The Ehrenstein-Orbison Illusion
When the German psychologist Walter Ludwig Ehrenstein wasn't criticizing Freud's psychoanalysis as unscientific, he studied perception. There are several visual illusions named after Ehrenstein, most of which deal with illusory contours. The Ehrenstein Illusion on this page does not involve illusory contours.
The Ehrenstein-Orbison illusion occurs when straight lines (often in the shape of a square) on placed on a background of concentric circles. The straight lines can appear curved in the opposite direction of the curvature of the circles.
William Orbison's explanation of the illusion is an old-school Gestalt explanation of how the fields of force of the target (square) and inducers (circles / arcs) interact in the brain. It is not a very satisfying explanation by today's standards.
Unfortunately, there is no completely accepted theory of the Ehrenstein-Orbison illusion. One explanation is that the angle formed between the target (square) and the inducers (circles / arcs) as they intersect is misinterpreted by the visual system. If this is true, changing the orientation of the target (square) should have no effect on the illusion with circles as the inducers (the angle of intersection remains constant as the orientation of the square changes) but should affect the illusion with arcs as the inducers (the angle of interssection changes as the orientation of the square changes).
Below are two instances of the Ehrenstein-Orbison illusion. The upper one is the classical illusion with concentric circles for the background. The lower one is a variant with arcs centered at the four corners as the background. Notice how the squares appear -- in the upper illusion, the sides of the square appear to bow inward toward the center of the illusions while in the lower illusion, the sides of the square appear to bow outward toward the corners of the illusion.
You can toggle the background (circles / arcs) on and off to see the effect of the background on the perception of the squares (un-tick the Show circles / arcs to see that the squares are in fact straight edged). You can also change the density of the circles / arcs by sliding the slider left (higher density) or right (lower density). Can you make the illusion disappear by changing the density of the background? Can you make the illusion stronger (so that the straight lines appear even more bent) by changing the density of the background?
Does changing the orientation of the square influence the strength of the illusion? Rotate the square by sliding the Square Orientation slider to the left or right. What orientation produces the weakest illusion? What orientation produces the strongest? Does it matter whether the background is circles or arcs?
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Show circles / arcs Show square