Valence-dependent influence of serotonin depletion on model-based choice strategy

TLDR

Human decision‑making involves reflective and reflexive mechanisms that correspond to model‑based and model‑free learning systems. The study examined how dietary tryptophan depletion, which reduces serotonin transmission, affects performance on a two‑stage decision‑making task that distinguishes model‑based from model‑free strategies. A novel version of the task was used to assess choice balance for both monetary reward and punishment. Tryptophan depletion weakened model‑based behavior for rewards but strengthened it for punishments, indicating serotonin modulates goal‑directed learning through reward representation changes and has implications for disorders with impaired goal‑directed control.

Abstract

Abstract Human decision-making arises from both reflective and reflexive mechanisms, which underpin goal-directed and habitual behavioural control. Computationally, these two systems of behavioural control have been described by different learning algorithms, model-based and model-free learning, respectively. Here, we investigated the effect of diminished serotonin (5-hydroxytryptamine) neurotransmission using dietary tryptophan depletion (TD) in healthy volunteers on the performance of a two-stage decision-making task, which allows discrimination between model-free and model-based behavioural strategies. A novel version of the task was used, which not only examined choice balance for monetary reward but also for punishment (monetary loss). TD impaired goal-directed (model-based) behaviour in the reward condition, but promoted it under punishment. This effect on appetitive and aversive goal-directed behaviour is likely mediated by alteration of the average reward representation produced by TD, which is consistent with previous studies. Overall, the major implication of this study is that serotonin differentially affects goal-directed learning as a function of affective valence. These findings are relevant for a further understanding of psychiatric disorders associated with breakdown of goal-directed behavioural control such as obsessive-compulsive disorders or addictions.