(A) Prior to the conditioning phase of the experiment, participants in the Instructed Group saw each image and were informed about initial probabilities. Participants in the Uninstructed Group also saw the images prior to the experiment, but were not told about contingencies. (B) Participants in both groups underwent a Pavlovian fear conditioning task with serial reversals. There were three reversals across the duration of the task, leading to four continuous blocks of twenty trials. In each block, one image (the conditioned stimulus, or CS+) was paired with a shock (the unconditioned stimulus, or US) 30% of the time, leading to 4 reinforced trials and 8 unreinforced trials, whereas a second image (the CS-) was never paired with a shock. Images were presented for 4 s, followed by a 12-second inter-stimulus interval. (C) Upon each reversal, the Instructed Group was informed that contingencies had reversed. Button presses were included to ensure participants were paying attention to the instructions but had no effect on the task itself or task timing. Instructions were always immediately followed by at least two unreinforced presentations of each CS before the new CS+ was paired with a shock. The figure presents one of two pseudorandom trial orders used during the experiment (see Materials and methods).