Perception is in the Details: A Predictive Coding Account of the Psychedelic Phenomenon

Introduction

Earlier work has characterised the psychedelic state as an ‘‘entropic brain’’ in which neural activity occupies a larger repertoire of states than normal. At an implementational level, the consensus is that classic psychedelics act as agonists at serotonin 5-HT2A receptors, particularly those expressed on apical dendrites of layer V neocortical pyramidal neurons, and that this receptor activation alters network dynamics (for example by desynchronising slower rhythms). Pink-Hashkes and colleagues bring these computational- and implementational-level observations together, arguing that an increased repertoire of brain states can be formalised within the predictive coding framework. This paper sets out to formalise the Entropic Brain hypothesis using predictive coding concepts. Specifically, the authors propose that 5-HT2A receptor hyperactivation in layer V pyramidal cells decomposes broad, top-down categorical predictions into many more overly detailed (high-granularity) predictions. That decomposition raises prediction error and entropy, and the study aims to show how the brain’s compensatory mechanisms for reducing prediction error can account for a wide range of psychedelic phenomena (hallucinations, heightened sensory detail, synaesthesia, openness, ego dissolution, time dilation).