Figure 1.

Example trial procedure. Each trial began with a colored fixation dot (illustrated in gray here) at bottom-center, plus two black dots, jittered in location, in the upper left and right quadrants. The color of the fixation dot indicated the direction of saccade plan for that trial (upper left or right) and the exact position of the black dots indicated the location of the saccade target on the corresponding side for that trial (the dot on the other side was irrelevant). The peripheral dots disappeared after 1200 ms and were followed by a 1200 ms interval during which either only the fixation dot was visible, or else during which bilateral task-irrelevant checkerboard stimuli could appear for 215 ms at a random point during the interval (as shown in left ‘branch’ of third panels from top). Subjects were informed that any checkerboards were task-irrelevant. Next, an auditory tone signaled to the subject whether to make a saccade to the target (go trial) or to cancel the saccade and hold central fixation (no-go trial). Finally, a 2000-6000 ms blank interval followed before the start of the next trial.