Under the influence of mushrooms, overall brain activity drops, particularly in certain regions that are densely connected to sensory areas of the brain. When functioning normally, these connective “hubs” appear to help constrain the way we see, hear and experience the world, grounding us in reality. It seems that a lot of brain activity is used to keep the world very ordinary and very familiar. They are also the key nodes of a brain network linked to self-consciousness and depression. Psilocybin cuts activity in these nodes and severs their connection to other brain areas, allowing the senses to run free.
For the study, the authors recruited 15 volunteers to receive injections of psilocybin or placebo, in alternate sessions, while being scanned in an fMRI machine. Taken intravenously, psilocybin alters consciousness in a mere 60 seconds, as opposed to the 40 minutes it normally takes when administered orally. And the high lasts a half an hour, not the five hours that typical users experience.
Provisions were made for the possibility that the participants might panic while high in the noisy, claustrophic setting of the scanner, but none of the volunteers did so. In fact, once they’d become accustomed to the noise and small space, “they quite liked being enclosed and felt secure.” All of the participants were already experienced.
Reducing the brain’s activity interfered with its normal ability to filter out stimuli, allowing participants to see afresh what would ordinarily have been dismissed as irrelevant or as background noise. They described having wandering thoughts, dreamlike perceptions, geometric visual hallucinations and other unusual changes in their sensory experiences, like sounds triggering visual images.
The particular brain regions that were silenced or disconnected from each other by the drug also provided insight on the nature of psychedelic experience and the therapeutic potential of psilocybin. Two regions that showed the greatest decline in activity were the medial prefrontal cortex (mPFC) (area linked with obsessive thinking) and the posterior cingulate cortex (PCC) (plays a key role in consciousness and self-identity).
The coolest thing is that geometric visual hallucinations commonly seen by people on psychedelics (and by some sufferers of migraines) help reveal the architecture of the brain’s visual processing mechanism. “One hypothesis is that what you’re actually seeing is the functional organization of the visual cortex itself. The visual cortex is organized in a sort of fractal way [it repeats the same patterns in different sizes]. It’s the same way that fractals are everywhere in nature. You’re not seeing the cells themselves, but the way they’re organized — as if the brain is revealing itself to itself.