A core brain network called the default mode network (DMN) supports three distinct self-referential processes: remembering the past, imagining the future, and understanding others' minds. Functional MRI scans revealed a common pattern of neural activation across all three processes within the DMN. Autobiographical remembering and prospection more strongly engaged midline DMN structures, while theory-of-mind reasoning more strongly engaged lateral DMN areas. Activity in a key DMN node, the medial prefrontal cortex, correlated with activity in other DMN regions during all three tasks. The findings suggest the DMN provides a shared neural foundation for simulating internal experiences.
LSD and psilocybin flatten the brain's hierarchical organization, reducing the functional separation between sensory and higher-order cognitive networks. Using a non-linear dimensionality reduction technique on resting-state fMRI data, the authors found that both drugs compressed the principal gradient of cortical connectivity, which normally spans from unimodal (sensory) to transmodal (association) cortex. This flattening was driven by decreased differentiation at both ends of the hierarchy—default and frontoparietal networks at the upper end and somatomotor networks at the lower end—and was accompanied by increased crosstalk between unimodal and transmodal regions. Changes in the principal gradient under LSD tracked self-reported ego-dissolution. The findings support a mechanistic model of the psychedelic state and demonstrate that macroscale connectivity gradients are sensitive to serotonergic modulation.