Far from expanding your mind, the
hallucinogenic chemical found in magic mushrooms induces widespread decreases in
brain activity, researchers report today in the Proceedings of the National
Academy of Sciences.
Psilocybin has been revered for centuries for its ability to induce mystical
experiences, and has potential therapeutic value for various psychiatric
conditions. The drug is known to activate serotonin receptors, but how this
produces its effects is little understood.
In one of the few studies of its kind, David Nutt, a
neuropsychopharmacologist at Imperial College London, and his colleagues used
functional magnetic resonance imaging to monitor the changes in brain activity
during the transition from normal consciousness to the psychedelic state induced
by psilocybin.
The researchers recruited 30 volunteers, all experienced users of
hallucinogenic drugs, and scanned their brains twice: once after the
participants had been given a salt-water placebo and once after the injection of
a moderate dose of psilocybin, which produced a short ‚trip‘ that came on
within seconds.
“Psychedelics are thought of as ‘mind-expanding’ drugs, so it has commonly
been assumed that they work by increasing brain activity,” says Nutt.
“Surprisingly, we found that psilocybin actually caused activity to decrease
in areas that have the densest connections with other areas.”
Power down
The largest decreases were observed in the the medial prefrontal cortex (mPFC)
and the anterior and posterior cingulate cortices (ACC and PCC, respectively).
The scans also showed a reduction in functional connectivity between the mPFC
and PCC, so that their normally synchronous activity was de-synchronized.
Furthermore, the intensity of the drug’s effects — as reported,
subjectively, by the participants — was closely related to the extent of
decreased ACC and mPFC activity, with the strongest effects associated with the
largest decreases.
The mPFC, PCC and thalamus are thought to act as ‚connector hubs‘ that
have a pivotal role in co-ordinating the flow of information through the brain,
and the researchers say that this accounts for the effects of hallucinogens,
which induce a state of “unconstrained cognition”.
his 1954 book The Doors of Perception, novelist Aldous Huxley, who
famously experimented with psychedelics, suggested that the drugs produce a
sensory deluge by opening a “reducing valve” in the brain that normally acts
to limit our perceptions.
The new findings are consistent with this idea, and with the free-energy
principle of brain function developed by Karl Friston of University College
London that states that the brain works by constraining our perceptual
experiences so that its predictions of the world are as accurate as possible.
Therapeutic potential
Nutt and his colleagues suggest their results could explain some of the
therapeutic effects of psilocybin. Depression involves hyperactivity in the
mPFC, leading to the pessimistic outlook and pathological brooding
characteristic of the condition, so mPFC deactivation could alleviate those
symptoms.
The researchers also observed reduced blood flow to the hypothalamus, and
suggest that this explains anecdotal reports that psychedelics alleviate
symptoms of cluster headaches, which are associated with increased hypothalamic
activity.
However, Nutt’s findings conflict with those of other studies.
“We have completed a number of similar studies and we always saw an
activation of these same areas,” says Franz Vollenweider at the University of
Zurich in Switzerland. “We gave the drug orally and waited an hour, but they
administered it intravenously just before the scans, so one explanation is that
the effects were not that strong.”
And according to Keith Laws, a neuropsychologist at the University of
Hertfordshire, UK, the results could be explained in another way.
“Deactivation of the mPFC and PCC are linked to anxiety and anticipation of
pleasant and unpleasant experiences,” he says. “This is a stressful
situation, even for experienced drug users, and I suspect that they measured
something to do with anxiety.”
Zdroj: Nature.com
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