Psilocybin, the active ingredient in magic mushrooms, may be a useful therapeutic tool for treating depression, anxiety and other psychiatric problems, scientists have suggested.
Brain scans of people under the influence of the psilocybin have given scientists the most detailed picture to date of how psychedelic drugs work.
The findings of two studies identify areas of the brain where activity is suppressed by psilocybin and suggest that it helps people to experience memories more vividly.
In the first study, 30 healthy volunteers had psilocybin infused into their blood while inside magnetic resonance imaging (MRI) scanners, which measure changes in brain activity.
The scans showed that activity decreased in "hub" regions of the brain - areas that are especially well connected with other areas.
The second study, found that psilocybin enhanced volunteers' recollections of personal memories, which the researchers suggest could make it useful as an adjunct to psychotherapy.
"Psychedelics are thought of as 'mind-expanding' drugs so it has commonly been assumed that they work by increasing brain activity, but surprisingly, we found that psilocybin actually caused activity to decrease in areas that have the densest connections with other areas," said Professor David Nutt, from the Department of Medicine at Imperial College London, the senior author of both studies.
"These hubs constrain our experience of the world and keep it orderly. We now know that deactivating these regions leads to a state in which the world is experienced as strange," he explained.
The intensity of the effects reported by the participants, including visions of geometric patterns, unusual bodily sensations and altered sense of space and time, correlated with a decrease in oxygenation and blood flow in certain parts of the brain.
The function of these areas, the medial prefrontal cortex (mPFC) and the posterior cingulate cortex (PCC), is the subject of debate among neuroscientists, but the PCC is proposed to have a role in consciousness and self-identity.
The mPFC is known to be hyperactive in depression, so psilocybin's action on this area could be responsible for some antidepressant effects that have been reported.
Similarly, psilocybin reduced blood flow in the hypothalamus, where blood flow is increased during cluster headaches, perhaps explaining why some sufferers have said symptoms improved under psilocybin.
"Psilocybin was used extensively in psychotherapy in the 1950s, but the biological rationale for its use has not been properly investigated until now. Our findings support the idea that psilocybin facilitates access to personal memories and emotions," said Dr Robin Carhart-Harris, from the Department of Medicine at Imperial College London, the first author of both papers.
"Previous studies have suggested that psilocybin can improve people's sense of emotional wellbeing and even reduce depression in people with anxiety.
"This is consistent with our finding that psilocybin decreases mPFC activity, as many effective depression treatments do. The effects need to be investigated further, and ours was only a small study, but we are interested in exploring psilocybin's potential as a therapeutic tool," he added.