The Entropic Brain Hypothesis proposes that conscious states correspond to the entropy (diversity and unpredictability) of spontaneous brain activity. A recent cross-condition fMRI study found an entropic gradient: propofol anaesthesia reduced entropy, while LSD, DMT, modafinil, and schizophrenia increased it, with schizophrenia showing the largest increase. The present project aims to test this hypothesis more broadly by examining fMRI datasets from LSD, psilocybin, modafinil, propofol anaesthesia, natural sleep, schizophrenia, Mild Cognitive Impairment, Alzheimer's disease, and Frontotemporal Dementia. It hypothesizes that diminished states (sleep, anaesthesia) will show low entropy, psychedelic and stimulant states high entropy, and neurodegenerative conditions low entropy. A second objective is to compare a topology-derived entropy measure with a brain-state-derived Shannon entropy measure to assess robustness across operationalizations.
A participant who can reliably enter a self-induced non-ordinary state of consciousness (NOC) characterized by vivid imagery, altered bodily perception, and a sense of unity underwent 20 fMRI sessions. Compared to a control group, during the transition into the NOC state, functional connectivity became more variable, indicating temporary destabilization of network organization. In the NOC state, connectivity between brain networks broadly decreased, especially visual cortex coupling with auditory, sensorimotor, and other regions, while frontoparietal and salience networks increased coupling with precuneus and temporal areas, matching reports of inward attention and absorption. Entropy and complexity measures tracked the experience and returned to baseline afterward.
States of consciousness can be ordered along a single dimension defined by the entropy of spontaneous neural activity, as proposed by the Entropic Brain Theory. Applying the same analytical pipeline to pharmacological (psychedelics, modafinil, propofol anaesthesia) and clinical (schizophrenia) fMRI datasets, the temporal irregularity of brain network topology was quantified. Propofol anaesthesia occupied the low-entropy end; psychedelic states and schizophrenia occupied the high end. This ordering tracks combined modulations of the level and content of consciousness, from reduced awareness under anaesthesia to heightened arousal and expanded experience under psychedelics and disorganised processing in schizophrenia. The result was not reducible to fluctuations in mean functional connectivity and was supported by convergent reorganisation of higher-order association cortex.