Your eyes invent triangles that were never drawn
Richard Gregory sorts visual illusions into physical, physiological, and cognitive classes—each with ambiguities, distortions, paradoxes, and fictions. A stick bends in water; afterimages burn; the brain’s unconscious inferences invent Kanizsa’s floating white triangle from Pac-Man corners that were never drawn.
Physical tricks include aerial haze that makes clear-weather mountains look nearer, and refraction that Ptolemy already discussed around 150 CE. Physiological effects follow overstimulation: motion aftereffects, afterimages, Mach bands at luminance edges. Lateral inhibition once explained Hermann’s grid’s phantom gray spots, but that account has been disproved; newer empirical models pick up the slack. Cognitive illusions, Helmholtz argued in the nineteenth century, spring from silent assumptions about the world that the visual system never bothers to announce.
Ambiguous figures—Necker cube, Rubin vase, Sugihara’s “squircle”—flip interpretations. Geometrical distortions include the Café wall, Müller-Lyer fins that stretch equal lines, Ponzo’s converging rails that enlarge the upper bar, and Poggendorff’s misaligned transversals. Paradoxes such as the Penrose triangle and Escher’s Ascending and Descending staircase force edges to join wrongly. Fictions add contours that are not in the stimulus. Gestalt grouping—good form, continuity, proximity, similarity—helps the brain finish puzzles and also builds impossible objects from partial cues.
Depth shortcuts fool even flat pictures: the vertical–horizontal illusion stretches upright lines; monocular cues drive dioramas and Escher’s Waterfall. Film animation and the phi phenomenon stitch stills into motion; the spinning dancer’s clockwise or counterclockwise spin tracks parietal cortical fluctuations on fMRI. Magritte’s 1938 Time Transfixed pairs a locomotive with a fireplace so smoke links industrial and domestic into an unpictured third idea—an artistic cousin of cognitive inference. Arthur Siegel’s 1953 Plaster Wall, Optical Illusion titles the same game in dye imbibition print.
Source: Optical illusion
