The Ehrenstein illusion is an

optical illusion Within visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual perception, percept that arguably appears to differ from reality. Illusions come in a wide v ...

studied by the German psychologist (1899 – 1961) in which the sides of a square placed inside a pattern of concentric circles take an apparent curved shape. File:Ehrenstein.svg, Original Ehrenstein illusion, where a square appears curved when placed inside of concentric circles File:Ehrenstein Illusion.svg, Alternative Ehrenstein illusion, where the ends of the dark segments produce the illusion of circles or squares File:Ehrenstein figure.svg, The illusion of a bright central disk (above) is destroyed by adding a circle (bottom). The monochromity of the piece further adds to the effect of this square appearing to become curved, a common illusion many associate with the umbrella term of optical illusions. Sometimes the name "Ehrenstein" is associated with one of the illusory contour figures where the ends of the dark segments produce the illusion of circles. The apparent figures have the same color as the background, but appear brighter. A similar effect is obtained in the

Kanizsa triangle Illusory contours or subjective contours are visual illusions that evoke the perception of an edge without a luminance or color change across that edge. Illusory brightness and depth ordering often accompany illusory contours. Friedrich Schumann ...

References

Optical illusions {{psychology-stub