Fig. 1.

Schematic representation of the stop-signal (or countermanding) task. Black squares indicate stimulus location. No-stop trials began with a central fixation point. The fixation point was then extinguished, and a peripheral target was simultaneously presented at 1 of 2 possible locations. Subjects were instructed to fixate targets as quickly and accurately as possible. Stop trials began in the same way. However, after 1 of 6 possible time intervals following target onset, termed stop-signal delay (SSD), the fixation point was re-illuminated and subjects were instructed to withhold their response. Successful response inhibition resulted in canceled trials, whereas errant responses resulted in noncanceled trials. Dotted circles represent area of fixation. F, fixation point; T, target; RT, reaction time.