LSD and psilocybin produce opposite changes in cerebral blood flow and functional connectivity, study suggests

A study combining PET and MRI imaging examined how LSD affects the brains of 11 healthy volunteers and compared the findings with earlier data from 25 people who received psilocybin. LSD strongly occupied serotonin 2A receptors, increased cerebral blood flow across much of the brain, and reduced global functional connectivity. Psilocybin showed the opposite pattern, with decreased cerebral blood flow and increased global connectivity. The study also found that blood concentrations of LSD and subjective effects followed a time-dependent counterclockwise hysteresis pattern. These results clarify distinct physiological responses to the two psychedelics, but the researchers did not establish whether either pattern causes therapeutic benefits. Future work will examine neurovascular coupling and possible links between psychedelic effects, neuroplasticity, and psychotherapy or brain-stimulation strategies.