Experimental design.

Top row. In the urn experiment, the trial was initiated by moving on a gray start bar. Two point clouds appeared showing samples from two urns with different underlying ratios of blue and red balls. The risky urn was always displayed by 100 samples drawn from a 50 : 50 ratio. The ambiguous urn had a variable number of samples drawn from an unknown ratio. Subjects made their decision about the urn they believed to have a higher red ratio by crossing into the area highlighted in orange. In case of choosing the ambiguous urn, the composition of the urn was revealed. The payoff was given by a viscous force field, which was switched on with the probability determined by the chosen urn’s ratio for blue. Middle row. In the motor task, subjects had to decide to hit one of two targets by moving into the corresponding decision circle. In case they chose a (partially) occluded target, the target became fully visible after crossing into the highlighted area. When failing to hit the target, subjects had to move against a viscous force field. In both experiments subjects had to move towards the goal bar and back. The orange color is only for illustration and was not displayed during the experiment. Bottom row. Experiment 2. Subjects are presented with the same stimulus as in the motor task but perform an urn task where the random outcome is computer generated, in contrast to the motor task where the outcome is determined by the subjects behavior. After choosing between an ambiguous and a risky option a cloud of points appeared revealing the composition of the hidden urn that determined the payoff in the same way as in the urn task.