Vusl pronounced as Vi-sul
The visual system is the part of the nervous system which allows organisms to
see. It interprets the information from visible light to build a representation
of the world surrounding the body. The visual system has the complex task of (re)constructing
a three dimensional world from a two dimensional projection of that world. The
psychological manifestation of visual information is known as visual perception.
Introduction
This article mostly describes the visual system of mammals, although other
"higher" animals have similar visual systems. In this case, the visual system
consists of:
* The eye, especially the retina
* The optic nerve
* The optic chiasm
* The optic tract
* The lateral geniculate nucleus
* The optic radiation
* The visual cortex
Different species are able to see different parts of the light spectrum; for
example, bees can see into the ultraviolet , while pit vipers can accurately
target prey with their infrared imaging sensors .
The image projected onto the retina is inverted due to the optics of the eye.
Eye
The eye is a complex biological device. The functioning of a camera is often
compared with the workings of the eye, mostly since both focus light from
external objects in the visual field onto a light-sensitive medium. In the case
of the camera, this medium is film or an electronic sensor; in the case of the
eye, it is an array of visual receptors. With this simple geometrical
similarity, based on the laws of optics, the eye functions as a transducer, as
does a CCD camera.
Light entering the eye is refracted as it passes through the cornea. It then
passes through the pupil (controlled by the iris) and is further refracted by
the lens. The cornea and lens act together as a compound lens to project an
inverted image onto the retina.
Enlarge picture
S. Ramón y Cajal, Structure of the Mammalian Retina, 1900
Retina
The retina consists of a large number of photoreceptor cells which contain a
particular protein molecule called an opsin. In humans, there are two types of
opsins, rod opsins and cone opsins. Either opsin absorbs a photon (a particle of
light) and transmits a signal to the cell through a signal transduction pathway,
resulting in hyperpolarization of the photoreceptor. (For more information, see
photoreceptor).
Rods and cones differ in function. Rods are found primarily in the periphery of
the retina and are used to see at low levels of light. Cones are found primarily
in the center (or fovea) of the retina. There are three types of cones that
differ in the wavelengths of light they absorb; they are usually called short or
blue, middle or green, and long or red. Cones are used primarily to distinguish
color and other features of the visual world at normal levels of light.
In the retina, the photoreceptors synapse directly onto bipolar cells, which in
turn synapse onto ganglion cells of the outermost layer, who will then conduct
action potentials to the brain. A significant amount of visual processing arises
from the patterns of communication between neurons in the retina. About 130
million photoreceptors absorb light, yet roughly 1.2 million axons of ganglion
cells transmit information from the retina to the brain. The processing in the
retina includes the formation of center-surround receptive fields of bipolar and
ganglion cells in the retina, as well as convergence and divergence from
photoreceptor to bipolar cell. In addition, other neurons in the retina,
particularly horizontal and amacrine cells, transmit information laterally (from
a neuron in one layer to an adjacent neuron in the same layer), resulting in
more complex receptive fields that can be either indifferent to color and
sensitive to motion or sensitive to color and indifferent to motion.
The final result of all this processing is five different populations of
ganglion cells that send information to the brain: M cells, with large
center-surround receptive fields that are sensitive to depth, indifferent to
color, and rapidly adapt to a stimulus; P cells, with smaller center-surround
receptive fields that are sensitive to color and shape; K cells, with very large
center-only receptive fields that are sensitive to color and indifferent to
shape or depth; another population that is intrinsically photosensitive; and a
final population that is used for eye movements.
A recent University of Pennsylvania study calculated the approximate bandwidth
of human retinas as 8.75 megabits per second, whereas guinea pig retinas
transfer at 875 kilobits.
Photochemistry
In the vision system, retinal, technically called retinene1 or "retinaldehyde",
is a light-sensitive retinene molecule found in the photoreceptor cells of the
retina. Retinal is the fundamental structure involved in the transduction of
light into visual signals, i.e. nerve impulses in the ocular system of the
central nervous system. In the presence of light, the retinal molecule changes
configuration and as a result a nerve impulse is generated.
Fibers to thalamus
Optic nerve
Information flow from the eyes (top), crossing at the optic chiasma, joining
left and right eye information in the optic tract, and layering left and right
visual stimuli in the lateral geniculate nucleus. V1 in red at bottom of image.
(1543 image from Andreas Vesalius' Fabrica)
The information about the image via the eye is transmitted to the brain along
the optic nerve. Different populations of ganglion cells in the retina send
information to the brain through the optic nerve. About 90% of the axons in the
optic nerve go to the lateral geniculate nucleus in the thalamus. These axons
originate from the M, P, and K ganglion cells in the retina. This parallel
processing is important for reconstructing the visual world; each type of
information will go through a different route to perception. Another population
sends information to both the superior colliculus in the midbrain, which assists
in controlling eye movements (saccades).
A final population of photosensitive ganglion cells (containing melanopsin)
sends information to the pretectum (pupillary reflex), and to several structures
involved in the control of circadian rhythms and sleep such as the
suprachiasmatic nucleus (SCN, the biological clock), the ventrolateral preoptic
nucleus (VLPO, a region involved in sleep regulation).
The optic nerves from both eyes meet and cross at the optic chiasm, at the base
of the hypothalamus of the brain. At this point the information coming both eyes
is combined and then splits according to the visual field. The corresponding
halves of the field of view (right and left) are sent to the left and right
halves of the brain, respectively, to be processed. That is, the right side of
primary visual cortex deals with the left half of the field of view from both
eyes, and similarly for the left brain. A small region in the center of the
field of view is processed redundantly by both halves of the brain.
Optic tract
Information from the right visual field (now on the left side of the brain)
travels in the left optic tract. Information from the left visual field travels
in the right optic tract. Each optic tract terminates in the lateral geniculate
nucleus (LGN) in the thalamus.
The lateral geniculate nucleus (LGN) is a sensory relay nucleus in the thalamus
of the brain. The LGN consists of six layers in humans and other primates
starting from catarhinians, including cercopithecidae and apes. Layers 1, 4, and
6 correspond to information from one eye; layers 2, 3, and 5 correspond to
information from the other eye. Layer one (1) contains M cells, which correspond
to the M (magnocellular) cells of the optic nerve of the opposite eye, and are
concerned with depth or motion. Layers four and six (4 & 6) of the LGN also
connect to the opposite eye, but to the P cells (color and edges) of the optic
nerve. By contrast, layers two, three and five (2, 3, & 5) of the LGN connect to
the M cells and P (parvocellular) cells of the optic nerve for the same side of
the brain as its respective LGN. The six layers of the LGN are the area of a
credit card, but about three times the thickness of a credit card, rolled up
into two ellipsoids about the size and shape of two small birds eggs. In between
the six layers are smaller cells that receive information from the K cells
(color) in the retina. The neurons of the LGN then relay the visual image to the
primary visual cortex (V1) which is located at the back of the brain (caudal
end) in the occipital lobe in and close to the calcarine sulcus.
The optic radiations carries information from the thalamic lateral geniculate
nucleus to layer 4 of the visual cortex. The P layer neurons of the LGN relay to
V1 layer 4C β. The M layer neurons relay to V1 layer 4C α. The K layer neurons
in the LGN relay to large neurons called blobs in layers 2 and 3 of V1.
There is a direct correspondence from an angular position in the field of view
of the eye, all the way through the optic tract to a nerve position in V1. At
this juncture in V1, the image path ceases to be straightforward; there is more
cross-connection within the visual cortex.
Visual cortex
The visual cortex is the most massive system in the human brain and is
responsible for higher-level processing of the visual image. It lies at the rear
of the brain (highlighted in the image), above the cerebellum. The
interconnections between layers of the cortex, the thalamus, the cerebellum, the
hippocampus and the remainder of the areas of the brain are under active
investigation. Currently, much of what is known stems from patients with damage
to known areas of the brain, with a corresponding study of the cognitive
functions which have been spared. See visual modularity for a discussion of the
modular thesis of visual perception.
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
The visual system allows us to assimilate information from the environment to
help guide our actions. The act of seeing starts when the lens 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 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.
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" 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 scientific 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.
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 or the
perception of depth.
Gestalt theory
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.
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.
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