visual illusion in which retinal impressions persist after the removal of a stimulus, believed to be caused by the continued activation of the visual system. The afterimage may be positive, corresponding in colour or brightness to the original image, or negative, being less bright or of colours complementary to the original. A common afterimage is the spot of light one sees after a camera flash has been fired. The afterimage is the most readily observed of the class of phenomena known as aftersensations, or aftereffects.