Multi-metric evaluations of acute psychedelic effects on fMRI brain entropy
Nature Communications June 24, 2026 Drummond E-Wen Mcculloch, Anders S. Olsen, Brice Ozenne et al.
A prominent theory holds that psychedelics increase brain entropy, but past studies have used many different entropy measures. This work analyzed 121 fMRI scans from 28 healthy adults before and after psilocybin, testing 14 entropy metrics with two brain-parcellation methods and seven denoising pipelines. Five metrics—including Shannon entropy of spatial eigendistribution, path-length, instantaneous correlations, brain-state switching, and sample entropy at short time-scales—consistently showed positive associations with psychedelic effects. However, eight metrics showed no significant effects, and Lempel-Ziv complexity gave inconsistent positive results. The entropy measures correlated poorly with each other, indicating that brain entropy is not a single, unified phenomenon.