Concept of optical illusions

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Concept of optical illusions
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Three main types illusions

• literal optical illusions
• Physiological illusions
• Cognitive illusions

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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.
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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.
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Café wall illusion
Reversible figure and vase?
Duck or Rabbit?
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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.
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The M. C. Escher painting Waterfall
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• 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. "Its 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.
  Its a similar process to photography - when you
  develop it, whatever hasnt 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 Ergins 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.

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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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Thank you for your kind attention