A New Theory of Psychosis?

Neuroskeptic iconNeuroskepticBy NeuroskepticOct 13, 2012 6:56 PM


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A team of British neuroscientists led by the (in)famous David Nutt says that magic mushrooms offer a new theory of psychosis: Functional Connectivity Measures After Psilocybin Inform a Novel Hypothesis of Early Psychosis

It's a reanalysis of a study from earlier this year, which got quite a lotof attention, in which 15 volunteers were injected with psilocybin - the major active hallucinogenic ingredient in 'magic mushrooms' - during an fMRI scan.

In a nutshell, the rather interesting proposal in the new paper is that psilocybin may cause mind-altering effects by blurring the difference between the brain networks responsible for 'internal' and 'external' thought.

Activity in the internal "default mode network" (DMN) is generally anti-correlated with the "task-positive network" (TPN) - when one is higher, the other's lower. The DMN is active when you're not doing much - hence 'default' while the TPN comes online when you're engaged in a particular mental activity.

Nutt's team say, however, that their functional connectivity fMRI data show that after psilocybin, activity in these two networks becomes positively correlated - an unusual pattern. They write:

Increased DMN-TPN coupling has been found in people at high risk of psychosis and an inability to distinguish between one’s internal world and the external environment, sometimes referred to as “disturbed ego boundaries,” is a hallmark of early psychoses and the psychedelic state.

One of our volunteers reported the following after psilocybin: “It was quite difficult at times to know where I ended and where I melted into everything around me.”

To be honest I'd need to see a replication before I put too much faith in this, because this kind of post-hoc reanalysis of fMRI data is very flexible and therefore prone to false positives, but it's an interesting idea, and at least it provides a clear theory for further research.

Carhart-Harris RL, Leech R, Erritzoe D, Williams TM, Stone JM, Evans J, Sharp DJ, Feilding A, Wise RG, and Nutt DJ (2012). Functional Connectivity Measures After Psilocybin Inform a Novel Hypothesis of Early Psychosis. Schizophrenia bulletin PMID: 23044373

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