Figure 1.

Panel A = Sequence of events on each trial. The task required a present/absent response regarding the presence of a target shape singleton. In the example, the target is a vertical bar. In the constant search condition, the target was known in advance and remained constant during all trials, and was never a distractor. In the varied search condition, the target was also a shape singleton but varied unpredictably among hexagon, trapezoid, and vertical bar shapes. In the experiment, the target and nontarget items were green outline shapes against a black background. On half of the trials (both target-present and target-absent) within each search condition, there was a red color singleton distractor, illustrated in the figure as the darker outlined shapes. The color singleton was never the target. Panel B = fMRI task design. Each of five runs contained two on-task periods, one constant and one varied search condition, each containing 32 trials.