After adaptation by an observer to a patch of gradually increasing (or decreasing) luminance, a steady test patch appeared to be gradually dimming (or brightening). These aftereffects did not transfer interocularly. Adaptation to a checkerboard, in which the white squares gradually dimmed while the black squares gradually brightened, gave an aftereffect that was a pattern of intersecting diagonal lines, that is, an extremely blurred checkerboard. The larger the squares of the checkerboard were, the farther into the periphery the aftereffect extended, because small squares were blurred out by the summation areas of the underlying visual channels, which were larger at increasing eccentricities and had diameters of 20 times the resolvable dot separation. The estimated visual acuity of these channels was as low as 20/400. These estimates were confirmed by manipulating separately the local and space-averaged luminances of the adapting stimulus.