Abstract
A gray target can appear lighter or darker depending on its surrounding spatial context. We examined the effect of exposure time on three such examples (simultaneous lightness contrast, dungeon illusion, and the two-room arrangement), finding very different results with exposure time as brief as 15 ms: the simultaneous lightness contrast was much stronger, the effect of the dungeon illusion was reversed, and the lightness difference between the two isoluminant patches in the two-room arrangement disappeared. These suggest that local luminance ratios dominate lightness perception in a brief flash.