Fig. 2

Two-step task designed to assess model free and model based learning. A Stage 1 was a choice between two spaceships. This choice determined the transition to the next stage according to a fixed probability scheme: each spaceship was predominantly associated with one or the other Stage 2 states (i.e. planets) and led there 70% of the time. In Stage 2. Participants selected one of two aliens to learn if they would be rewarded. Each alien was associated with a probability rewarded. Each alien was associated with a probability reward (range: 0.25–0.75) that changed gradually over time, based on Gaussian random walk. B Timing of stages within a single trial C Example of one of the sets of Gaussian random walks that determined the probability of reward at each of the four stage 2 options