The visual system is the part of the central nervous system The central nervous system is the part of the nervous system that coordinates the activity of all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish. It contains the majority of the nervous system and consists of the brain and the spinal cord, as well as which enables organisms to process visual detail Visual perception is the ability to interpret information and surroundings from the effects of 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, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of frequency, this corresponds to a band in the vicinity of 400-790 to build a representation of the surrounding world. The visual system accomplishes a number of complex tasks, including the reception of light and the formation of monocular representations; the construction of a binocular perception from a pair of two dimensional projections; the identification and categorization of visual objects; assessing distances to and between objects; and guiding body movements in relation to visual objects. The psychological manifestation of visual information is known as visual perception Visual perception is the ability to interpret information and surroundings from the effects of 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, a lack of which is called blindness Blindness is the condition of lacking visual perception due to physiological or neurological factors. Non-image forming visual functions, independent of visual perception, include the pupillary light reflex The pupillary light reflex is a reflex that controls the diameter of the pupil, in response to the intensity of light that falls on the retina of the eye. Greater intensity light causes the pupil to become smaller (allowing less light in), whereas lower intensity light causes the pupil to become larger (allowing more light in). Thus, the pupillary (PLR) and circadian photoentrainment In chronobiology, entrainment of a circadian system is the alignment of its own period and phase to the period and phase of an external rhythm. A common example is the entrainment of endogenous circadian rhythms to the daily light-dark cycle. Of the several possible cues, called zeitgeber (German for time-giver, synchronizer), which can contribute.

Introduction

This article mostly describes the visual system of mammals Mammals are a class of vertebrate, air-breathing animals whose females are characterized by the possession of mammary glands while both males and females are characterized by hair and/or fur, three middle ear bones used in hearing, and a neocortex region in the brain. Some mammals have sweat glands, but most do not, although other "higher" animals have similar visual systems. In this case, the visual system consists of:

• The eye The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world. The visual system accomplishes a number of complex tasks, including the, especially the retina The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world. The visual system accomplishes a number of complex tasks, including the
• The optic nerve The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world. The visual system accomplishes a number of complex tasks, including the
• The optic chiasma The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world. The visual system accomplishes a number of complex tasks, including the
• The optic tract The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world. The visual system accomplishes a number of complex tasks, including the
• The lateral geniculate body The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world. The visual system accomplishes a number of complex tasks, including the
• The optic radiation The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world. The visual system accomplishes a number of complex tasks, including the
• Visual cortex The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world. The visual system accomplishes a number of complex tasks, including the
• Visual association cortex The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world. The visual system accomplishes a number of complex tasks, including the

Different species In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are often used, such as similarity of DNA, morphology or are able to see different parts of the light spectrum The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object; for example, bees Bees are flying insects closely related to wasps and ants, and are known for their role in pollination and for producing honey and beeswax. Bees are a monophyletic lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila. There are nearly 20,000 known species of bees in seven to nine recognized families, can see into the ultraviolet Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3eV to 124 eV. It is so named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the colour violet,^[1] while pit vipers The Crotalinae, or crotalines, are a subfamily of venomous vipers found in Asia and the Americas. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on either side of the head. Currently, 18 genera and 151 species are recognized: 7 genera and 54 species in the Old World, against a greater can accurately target prey with their pit organs The ability to sense infrared thermal radiation evolved independently in several different families of snakes. Essentially, it allows these animals to “see” radiant heat at wavelengths between 5 and 30 μm to a degree of accuracy such that a blind rattlesnake can target vulnerable body parts of the prey at which it strikes. It was previously, which are sensitive to infrared radiation.^[2]

History

In the second half of the 19th century, many motifs of the nervous system The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous system contains the brain, spinal cord, and retina were identified such as the neuron doctrine The neuron doctrine is the now fundamental idea, formally proposed in 1891 by Heinrich Wilhelm Gottfried von Waldeyer-Hartz, that the nervous system is made up of discrete individual cells. The term neuron was itself coined by Waldeyer as a way of identifying the cells in question. The neuron doctrine, as it became known, served to position and brain localisation, which related to the neuron A neuron (pronounced /ˈnjʊərɒn/ NOOR-on, also known as a neurone or nerve cell) is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core being the basic unit of the nervous system and functional localisation in the brain, respectively. These would become tenets of the fledgling neuroscience Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. Nevertheless, it is currently an interdisciplinary science that involves other disciplines such as psychology, computer science, mathematics, physics, philosophy, and medicine. As a result, the scope of neuroscience has and would support further understanding of the visual system.

The notion that the cerebral cortex The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different composition in terms of neurons and connectivity. The human is divided into functionally distinct cortices now known to be responsible for capacities such as touch The somatosensory system is a diverse sensory system comprising the receptors and processing centres to produce the sensory modalities such as touch, temperature, proprioception , and nociception (pain). The sensory receptors cover the skin and epithelia, skeletal muscles, bones and joints, internal organs, and the cardiovascular system. While (somatosensory cortex The lateral postcentral gyrus is a prominent structure in the parietal lobe of the human brain and an important landmark. It was initially defined from surface stimulation studies of Penfield, and parallel surface potential studies of Bard, Woolsey, and Marshall. Although initially defined to be roughly the same as Brodmann areas 3, 1 and 2, more), movement In physics, motion is change of location or position of an object with respect to time. Change in motion is the result of an applied force. Motion is typically described in terms of velocity also seen as speed, acceleration, displacement, and time. An object's velocity cannot change unless it is acted upon by a force, as described by Newton's (motor cortex Motor cortex is a term that describes regions of the cerebral cortex involved in the planning, control, and execution of voluntary motor functions), and vision (visual cortex The term visual cortex refers to the primary visual cortex and extrastriate visual cortical areas such as V2, V3, V4, and V5. The primary visual cortex is anatomically equivalent to Brodmann area 17, or BA17. The extrastriate cortical areas consist of Brodmann area 18 and Brodmann area 19. There is a visual cortex for each hemisphere of the brain), was first proposed by Franz Joseph Gall Franz Joseph Gall was a neuroanatomist, physiologist, and pioneer in the study of the localization of mental functions in the brain in 1810.^[3] Evidence for functionally distinct areas of the brain (and, specifically, of the cerebral cortex) mounted throughout the 19th century with discoveries by Paul Broca Pierre Paul Broca was a French physician, anatomist, and anthropologist. He was born in Sainte-Foy-la-Grande, Gironde. He is best known for his research on Broca's area, a region of the frontal lobe that has been named after him of the language center (1861), and Gustav Fritsch and Edouard Hitzig Eduard Hitzig was a German neurologist and neuropsychiatrist born in Berlin of the motor cortex (1871).^[3][4] Based on selective damage to parts of the brain and the functional effects this would produce (lesion studies), David Ferrier Sir David Ferrier was a pioneer Scottish neurologist and psychologist proposed that visual function was localised to the parietal lobe The parietal lobe is a lobe in the brain. It is positioned above the occipital lobe and behind (posterior to) the frontal lobe of the brain in 1876.^[4] In 1881, Hermann Munk Hermann Munk was a Jewish German physiologist. He was born at Posen, studied at Berlin and Göttingen, and in 1862 became docent in the former university. Seven years afterward he was promoted to assistant professor, and in 1876 to professor of physiology at the veterinary college at Berlin. Besides studies on the productive methods of threadworms more accurately located vision in the occipital lobe The occipital lobe is the visual processing center of the mammalian brain containing most of the anatomical region of the visual cortex. The primary visual cortex is Brodmann area 17, commonly called V1 . Human V1 is located on the medial side of the occipital lobe within the calcarine sulcus; the full extent of V1 often continues onto the, where the primary visual cortex The term visual cortex refers to the primary visual cortex and extrastriate visual cortical areas such as V2, V3, V4, and V5. The primary visual cortex is anatomically equivalent to Brodmann area 17, or BA17. The extrastriate cortical areas consist of Brodmann area 18 and Brodmann area 19. There is a visual cortex for each hemisphere of the brain is now known to be.^[4]

Biology of the visual system

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 field of view The field of view is the (angular or linear or areal) extent of the observable world that is seen at any given moment 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 A transducer is a device that converts one type of energy to another. The conversion can be to/from electrical, electro-mechanical, electromagnetic, photonic, photovoltaic, or any other form of energy. While the term transducer commonly implies use as a sensor/detector, any device which converts energy can be considered a transducer, as does a CCD camera A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time. Technically, CCDs are implemented as shift.

Light entering the eye is refracted Refraction is the change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one medium to another. Refraction of light is the most commonly observed example, but any type of wave can refract when it interacts with a medium, for example when sound waves pass from one medium into another or as it passes through the cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43 dioptres. While the cornea contributes most of the eye'. It then passes through the pupil The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In optical terms, the anatomical pupil is the eye's (controlled by the iris The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and the amount of light reaching the pupil. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel (light brown). In response to the amount of light entering the eye, muscles) and is further refracted by the lens The lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a sharp real image of the object of interest to be formed on the. The cornea and lens act together as a compound lens to project an inverted image onto the retina.

Retina

The retina consists of a large number of photoreceptor cells which contain particular protein molecules called opsins. In humans, two types of opsins are involved in conscious vision: rod opsins and cone opsins. (A third type, melanopsin in some of the retinal ganglion cells (RGC), part of the body clock mechanism, is probably not involved in conscious vision, as these RGC do not project to the lateral geniculate nucleus (LGN) but to the pretectal olivary nucleus (PON).^[5]) An 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 cell).

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.^{[citation needed]} 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.^{[citation needed]}

In the retina, the photoreceptors synapse directly onto bipolar cells, which in turn synapse onto ganglion cells of the outermost layer, which 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.^{[citation needed]}

The final result of all this processing is five different populations of ganglion cells that send visual (image-forming and non-image-forming) information to the brain:

1. M cells, with large center-surround receptive fields that are sensitive to depth, indifferent to color, and rapidly adapt to a stimulus;
2. P cells, with smaller center-surround receptive fields that are sensitive to color and shape;
3. K cells, with very large center-only receptive fields that are sensitive to color and indifferent to shape or depth;
4. another population that is intrinsically photosensitive; and
5. a final population that is used for eye movements.^{[citation needed]}

A 2006 University of Pennsylvania study calculated the approximate bandwidth of human retinas to be about 8960 kilobits per second, whereas guinea pig retinas transfer at about 875 kilobits.^[6]

In 2007 Zaidi and co-researchers on both sides of the Atlantic studying patients without rods and cones, discovered that the novel photoreceptive ganglion cell in humans also has a role in conscious and unconscious visual perception.^[7][8][9] The peak spectral sensitivity was 481 nm. This shows that there are two pathways for sight in the retina – one based on classic photoreceptors (rods and cones) and the other, newly discovered, based on photoreceptive ganglion cells which act as rudimentary visual brightness detectors.

Photochemistry

In the visual system, retinal, technically called retinene₁ or "retinaldehyde", is a light-sensitive retinene molecule found in the rods and cones 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.^{[citation needed]}

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