Fig. 3.

Experiment 2 Study Design – Model-Based Learning Task. (a) On each trial, subjects chose between two fractals, which probabilistically transition to either an orange or blue state (pictured here in greyscale) where they must make another choice. In this schematic, the fractal on the left had a 70% chance of transitioning to the blue state, what is called a ‘common’ transition, and a 30% chance of transitioning to the orange state, i.e. a ‘rare’ transition. In the second orange or blue state, subjects again chose between two fractals, each of which was associated with a probability of reward (a pound coin). Unlike the transition structure, these reward probabilities drifted slowly over time (0.25 < p < 0.75). This meant that subjects were required to dynamically track which of the fractals in the orange and blue states were currently best. The reward probabilities depicted (34%, 68%, 72% and 67%) refer to the probability of reward for each of the 4 options presented in an example trial at a certain point along the reward probability drifts. Model-based planning on this task is operationalised as the extent to which subjects' decision to repeat an action at the first stage, depend on (i) whether this action was rewarded on the previous trial and (ii) and whether the path from action to outcome was expected (‘common’). (b) Physiological response to anxiety induction. Heart rate was elevated significantly during the gas condition, F_(1,49) = 10.72, p = 0.002. Error bars represent s.e. (c) Psychological response to anxiety induction. Self-reported anxiety levels were also significantly elevated during the inhalation of gas compared with air, F_(1,49) = 57.47, p < 0.001. Error bars represent s.e. ***, p < 0.001.