Psychedelics distinctly alter brain entropy and complexity compared to psychostimulants

Classical psychedelics have regained interest for their potential to treat psychiatric and neurological disorders, and explore neural mechanisms supporting perception, cognition, and mood. Acute psychedelic effects have been linked to increases in brain complexity and entropy, but it is unclear if these changes are specific to psychedelics or reflect more general psychoactive effects. Here, we examined whether brain complexity and entropy metrics can identify features specific to the psychedelic state. Using resting-state fMRI from three placebo-controlled crossover trials (N=79; 255 sessions), we compared LSD, psilocybin, and mescaline with psychostimulants MDMA and d-Amphetamine. Compared to stimulants, psychedelics produced significant increases in meta-state complexity, short-timescale multiscale entropy, and dynamic conditional correlation entropy. Both drug classes increased Lempel-Ziv and spatial complexity, and decreased absolute modularity. Our findings highlight psychedelic-specific effects on brain signals that distinguish the acute psychedelic state from other psychoactive drug effects and may be relevant for understanding their therapeutic potential.