Task description.

<p>(A) Illustration of the timeline of events within a trial. Initially a black screen is presented, and the subject can choose between pressing A1, or A2 (first stage choice). After a key is pressed, one of the slot machines is presented, and the subject can again choose between pressing A1, and A2 (second stage choice). Choices at the second stage are reinforced by monetary reward. (B) Structure of the task. One of the key presses commonly leads to one of slot machines (70% of the time), and the other key commonly leads to the other slot machine. Choices at the second stage are reinforced either by a high probability (0.7) or a low probability (0.2). With a small probability (1/7), the rewarding probability of each key changes randomly to either the high or low probability.