Fig. 4.

Results from Experiment 2. (a) Stay/switch behaviour for subjects in air condition as a function of whether or not the last trial was rewarded/unrewarded and followed a rare/common transition. Error bars represent s.e. (b) The same plot, showing the group average behaviour under CO₂. In both plots, subjects showed the classic signatures of both model-based and model-free planning, indexed by a significant reward × transition interaction (β = 0.28, s.e. = 0.06, p < 0.001) and a main effect of reward (β = 0.55, s.e. = 0.08, p < 0.001). Error bars represent s.e.