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See Inside Scientific American Mind Volume 23, Issue 2

This Is Your Brain on Drugs

To the great surprise of many, psilocybin, a potent psychedelic, reduces brain activity

Hemodynamic activity as registered by fMRI scanners is tightly linked to neuronal activity. A standard reading of Nutt’s fMRI data seems to imply that expanding your mind by taking magic mushrooms turns many brain circuits down rather than up. Suddenly, Timothy Leary’s famous admonition to hippies to “turn on, tune in and drop out” acquires a whole new meaning.

The ACC and parts of the mPFC inhibit limbic and other structures. Thus, their downregulation, or reduction in response, would allow the content of the limbic systems that process emotion and perhaps sensory cortices to play a relatively more dominant role. It is not that enhanced hemodynamic, or even neuronal, activity by itself gives rise to perception and thought. After all, epileptic seizures are hypersynchronized discharges that engulf the entire cortex in massive rhythmic activity that renders the patient unconscious. It is the pattern of spiking across heterogeneous populations of neurons that carries the specific information, the messages, that are represented in consciousness.

At this point, this is all pure speculation because the detailed biophysical mechanisms and the effects of psilocybin on different neurons remain to be worked out.

Any such remarkable finding needs to be replicated by other groups before it becomes part of textbook knowledge. Moreover, the discrepancy with the earlier PET experiments needs to be explained. Two major differences are the mode of taking the drug (intravenously versus orally) and the time of measurement (immediately versus an hour later).

What is intriguing is that the regions that show the strongest reduction in activity are among the most heavily interconnected in the brain. They act like traffic circles or hubs that link disparate regions. Thus, the brain on psilocybin becomes more disconnected, more fragmented, which might explain some of the dissociative aspects of acid trips. Yet why this state should cause the mind-expanding effects that are the prime reason these drugs are treasured is utterly unclear. The study once again highlights how elusive our knowledge of the mind-brain hinge remains.

This article was published in print as "This is Your Brain on Drugs."

(Further Reading)
  • The Neurobiology of Psychedelic Drugs: Implications for the Treatment of Mood Disorders. Franz X. Vollenweider and Michael Kometer in Nature Reviews Neuroscience, Vol. 11, pages 642–651; September 2010.
  • Neural Correlates of the Psychedelic State as Determined by fMRI Studies with Psilocybin. Robin L. Carhart-Harris et al. in Proceedings of the National Academy of Sciences USA , Vol. 109, No. 6, pages 2138–2143; February 7, 2012.
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