Figure 1. Study and task design.

(A) Illustration of within-subjects design. On each of 2 testing days, approximately 7 days apart, participants started with either a medical screening and brief physical exam (day 1) or a working memory test (day 2). Subsequently they drank an orange squash containing either levodopa ("D") or placebo ("P"). (B) Task structure of the Magic Castle Game. Following a choice of vehicle, participants ‘travelled’ to two associated destinations. Each vehicle shared a destination with another vehicle. At each destination, participants could win a reward (10 pence) with a probability that drifted slowly as Gaussian random walks, illustrated in (C). (D) Depiction of trial types and sequences. (1) On standard trials (2/3 of the trials), participants made a choice out of two options in trial n (max. choice 2 s). The choice was then highlighted (0.25 s) and participants subsequently visited each destination (0.5 s displayed alone). Reward, if obtained, was overlaid to each of the destinations for 1 s. (2) On uncertainty trials, participants made a choice between two pairs of vehicles. Subsequently, the ghost nominates, unbeknown to the participant, one vehicle out of the chosen pair. Firstly, the participant is presented the destination shared by the chosen pair of vehicles (here the forest) and this destination is therefore non-informative about the ghost’s nominee. Secondly, the destination unique to the ghost-nominated vehicle is then shown (the highway). This second destination is informative because it enables inference of the ghost’s nominee with perfect certainty based on a model-based (MB) inference that relies on task transition structure. Trial timing was identical for standard and uncertainty trials.