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
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
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