Difference between revisions of "Ebbinghaus illusion" - New World Encyclopedia

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Revision as of 21:00, 7 September 2007


The two orange circles are exactly the same size; however, the one on the left seems smaller.
File:Opsphere3.jpg
Another example of the Ebbinghaus Illusion. Although, in perspective the sphere to the right seems larger, in reality both spheres are exactly the same size.

The Ebbinghaus illusion 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).

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 visual cortex). 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 (Goodale & Milner, 1992). However, recent work (Franz et al, 2005) 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.

References
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  • Franz VH, Scharnowski F, Gegenfurtner (2005) Illusion effects on grasping are temporally constant not dynamic. J Exp Psychol Hum Percept Perform. 31(6), 1359-78
  • Goodale & Milner (1992) Separate pathways for perception and action. Trends in Neuroscience, 15, 20-25.


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