Robin Carhart‐Harris
Introduced in 2014 and revised in 2018, the entropic brain hypothesis has accrued a wealth of supportive evidence. The hypothesis states that-along a dimension of the size of phenomenal consciousness-expansive states reliably exhibit increased brain entropy whereas the inverse applies for states of no or reduced consciousness. Examples of expansive states include expert meditation, flicker light stimulation, near-death-like experiences, atypical breathing, rapid-eye-movement sleep, the pre-ictal aura, unmedicated early psychosis and psychedelic drug states. Examples of states of no or reduced consciousness with low brain entropy, include disorders of consciousness, deep sleep, the anesthetized state, seizure, post-stroke, ageing, cognitive impairment, and neurodegenerative illness. It is shown that the entropic brain has convergent, correlative, predictive, discriminative and external validity. Regarding its predictive validity, increased brain entropy under psilocybin (in a supportive context) predicts subsequent improvements in mental health (improved wellbeing 1-month post-dose). Regarding its discriminative validity, changes in brain entropy selectively index the breadth of subjective experience versus alternative dimensions, such as arousal. Regarding portability/external validity, an entropy-related function is applied in generative artificial intelligence. In conclusion, the entropic brain is a useful model of conscious states.