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Brain researcher Robert Carhart-Harris and physicist Karl Friston suggest that psychedelics will save humanity. They are, of course, far from the first people to do so. But they’re the first to explain how psychedelics will save humanity using the theory of predictive processing, which was developed in large part by Friston himself. And so with my interest sufficiently piqued, allow me to condense a decade of research on the Entropic Brain Hypothesis into a few paragraphs.

Our brains evolved to model the environment and minimize surprise and uncertainty. Since our environment is complex and dynamic, so are more evolved brains. fMRI allows us to measure brain entropy, how unpredictable is one’s brain state in the future based on its current state. It’s a proxy for a brain’s flexibility and complexity.

Human brains are more entropic than those of our animal relatives, which in turn have more entropic brains than phylogenetically distant species. But humans have also developed a brain structure that suppresses entropy: the default mode network (DMN). According to Friston, optimal prediction is achieved when the brain is finely poised between order and disorder, rigidity and entropy. The DMN is less developed in children, and is suppressed in REM sleep, the onset of psychosis, and by psychedelics — all the above states are characterized by wandering thoughts, creativity, and magical thinking (hello, Ribbonfarm!) that isn’t strongly constrained by reason and prior experiences.

The DMN is also overactive in people with depression. This manifests in two features of depression: “depressive realism” (a capacity to judge reality more accurately) and rigidity of thought (the mind is stuck in a negative bias that doesn’t respond to changes in the environment). In fact, many other mental disorders can be thought of as disorders of mental rigidity. For example: addiction (the brain is stuck in loops of craving and indulgence), autism, PTSD, and schizophrenia. Psychedelics increase brain entropy and “shake it out” of its rigid habits, allowing it to settle into more salubrious patterns (especially if guided by a good therapist).

There’s another effect that psychedelics cause by inhibiting the DMN — the dissolution of ego and the sense of self. Should we think of self-identity too as a disorder of mental rigidity? Stay tuned.