Illusory Contours

Illusory contours, also called subjective contours, are a visual illusion that evokes the perception of an edge where no actual change in luminance or color exists across that edge, typically accompanied by an illusory sense of brightness and depth ordering around the perceived edge. The phenomenon has historical roots stretching back centuries, with examples appearing in medieval artwork, and Friedrich Schumann is generally credited with first describing illusory contours scientifically around 1900. Modern scientific interest in the phenomenon was reignited by the Italian psychologist Gaetano Kanizsa's 1976 paper in Scientific American, which popularized the now-famous demonstration in which three pac-man-like circles and angled line segments, arranged just so, cause the visual system to construct the crisp edges of a bright triangle that is not actually drawn anywhere in the image, a figure often called the Kanizsa triangle.