Brain dynamics under LSD become more disordered and complex, moving further from the critical point that characterizes healthy brain function. Using Ising spin models fitted to fMRI data from fifteen participants, the authors show that LSD reduces interhemispheric connectivity, especially between corresponding regions in opposite hemispheres. Ising temperatures were significantly higher under LSD than placebo, indicating a shift into a more disordered (paramagnetic) state. Algorithmic complexity of brain activity, measured by block decomposition, correlated with both Ising temperature and condition, supporting the entropic brain hypothesis that psychedelics increase neural disorder.
Using fMRI data from fifteen people who took LSD or a placebo, researchers modeled brain dynamics with an Ising spin model to test whether psychedelics push the brain into a more disordered state. LSD increased the Ising temperature of brain activity, moving it further away from a critical point (the edge between order and disorder) into a more disordered, paramagnetic phase. This shift was accompanied by a decrease in interhemispheric connectivity, especially between corresponding regions in the two hemispheres. Algorithmic complexity of brain signals also increased with LSD. The findings suggest LSD loosens homotopic connections, driving the brain into a more flexible, complex state, consistent with theories that psychedelics increase neural entropy.