Optical Illusions

How Many People
How Many People in This Picture?

An optical illusion is a type of illusion characterized by visually perceived images that are deceptive or misleading. Information gathered by the eye is interpreted by the brain to give the perception that something is present when it is not. There are physiological illusions and cognitive illusions. Optical illusions can be a natural consequence of specific optical tricks that show particular assumptions in the human perceptual system.

1) Developed optical illusions include phenomena such as the Necker cube and the Scintillating/Hermann grid. They could also be called discovered illusions. Understanding these phenomena is useful in order to understand the limitations of the human visual system.

2) Physiological optical illusions, such as the afterimages following bright lights or adapting stimuli of excessively longer alternating patterns (contingent perceptual aftereffect, CAE), are the effects on the eyes or brain of excessive stimulation of a specific type - brightness, tilt, colour, movement, and so on. The theory is that stimuli have individual dedicated neural paths in the visual outer wall of an organism for the early stages of visual processing; repetitive stimulation of only a few channels misleads the visual system.

3) Cognitive optical illusions are often more well-known. Instead of demonstrating a physiological base they interact with different levels of perceptual processing, in-built assumptions or 'knowledge' are misdirected. Cognitive illusions are commonly divided into ambiguous illusions, distorting illusions, paradox illusions, or fiction illusions. They often exploit the predictive hypotheses of early visual processing. Stereograms are based on a cognitive visual illusion.

4) Ambiguous optical illusions are pictures or objects that offer significant changes in appearance. Perception will 'switch' between the alternates as they are considered in turn as available data does not confirm a single view. The Necker cube is a well known example, the motion parallax due to movement is being misinterpreted, even in the face of other sensory data. Another popular is the Rubin vase.

5) Distorting optical illusions are the most common, these illusions offer distortions of size, length, or curvature. They were simple to discover and are easily repeatable. Many are physiological illusions, such as the Café wall illusion which exploits the early visual system encoding for edges. Other distortions, such as converging line illusions, are more difficult to place as physiological or cognitive as the depth-cue challenges they offer are not easily placed. All pictures that have perspective cues are in effect illusions. Visual judgements as to size are controlled by perspective or other depth-cues and can easily be wrongly set.

6) Paradox optical illusions offer objects that are paradoxical or impossible, such as the Penrose triangle or impossible staircases seen, for example, in the work of M. C. Escher. The triangle is an illusion dependent on a cognitive misunderstanding that adjacent edges must join. They occur as a byproduct of perceptual learning.

7) Fiction optical illusions are the perception of objects that are genuinely not there to all but a single observer, such as those induced by schizophrenia or hallucinogenic drugs.

7) Text optical illusions are the perception of objects that are created from the strategic placement of letters and numbers.  Shift the letters and numbers in a can give the object animation.


More Optical Illusions

Blivet Illusion

Ehrenstein Illusion

Grid Illusion

Hering Illusion

Kanizs Illusion

Missing Square Illusion

Orbison Illusion

Poggendorff Illusion

Ponzo Illusion

White's Illusion

Wundt Illusion

Zollner Illusion

Optical Illusions


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straight lines

people count

Read the color names

inverted cubes

fork illusion

checker shadow

seeing double

moving circle

strange picture

straight lines

moving picture

moving dots

Triangle area

Circle Illusions

Impossible Seat

Jesus Illusion

Young Woman or Old Lady
Eskimo or Indian Invisible Dots


This Optical Illusions Site Created by Chuck Ayoub © 2002 - 2005