An Introduction to Optical Illusions
by Tauqeer Hassan

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Optical illusions are considered as malfunction of the visual system. This phenomenon is also viewed as bringing out particular good adaptations of visual system to particularly standard viewing situations. These adaptations are considered to be hard-wired in brain's system. Consequently, inappropriate interpretations of visuals scene are causes. Teuber (1960) defined it as illusions of the senses tell us the truth about perception. Further, an optical illusion which is also called a visual illusion is characterized by visually perceived images. These images differ from objective reality. In optical illusions, the information gathered by the eye is processed in the brain to give a percept. This perception does not tally with a physical measurement of the stimulus source. 

Types of Optical Illusions
There are basically three types of optical illusions literal, physiological and cognitive.

1. Literal Illusions
Literal optical illusions that create images that are different from the objects that make them. 



2. Physiological Illusions
Physiological ones are the effects on the eyes and brain of excessive stimulation of a specific type (brightness, tilt, color, movement). Such as the afterimages following bright lights, or adapting stimuli of excessively longer alternating patterns, theses illusions are contingent perceptual aftereffect which 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 behind these illusions 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.

3. Cognitive
Cognitive illusions are those where the eye and brain make unconscious inferences. 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 are suppose to elicit a perceptual switch between the alternative interpretations. The Necker cube 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 Cafe wall illusion. Another example is the famous Muller-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. 
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