The Ehrenstein illusion is an optical illusion in which a circle appears at the end points of a series of lines. The Ehrenstein illusion is one of the most popular subjective contour illusions—illusions that create the impression of a shape even though a large portion of the contour is nonexistent. A similar effect can be found in the Kanizsa triangle. Like many other visual and perceptual illusions, study of the Ehrenstein illusion, and other subjective contour illusions, has led to greater understanding of how the brain and eyes perceive visual information. Such figures are also used to great effect by artists, reflecting the endless creativity of human nature and the equally endless ability we have to appreciate creative and novel phenomena.
The Ehrenstein illusion is named for German psychologist Walter Ehrenstein, who published his findings in 1941 in the German psychology journal Zeitschrift für Psychologie (150, 83-91). Ehrenstein published his findings as a modification of the Hermann grid illusion.
The classic Ehrenstein illusion consists of a grid in which the lines at an intersection terminate a short distance away from the intersection. When looking at the illusion, the viewer tends to see a circle that is slightly brighter than the background. If the space at the intersections is imagined to be square, it can also be perceived that way.
The "phantom edge phenomena" (seeing an outline that is not actually there) is due to what neuropsychologists call the "T-effect." Groups of neural cells see breaks in lines, and if given no further input, will assume that there is a figure in front of the lines.
Scientists believe that this happens because the brain has been trained to view the break in lines as an object that could pose a potential threat. With lack of additional information, the brain errs on the side of safety and perceives the space as an object. The circle is the most simple and symmetrical object, so the mind usually sees a circle unless active effort is made to see an alternate shape.
Like many other perceptual illusions, the Ehrenstein illusion helps neuroscientists study how the brain processes visual information. Artists have used this ability we have to construct objects to great effect in their works. There are many examples where artists, including Isoda Koryusai's Crow and Heron in the Snow where subjective contours create the shapes and enhance the blackness and whiteness of the birds, Albrecht Durer's The Satyr Family, and many more.
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