A perceptual phenomenon in which what a person sees departs from the true physical properties of the stimulus in front of them, produced by the way the visual system processes edges, color, brightness, motion, faces and letters rather than by any defect in the eye itself. This group gathers the visual perception and illusion phenomena recorded for this atlas, spanning classic geometric and brightness illusions, gestalt organization principles that describe how the visual system groups shapes and figures, and closely related effects in how faces, letters and ambiguous images are seen and recognized.