Neural correlates of the psychedelic state as determined by fMRI studies with psilocybin.
Level 2 - randomized trial
Placebo-controlled crossover neuroimaging trial in healthy humans
PubMed 22308440 · doi:10.1073/pnas.1119598109
What was done
Thirty healthy volunteers underwent task-free functional MRI before and after intravenous infusions of placebo and psilocybin. Fifteen participants were scanned using arterial spin labeling perfusion to measure cerebral blood flow, and a separate cohort of 15 participants was scanned using blood-oxygen level-dependent (BOLD) fMRI. Functional connectivity was evaluated using seed-based analyses with a medial prefrontal cortex seed.
What was found
Psilocybin produced profound alterations in consciousness accompanied exclusively by decreases in cerebral blood flow and BOLD signal, which were maximal in connector hubs including the thalamus, anterior cingulate cortex (ACC), and posterior cingulate cortex (PCC). The magnitude of decreased activity in the ACC/medial prefrontal cortex (mPFC) predicted the intensity of subjective effects. Seed-based functional connectivity analysis showed a significant decrease in positive coupling between the mPFC and PCC. No exact numerical values or effect sizes were reported in the abstract.
Why it matters
This study provided early fMRI evidence that the psychedelic state is characterized by decreased activity and desynchronization within major connector hubs rather than global brain excitation.
Limits
The total sample size was small (two independent groups of 15 healthy volunteers), limiting generalizability. Intravenous drug administration differs pharmacokinetically from typical oral consumption. The abstract does not provide exact numerical effect sizes, variance estimates, or p-values, and task-free scanning does not measure objective cognitive or behavioral performance.