Psilocybin and altered states

consciousnessNeural correlates of the psychedelic state as determined by fMRI studies with psilocybin

www.pnas.org/cgi/doi/10.1073/pnas.1119598109

This paper further undermines the persistent claim within mainstream consciousness studies that all that needs to be done is to deconstruct the self (a relatively easy process) and then declare the consciousness problem solved. Altered states of consciousness have always appeared to contradict this claim, but the evidence of this was at a rather anecdotal level. Compiled by several prominent universities, this paper demonstrates that it is  likely that the self can be deactivated by a drug while the subject continues to have conscious experiences.

In this paper, the authors use psilocybin, the active compound in so-called ‘magic mushrooms’ to study the transition from a normal to an altered state of consciousness. The authors were surprised that a markedly altered state correlated with a reduction in blood flow and BOLD signal, rather than their expectation of increased neural activity. The biggest reductions in blood flow and BOLD were observed in the thalamus and the cingulate cortex. The larger the decrease observed, the greater was the reported strength of the subjective experiences. In particular, psilocybin caused a large decrease in interaction between the medial prefrontal cortex and the posterior cingulate cortex. This is taken by the authors to imply that the altered state correlates with decreased connectivity between hubs involved in connecting and organising the brain.

In this study, subjects receiving psilocybin were compared to a control group that received a placebo. The psilocybin group demonstrated a significant decrease in cerebral blood flow in parts of the thalamus, the posterior and anterior cingulate cortex, the medial prefrontal cortex, the orbitofrontal, the frontal operculum and a number of other brain regions. The decreases were localised in associative regions or hub/connector regions such as the thalamus. In each area of the brain that was studied the decrease in blood flow correlated to the reported intensity of the subjective effect. A separate study based on the BOLD signal showed regional decreases in the same areas as those that saw a decrease in cerebral blood flow. The  cingulate cortex and the medial prefrontal are seen as being particularly implicated in the action of psilocybin. A study with rats showed a decrease in local field potentials after receiving psilocybin.

The authors consider the results of their study unexpected and therefore in need of some explanation. Previous studies had shown an increase in brain activity in terms of glucose metabolism, and there has been as assumed connection between psychedelics and increased neural activity. The authors suggest that stimulation of serotonin transmission by the drug, leading to increases in GABA transmission, could in turn lead to the inhibition of pyramidal cells, and the observed deactivation in some brain areas.

The posterior cingulate and the medial prefrontal showed the most consistent deactivation under psilocybin and are also areas that have a 20% higher rate of metabolism than the rest of the brain. The study showed a decrease in interaction between the posterior cingulate, and some theorists have suggested that the posterior cingulate and the associated default-mode network have a role in the experience of the ‘self’ or self-consciousness. The default network of which the posterior cingulate is part also involves the largest concentration of cortico-cortical connections in the brain. Deactivation of such connections may relate to alterations in conscious states. This idea is seen by the authors as being consistent with Aldous Huxley’s idea of the brain as a reducing valve. One possibility suggested is that deactivation in frontal areas such as the posterior cingulate leads to enhanced influence from sensory areas such as the parietal cortex.

From the point of view of consciousness studies, this paper further undermines the persistent claim in mainstream works that all that needs to be done was to deconstruct the self (a relatively easy process), and then declare the consciousness problem solved. Altered states of consciousness have always appeared to contradict this claim, but the evidence of this was at a rather anecdotal level. This paper based on research in prominent universities including Imperial College London indicates that it is likely that the self can be wholly or partly deactivated by a drug while the subject continues to have conscious experiences.

 

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