Under LSD, people update their beliefs more quickly in response to surprising outcomes, and this behavioral marker of relaxed beliefs correlates with specific brain changes: lower alpha brainwave power and greater disintegration of the default mode network, particularly in the medial prefrontal cortex. These neural signatures support the REBUS model, which proposes that psychedelics relax the confidence in prior beliefs, making the brain more sensitive to prediction errors. The findings link a computational measure of surprise sensitivity to objective brain activity patterns, providing evidence for how psychedelics may alter belief updating and cognitive flexibility.
A neurobiologically realistic computational model of whole-brain haemodynamic signals, perturbed to simulate loss of consciousness, reveals two distinct neurobiological paths to unconscious brain activity. Incorporating PET data on GABA receptor distribution shows that spatially-specific local inhibition reproduces fMRI activity observed during propofol anaesthesia. Incorporating diffusion MRI data from patients with disorders of consciousness shows that randomized neuroanatomical connectivity can also produce the dynamics characteristic of loss of consciousness. The results generalize across anaesthesia and injury datasets, suggesting that increased inhibition and connectome perturbation are distinct routes to the same functional brain dynamics.
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.