Figure 1. Behavioral tasks used in the two experiments.

Both studies used a slow event-related design (the timing of the 16 scans sampling each trial is depicted in parallel with the events of a trial). Each trial consisted of a brief presentation of a sample stimulus, a sample-target interval, and a brief presentation of the target stimulus. Immediately after target offset, the feedback screen was presented (transparent rectangles in the figure), followed by the inter-trial interval. At the beginning of each block of trials, subjects were given the response rule for that block: identify the location of the sample star in the target and respond on the same side (low control trial, LC), or to identify the location of the sample star but respond on the opposite side (high control condition, HC, due to the need to suppress the spatial response bias). The color of the stimuli changed randomly from trial to trial, indicating the monetary consequences of the response to the target. In Experiment 1, two colors were used, coding for Reward or No-Reward trials. Subjects only had one second to respond to the target (i.e. late responses counted as errors). In Experiment 2, three colors coded for Reward, No-Reward, and potential Penalties (i.e. incorrect or slow responses resulted in a loss). Unbeknownst to the subjects, the response deadline in Experiment 2 was individually titrated in order to avoid differential speed-accuracy tradeoffs in the incentive trials (see Methods and Behavioral results).