“Cognitive illusions are assumed to arise by interaction with assumptions about the world, leading to “unconscious inferences”, an idea first suggested in the 19th century by Hermann Helmholtz. Cognitive illusions are commonly divided into four categories:

  • Ambiguous illusions are pictures or objects that elicit a perceptual ‘switch’ between the alternative interpretations. The Necker cube (above) is a well known example; another instance is the Rubin vase.
  • Distorting illusions are characterized by distortions of size, length, or curvature. A striking example is the Café wall illusion. Another example is the famous Müller-Lyer illusion.
  • Paradox illusions are generated by objects that are paradoxical or impossible, such as the Penrose triangle or impossible staircases seen, for example, in M. C. Escher‘s Ascending and Descending and Waterfall. The triangle is an illusion dependent on a cognitive misunderstanding that adjacent edges must join.
  • Fictional illusions are defined as the perception of objects that are genuinely not there to all but a single observer, such as those induced by schizophrenia or a hallucinogen. These are more properly called hallucinations.” <wiki>

Al Seckel, a cognitive neuroscientist and master of visual illusions, explores some of the perceptual illusions that fool our eyes and our brains. Running through example after example of images that buck our expectations, he asks why such tricks make us so happy.

Although there are many kinds of illusions the ones that are most interesting to me are cases in which significance is attributed where none (otherwise) exists. This phenomena is generally referred to as pareidolia, and is actually a subject which I have blogged about before (see Strange Beliefs). A specific case of pareidolia is the ability to see hidden faces in complex images and scenes. This is quite common when it comes to religious icons (the Virgin Mary on toasted cheese sandwiches, for example). It always astounds me that, not only do people believe that this is the preferred medium by which the divine chooses to interact with us mere mortals, but that people readily ascribe absolute confidence in their perfect knowledge of what these characters would actually look like (caucasian…?).

For those of you that would like to explore optical illusions further, here are some really great websites for fun and amazement:

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