Gilger, Maximilian D, Hellrung, Lydia; https://orcid.org/0000-0003-0317-4770, Neukam, Philipp T; https://orcid.org/0000-0002-9442-0043, Kroemer, Nils B, Nebe, Stephan; https://orcid.org/0000-0003-3968-9557, Pooseh, Shakoor, Deza-Lougovski, Yacila I, Smolka, Michael N; https://orcid.org/0000-0001-5398-5569, Gilger, Maximilian D, Hellrung, Lydia; https://orcid.org/0000-0003-0317-4770, Neukam, Philipp T; https://orcid.org/0000-0002-9442-0043, Kroemer, Nils B, Nebe, Stephan; https://orcid.org/0000-0003-3968-9557, Pooseh, Shakoor, Deza-Lougovski, Yacila I, and Smolka, Michael N; https://orcid.org/0000-0001-5398-5569
Background: Serotonin has been suggested to modulate decision-making by influencing the arbitration between model-based and model-free control. Disruptions in these control mechanisms are involved in mental disorders such as drug dependence or obsessive-compulsive disorder. While previous reports indicate that lower brain serotonin levels reduce model-based control, it remains unknown whether increases in serotonergic availability might thus increase model-based control. Moreover, the mediating neural mechanisms have not been studied yet. Aim: The first aim of this study was to investigate whether increased/decreased tonic serotonin levels affect the arbitration between model-free and model-based control. Second, we aimed to identify the underlying neural processes. Methods: We employed a sequential two-stage Markov decision-task and measured brain responses during functional magnetic resonance imaging in 98 participants in a randomized, double-blind cross-over within-subject design. To investigate the influence of serotonin on the balance between model-free and model-based control, we used a tryptophan intervention with three intervention levels (loading, balanced, depletion). We hypothesized that model-based behaviour would increase with higher serotonin levels. Results: We found evidence that neither model-free nor model-based control were affected by changes in tonic serotonin levels. Furthermore, our tryptophan intervention did not elicit relevant changes in Blood-Oxygenation-Level Dependent activity.