Brain Networks and the Default Mode

The brain as a network: small-world wiring, hubs and the rich club, large-scale networks such as the default mode, salience and control networks, and what network science reveals about health and disease.

The brain as a graph

Network science represents the brain as nodes (regions or neurons) linked by edges (connections). Analyses of anatomical and functional data reveal common principles. Brain networks are small-world: most connections are local, but a few long-range shortcuts mean any two regions are linked by few steps — efficient both for specialised local processing and global integration (a concept introduced by Duncan Watts and Steven Strogatz in 1998). Brain networks have hubs — highly connected regions such as the precuneus and parts of prefrontal and parietal cortex — which connect densely with one another in a "rich club". Hubs are costly, metabolically active and, perhaps for that reason, vulnerable in many diseases.

Large-scale networks

Resting-state fMRI reveals regions whose spontaneous activity rises and falls together, forming reproducible networks: visual, somatomotor, auditory, dorsal attention, salience (anterior insula and anterior cingulate, detecting important events), frontoparietal control (flexible goal-directed control) and the default mode network.

The default mode network

In 2001 Marcus Raichle and colleagues described a set of regions — including the medial prefrontal cortex, posterior cingulate cortex/precuneus and angular gyrus — that are consistently more active during rest than during focused external tasks. This default mode network (DMN) is engaged during mind-wandering, remembering the past, imagining the future, thinking about oneself and others, and understanding stories. It typically decreases when attention turns outward, and its balance with task networks may be important for attention.

Networks and disorders

Altered network organisation has been linked to many conditions: reduced DMN integrity in Alzheimer's disease (whose amyloid deposits concentrate in DMN regions), altered DMN and salience network interactions in depression (ruminative thinking) and schizophrenia, and attention network differences in ADHD. Psychedelics temporarily reduce DMN integrity and increase cross-network communication. Lesion network mapping uses connectomes to explain how damage in different places can produce the same symptom.