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.
Highlights
Flexible behaviour is crucial for adapting to the environment.When choosing an action, we use multiple strategies to obtain potential reward and to avoid potential punishment
Post-procedure biochemical analysis showed that tryptophan depletion (TD) robustly decreased the TRP:ΣLNAAs ratio relative to the BAL group (main effect of group: F(1,42) = 41.595, P o 0.0001; main effect of time: F(1,42) = 5.402, P = 0.025; group × time interaction: F(1,42) = 41.916, Po 0.0001)
Using mixedmeasures analysis of variance (ANOVA), we examined the probability of staying or shifting at the first task stage dependent on the between-subjects factor of group (TD or BAL) and within-subject factors of task valence, outcome and transition probability on the previous trial (common (70%) or rare (30%))
Summary
We use multiple strategies to obtain potential reward and to avoid potential punishment. Studies on humans and other animals suggest the existence of ‘reflective’ or goal-directed responses that depend on prospective consideration of future actions and their consequent outcomes in contrast to ‘reflexive’ or habitual responses that relies on retrospective experience with good and bad outcomes.[1,2,3]. Two behavioural control systems have been proposed to arise from different learning algorithms, model-based and model-free learning.[3,4] a model-based strategy was linked to the goal-directed behavioural control, whereas a model-free strategy, which presumes choices based on previously reinforced actions, suggests shared similarities with habitual control.[3] it is likely that habitual behaviour exceeds a simple reinforcement learning model-free mechanism.[5]
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