6 ILLUSION AND IMPRESSION Color Illusions / The Illusion of Depth / Spatial Effects of Colors / Transparence Illusion / Fluting / Vibration / Vanishing Boundaries / Luminosity / Bezold Effect / Optical Mixes

Reality is an illusion, albeit a very persistent one.

—Albert Einstein

An optical illusion is a seemingly magical event that takes place when an image is misperceived or misinterpreted by a viewer. An illusion can only be seen. It cannot be confirmed by any of the other senses. It is an entirely visual experience that is mistaken for reality.

The simplest illusions are initiated by involuntary responses of the eyes to overstimulation by something specific, like a brilliant hue or extreme contrast. Afterimages and the color shifts caused by simultaneous contrast, complementary contrast, or ground subtraction are examples of this kind of direct response. Other aspects of vision, like lateral inhibition or involuntary eye movements, can also play a part. Traditionally, these have been considered to be illusions with a physiological basis. A different kind of illusion is considered cognitive (knowing). Cognitive illusions occur when an observer makes unconscious assumptions or inferences about something when the actual image is different.

Seeing requires an elaborate interplay between the eyes and the brain. Illusions occur when an image is interpreted by a brain that is processing visual information—and processing it inaccurately. The current thinking among vision scientists ...

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