A network of brain regions called the default mode network breaks down during anesthesia and after brain damage causing disorders of consciousness. The neurochemical reasons for this breakdown were unclear. Using functional MRI, researchers found that the ventral tegmental area, a dopamine-producing brainstem region, disconnects from key default mode network nodes (precuneus and posterior cingulate) during both propofol sedation and disorders of consciousness. Stronger connectivity between the ventral tegmental area and these nodes was associated with a more awake-like configuration of the default mode network. In patients with disorders of consciousness who later improved behaviorally, this connectivity increased toward healthy levels. In a separate group of traumatic brain injury patients, the drug methylphenidate significantly strengthened this connection. The findings suggest that dopamine modulation may be central to maintaining consciousness.
Consciousness relies on spatio-temporal interactions between brain integration and functional diversity. Combining graph theory and dynamic functional connectivity, resting-state fMRI data from awake volunteers, propofol-anaesthetised volunteers, and patients with disorders of consciousness showed that cortical networks are especially affected during loss of consciousness in temporal states of high integration, exhibiting reduced functional diversity and compromised informational capacity, while thalamo-cortical functional disconnections emerge during states of higher segregation. Posterior regions of the brain's default mode network show reductions in both functional diversity and integration during unconsciousness. These overlapping reductions in diversity and integration may represent a generalisable biomarker of loss of consciousness.