Gabor patches were created using Matlab (The Mathworks, Inc.) and the psychophysics toolbox (Brainard, 1997). They were presented on a 17-inch monitor, which refreshed at 75 Hz. Stimuli were centrally presented on a gray background (average luminance = 14.4 cd/m2). The space-average luminance of each grating was 16.3 cd/m2, and the Michelson contrast, defined by (Lmax − Lmin)/(Lmax + Lmin) was 68%. The slight difference between the average luminance of the background and the stimuli was not visibly apparent and did not lead to any visible edges around the stimuli. At a viewing distance of 114 cm the patches subtended 6.78° by 6.78° of visual angle. All patches were presented in diagonal (45°), orientation, had a gaussian envelope with SD of 0.68°, and with spatial frequency modulation of 0.5, 1, 4, or 8 cycles/degree. An additional stimulus, a gray-scaled image of a zebra was presented. Participants were instructed to respond by pressing a response button with the index finger of their dominant hand as quickly as possible whenever they saw the zebra.
