Kanizsa Triangle

The Kanizsa triangle is a classic optical illusion in which viewers perceive a solid white triangle, appearing brighter than the surrounding background and bounded by crisp edges, even though no such triangle is actually drawn anywhere in the image; the illusory shape instead emerges from the arrangement of three pac-man-like circles with wedge-shaped notches cut out, positioned so their notches align, along with three V-shaped angle outlines suggesting the triangle's far corners. The Italian psychologist Gaetano Kanizsa introduced the figure in a 1955 paper and it quickly became one of the best-known demonstrations in the study of visual perception. The effect is explained within Gestalt psychology as a striking example of the principle of closure, the visual system's general tendency to perceive a complete, whole shape out of fragmented or incomplete cues, combined with what researchers call amodal completion and illusory or subjective contours, in which the brain actively constructs sharp edges and even a perceived difference in brightness where physically there is none, treating the incomplete inducing shapes as though a solid object were simply occluding parts of them. Because it isolates how much of ordinary visual perception is actively constructed by the brain rather than passively registered from the raw image on the retina, the Kanizsa triangle has been used extensively in later vision-science and neuroscience research investigating which stages of visual cortical processing are responsible for generating illusory contours.