Fig. 1.

Sequence of trial events. (A) Training phase for Experiments 1–3. Each trial began after the participant fixated the white box located in the center of the screen for 500 ms. The target was defined by color (red or green, exactly one of which was present on each trial). Correct responses were followed by the delivery of monetary reward feedback. (B) Test phase of Experiment 1. Participants were informed whether it was or was not possible to receive an electric shock during the next block of 20 trials. Each trial began after the participant fixated the image corresponding to each block at the center of the screen for 500 ms. The target was defined as the unique shape, and no reward feedback was provided. (C) Test phase of Experiment 2, which mirrored Experiment 1 except that each block was the same and there was no reference to shock. (D) In Experiment 3, each of two 120-trial blocks of the test phase was preceded by a separate training phase. The stimuli were identical to Experiment 1. (E) In Experiment 4, there was no training phase and participants completed only the shape singleton search task with a color singleton as a distractor. The block design and procedure were identical to Experiment 1. (F) In Experiment 5, the same shape singleton search task as in Experiment 4 was completed without reference to shock.