Figure 2.

Stimuli and trial sequence. a, Three segmented (Big Gap, Small Gap, and Out-of-Phase) and one unsegmented (In-Phase) target-background configurations were used. The diameter of the central target was 1.5° (0.25° gap), 1.8° (0.1° gap), and 2° (no gap) in these four configurations. Note that the size of the surround is constant in these configurations; the size of the target is reduced for the two gap conditions. b, On each trial, two center-surround stimuli were simultaneously presented 4.5° to the left and right of fixation and 1.5 ° below it. Surround gratings flickered at 16.67 Hz on the left and at 12.5 Hz on the right. Each trial was initiated by a button press. The trial lasted 2.4 s and started with the appearance of the cue at the fixation point indicating the location (left or right) of the increment. The target and grating stimuli came on and went off 720 ms before the start and end of the trial to eliminate onset and offset transients. The contrast increment (if present) appeared 1.2 s after the cue and lasted for 240 ms. Only 40% of the trials had an increment. Observers indicated the presence/absence of the increment with a key press at the end of the trial.