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Visual perception is one of the senses, consisting of the ability to detect light and interpret (see) it as the perception known as sight or naked eye vision. Vision has a specific sensory system, the visual system. There is disagreement as to whether or not this constitutes one, two or even three distinct senses. Some people make a distinction between "black and white" vision and the perception of colour, and others point out that vision using rod cells uses different physical detectors on the retina from cone cells. Some argue that the perception of depth also constitutes a sense, but others argue that this is really cognition (that is, post-sensory) function derived from having stereoscopic vision (two eyes) and is not a sensory perception as such. Many people are also able to perceive the polarization of light.
The visual systemEditThe eye is the light-sensitive organ that is the first component of the visual system. The eye's retina performs the first stages of visual perception processing.
- Main article: Visual system
Our eyes are our bodies' most highly developed sensory organs.[How to reference and link to summary or text] Light rays enter the eye by first crossing the clear cornea. Nearly two-thirds of the eye's focusing power occurs along the front surface. A normal cornea should have a round contour like a soup spoon, allowing the eye to create a single focused image.
The visual system is highly efficient in providing a rapid assimilation of information from the environment to help guide our actions. The act of seeing starts when the cornea and lens 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.
- Main article: Biochemistry of the retina
The lens of the eye focuses light on the photoreceptive cells of the retina, which detect the photons of light via the visual cycle and respond by producing neural impulses.Light is absorbed by photopigment into two classes of receptors, rods and cones. There are approximately one hundred million rods and five million cones in the human retina. The rods are active under scotopic, or dim lighting, and the cones are active under photopic, or daylight settings. There are two opponent colour systems; blue-yellow and red-green. The three streams (luminance, B-Y and R-G) are initially processed in parallel. The retina contains three different types of cones each with visual pigments of differing peak spectral sensitivity, Red: (560nm), green (530 nm) and blue (430 nm). Both the red and green pigments are encoded on the x chromosome, and the blue con pigment is found on chromosome seven.
The neuropsychology of visionEdit
After light passes through the cornea it then moves through the lens to the retina. Signals from the retina are processed in a hierarchical fashion by different parts of the brain, such as the lateral geniculate nucleus, and the primary and secondary visual cortex of the brain.
- Main article: Neuropsychology of visual perception
Areas of study in visual perceptionEdit
- Main article: Visual object processing
- Main article: Shape processing
Sources of informationEdit
To perform its task, visual perception takes into account not only patterns of illumination on the retina, but also our other senses and our past experiences. Consider the task of bird sighting (an instance of object recognition): to be able to identify a bird against a background of tree and brushes, one needs prior exposure to general properties of the bird category. From past experiences, we expect birds to have a certain shape, color, etc. Hearing a sound that is characteristic of birds, a song for example, will help us locate one: information from the other senses is used in visual perception. In this case, locational information from the auditory domain is used.
The development of visual perceptionEdit
Visual perception develops through interaction with the environment and is a developmental process
- Main article: Development of visual perception
Individual and group differences in visual perceptionEdit
Most of the general processes of visual perception have been shown to be universal, as opposed to being dependant on culture, although there are specific instances where cultural variability appears to come into play.
It has also been shown that certain individual differences such as impairment of sight and spatial skills can also affect our visual perception. There are also other factors that influence how we perceive things such as personality, cognitive styles, gender, occupation, age, values, attitudes, motivation, religious beliefs, economic status, education and habits.
Theoretical perspectives in the study of visual perceptionEdit
Study of visual perception Edit
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 Edit
There were two major ancient Greek schools, providing a primitive explanation of how vision is carried out in the body.
The first was the "emission theory" 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. This theory was championed by scholars like Euclid and Ptolemy and their followers.
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 light did not play any role in this theory and it remained only a speculation lacking any experimental foundation.
Ibn al-Haytham (also known as Alhacen or Alhazen), the "father of optics", was the first to reconcile both schools of thought in his influential Book of Optics (1021). He argued that vision is due to light from objects entering the eye, and he developed an early scientific method emphasizing extensive experimentation in order to prove this. He pioneered the scientific study of the psychology of visual perception, being the first scientist to argue that vision occurs in the brain, rather than the eyes. He pointed out that personal experience has an effect 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. For a little child however ugly a mother is, it does not matter to it as the definition of beauty is not that well defined for the little child as it is with any other adult. 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. This can be easily related to the famous saying "beauty lies in the eye of the beholder," which is to say that a flower which may appear beautiful to one person may not appeal that much to another. Al-Haytham carried out many investigations and experiments on visual perception, extended the work of Ptolemy on binocular vision, and commented on the anatomical works of Galen.
Leonardo DaVinci,1452-1519, was the first to recognize the special optical qualities of the eye. He wrote "The function of the human eye, ... was described by a large number of authors in a certain way. But I found it to be completely different." His main experimental finding was that there is only a distinct and clear vision at the line of sight, the optical line that ends at the fovea. Although he did not use these words literally he actually is the father of the modern distinction between foveal vision and peripheral vision.
Hermann von Helmholtz is often credited with the first study of visual perception in modern times. Helmholtz examined the human eye and concluded that it was, optically, rather poor. The poor quality information gathered via the eye seemed to him to make vision impossible. He therefore concluded that vision could only be the result of some form of unconscious inferences: a matter of making assumptions and conclusions from incomplete data, based on previous experiences.
Inference requires prior experience of the world: examples of well-known assumptions - based on visual experience - are:
- light comes from above
- objects are normally not viewed from below
- faces are seen (and recognized) upright 
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.
Another type of the unconscious inference hypothesis (based on probabilities) 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 or the perception of depth.
- Main article: Gestalt psychology
Gestalt psychologists 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.
One of the reasons why Gestalt laws have often been disregarded by cognitive psychologists is a lack of understanding the nature of peripheral vision. It is true that visual perception only takes place during fixations.
But during fixations not only the high definition foveal vision at the fixation point, but also the peripheral vision is functioning. Due to its lack of acuity and relative independence of eye position (due to its extreme wide angle) it is an image compressing system.
While foveal vision is very slow (only 3 to 4 high quality telescopic images per second), peripheral vision is very inaccurate but also very fast (up to 90 images per second - permitting one to see the flicker of the European 50Hz TV images). Elements of the visual field are thus grouped automatically according to laws like Proximity, Similarity, Closure, Symmetry, Common fate and Continuity.
Analysis of eye-movementsEdit
During the 1960s the technical development permitted the continuous registration of eye movements during reading in picture viewing  and later in visual problem solving  and when headset-cameras became available, also during driving.
The picture to the left shows what may happen during the first two seconds of visual inspection. While the background is out of focus, representing the peripheral vision, the first eye movement goes to the boots of the man (just because they are very near the starting fixation and have a reasonable contrast).
The following fixations jump from face to face. They might even permit comparisons between faces.
It may be concluded that the icon face is a very attractive search icon within the peripheral field of vision. The foveal vision adds detailed information to the peripheral first impression.
The cognitive and computational approachesEdit
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 models of vision have had more success in explaining visual phenomena and have largely superseded Gestalt theory. More recently, the computational models of visual perception have been developed for Virtual Reality systems - these are closer to real life situation as they account for motion and activities which populate the real world. Regarding Gestalt influence on the study of visual perception, Bruce, Green & Georgeson conclude:
- "The physiological theory of the Gestaltists has fallen by the wayside, leaving us with a set of descriptive principles, but without a model of perceptual processing. Indeed, some of their "laws" of perceptual organisation today sound vague and inadequate. What is meant by a "good" or "simple" shape, for example?" 
In the 1980s David Marr developed a multi-level theory of vision, which analysed the process of vision at different levels of abstraction. In order to focus on the understanding of specific problems in vision, he identified (with Tomaso Poggio) three levels of analysis: the computational, algorithmic and implementational levels.
The computational level addresses, at a high level of abstraction, the problems that the visual system must overcome. The algorithmic level attempts to identify the strategy that may be used to solve these problems. Finally, the implementational level attempts to explain how these problems are overcome in terms of the actual neural activity necessary.
Marr suggested that it is possible to investigate vision at any of these levels independently. Marr described vision as proceeding from a two-dimensional visual array (on the retina) to a three-dimensional description of the world as output. His stages of vision include:
- a 2D or primal sketch of the scene, based on feature extraction of fundamental components of the scene, including edges, regions, etc. Note the similarity in concept to a pencil sketch drawn quickly by an artist as an impression.
- a 2-1/2 D sketch of the scene, where textures are acknowledged, etc. Note the similarity in concept to the stage in drawing where an artist highlights or shades areas of a scene, to provide depth.
- a 3 D model, where the scene is visualized in a continuous, 3-dimensional map.
Psychologist James J. Gibson developed a theoretical perspective on vision that is radically different from that of Helmholtz. Gibson considers that enough visual perception is available in normal environments to allow for veridical perception (accurate perception of the world). Gibson replaces inference with information pickup. Although most researchers today feel closer to Helmholtz's unconscious inference theory, Gibson has done much in identifying what sort of information is available to the visual system.
Development of visual perceptionEdit
- Main article: Development of visual perception
Types of visual perceptionEdit
- Black and white vision
- Brightness perception
- Colour perception
- Color vision
- Depth perception
- Face perception
- Object perception
- Motion perception
- Multimodal perception
- Texture perception
See aberration in optical systems but this needs rewriting for the eye
- Chromatic aberration
- Color blindness
- Eye disorders
- Scotopic Sensitivity Syndrome
- Spherical aberration
- Acquired vision
- Attentional blink
- Autokinetic illusion
- Binocular vision
- Binocular rivalry and Multistable perception
- Brightness and Contrast
- Color, Color circle, and Color vision
- Consciousness and visual qualia
- Corrective lens
- Dark adaptation
- Entoptic phenomenon
- Eye dominance
- Eye fixation
- Eye tracking
- Foveal vision
- Flicker fusion and the Persistence of vision
- Interocular transfer
- Visual threshold
- Mirror image
- Monocular vision
- Night vision
- Optic flow
- Optical illusion
- Peripheral vision
- Perspective (visual)
- Phi phenomenon
- Philosophy of perception
- Pattern recognition and Computer vision
- Primary sensory cortex
- Repetition blindness
- Retinal eccentricity
- Stereoscopic vision
- Visual perception in dreams
- Vestibulo-ocular reflex
- Visual acuity
- Visual aid
- Visual cortex
- Visual contrast
- Visual deprivation
- Visual discrimination
- Visual feedback
- Visual field
- Visual fixation
- Visual memory
- Visual pathway
- Visual photosensitivity
- Visual phototransduction
- Visual pigment
- Visual receptive fieldslds]]
- Visual skills
- Visual stimulus
- Visual tectum
- Visual threshold
- Visual tracking
- Visuospatial ability
References & BibliographyEdit
- Farah, M. J. & RatcliffG. (1994)(Eds.), The Neuropsychology of High-level Vision. Hillsdale, N. J.: Erlbaum. ISBN 0805809112
- Humphreys, G. W. (1992) (Ed.) Understanding vision: An inter-disciplinary approach. Oxford: Blackwells.
- Spillman, L., and Werner, J.(1990) (Eds.) Visual Perception: The Neurophysiological Foundations, New York: Academic Press.
- Rudolph Arnheim (1954). Art and Visual Perception: A Psychology of the Creative Eye. Berkeley: University of California Press.
- Lothar Kleine-Horst (2001). Empiristic Theory of Visual Gestalt Perception. Hierarchy and Interactions of Visual Functions. Koeln: Enane. ISBN 3-928955-42X
- Barlow H, Blakemore C (1990/1991) Images and Understanding, Cambridge, UK, Cambridge University Press.
- Helmholtz Hermann Von (2000), reprinted from 1865/1866 edition, The Treatise On Physiolological Optics, Thoemmes Continuum.
- Kleine-Horst Lothar (2001). Empiristic Theory of Visual Gestalt Perception. Hierarchy and Interactions of Visual Functions. Koeln: Enane. ISBN 3-928955-42X
- Palmer Stephen E., (1999) Vision Science: Photons To Phenomenology, Bradford Books.
- Purves D, Lotto B, (2003) Why We See What We Do: An Empirical Theory of Vision, Sunderland, MA: Sinauer Associates.
- Rodieck RW, (1998) The First Steps In Seeing, Sunderland, MA: Sinauer Associates.
- Empiristic theory of visual gestalt perception
- Visual Perception 3 - Cultural and Environmental Factors
- Gestalt Laws
- Visual Perception 4 - Individual Differences, Purposes and Needs
- VisionSimulations.com | Vision simulators for various diseases and conditions of the eye
- Summary of Kosslyn et al.'s theory of high-level vision
- The Organization of the Retina and Visual System
- San Diego Vision Group
- Effect of Detail on Visual Perception by Jon McLoone, the Wolfram Demonstrations Project.
- The Joy of Visual Perception: A Web Book by Peter K. Kaiser York University
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