Mechanism of generating visual signals : The retina adapts to change in light through the use of the rods. In the dark, the chromophore retinal has a bent shape called cis-retinal referring to a cis conformation in one of the double bonds. When light interacts with the retinal, it changes conformation to a straight form called trans-retinal and breaks away from the opsin. This is called bleaching because the purified rhodopsin changes from violet to colorless in the light.
At baseline in the dark, the rhodopsin absorbs no light and releases glutamate which inhibits the bipolar cell.
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This inhibits the release of neurotransmitters from the bipolar cells to the ganglion cell. When there is light present, glutamate secretion ceases thus no longer inhibiting the bipolar cell from releasing neurotransmitters to the ganglion cell and therefore an image can be detected. 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 University of Pennsylvania study calculated the approximate bandwidth of human retinas to be about kilobits per second, whereas guinea pig retinas transfer at about kilobits.
In 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. 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 photo-receptive ganglion cells which act as rudimentary visual brightness detectors. 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 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. In the visual system, retinal , technically called retinene 1 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.
In the presence of light, the retinal molecule changes configuration and as a result a nerve impulse is generated. 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. These axons originate from the M, P, and K ganglion cells in the retina, see above. 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 the superior colliculus in the midbrain , which assists in controlling eye movements saccades  as well as other motor responses. A final population of photosensitive ganglion cells , containing melanopsin for photosensitivity, sends information via the retinohypothalamic tract RHT to the pretectum pupillary reflex , to several structures involved in the control of circadian rhythms and sleep such as the suprachiasmatic nucleus SCN, the biological clock , and to 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 from 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. 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.
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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 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 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.
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 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 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 and subcortical layers and reciprocal innervation from the visual cortex.
The optic radiations , one on each side of the brain, carry information from the thalamic lateral geniculate nucleus to layer 4 of 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. After that, the visual pathway is roughly separated into a ventral and dorsal pathway.
The visual cortex is the largest system in the human brain and is responsible for processing the visual image. It lies at the rear of the brain highlighted in the image , above the cerebellum. The region that receives information directly from the LGN is called the primary visual cortex , also called V1 and striate cortex.
Visual information then flows through a cortical hierarchy. 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. As visual information passes forward through the visual hierarchy , the complexity of the neural representations increases. Whereas a V1 neuron may respond selectively to a line segment of a particular orientation in a particular retinotopic location, neurons in the lateral occipital complex respond selectively to complete object e.
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 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 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 as a theoretical model of sensory coding in the brain.
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. 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. Newborn infants have limited color perception. After one month performance "improved somewhat.
The pediatricians are able to perform non-verbal testing to assess visual acuity of a newborn, detect nearsightedness and astigmatism , and evaluate the eye teaming and alignment. All this is happening because the nerve cells in their retina and brain that control vision are not fully developed. Depth perception , focus, tracking and other aspects of vision continue to develop throughout early and middle childhood. From recent studies in the United States and Australia there is some evidence that the amount of time school aged children spend outdoors, in natural light, may have some impact on whether they develop myopia.
The condition tends to get somewhat worse through childhood and adolescence, but stabilizes in adulthood. More prominent myopia nearsightedness and astigmatism are thought to be inherited. Children with this condition may need to wear glasses. Eyesight is often one of the first senses affected by aging. A number of changes occur with aging:.
Along with proprioception and vestibular function , the visual system plays an important role in the ability of an individual to control balance and maintain an upright posture.
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When these three conditions are isolated and balance is tested, it has been found that vision is the most significant contributor to balance, playing a bigger role than either of the two other intrinsic mechanisms. Anything that affects any of these variables can have a negative effect on balance and maintaining posture.
According to Pollock et al. 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. 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 farsightedness. 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.
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 FFA.
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. 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 pit organs , which are sensitive to infrared radiation. The eye of the mantis shrimp holds 16 color receptive cones, whereas humans only have three.
The variety of cones enables them to perceive an enhanced array of colors as a mechanism for mate selection, avoidance of predators, and detection of prey. The eye of a swordfish can generate heat to better cope with detecting their prey at depths of feet. Many fan worms , 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 G proteins in the fan worm's eyes, which were previously only seen in simple ciliary photoreceptors in the brains of some invertebrates, as opposed to the rhabdomeric receptors in the eyes of most invertebrates. Only higher primate Old World African monkeys and apes macaques , apes , orangutans have the same kind of three-cone photoreceptor color vision humans have, while lower primate New World South American monkeys spider monkeys , squirrel monkeys , cebus monkeys have a two-cone photoreceptor kind of color vision.
In the second half of the 19th century, many motifs of the nervous system were identified such as the neuron doctrine and brain localization, which related to the neuron being the basic unit of the nervous system and functional localisation in the brain, respectively. These would become tenets of the fledgling neuroscience and would support further understanding of the visual system.
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 From Wikipedia, the free encyclopedia. This article is about the physiological components involved in vision. For the ability to interpret the surrounding environment, see Visual perception.
For electronic visual sensors, see Visual sensor network. The visual system includes the eyes, the connecting pathways through to the visual cortex and other parts of the brain.
The illustration shows the mammalian system. Main articles: Eye and Anterior segment of eyeball. Main article: Retina. Main article: Visual cycle. Main article: Optic nerve. Main article: Optic chiasm. Main article: Optic tract.
Main article: Lateral geniculate nucleus. Main article: Optic radiation. Main article: Visual cortex. Main article: Two Streams hypothesis. See also: Infant vision. Achromatopsia Akinetopsia Apperceptive agnosia Associative visual agnosia Asthenopia Astigmatism Color blindness Echolocation Computer vision Helmholtz—Kohlrausch effect — how color balance affects vision Magnocellular cell Memory-prediction framework Prosopagnosia Scotopic sensitivity syndrome Recovery from blindness Visual agnosia Visual modularity Visual perception Visual processing.
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