| Visual system | |
|---|---|
 The visual system includes the eyes, the connecting pathways through to the visual cortex and other parts of the brain (human system shown). | |
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| Identifiers | |
| FMA | 7191 |
| Anatomical terminology | |
Thevisual system is the physiological basis ofvisual perception (the ability todetect and processlight). The system detects,transduces and interprets information concerninglight within thevisible range to construct animage and build amental model of the surrounding environment. The visual system is associated with theeye and functionally divided into theoptical system (includingcornea andlens) and theneural system (including theretina andvisual cortex).
The visual system performs a number of complex tasks based on theimage forming functionality of the eye, including the formation of monocular images, the neural mechanisms underlyingstereopsis and assessment of distances to (depth perception) and between objects,motion perception,pattern recognition, accuratemotor coordination under visual guidance, andcolour vision. Together, these facilitate higher order tasks, such asobject identification. Theneuropsychological side of visual information processing is known asvisual perception, an abnormality of which is calledvisual impairment, and a complete absence of which is calledblindness. The visual system also has several non-image forming visual functions, independent of visual perception, including thepupillary light reflex and circadianphotoentrainment.
This article describes the human visual system, which is representative ofmammalian vision, and to a lesser extent thevertebrate visual system.
Together, thecornea andlens refract light into a small image and shine it on theretina. The retinatransduces this image into electrical pulses usingrods andcones. Theoptic nerve then carries these pulses through theoptic canal. Upon reaching theoptic chiasm the nerve fibers decussate (left becomes right). The fibers then branch and terminate in three places.[1][2][3][4][5][6][7]
Most of the optic nerve fibers end in thelateral geniculate nucleus (LGN). Before the LGN forwards the pulses to V1 of the visual cortex (primary) it gauges the range of objects and tags every major object with a velocity tag. These tags predict object movement.
The LGN also sends some fibers to V2 and V3.[8][9][10][11][12]
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-upsaliency map to guide attention orgaze shift.[13]
V2 both forwards (direct and viapulvinar) pulses to V1 and receives them. Pulvinar is responsible forsaccade and visual attention. V2 serves much the same function as V1, however, it also handlesillusory 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 theinferior temporal cortex.[14][15]
V4 recognizes simple shapes, and gets input from V1 (strong), V2, V3, LGN, and pulvinar.[16] V5's outputs include V4 and its surrounding area, and eye-movement motor cortices (frontal eye-field andlateral 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 thetopographical map for vision. V6 outputs to the region directly around it (V6A). V6A has direct connections to arm-moving cortices, including thepremotor cortex.[17][18]
Theinferior temporal gyrus recognizes complex shapes, objects, and faces or, in conjunction with thehippocampus, creates newmemories.[19] Thepretectal area is seven uniquenuclei. Anterior, posterior and medial pretectal nuclei inhibit pain (indirectly), aid inREM, and aid theaccommodation reflex, respectively.[20] TheEdinger-Westphal nucleus moderatespupil dilation and aids (since it provides parasympathetic fibers) in convergence of the eyes and lens adjustment.[21] Nuclei of the optic tract are involved in smooth pursuit eye movement and the accommodation reflex, as well as REM.
Thesuprachiasmatic nucleus is the region of thehypothalamus that halts production ofmelatonin (indirectly) at first light.[22]
These are components of thevisual pathway, also called theoptic pathway,[23] that can be divided intoanterior and posterior visual pathways. The anterior visual pathway refers to structures involved in vision before thelateral geniculate nucleus. The posterior visual pathway refers to structures after this point.
Light entering the eye isrefracted as it passes through thecornea. It then passes through thepupil (controlled by theiris) and is further refracted by thelens. The cornea and lens act together as a compound lens to project an inverted image onto the retina.
The retina consists of manyphotoreceptor cells which contain particularproteinmolecules calledopsins. In humans, two types of opsins are involved in conscious vision:rod opsins andcone opsins. (A third type,melanopsin in someretinal ganglion cells (RGC), part of thebody clock mechanism, is probably not involved in conscious vision, as these RGC do not project to thelateral geniculate nucleus but to thepretectal olivary nucleus.[24]) An opsin absorbs aphoton (a particle of light) and transmits a signal to thecell through asignal transduction pathway, resulting in hyper-polarization of the 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. Each human eye contains 120 million rods. Cones are found primarily in the center (orfovea) of the retina.[25] There are three types of cones that differ in thewavelengths of light they absorb; they are usually called short or blue, middle or green, and long or red. Cones mediate day vision and can distinguishcolor 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).[25]
In the retina, the photoreceptorssynapse directly ontobipolar cells, which in turn synapse ontoganglion cells of the outermost layer, which then conductaction potentials to thebrain. A significant amount ofvisual processing arises from the patterns of communication betweenneurons in the retina. About 130 million photo-receptors absorb light, yet roughly 1.2 millionaxons of ganglion cells transmit information from the retina to the brain. The processing in the retina includes the formation of center-surroundreceptive 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, particularlyhorizontal andamacrine 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 tomotion or sensitive to color and indifferent to motion.[26]
The retina adapts to change in light through the use of the rods. In the dark, thechromophoreretinal has a bent shape called cis-retinal (referring to acis 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 purifiedrhodopsin changes from violet to colorless in the light. At baseline in the dark, the rhodopsin absorbs no light and releasesglutamate, which inhibits the bipolar cell. 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.[27][28]
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:[26]
A 2006University of Pennsylvania study calculated the approximatebandwidth of human retinas to be about 8,960kilobits per second, whereasguinea pig retinas transfer at about 875 kilobits.[29]
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.[30] The peakspectral 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.
The functioning of acamera is often compared with the workings of the eye, mostly since both focus light from external objects in thefield 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 atransducer, as does aCCD camera.
In the visual system,retinal, technically calledretinene1 or "retinaldehyde", is a light-sensitive molecule found in the rods and cones of theretina. Retinal is the fundamental structure involved in the transduction oflight into visual signals, i.e. nerve impulses in the ocular system of thecentral nervous system. In the presence of light, the retinal molecule changes configuration and as a result, anerve impulse is generated.[26]
The information about the image via the eye is transmitted to the brain along theoptic nerve. Different populations of ganglion cells in the retina send information to the brain through the optic nerve. About 90% of theaxons in the optic nerve go to thelateral geniculate nucleus in thethalamus. These axons originate from the M, P, and K ganglion cells in the retina, see above. Thisparallel processing is important for reconstructing the visual world; each type of information will go through a different route toperception. Another population sends information to thesuperior colliculus in themidbrain, which assists in controlling eye movements (saccades)[31] as well as other motor responses.
A final population ofphotosensitive ganglion cells, containingmelanopsin forphotosensitivity, sends information via theretinohypothalamic tract to thepretectum (pupillary reflex), to several structures involved in the control ofcircadian rhythms andsleep such as thesuprachiasmatic nucleus (the biological clock), and to theventrolateral preoptic nucleus (a region involved insleep regulation).[32] 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.[30]
The optic nerves from both eyes meet and cross at the optic chiasm,[33][34] at the base of thehypothalamus of the brain. At this point, the information coming from both eyes is combined and then splits according to thevisual field. The corresponding halves of the field of view (right and left) are sent to the left and righthalves of the brain, respectively, to be processed. That is, the right side ofprimary visual cortex deals with the left half of thefield of view from both eyes, and similarly for the left brain.[31] A small region in the center of the field of view is processed redundantly by both halves of the brain.
Information from the rightvisual field (now on the left side of the brain) travels in the left optic tract. Information from the leftvisual field travels in the right optic tract. Each optic tract terminates in thelateral geniculate nucleus (LGN) in the thalamus.
Thelateral geniculate nucleus (LGN) is a sensory relay nucleus in the thalamus of the brain. The LGN consists of six layers inhumans and otherprimates starting fromcatarrhines, includingcercopithecidae andapes. 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 toinformation 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 acredit card and about three times its thickness. The LGN is rolled up into twoellipsoids 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 theprimary visual cortex (V1) which is located at the back of the brain (posterior end) in theoccipital lobe in and close to thecalcarine sulcus. The LGN is not just a simple relay station, but it is also a center for processing; it receives reciprocal input from thecortical and subcortical layers andreciprocal innervation from the visual cortex.[26]
Theoptic radiations, one on each side of the brain, carry information from the thalamiclateral geniculate nucleus to layer 4 of thevisual 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.[26]
There is a direct correspondence from an angular position in thevisual 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 aventral and dorsal pathway.
The visual cortex is responsible for processing the visual image. It lies at the rear of the brain (highlighted in the image), above thecerebellum. The region that receives information directly from the LGN is called theprimary visual cortex (also called V1 and striate cortex). It creates a bottom-up saliency map of the visual field to guide attention or eye gaze to salient visual locations.[35][clarification needed] Hence selection of visual input information by attention starts at V1[36] along the visual 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.[37]
Heider, et al. (2002) found that neurons involving V1, V2, and V3 can detect stereoscopicillusory contours; they found that stereoscopic stimuli subtending up to 8° can activate these neurons.[38]
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 particularretinotopic location, neurons in the lateral occipital complex respond selectively to a complete object (e.g., a figure drawing), and neurons in the visual association cortex may respond selectively to human faces, or to a particular object.
Along with this increasing complexity of neural representation may come a level of specialization of processing into two distinct pathways: thedorsal stream and theventral stream (theTwo Streams hypothesis,[39] 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.[40]
Horace Barlow proposed theefficient coding hypothesis in 1961 as a theoretical model ofsensory coding in thebrain.[41] Limitations in the applicability of this theory in theprimary visual cortex (V1) motivated theV1 Saliency Hypothesis that V1 creates a bottom-up saliency map to guide attention exogenously.[35] With attentional selection as a center stage, vision is seen as composed of encoding, selection, and decoding stages.[42]
Thedefault 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 duringresting state fMRI:Fox, et al. (2005) found that "the human brain is intrinsically organized into dynamic, anticorrelated functional networks",[43] in which the visual system switches from resting state to attention.
In theparietal lobe, thelateral and ventral intraparietal cortex are involved in visual attention and saccadic eye movements. These regions are in theintraparietal sulcus (marked in red in the adjacent image).
Newborn infants have limitedcolor perception.[44] One study found that 74% of newborns can distinguish red, 36% green, 25% yellow, and 14% blue. After one month, performance "improved somewhat."[45] Infant's eyes do not have the ability toaccommodate. Pediatricians are able to perform non-verbal testing to assessvisual acuity of a newborn, detectnearsightedness andastigmatism, and evaluate the eye teaming and alignment. Visual acuity improves from about 20/400 at birth to approximately 20/25 at 6 months of age. This happens because the nerve cells in theretina 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 theUnited States andAustralia there is some evidence that the amount of time school aged children spend outdoors, in natural light, may have some impact on whether they developmyopia. 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.
Vision is often one of the first senses affected by aging. A number of changes occur with aging:
Along withproprioception andvestibular function, the visual system plays an important role in the ability of an individual to control balance and maintain an upright posture. 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.[46] The clarity with which an individual can see his environment, as well as the size of the visual field, the susceptibility of the individual to light and glare, and poor depth perception play important roles in providing a feedback loop to the brain on the body's movement through the environment. Anything that affects any of these variables can have a negative effect on balance and maintaining posture.[47] This effect has been seen in research involving elderly subjects when compared to young controls,[48] inglaucoma patients compared to age matched controls,[49]cataract patients pre and post surgery,[50] and even something as simple as wearing safety goggles.[51]Monocular vision (one eyed vision) has also been shown to negatively impact balance, which was seen in the previously referenced cataract and glaucoma studies,[49][50] as well as in healthy children and adults.[52]
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.[53]
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 causesfarsightedness. The eye's lens becomes too inflexible toaccommodate 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 intunnel 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.[54]
Scotoma is a type of blindness that produces a smallblind 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 thefusiform 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 theventral stream.
Differentspecies are able to see different parts of thelight spectrum; for example,bees can see into theultraviolet,[55] whilepit vipers can accurately target prey with theirpit organs, which are sensitive to infrared radiation.[56] Themantis shrimp possesses arguably the most complex visual system of any species. 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.[57] Swordfish also possess an impressive visual system. The eye of aswordfish can generateheat to better cope with detecting theirprey at depths of 2000 feet.[58] Certainone-celledmicroorganisms, thewarnowiiddinoflagellates have eye-likeocelloids, with analogous structures for the lens and retina of the multi-cellular eye.[59] The armored shell of thechitonAcanthopleura granulata is also covered with hundreds ofaragonite crystalline eyes, namedocelli, which can formimages.[60]
Manyfan worms, such asAcromegalomma interruptum which live in tubes on the sea floor of theGreat 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 andG proteins in the fan worm's eyes, which were previously only seen in simpleciliary photoreceptors in the brains of someinvertebrates, as opposed to therhabdomeric receptors in the eyes of most invertebrates.[61]
Onlyhigher primateOld World (African)monkeys and apes (macaques,apes,orangutans) have the same kind of three-conephotoreceptor color vision humans have, while lower primateNew World (South American) monkeys (spider monkeys,squirrel monkeys,cebus monkeys) have a two-cone photoreceptor kind of color vision.[62]
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 ofpredatory birds.[63][64] Other animals such asdogs are thought to rely more on senses other than vision, which in turn may be better developed than in humans.[65][66]
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 theneuron being the basic unit of the nervous system and functional localisation in the brain, respectively. These would become tenets of the fledglingneuroscience and would support further understanding of the visual system.
The notion that thecerebral cortex is divided into functionally distinct cortices now known to be responsible for capacities such astouch (somatosensory cortex),movement (motor cortex), and vision (visual cortex), was first proposed byFranz Joseph Gall in 1810.[67] Evidence for functionally distinct areas of the brain (and, specifically, of the cerebral cortex) mounted throughout the 19th century with discoveries byPaul Broca of thelanguage center (1861), andGustav Fritsch andEduard Hitzig of the motor cortex (1871).[67][68] Based on selective damage to parts of the brain and the functional effects of the resultinglesions,David Ferrier proposed that visual function was localized to theparietal lobe of the brain in 1876.[68] In 1881,Hermann Munk more accurately located vision in theoccipital lobe, where theprimary visual cortex is now known to be.[68]
In 2014, a textbook "Understanding vision: theory, models, and data"[42] illustrates how to link neurobiological data and visual behavior/psychological data through theoretical principles and computational models.
