This study examined the effects of psilocybin on the brain using fMRI. The researchers discovered that psilocybin significantly decreased blood flow and venous oxygen levels in the brain. These results correlated with the subjective effects the participants experienced. Also, there was a significant decrease in the positive coupling between the medial prefrontal cortex and the posterior cingulate cortex. In summary, “These results strongly imply that the subjective effects of psychedelic drugs are caused by decreased activity and connectivity in the brain’s key connector hubs, enabling a state of unconstrained cognition.”