Figure 10.

Time courses of trials. Experiment 1: before the start of each trial, participants fixated a central cross. One Cornsweet and one real luminance step stimulus appeared for 200 ms each, with a 200 ms interval between them. The order of the stimuli, their orientation and the side on which each appeared were randomized (although they were constrained to appear on opposite sides within each trial). Participants then had 1750 ms to report the side on which the stimulus with the greatest contrast had appeared (using the arrow keys of the keyboard). Experiment 2: each trial stared with fixation. A Cornsweet stimulus then appeared for 200 ms with a random orientation to the left or right of fixation. Participants had 1750 ms to report, with the “Y” and “N” keys, if they perceived Mach bands.