Default mode network and consciousness
Explanation
Marcus Raichle and others, in the early 2000s, discovered through functional neuroimaging a brain network that activates when subjects are not performing any specific task: the Default Mode Network (DMN). Initially intriguing, this network has turned out to be central to understanding self-reference, narrative consciousness and various mental disorders.
The DMN includes several regions: the medial prefrontal cortex, the posterior cingulate cortex, the precuneus, parts of the inferior parietal lobe and the medial temporal cortex. These regions are more active during rest (lying down, eyes closed, no task) than during the performance of externally goal-directed tasks, which led them to be called the default network.
What does the brain do during rest? It was found to engage in mind wandering: autobiography, personal memories, future planning, speculation about the thoughts of others, narratives about oneself. The DMN seems to be the neural substrate of the narrative self, of the stream of self-referential thoughts that constitutes much of everyday mental life.
Studies of meditation have shown that deep meditative practice reduces DMN activity. Expert meditators show characteristic patterns: less mind wandering, more moment-to-moment presence, less identification with self-referential narratives. This correlates with phenomenological reports of ego disidentification in advanced meditation.
Studies with psychedelics (Robin Carhart-Harris and others) have shown that substances such as psilocybin and LSD dramatically deactivate the DMN. This deactivation correlates with the subjective experiences of ego dissolution reported with these substances. The DMN would then be the neural substrate of the sense of self, and its temporary disorganisation is what produces mystical or self-transcendent experiences.
The DMN is central in many pathologies. It is hyperactive in depression (self-critical rumination), in anxiety (constant worries about oneself), in autism (altered). It is hypoactive in Alzheimer's (loss of coherent self-reference), in vegetative states, and during deep sleep. It is one of the most important discoveries in contemporary cognitive neuroscience regarding self-referential consciousness.
Strengths
- A network robustly replicable in neuroimaging.
- Consistent association with self-referential thought.
- Foundation for psychedelic research.
- Clinical implications.
Main critiques
- Specific function not entirely clear.
- Role in consciousness disputed.
- Risk of reifying a 'network of the self'.
- The DMN also activates during specific tasks.