Ehrenstein illusion

The Ehrenstein illusion is an optical illusion of brightness or colour perception. The visual phenomena was studied by the German psychologist Walter H. Ehrenstein (1899- 1961) who wanted to modify the theory behind the Hermann grid illusion. In the original discovery of the optical illusion, Ehrenstein found that grating patterns that stop at a certain point have an apparent increase in brightness at the centre area between the end of the lines, compared to the other areas.

Ehrenstein published his book 'Modifications of the Brightness Phenomenon of L. Hermann' to disprove Hermann's phenomenon, arguing the illusion was not caused by a contrast effect but rather a brightness effect which needed to be further explored.

The effect

The Ehrenstein illusion is a brightness illusion, whereby the borders of shapes or images affects the perception of the surface's luminance. The effects of brightness illusions vary from person to person and can be enhanced by changing the background of the configuration and the surroundings of the image surface. We perceive the luminance of a surface to be brighter, despite it being identical to the background.

• Ehrenstein Illusion Variants
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  Original Ehrenstein illusion, where a square appears curved when placed inside concentric circles
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  The illusion of a bright central disk (above) is destroyed by adding a circle (bottom).
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  Alternative Ehrenstein illusion, where the ends of the dark segments produce the illusion of circles or squares

Illusory contour

Sometimes the "Ehrenstein" is associated with one of the illusory contour figures where the ends of the dark segments produce the illusion of circles. The apparent circular figures at the centre of the configuration are the same colour as the background, but appear brighter. This fits under the characteristics of an optical illusion, as there are no physical origins to the asymmetrical perceptual sensations we perceive.

A similar effect of illusory contour is seen in figures such as the Kanizsa triangle.

The monochromity of the first illustration above further adds to the effect of the square appearing to have become curved, a common illusion many associate with the umbrella term of optical illusions.

Variations

Ehrenstein carried out variations of the original illusion to test out how the strength of perception changed. In one variation, he changed the thickness of the lines. He found that the lines could be very thin and the illusion would remain. The brightness of the centre increases with the thickness of the lines. However when the lines become so broad that the central white line in enclosed, the illusion loses its bright appearance.

Paradox of shape perception

In 1954, further variations of the original Ehrenstein illusion found that the sides of a square placed inside a pattern of concentric circles takes an apparent curved shape. This is an illusory contour that does not use illusory brightness, but instead the geometric factors affect the illusory contours of the shape. For example, when Ehrenstein reduces the image size of the overall figure, he enhances the paradox and makes the contour appear thicker. The shapes in the image remain constant despite small changes in the overall characteristics of the configuration.

Theories and explanations

Gestalt psychology explains illusory contours using a bottom up approach. It argues human perception of the physical stimulus is constructed from our expectations of what we already know, rather than what we actually see. When we look at optical illusions or configurations, we intrinsically construct the image as whole again.