The Ebbinghaus illusion or Titchener circles is an optical illusion of relative size perception. In the best-known version of the illusion, two circles of identical size are placed near to each other and one is surrounded by large circles while the other is surrounded by small circles; the first central circle then appears smaller than the second central circle.
It was named for its discoverer, the German psychologist Hermann Ebbinghaus (1850-1909) it was popularised in the English-speaking world by Titchener in a 1901 textbook of experimental psychology, hence its alternative name "Titchener circles".[1]
Although commonly thought of as an illusion of size, recent work suggests that the critical factor in the illusion is the distance of the surrounding circles and the completeness of the annulus, making the illusion a variation of the Delboeuf illusion. If the surrounding circles are near to the central circle it appears larger, while if they are far away it appears smaller. Obviously, the size of the surrounding circles dictates how near they can be to the central circle, resulting in many studies confounding the two variables.[1]
The Ebbinghaus illusion has played a crucial role in the recent debate over the existence of separate pathways in the brain for perception and action (for more details see Two Streams hypothesis). It has been argued that the Ebbinghaus illusion distorts perception of size, but when a subject is required to respond with an action, such as grasping, no size distortion occurs.[2] However, recent work[3] suggests that the original experiments were flawed. The original stimuli limited the possibility for error in the grasping action, therefore making the grasp response more accurate, and presented the large and small versions of the stimulus in isolation—which results in no illusion because there is no second central circle to act as a reference. Franz et al. conclude that both the action and perception systems are equally fooled by the Ebbinghaus illusion. Other recent research suggests that an individual's receptivity to this illusion, as well as the Ponzo illusion, may be inversely correlated with the size of that individual's primary visual cortex.[4]