======================================================================
=                             Illusions_                             =
======================================================================

                            Introduction
======================================================================
An illusion is a distortion of the senses, which can reveal how the
mind normally organizes and interprets sensory stimulation. Although
illusions distort the human perception of reality, they are generally
shared by most people.

Illusions may occur with any of the human senses, but visual illusions
(optical illusions) are the best-known and understood. The emphasis on
visual illusions occurs because vision often dominates the other
senses. For example, individuals watching a ventriloquist will
perceive the voice as coming from the dummy since they are able to see
the dummy mouth the words.

Some illusions are based on general assumptions the brain makes during
perception. These assumptions are made using organizational principles
(e.g., Gestalt theory), an individual's capacity for depth perception
and motion perception, and perceptual constancy. Other illusions occur
due to biological sensory structures within the human body or
conditions outside the body within one's physical environment.

The term 'illusion' refers to a specific form of sensory distortion.
Unlike a hallucination, which is a distortion in the absence of a
stimulus, an illusion describes a misinterpretation of a true
sensation. For example, hearing voices regardless of the environment
would be a hallucination, whereas hearing voices in the sound of
running water (or another auditory source) would be an illusion. So,
it should not be wrong to consider that illusions are just
"misinterpretations" on how our brain perceives something that exists
(unlike a hallucination where a stimulus is absent).


                               Visual
======================================================================
A visual illusion or optical illusion is characterized by visually
perceived images that are deceptive or misleading. Therefore, the
information gathered by the visual sense is processed to create a
percept that does not tally with information from other senses or
physical measurements.

The visual system, which includes the eyes (namely the retinas) and
the central nervous system (namely the brain's visual cortex),
constructs reality through both perceptual and cognitive neural
pathways. Visual illusions are (at least in part) thought to be caused
by excessive competing stimuli. Each stimulus follows a dedicated
neural path in the early stages of visual processing, and
intense/repetitive activity or interaction with active adjoining
channels (perceptual neural circuits, usually at the same level)
causes a physiological imbalance that alters perception. During
low-level visual processing, the retinal circuit arranges the
information in the photoreceptors, by creating initial visual percepts
from the patterns of light which fall on the retina. The Hermann grid
illusion and Mach bands are two illusions that are widely considered
to be caused by a biological phenomenon named lateral inhibition,
where the receptor signal in the retina's receptive fields from light
and dark areas compete with one another.

The assembly of visual elements into a collective percept, that
distinguishes objects from backgrounds, takes part during
intermediate-level visual processing. Many common visual illusions are
a consequence of the percept constructed during this processing stage,
as the elements first captured during low-level processing might
easily be interpreted to form an image that differs from objective
reality. An example is when two objects of the same size are placed on
a certain background which conditions us to believe that one object
might be larger than the other, and when the background is removed or
replaced our perception immediately changes to the correct scenario
(effectively concluding that both objects have equal dimensions).

High-level visual processing consolidates information gathered from
various sources to apply cognitive influences that create a conscious
visual experience. Thus, allowing us to recognize the complex identity
of different elements, and the disparate relations between them
through cognitive processes. Visual illusions are also often a product
of this processing stage, and it is during this stage that we might
ultimately become conscious of any optical illusion. There are two
crucial properties of our visual system related mostly to high-level
visual processing, referred to as selectivity and invariance (which we
have consistently attempted to replicate in image recognition computer
algorithms). Selectivity refers to the identification of particular
features that are relevant to recognize a specific element or object,
while abstracting from other features that are not fundamental to
performing the same recognition (e.g. when we see the shape of a
house, certain contours that are essential for us to recognize it
while other contours or image properties are not, such as color). On
the other hand, invariance refers to the ability to be indifferent to
small variations of a given feature, effectively identifying all those
variations as simply being different versions of the same feature
(e.g. we can recognize a given handwritten letter of the alphabet,
written by different people with distinct styles of calligraphy).

The whole process that constructs our visual experience is extremely
complex (with multiple qualities that are unmatched by any computer or
digital system). It is organized by many sequential and parallel
sub-processes, each of which is essential in building our conscious
image of the world. Our whole visual system seeks to simplify and
categorize the unstructured low-level visual information, through both
selectivity and invariance. Thus, while trying to organize an image by
"filling in the gaps" through assumptions, we become vulnerable to
misinterpretation.


                              Auditory
======================================================================
An auditory illusion is an illusion of hearing, the auditory
equivalent of a visual illusion: the listener hears either sounds
which are not present in the stimulus, or "impossible" sounds. In
short, audio illusions highlight areas where the human ear and brain,
as organic, makeshift tools, differ from perfect audio receptors (for
better or for worse). One example of an auditory illusion is a Shepard
tone.


                              Tactile
======================================================================
Examples of tactile illusions include phantom limb, the thermal grill
illusion, the cutaneous rabbit illusion and a curious illusion that
occurs when the crossed index and middle fingers are run along the
bridge of the nose with one finger on each side, resulting in the
perception of two separate noses. The brain areas activated during
illusory tactile perception are similar to those activated during
actual tactile stimulation. Tactile illusions can also be elicited
through haptic technology. These "illusory" tactile objects can be
used to create "virtual objects".


                              Temporal
======================================================================
A temporal illusion is a distortion in the perception of time, which
occurs when the time interval between two or more events is very
narrow (typically less than a second). In such cases, a person may
momentarily perceive time as slowing down, stopping, speeding up, or
running backward.


                            Intersensory
======================================================================
Illusions can occur with the other senses including those involved in
food perception. Both sound and touch have been shown to modulate the
perceived staleness and crispness of food products. It was also
discovered that even if some portion of the taste receptor on the
tongue became damaged that illusory taste could be produced by tactile
stimulation. Evidence of olfactory (smell) illusions occurred when
positive or negative verbal labels were given prior to olfactory
stimulation. The McGurk effect shows that what we hear is influenced
by what we see as we hear the person speaking; the auditory component
of one sound is paired with the visual component of another sound,
leading to the perception of a third sound.


                             Disorders
======================================================================
Some illusions occur as a result of an illness or a disorder. While
these types of illusions are not shared with everyone, they are
typical of each condition. For example, people with migraines often
report fortification illusions.


                            Neuroscience
======================================================================
Perception is linked to specific brain activity and so can be elicited
by brain stimulation. The (illusory) percepts that can be evoked range
from simple phosphenes (detections of lights in the visual field) to
high-level percepts. In a single-case study on a patient undergoing
presurgical evaluation for epilepsy treatment, electrical stimulation
at the left temporo-parietal junction evoked the percept of a nearby
(illusory) person who "closely 'shadowed' changes in the patient's
body position and posture".


                              See also
======================================================================
*.
*.
*.
*
*.
*.
*.
*
*Hallucination - A vivid perception in the absence of external
stimulus that has qualities of real perceptions.
*.
*.

*,

*.
*.
*.
*.
*.


                           External links
======================================================================
*[http://www.virtuescience.com/universalveiling.html Universal Veiling
Techniques]
* by J.R. Block.
*[http://www.michaelbach.de/ot Optical illusions and visual phenomena]
by Michael Bach
*[https://web.archive.org/web/20081227025626/http://www.brl.ntt.co.jp/IllusionForum/basics/auditory/index-e.html
Auditory illusions]
*[https://web.archive.org/web/20181026055948/http://www.roblesdelatorre.com/gabriel/haptics.htm
Haptic Perception of Shape] - touch illusions, forces and the geometry
of objects, by Gabriel Robles-De-La-Torre.
*[http://visionlab.harvard.edu/silencing/ Silencing awareness of
visual change by motion ]


License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL: http://creativecommons.org/licenses/by-sa/3.0/
Original Article: http://en.wikipedia.org/wiki/Illusions_

You are viewing proxied material from gopherpedia.com. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.