Psilocybin increases optimistic engagement over time: computational modelling of behaviour in rats

Psilocybin increases optimistic engagement in rats, with effects that build over time after dosing. Researchers used a two-armed bandit reversal learning task to measure how rats choose between poking for reward, poking for loss, or staying in the cage. Computational modelling of rat choices (active inference and reinforcement learning) suggested that psilocybin led to more reward-seeking via increased task engagement. The model attributed this to changes in belief updating, specifically modified forgetting rates and reduced loss aversion. The findings frame psilocybin as potentially creating an “optimism bias” driven by altered information processing, with translational relevance for depressive states characterized by low optimism and reduced engagement with rewarding experiences.