Oppel-Kundt Illusion

The Oppel-Kundt illusion is a geometric optical illusion in which a section of an image that is filled with visual distractors, such as a series of short parallel lines or tick marks, appears longer than an unfilled section of exactly the same length placed next to it; the effect is also known as the filled-space illusion or the illusion of interrupted extent. The illusion is named for the German physicists Johann Joseph Oppel, who first documented and systematically studied the effect in 1860, and August Kundt, who extended that study in 1863, and it can be produced through a wide variety of different graphic arrangements of filling elements, arrangements that differ considerably in how strong a distortion they produce. Despite over a century of study, no single explanation for why the illusion occurs has become generally accepted; researchers have proposed a range of competing accounts, including phenomenological models, potential theory, physiological approaches based on patterns of neural activation, accounts based on the density of contours within the filled section, models based on spatial-frequency filtering, and models based on spatial coding.