We physically implemented the task using a set of four nose-poke ports: top and bottom ports in the center, flanked by left and right ports (Figure 1A). Each trial started with the central ports lighting up, requiring a choice between top and bottom ports. The choice of a central port led probabilistically to a “left-active” or “right-active” state, in which respectively the left or right port was illuminated. The subject then poked the illuminated left or right port to gain a probabilistic water reward (Figures 1A and 1B). Pokes to non-illuminated ports were ignored, so at the first step only pokes to the top or bottom ports, and at the second step only pokes to the illuminated side port, affected the task. A 1 second inter-trial interval started when the subject exited the side port. Subjects rarely poked either side port at the time of first-step choice, or the inactive side port at the second step (Figure S2), indicating that they understood the trial structure.
