Concept of optical illusions Three main types illusions

Concept of optical illusions

Three main types illusions 1. literal optical illusions 2. Physiological illusions 3. Cognitive illusions

Physiological illusions. A scintillating grid illusion. Shape, position, colour, and 3 D contrast converge to produce the illusion of black dots at the intersections. Physiological illusions, such as the afterimages following bright lights, or adapting stimuli of excessively longer alternating patterns (contingent perceptual aftereffect), are presumed to be the effects on the eyes or brain of excessive stimulation of a specific type - brightness, tilt, color, movement, etc. The theory is that stimuli have individual dedicated neural paths in the early stages of visual processing, and that repetitive stimulation of only one or a few channels causes a physiological imbalance that alters perception.

Cognitive illusions are assumed to arise by interaction with assumptions about the world, leading to "unconscious inferences", an idea first suggested in the 19 th century by Hermann Helmholtz. Cognitive illusions are commonly divided into ambiguous illusions, distorting illusions, paradox illusions, or fiction illusions. Ambiguous illusions are pictures or objects that elicit a perceptual 'switch' between the alternative interpretations. The Necker cube is a well known example.

Café wall illusion Reversible figure and vase? Duck or Rabbit?

Depth and motion perception Illusions can be based on an individual's ability to see in three dimensions even though the image hitting the retina is only two dimensional. The Ponzo illusion is an example of an illusion which uses monocular cues of depth perception to fool the eye. In the Ponzo illusion the converging parallel lines tell the brain that the image higher in the visual field is farther away therefore the brain perceives the image to be larger, although the two images hitting the retina are the same size. The Optical illusion seen in a diorama/false perspective also exploits assumptions based on monocular cues of depth perception.

The M. C. Escher painting Waterfall

The Enigma of Op Art In a study due to be published in the Proceedings of the National Academy of Sciences, they provide strong evidence that microsaccades are essential for the illusion produced by Isia Leviant's painting Enigma (above). This consists of several concentric circles of the same colour superimposed on a pattern of radiating black and white lines, and creates an intereference effect known as the moiré sensation, a shimmering effect and a sensation of circular motion. In the study, 3 participants were presented with Leviant's painting. They were asked to press a button each time they perceived a slowing down of the circular movements and to release the button when it appeared to speed up again. As they did this, their eye movements were recorded 500 times per second with a camera. A strong correlation was found between the intensity of the illusion and the frequency of the eye movements - the more frequent the microsaccades, the more intense the illusion. Martinez-Conde says that this is a likely explanation for other illusions, such as that in Riley's Fall. Exactly how the brain generates microsaccades is, however, still unclear, as is whether or not they are generated at random. How these movements are linked to the brain mechanisms underlying the illusions, and why they do not produce similar illusions when we view other patterns such as straight gratings or checkerboards, is also poorly understood. What is clear though, is that while these eye movements are essential for the maintenance of proper vision, they are also at least partly responsible for producing perceptions that are inconsistent with the image being viewed.

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