Fig. 1.

Participants were administered a spatial working memory task in which either two or three memory stimuli were sequentially presented on the screen. Each stimulus was presented at one of sixteen possible locations configured circularly around a central fixation. These stimuli were either “targets” (black circles) or “distractors” (black squares); a maximum of one distractor could appear per trial. After the memory stimuli were presented, a probe stimulus (a green circle) was subsequently presented, and participants were asked to indicate whether or not the probe stimulus appeared in the position of a previous target stimulus; if the probe appeared in the position of a previous distractor, the participants were instructed to respond “no.” The task included 2 two-stimulus blocks and 6 three-stimulus blocks of 36 trials each, presented in a pseudo-randomized order. Participants who performed at less than 60% accuracy on two-stimulus trials were excluded from behavioral and ERP analyses. Stimuli were presented in one of 16 locations on an invisible circle with a radius subtending a visual angle of 9.3 degrees. Stimuli subtended a visual angle of 1.6 degrees, and potential locations were separated by 22.5 degrees of arc around the circumference of the circle.