Modelling Rumination as a State-Inference Process
Rumination is a kind of repetitive negative thinking that involves prolonged sampling of negative episodes from one’s past, typically prompted by a present negative experience. We model rumination as an attempt at hidden-state inference, formalized as a partially-observable Markov decision process (POMDP). Using this allegorical model, we demonstrate conditions under which continuous, prolonged collection of samples from memory is the optimal policy. Consistent with phenomenological observations from clinical and experimental work, we show that prolonged sampling (i.e., chronic rumination), formalized as needing to sample more evidence before selecting an action, is required when possible negative outcomes increase in magnitude, when states of the world with negative outcomes are a priori more likely, and when samples are more variable than expected. By demonstrating that prolonged sampling may allow for optimal action selection under certain environmental conditions, we show how rumination may be adaptive for solving particular problems.