Concept of optical illusions

Three main types illusions

1.1.literal optical illusionsliteral optical illusions2.2.Physiological illusions Physiological illusions

3.3.Cognitive illusionsCognitive illusions

Physiological illusions. A scintillating grid illusion. Shape, position, colour, and 3D 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. 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 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

• Researchers studying at the Karlsruhe Institute of Technology in Germany have taken a tiny step toward being able to actually create a cloaking device. They were able to make invisible a tiny bump located in a layer of gold - preventing it from being detected at infrared frequencies that were nearly visible to the human eye. Previously developed similar devices, created by a team at Duke University, worked in only two dimensions to cloak objects from microwaves, so objects could be hidden from light traveling only in one direction. The new device works in three, meaning that objects can be viewed from any angle and remain hidden.Lead researcher Tolga Ergin explained that the cloak is composed of a grouping of crystals with air spaces between them, similar to a pile of wood. As light passes through the crystals, it bends, hiding the bump in the layer of gold beneath. The bump was extremely tiny - only .00004" high and .0005" wide - and could be seen only with a magnifying lens. The team designed what Ergin called a "photonic metamaterial" that was able to influence the behavior of light rays. The design is based on a "carpet cloak" that was first suggested by Professor Sir John Pendry of Imperial College London. Pendry proposed the theory of having an object hidden beneath a bump, and then making the bump invisible. "It’s like a carpet mirror," Ergin said. "If you hide an object under it, there is a bump, so you see a distortion in the reflected image. We put the carpet cloak on top of that bump and it bends the light so that the distortions disappear."Under this experiment, the reflective surface (carpet) appeared to be flat. The team used "laser writing" to create the cloak, using a tightly focused laser beam to "write" on light-sensitive material. Ergin explained, "Wherever you put the focus spot into the material, it will harden. It’s a similar process to photography - when you develop it, whatever hasn’t been exposed to the laser will be washed away."Invisibility cloaks use special materials to deflect light, radar, or other waves that flow around an object, similar to water flowing around a smooth pebble in a babbling brook. Invisibility is different from stealth technology, which is not designed to make an airplane invisible, but instead to reduce the cross-section image available to radar. Professor Ortwin Hess, of the University of Surrey, called the study a "huge step forward" in demonstrating invisibility in three dimensions. He said this great achievement and the photonic materials developed by the team could be used in the future development of lenses, as well as in optical circuitry. Hess acknowledged that a true invisibility cloak is a long way off, but Ergin’s results showed a remarkable proof of the principle. The new research is a significant step toward the future possibility of developing a true cloak of invisibility. The experiment dealt with an extremely tiny particle, but according to Ergin, there is no limit to the size of an object you could hide with this technology, in theory. "You could blow this up and hide a house," Ergin said. But it took his team three hours to make this microscopic structure, so to make one that is even a millimeter in size it would take a very long time. So the development of a human-sized invisibility cloak still remains a sci-fi theory - for now.

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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