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Visual perception is the ability to interpret information and surroundings from visible light reaching the eye. The resulting perception is also known as eyesight, sight or vision. The various physiological components involved in vision are referred to collectively as the visual system, and are the focus of much research in psychology, cognitive science, neuroscience and molecular biology.

Visual system

The visual system in humans allows individuals to assimilate information from the environment. The act of seeing starts when the lens of the eye focuses an image of its surroundings 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^{[citation needed]} for the conversion of patterns of light into neuronal signals. The lens of the eye focuses light on the photoreceptive cells of the retina, which detect the photons of light and respond by producing neural impulses. These signals are processed in a hierarchical 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. Signals from the retina can also travel directly from the retina to the Superior colliculus.

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

The visual dorsal stream (green) and ventral stream (purple) are shown. Much of the human cerebral cortex is involved in vision.

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 '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 some contact with modern theories of what vision really is, but it remained only a speculation lacking any experimental foundation.

Both schools of thought relied upon the principle that "like is only known by like," and thus upon the notion that the eye was composed of some "internal fire" which interacted with the "external fire" of visible light and made vision possible. Plato makes this assertion in his dialogue, Timaeus; as does Aristotle, in his De Sensu.^[1]

Leonardo DaVinci: The eye has a central line and everything that reaches the eye through this central line can be seen distinctly.

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Ibn al-Haytham (also known as Alhacen or Alhazen), the "father of optics", was the first to resolve this argument, by refining intromission theory into what is now the modern accepted theory of vision in his influential Book of Optics (1021). He argued that vision is due to light from objects entering the eye. However, he maintained that the part of the eye responsive to light was the lens, whereas we now know it was the retina. He developed an early scientific method emphasizing extensive experimentation. 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. He also gives an example of an adult that can make mistakes in vision because of how one's experience suggests that they are seeing one thing, when they are really seeing something else. This can be easily related to the famous saying "beauty lies in the eye of the beholder".^[2] 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.^[3][4]

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 and peripheral vision.

Unconscious inference

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 ^[5]

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.^[6][7]

Gestalt theory

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 are often disregarded by cognitive psychologists is their inability to explain the nature of peripheral vision. In Gestalt theory, visual perception only takes place during fixations.

However, during fixations both the high definition foveal vision at the fixation point and the peripheral vision are functioning. Because of its lack of acuity and relative independence of eye position (due to its extreme wide angle), human vision is an image compressing system.

While foveal vision is very slow (from only three to four 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 movement

Eye movement first 2 seconds (Yarbus, 1967)

During the 1960s, technical development permitted the continuous registration of eye movement during reading^[8] in picture viewing ^[9] and later in visual problem solving ^[10] and when headset-cameras became available, also during driving.^[11]

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 approaches

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.^[12] 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?" ^[13]

In the 1970s 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.^[14]

See also

• Color vision
• Depth perception
• Lateral masking
• Motion perception
• Visual illusion

Disorders/Dysfunctions

• Achromatopsia
• Astigmatism
• Color blindness
• Scotopic Sensitivity Syndrome

Related disciplines

• Cognitive science
• Neuroscience
• Optometry
• Ophthalmology
• Psychophysics

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
• Effect of Detail on Visual Perception by Jon McLoone, the Wolfram Demonstrations Project.