A white square appeared on a gray background and subtended 1° × 1° of visual angle (Michelson contrast = .8). On each trial this square was presented in one of 16 possible locations, with its center positioned along the horizontal or vertical meridian at 1.5°, 3.5°, 5.5°, or 7.5° from the fixation point. A gap of one of three sizes, 3.1′, 3.9′, or 7.8′, was embedded equally often in the middle of the square’s left or right side (
Figure 2). On one half of the total trials, a 100% valid precue appeared 0.37° above the top of the Landolt-square (peripheral cue trials). The precue was a green horizontal bar, subtending 0.68° width × 0.26° height of visual angle. On the other half (central-neutral cue trials), instead of the bar, a green circle, whose diameter subtended 0.39° of visual angle, appeared in the center of the display. For both cues, the left- and right-side gaps occurred equally often. Both cues signaled the target onset but did not indicate on which side of the square the gap would appear. Whereas the peripheral cue indicated the location where the Landolt-square would appear, the neutral cue indicated that the Landolt-square had equal probability of appearing at any location. A small fixation dot was present in the center of the screen throughout the experiment. A plus (0.5° height × 0.5° width) or a minus (0.5° width × 0.1° height) sign served as the feedback, and was presented in the center of the screen. In one half of the blocks, a 1.6° wide × 1.5°-high rectangular local postmask composed of randomly oriented lines was presented at the target location.