Visual Perception

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In psychology, visual perception is the ability to interpret visible light information reaching the eyes which is then made available for planning and action. The resulting perception is also known as eyesight, sight or vision. The various components involved in vision are known as the visual system.

Visual system (green) and ventral stream (purple) are shown. Much of the human cerebral cortex is involved in vision.The visual system allows us to assimilate information from the environment to help guide our actions. The act of seeing starts when the lens (anatomy) of the eye focus an image of the outside world onto a light-sensitive membrane in the back of the eye, called the retina. The retina is actually part of the brain that is isolated to serve as a transducer for the conversion of patterns of light into neuronal signals. The lens (anatomy) of the eye focuses light on the photoreceptor cells of the retina, which detect the photons of light and respond by producing Action potential. These signals are processed in a Hierarchy fashion by different parts of the brain, from the retina to the lateral geniculate nucleus, to the primary and secondary visual cortex of the brain.

Study of visual perception The major problem in visual perception is that what people see is not simply a translation of retinal stimuli (i.e., the image on the retina). Thus people interested in perception have long struggled to explain what visual processing does to create what we actually see.

Early studies on visual perception There were two major Grecian schools, providing a primitive explanation of how vision is carried out.

The first was the "Emission theory (vision)" which maintained that vision occurs when rays emanate from the eyes and are intercepted by visual objects. If we saw an object directly it was by 'means of rays' coming out of the eyes and again falling on the object. A refracted image was, however, seen by 'means of rays' as well, which came out of the eyes, traversed through the air, and after refraction, fell on the visible object which was sighted as the result of the movement of the rays from the eye. Although this theory was championed by scholars like Euclid and Ptolemy and their followers, and was believed by Descartes.

The second school advocated the so called the 'intromission' approach which sees vision as coming from something entering the eyes representative of the object. With its main propagators Aristotle, Galen and their followers, this theory seems to have touched a little sense on what really vision is, but remained only a speculation lacking any experimental foundation.

The breakthrough came with Ibn al-Haytham (Alhacen), the "father of optics", pioneered the science study of the psychology of visual perception in his influential Book of Optics in the 1000s, being the first scientist to argue that vision occurs in the brain, rather than the eyes. He pointed out that personal experience has an affect on what people see and how they see, and that vision and perception are subjective. He explained possible errors in vision in detail, and as an example, describes how a small child with less experience may have more difficulty interpreting what he/she sees. He also gives an example of an adult that can make mistakes in vision because of how one's experience suggests that he/she is seeing one thing, when he/she is really seeing something else.Bradley Steffens (2006). Ibn al-Haytham: First Scientist, Chapter 5. Morgan Reynolds Publishing. ISBN 1599350246.

Ibn al-Haytham's investigations and experiments on visual perception also included sensation, variations in sensitivity, sensation of touch, perception of colours, perception of darkness, the psychological explanation of the moon illusion, and binocular vision.

Unconscious inference

Hermann von Helmholtz is often credited with the first study of visual perception in Modern Times. Helmholtz held vision to be a form of unconscious inference: vision is a matter of deriving a probable interpretation for incomplete data.

Inference requires prior assumptions about the world: two well-known assumptions that we make in processing visual information are that light comes from above, and that objects are viewed from above and not below. The study of visual illusions (cases when the inference process goes wrong) has yielded much insight into what sort of assumptions the visual system makes.

The unconscious inference hypothesis has recently been revived in so-called Bayesian studies of visual perception. Proponents of this approach consider that the visual system performs some form of Bayesian inference to derive a perception from sensory data. Models based on this idea have been used to describe various visual subsystems, such as the perception of motion (physics) or the perception of depth perception.Mamassian, Landy & Maloney (2002) A Primer on Probabilistic Approaches to Visual Perception

Gestalt theory Gestalt psychology working primarily in the 1930s and 1940s raised many of the research questions that are studied by vision scientists today.

The Gestalt Laws of Organization have guided the study of how people perceive visual components as organized patterns or wholes, instead of many different parts. Gestalt is a German word that translates to "configuration or pattern". According to this theory, there are six main factors that determine how we group things according to visual perception: Proximity, Similarity, Closure, Symmetry, Common fate and Continuity.

The major problem with the Gestalt laws (and the Gestalt school generally) is that they are descriptive not explanatory. For example, one cannot explain how humans see continuous contours by simply stating that the brain "prefers good continuity". Computational neuroscience of vision have had more success in explaining visual phenomena Computational models of contour integration, by Steve Dakin and have largely superseded Gestalt theory.

See also

Color vision
Motion perception
Depth perception
Visual illusion

Disorders/Dysfunctions

Achromatopsia
Color blindness
Scotopic Sensitivity Syndrome

Related Disciplines

Psychophysics
Neuroscience
Cognitive Science
Optometry
Ophthalmology

References External links

Empiristic theory of visual gestalt perception
Visual Perception 3 - Cultural and Environmental Factors
Gestalt Laws
Summary of Kosslyn et al.'s theory of high-level vision
The Organization of the Retina and Visual System
Dr Trippy's Sensorium A website dedicated to the study of the human sensorium and organisational behaviour

Visual Perception Notes
These are lecture notes on visual perception as a process of mediation. They support an undergraduate course run by Daniel Chandler at the University of Wales, Aberystwyth.

Visual Perception 4
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In psychology, visual perception is the ability to interpret information from visible light reaching the eyes. The resulting perception is also known as eyesight, sight or vision.

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