Psychedelics Relax Priors and Reshape Orbitofrontal Dynamics

Psychedelics such as psilocybin and ketamine are gaining attention as rapid-acting treatments for psychiatric disorders, yet the mechanisms by which they alter cognition remain unclear. A key hypothesis-the REBUS model-proposes that psychedelics relax high-level priors, allowing bottom-up sensory information to exert greater influence over perception and behavior. Here, we test this model in mice performing a free-response perceptual decision-making task that disambiguates prior-driven and sensory-driven decision strategies. Acute administration of psilocybin or ketamine significantly slowed decision times and improved accuracy. Behavioral modeling that combined drift diffusion and GLM-HMM frameworks revealed that these changes were mediated by increased decision thresholds and a marked shift into sensory-engaged cognitive states. Whole-brain c-Fos mapping identified a distributed decision-making network, with psychedelics selectively modulating cortical and subcortical nodes. Calcium imaging in the orbitofrontal cortex (OFC)-a key region for integrating priors and sensory inputs-revealed preserved decision-related selectivity under psychedelics, while exhibiting reduced neuronal correlations-population-level signatures of weakened top-down influence and relaxed priors. Together, these results provide circuit-level support for the REBUS model, showing that psychedelics reconfigure brain-wide and local dynamics to promote more deliberate, flexible, and sensory-driven decision policies.