All stimuli were presented on a computer monitor using the E-prime software system (Psychology Software Tools Inc., Pittsburgh, PA). Stimuli were presented on a black screen for 200 msec, followed by a delay varying between 1000 and 1100 msec before the next stimulus. Three different types of visual stimuli were presented: (1) standard nontarget stimuli, a small hollow white square; (2) target stimuli, a small white X; and (3) novel rare nontarget stimuli, different shapes of various colors. Participants responded with the index finger of their dominant hand and were instructed to press a response box button only when they saw target stimuli. Stimuli were presented in a predetermined order, with standard stimuli appearing 210 times, target stimuli appearing 35 times, and rare nontarget stimuli appearing 35 times over approximately 6.5 min. Each participant was shown an example of the target stimulus before the task began.
