George writes about LSD and the Anarchic Brain for the Psychedelic Science Review
Dr. Robin Carhart-Harris‘s REBUS model has received much attention for its purportedly unifying account of the effects of psychedelic drugs on global brain dynamics.1 REBUS is an eloquent marriage of the entropic brain hypothesis and hierarchical predictive coding. The entropic brain hypothesis proposes a kernel function of the brain to be the restriction of entropy associated with spontaneous neural activity. Importantly, it states perhaps the defining act of psychedelic drugs is the elevation of this entropy, eliciting a corresponding increase in the richness of the content of conscious experience.2,3
Predictive coding is the view that the brain is a hierarchically organised system that functions as a prediction engine.1 Under predictive coding, our perception is a generative mental model that is algorithmically optimised through a prediction error minimisation scheme. Prediction errors are the difference between the higher-level priors (expectations) a brain has about the world and the conflicting bottom-up information about the world the brain receives through sensory channels.4-6
REBUS postulates that the 5-HT2A receptor, the holy grail of classical psychedelic pharmacology, possesses a perhaps unique expression profile on cells across cortical regions deemed to be at the uppermost ranks of the brains hierarchy.1 As shown in Figure 1, it crucially suggests psychedelics act by deweighting the precision of higher-level priors, which concomitantly liberates the influence of information ascending the hierarchy, resulting in a situation where the functional hierarchy is flattened. In other words, the brain becomes anarchic, and a corollary of this is deemed to be that the variational free-energy landscape of the brain is flattened. That is to say that the brain is able to explore its repertoire of possible states with greater ease, as suggested by more temporally diverse or entropic brain activity.
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