Abstract
Psychoactive drugs profoundly alter consciousness, but the neural mechanisms by which they do so remain poorly understood. Global workspace theory may provide a framework for investigating these mechanisms. The global workspace, a candidate neural substrate of consciousness, is a set of brain regions that integrate and disseminate information to the rest of the brain. According to a recent model, the integrators, or 'gateways,' are characterized by synergistic interactions with other brain regions, while the disseminators, or 'broadcasters,' exhibit redundant interactions. Here, we measured the effects of various psychoactive drugs, including 'classic' psychedelics like LSD, mescaline, and psilocybin, 'atypical' psychedelics (entactogens) like MDMA, and the stimulant D-amphetamine, on the global workspace. In a secondary analysis of resting-state functional magnetic resonance imaging (fMRI) data on all these drugs, we found that classic psychedelics are associated with significant increases in the number of gateways in the frontoparietal control and dorsal attention networks. Meanwhile, MDMA and amphetamine reduced the number of gateways in the visual and somatomotor networks. The regional topographies of both synergy and redundancy distinguished classic psychedelics from other drugs. Finally, drugs producing similar changes in synergy and redundancy also produced similar profiles of subjective effects. Overall, our findings provide the first direct evidence that psychedelics may alter the synergistic and redundant interactions that characterize the global workspace of consciousness.