A team of British researchers has examined the effects of psilocybin on the brain using MRI scanners and found that the drug shuts down certain areas of the brain that serve as "reducing valves" designed to filter out external stimuli so that we might focus on survival skills and avoiding predators. They posit that the drug actually slows down brain function instead of speeding it up, as most of us had previously believed.
According to their results (in an on-line Time magazine article), certain areas of the human brain seem to limit the amount of information we take in so that we can sort it out and make rational decisions from what we see and hear. Psilocybin and other hallucinogens slow down brain function in those filtering areas, allowing users to see, hear, taste, and smell the world through a broader window with senses freed.
Fifteen volunteers were tested using psilocybin via intravenous injections, rather than normal consumption of the mushrooms. Effects on those not given the placebo were noticed within 60 seconds, and the "high" only lasted for thirty minutes. Normally, psilocybin takes much longer to take effect, and its effects last for hours.
Here's a snippet from the article:
Researchers had assumed that the hallucinations and bizarre sensations caused by psilocybin would have at least one part of the brain working overtime. But instead they found the opposite.
“The decline in activity was the most surprising finding,” says Carhart-Harris, “and anything that’s of surprise is usually important.”
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.
Indeed, if we always paid attention to every perceptible sensation or impulse like this, we’d be incapable of focusing at all. This is why it’s difficult to sit still and try to tune in all the feelings and perceptions we normally tune out, but why also, like psychedelic drugs, meditation can make the world seem strange and new.