrosplenial cortex and parts of the ventromedial prefrontal cortex (vmPFC) have high metabolic demands at rest, and are densely interconnected by white matter tracts that form part of the brain’s core structural network. These regions are located at the centre of many distributed brain networks, suggesting an important cognitive function. Abnormalities of the DMN are seen across a range of neurological and psychiatric disorders, and a detailed understanding of the effects of disease on the DMN may be particularly important for understanding its impact on behavior.
The DMN is more active in the absence of specific goal-directed behavior. Activity in parts of the DMN is higher when thoughts are directed internally, for example when autobiographical memories are retrieved. This has led to the proposal that the network supports mental activity that is internally directed. However, activity within the DMN has also been shown to correlate positively with the efficiency of processing external information, and the interaction of the DMN with networks engaged by external tasks predicts aspects of behavior. This suggests that parts of the DMN may be involved in regulating a wider range of behaviors.
It may be possible to reconcile these competing theories by recognizing that the DMN is functionally heterogeneous. For example, despite showing a similar pattern of relative deactivation in many situations, the PCC has sub-regions with distinct cytoarchitectonics, varying structural and functional connectivity, and different electrophysiological responses. This suggests that a detailed analysis of these sub-regions and their connectivity will be needed for a complete description of the DMNs function.
Despite a growing body of detailed work there is no unifying model of DMN function. Key issues for future work are likely to be a more detailed understanding of how the DMN interacts with other cognitive networks, and how its interactions change in different behavioural contexts. The development of computational models hold the promise of showing how the DMN might integrate with other high-level cognitive systems, but future insights are likely to come from the combination of different experimental approaches. We welcome contributions that address these issues, and particularly welcome papers, which focus on the role of the DMN in cognition.
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