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

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.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1

Two-stage decision-making task. Task. (a) On each trial (first stage), the initial choice between two stimuli (left-right randomised) led with fixed probabilities (transition) to one of two pairs of stimuli in stage 2. Each of the four second-stage stimuli was associated with probabilistic outcome: monetary reward in the reward or loss in the punishment version of the task. All stimuli in second stage were associated with probabilistic outcome, which changed slowly and independently across the trials. (b) Model-based and model-free strategies predict different choice patterns by which outcome obtained after the second stage affected subsequent first-stage choices. In the model-free system, the choices are driven by the reward or the no loss, which increase the chance of choosing the same stimulus on the next trial independently of the type of transition (upper row). In a model-based system, the choices of the stimuli on the next trial integrate the transition type (lower row).

Figure 2

(a) Factorial (stay-shift) behavioural results. Separate analysis of task valence showed a mixed choice strategy in BAL and a shift to a model-free choice strategy in the TD group in the reward condition. In the loss condition, the significant interaction between outcome × transition in the TD group indicates a shift of behavioural choice towards a model-based strategy. (b) Computationally fitted behavioural results before arscin transformation. Compared with BAL, the TD group showed a significant difference in the weighting factor ω in reward condition. BAL=control group; TD=TRP-depleted group. *P<0.05.