A brain pattern of high integration and low segregation is recurrently observed after psilocybin administration.

Psychedelic drugs have been used throughout history as a means of altering conscious experience and nowadays are tools in clinical context. Psilocybin is one such a psychoactive substance, which induces profound distortions in subjective experience typically accompanied by increased brain connectivity as revealed by stationary connectivity analysis. Here we study whether and how the brain reconfigure in more functional brain states and what the inter-state dynamics look like under psilocybin. In a double-blind, placebo-controlled, parallel-group design, we utilized ultra-high field (7T) fMRI where participants were randomized to receive a single dose of psilocybin (0.17mg/kg, n=22) or placebo (n=27). Connectivity analysis revealed that, compared to placebo, there was an overall increase in the whole-brain connectivity after usage of psilocybin, This effect was related to an increased connectivity between visual network and default mode and executive control networks. Time-varying connectivity further showed that a state of global cortico-cortical positive connectivity re-occurred more often in the psychedelic state. Inter-state dynamic analysis showed that pattern transitions were more probable in the psilocybin group, but the preferred pattern remained this overall positive connectivity pattern. This was proved by observing the significantly higher "ignition" power of cortical regions in psychedelic states which showed their unusually higher tendency to make functional connections with each other. Together, these results indicate that in a psychedelic state our brain recurrently supports a highly integrated and less segregated configuration, leading to such a profound altered experiential state.