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Optical Illusions, none of these pictures are animated

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The Hollow Face Illusion (Dragon Illusion)

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Artistic Illusions

Blind Spot

It is the only area of the retina which does not see, because of the lack of photoreceptors. It corresponds to the place where the optic nerve and the blood vessels leave the eye.
It is a spot in the visual field where objects are not seen.
It is at the XVIIth century, while proceeding to the dissection of a human eye, that the French physicist Edme Mariotte discovered the blind spot, area of the retina where the optic nerve is attached to the ocular sphere. It suspects that being deprived of photoreceptors cells, the light does not stimulate this area and that, consequently, each eye have a "blind spot", a small area of the visual field where it is blind. It shows it with the illustration which follows.

An optical illusion is always characterized by visually perceived images that, at least in common sense terms, are deceptive or misleading. Therefore, the information gathered by the eye is processed by the brain to give, on the face of it, a percept that does not tally with a physical measurement of the stimulus source. A conventional assumption is that there are physiological illusions that occur naturally and cognitive illusions that can be demonstrated by specific visual tricks that say something more basic about how human perceptual systems work.

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.

(a). Ambiguous illusions are pictures or objects that elicit a perceptual 'switch' between the alternative interpretations. The Necker cube is a well known example; another instance is the Rubin vase.

(b). Distorting illusions are characterized by distortions of size, length, or curvature. A striking example is the Café wall illusion. Another example is the famous Mueller-Lyer illusion.

(c). 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.

(d). 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.

Many artists have worked with optical illusions, including M.C. Escher, Bridget Riley, Salvador Dalí, Giuseppe Arcimboldo, Marcel Duchamp, Oscar Reutersvärd, and Charles Allan Gilbert. Also some contemporary artists are experimenting with optical illusion, including: Dick Termes, Shigeo Fukuda, Patrick Hughes, István Orosz, Rob Gonsalves and Akiyoshi Kitaoka. Optical illusion is also used in film by the technique of forced perspective.

Some visual illusions such as the Ponzo illusion and the Vertical-horizontal illusions can also occur when using an auditory-to-vision sensory substitution device.

Perceptual organization

In order to make sense of the world it is necessary to organize incoming sensations into information which is meaningful. Gestalt psychologists believe one way this is done is by perceiving individual sensory stimuli as a meaningful whole.

Gestalt organization can be used to explain many illusions including the Duck-Rabbit illusion where the image as a whole switches back and forth from being a duck then being a rabbit and why in the figure-ground illusion the figure and ground are reversible.

In addition, Gestalt theory can be used to explain the illusory contours in the Kanizsa Triangle. Here a floating white triangle, which does not exist, is seen. The brain has a need to see familiar simple objects and has a tendency to created a "whole" image from individual elements. Gestalt means "whole" in German. However, another explanation of the Kanizsa Triangle is based in evolutionary psychology and the fact that in order to survive it was important to see form and edges. The use of perceptual organization to create meaning out of stimuli is the principle behind other well-known illusions including impossible objects ‎and sounds like the auditory illusion, the Shepard tone. Our Brain makes since of shapes and symbols putting them together like a jigsaw puzzle. Formulating that which isn't there to that which is believable.

Depth and motion perception

Illusions can be based on an individual's ability to see in three dimensions even through 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 tells the brain the image higher in the visual field is further 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 exploits rules of depth and proximity and our understand of the physical world to create an impossible illusion.

Like depth perception, motion perception is responsible for a number of sensory illusions. Film animation is based on the illusion that the brain perceives a series of slightly varied images produced in rapid succession as a moving picture. Likewise, when we are moving, as we would be while riding in a vehicle, stable surrounding objects may appear to move. We may also perceive a large object, like an airplane, to move more slowly, than smaller objects, like a car, although the larger object is actually moving at a faster rate. The Phi phenomenon is yet another example of how the brain perceives motion, which is most often created by blinking lights in close succession.

Perceptual constancies

Perceptual constancies are sources of many illusions. Color constancy and brightness constancy are responsible for the fact that a familiar object will appear the same color regardless of the amount of light reflecting from it. An illusion of color difference can be created, however, when the luminosity of the area surrounding an unfamiliar object is changed. The color of the object will appear darker against a black field which reflects less light compared to a white field even though the object itself did not change in color. Like color, the brain has the ability to understand familiar objects as having a consistent shape or size. For example a door is perceived as rectangle regardless as to how the image may change on the retina as the door is opened and closed. Unfamiliar objects, however, do not always follow the rules of shape constancy and may change when the perspective is changed. The Shepard illusion of the changing table is an example of an illusion based on distortions in shape constancy.