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The visual system is the physiological basis of visual perception (the ability to perception light). The system detects, transduces and interprets information concerning light within the visible range to construct an imaging and build a mental model of the surrounding environment. The visual system is associated with the eye and functionally divided into the optics system (including cornea and crystalline lens) and the nervous system system (including the retina and visual cortex).
The visual system performs a number of complex tasks based on the image forming functionality of the eye, including the formation of monocular images, the neural mechanisms underlying stereopsis and assessment of distances to (depth perception) and between objects, motion perception, pattern recognition, accurate motor coordination under visual guidance, and colour vision. Together, these facilitate higher order tasks, such as object identification. The neuropsychological side of visual information processing is known as visual perception, an abnormality of which is called visual impairment, and a complete absence of which is called blindness. The visual system also has several non-image forming visual functions, independent of visual perception, including the pupillary light reflex and circadian photoentrainment.
This article describes the human visual system, which is representative of mammalian vision, and to a lesser extent the vertebrate visual system.
The LGN also sends some fibers to V2 and V3.Card, J. Patrick, and Robert Y. Moore. "Organization of Lateral Geniculate-hypothalamic Connections in the Rat." Wiley Online Library. 1 June. 1989. Web. 27 March 2016.
V1 performs edge-detection to understand spatial organization (initially, 40 milliseconds in, focusing on even small spatial and color changes. Then, 100 milliseconds in, upon receiving the translated LGN, V2, and V3 info, also begins focusing on global organization). V1 also creates a bottom-up saliency map to guide attention or gaze shift.
V2 both forwards (direct and via Pulvinar nuclei) pulses to V1 and receives them. Pulvinar is responsible for saccade and visual attention. V2 serves much the same function as V1, however, it also handles illusory contours, determining depth by comparing left and right pulses (2D images), and foreground distinguishment. V2 connects to V1 - V5.
V3 helps process 'global motion' (direction and speed) of objects. V3 connects to V1 (weak), V2, and the inferior temporal cortex.Catani, Marco, and Derek K. Jones. "Brain." Occipito‐temporal Connections in the Human Brain. 23 June 2003. Web. 27 March 2016.
V4 recognizes simple shapes, and gets input from V1 (strong), V2, V3, LGN, and pulvinar. V5's outputs include V4 and its surrounding area, and eye-movement motor cortices (frontal eye-field and lateral intraparietal area).
V5's functionality is similar to that of the other V's, however, it integrates local object motion into global motion on a complex level. V6 works in conjunction with V5 on motion analysis. V5 analyzes self-motion, whereas V6 analyzes motion of objects relative to the background. V6's primary input is V1, with V5 additions. V6 houses the Topographic map for vision. V6 outputs to the region directly around it (V6A). V6A has direct connections to arm-moving cortices, including the premotor cortex.
The inferior temporal gyrus recognizes complex shapes, objects, and faces or, in conjunction with the hippocampus, creates new memories.Moser, May-Britt, and Edvard I. Moser. "Functional Differentiation in the Hippocampus." Wiley Online Library. 1998. Web. 27 March 2016. The pretectal area is seven unique nuclei. Anterior, posterior and medial pretectal nuclei inhibit pain (indirectly), aid in REM, and aid the accommodation reflex, respectively. The Edinger-Westphal nucleus moderates pupil dilation and aids (since it provides parasympathetic fibers) in convergence of the eyes and lens adjustment.Reiner, Anton, and Harvey J. Karten. "Parasympathetic Ocular Control — Functional Subdivisions and Circuitry of the Avian Nucleus of Edinger-Westphal."Science Direct. 1983. Web. 27 March 2016. Nuclei of the optic tract are involved in smooth pursuit eye movement and the accommodation reflex, as well as REM.
The suprachiasmatic nucleus is the region of the hypothalamus that halts production of melatonin (indirectly) at first light.
These are components of the visual pathway, also called the optic pathway, that can be divided into anterior and posterior visual pathways. The anterior visual pathway refers to structures involved in vision before the lateral geniculate nucleus. The posterior visual pathway refers to structures after this point.
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. Each human eye contains 120 million rods. Cones are found primarily in the center (or Fovea centralis) 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 mediate day vision and can distinguish color and other features of the visual world at medium and high light levels. Cones are larger and much less numerous than rods (there are 6-7 million of them in each human eye).
In the retina, the photoreceptors synapse directly onto bipolar cells, which in turn synapse onto ganglion cells of the outermost layer, which then conduct action potentials to the brain. A significant amount of visual processing arises from the patterns of communication between in the retina. About 130 million photo-receptors 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 cell and , 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 visual (image-forming and non-image-forming) information to the brain:
A 2006 University of Pennsylvania study calculated the approximate bandwidth of human retinas to be about 8,960 Kilobit per second, whereas guinea pig retinas transfer at about 875 kilobits.
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. The peak spectral sensitivity was 481 nm. This shows that there are two pathways for vision in the retina – one based on classic photoreceptors (rods and cones) and the other, newly discovered, based on photo-receptive ganglion cells which act as rudimentary visual brightness detectors.
In the visual system, retinal, technically called retinene1 or "retinaldehyde", is a light-sensitive 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.
A final population of photosensitive ganglion cells, containing melanopsin for photosensitivity, sends information via the retinohypothalamic tract to the pretectum (pupillary reflex), to several structures involved in the control of circadian rhythms and sleep such as the suprachiasmatic nucleus (the biological clock), and to the ventrolateral preoptic nucleus (a region involved in sleep regulation). A recently discovered role for photoreceptive ganglion cells is that they mediate conscious and unconscious vision – acting as rudimentary visual brightness detectors as shown in rodless coneless eyes.
The lateral geniculate nucleus (LGN) is a sensory relay nucleus in the thalamus of the brain. The LGN consists of six layers in and other starting from Catarrhini, including cercopithecidae and Ape. Layers 1, 4, and 6 correspond to information from the contralateral (crossed) fibers of the nasal retina (temporal visual field); layers 2, 3, and 5 correspond to information from the ipsilateral (uncrossed) fibers of the temporal retina (nasal visual field).
Layer one 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 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 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.
Spread out, the six layers of the LGN are the area of a credit card and about three times its thickness. The LGN is rolled up into two Ellipsoid 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 (posterior end) in the occipital lobe in and close to the calcarine sulcus. The LGN is not just a simple relay station, but it is also a center for processing; it receives reciprocal input from the Cortical layer and subcortical layers and reciprocal innervation from the visual cortex.
There is a direct correspondence from an angular position in the visual field of the eye, all the way through the optic tract to a nerve position in V1 up to V4, i.e. the primary visual areas. After that, the visual pathway is roughly separated into a ventral and dorsal pathway.
Visual information then flows through a cortical hierarchy. These areas include V2, V3, V4 and area V5/MT. (The exact connectivity depends on the species of the animal.) These secondary visual areas (collectively termed the extrastriate visual cortex) process a wide variety of visual primitives. Neurons in V1 and V2 respond selectively to bars of specific orientations, or combinations of bars. These are believed to support edge and corner detection. Similarly, basic information about color and motion is processed here.
Heider, et al. (2002) found that neurons involving V1, V2, and V3 can detect stereoscopic illusory contours; they found that stereoscopic stimuli subtending up to 8° can activate these neurons. Heider, Barbara; Spillmann, Lothar; Peterhans, Esther (2002) "Stereoscopic Illusory Contours— Cortical Neuron Responses and Human Perception" J. Cognitive Neuroscience 14:7 pp.1018-29 accessdate=2014-05-18
Along with this increasing complexity of neural representation may come a level of specialization of processing into two distinct pathways: the dorsal stream and the ventral stream (the Two Streams hypothesis, first proposed by Ungerleider and Mishkin in 1982). The dorsal stream, commonly referred to as the "where" stream, is involved in spatial attention (covert and overt), and communicates with regions that control eye movements and hand movements. More recently, this area has been called the "how" stream to emphasize its role in guiding behaviors to spatial locations. The ventral stream, commonly referred to as the "what" stream, is involved in the recognition, identification and categorization of visual stimuli.
However, there is still much debate about the degree of specialization within these two pathways, since they are in fact heavily interconnected.
Horace Barlow proposed the efficient coding hypothesis in 1961 as a theoretical model of sensory coding in the brain.Barlow, H. (1961) "Possible principles underlying the transformation of sensory messages" in Sensory Communication, MIT Press Limitations in the applicability of this theory in the primary visual cortex (V1) motivated the V1 Saliency Hypothesis that V1 creates a bottom-up saliency map to guide attention exogenously. With attentional selection as a center stage, vision is seen as composed of encoding, selection, and decoding stages.
The default mode network is a network of brain regions that are active when an individual is awake and at rest. The visual system's default mode can be monitored during resting state fMRI: Fox, et al. (2005) found that " the human brain is intrinsically organized into dynamic, anticorrelated functional networks", in which the visual system switches from resting state to attention.
In the parietal lobe, the lateral and ventral intraparietal cortex are involved in visual attention and saccadic eye movements. These regions are in the intraparietal sulcus (marked in red in the adjacent image).
According to Pollock et al. (2010) stroke is the main cause of specific visual impairment, most frequently visual field loss (homonymous hemianopia, a visual field defect). Nevertheless, evidence for the efficacy of cost-effective interventions aimed at these visual field defects is still inconsistent.
Proper function of the visual system is required for sensing, processing, and understanding the surrounding environment. Difficulty in sensing, processing and understanding light input has the potential to adversely impact an individual's ability to communicate, learn and effectively complete routine tasks on a daily basis.
In children, early diagnosis and treatment of impaired visual system function is an important factor in ensuring that key social, academic and speech/language developmental milestones are met.
Cataract is clouding of the lens, which in turn affects vision. Although it may be accompanied by yellowing, clouding and yellowing can occur separately. This is typically a result of ageing, disease, or drug use.
Presbyopia is a visual condition that causes Far-sightedness. The eye's lens becomes too inflexible to accommodate to normal reading distance, focus tending to remain fixed at long distance.
Glaucoma is a type of blindness that begins at the edge of the visual field and progresses inward. It may result in tunnel vision. This typically involves the outer layers of the optic nerve, sometimes as a result of buildup of fluid and excessive pressure in the eye.Harvard Health Publications (2025). 9781935555162, Harvard Health Publications. . ISBN 9781935555162
Scotoma is a type of blindness that produces a small blind spot in the visual field typically caused by injury in the primary visual cortex.
Homonymous hemianopia is a type of blindness that destroys one entire side of the visual field typically caused by injury in the primary visual cortex.
Quadrantanopia is a type of blindness that destroys only a part of the visual field typically caused by partial injury in the primary visual cortex. This is very similar to homonymous hemianopia, but to a lesser degree.
Prosopagnosia, or face blindness, is a brain disorder that produces an inability to recognize faces. This disorder often arises after damage to the fusiform face area.
Visual agnosia, or visual-form agnosia, is a brain disorder that produces an inability to recognize objects. This disorder often arises after damage to the ventral stream.
Many , such as Acromegalomma interruptum which live in tubes on the sea floor of the Great Barrier Reef, have evolved compound eyes on their tentacles, which they use to detect encroaching movement. If movement is detected, the fan worms will rapidly withdraw their tentacles. Bok, et al., have discovered opsins and in the fan worm's eyes, which were previously only seen in simple Cilium photoreceptors in the brains of some Invertebrate, as opposed to the rhabdomeric receptors in the eyes of most invertebrates. cited by Evolution of fan worm eyes (August 1, 2017) Phys.org
Only higher primate Old World (African) Monkey and apes (, , ) have the same kind of three-cone photoreceptor color vision humans have, while lower primate New World (South American) monkeys (, , Capuchin monkey) have a two-cone photoreceptor kind of color vision.
Biologists have determined that humans have extremely good vision compared to the overwhelming majority of animals, particularly in daylight, surpassed only by a few large species of predatory birds. Other animals such as are thought to rely more on senses other than vision, which in turn may be better developed than in humans.
The notion that the cerebral cortex is divided into functionally distinct cortices now known to be responsible for capacities such as touch (somatosensory cortex), movement (motor cortex), and vision (visual cortex), was first proposed by Franz Joseph Gall in 1810. Evidence for functionally distinct areas of the brain (and, specifically, of the cerebral cortex) mounted throughout the 19th century with discoveries by Paul Broca of the language center (1861), and Gustav Fritsch and Eduard Hitzig of the motor cortex (1871). Based on selective damage to parts of the brain and the functional effects of the resulting , David Ferrier proposed that visual function was localized to the parietal lobe of the brain in 1876. In 1881, Hermann Munk more accurately located vision in the occipital lobe, where the primary visual cortex is now known to be.
In 2014, a textbook "Understanding vision: theory, models, and data" illustrates how to link neurobiological data and visual behavior/psychological data through theoretical principles and computational models.
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