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In
neuroanatomy Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defi ...
, the lateral geniculate nucleus (LGN; also called the lateral geniculate body or lateral geniculate complex) is a structure in the
thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all direct ...
and a key component of the mammalian
visual pathway The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the a ...
. It is a small, ovoid, ventral projection of the thalamus where the thalamus connects with the
optic nerve In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived fro ...
. There are two LGNs, one on the left and another on the right side of the thalamus. In humans, both LGNs have six layers of
neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa ...
s (
grey matter Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil ( dendrites and unmyelinated axons), glial cells ( astrocytes and oligodendrocytes), synapses, and capillaries. Grey matter is ...
) alternating with optic fibers (
white matter White matter refers to areas of the central nervous system (CNS) that are mainly made up of myelinated axons, also called tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distribu ...
). The LGN receives information directly from the ascending
retinal ganglion cell A retinal ganglion cell (RGC) is a type of neuron located near the inner surface (the ganglion cell layer) of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and reti ...
s via the optic tract and from the
reticular activating system The reticular formation is a set of interconnected nuclei that are located throughout the brainstem. It is not anatomically well defined, because it includes neurons located in different parts of the brain. The neurons of the reticular formati ...
. Neurons of the LGN send their axons through the
optic radiation In neuroanatomy, the optic radiation (also known as the geniculocalcarine tract, the geniculostriate pathway, and posterior thalamic radiation) are axons from the neurons in the lateral geniculate nucleus to the primary visual cortex. The op ...
, a direct pathway to the primary visual cortex. In addition, the LGN receives many strong feedback connections from the primary
visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus ...
. In
humans" \n\n\n\n\nThe robots exclusion standard, also known as the robots exclusion protocol or simply robots.txt, is a standard used by websites to indicate to visiting web crawlers and other web robots which portions of the site they are allowed to visi ...
as well as other
mammals Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...
, the two strongest pathways linking the eye to the brain are those projecting to the dorsal part of the LGN in the thalamus, and to the
superior colliculus In neuroanatomy, the superior colliculus () is a structure lying on the roof of the mammalian midbrain. In non-mammalian vertebrates, the homologous structure is known as the optic tectum, or optic lobe. The adjective form '' tectal'' is common ...
.


Structure

Both the left and right
hemisphere Hemisphere refers to: * A half of a sphere As half of the Earth * A hemisphere of Earth ** Northern Hemisphere ** Southern Hemisphere ** Eastern Hemisphere ** Western Hemisphere ** Land and water hemispheres * A half of the (geocentric) celes ...
of the
brain A brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as Visual perception, vision. I ...
have a lateral geniculate nucleus, named after its resemblance to a bent knee (''genu'' is Latin for "knee"). In
human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...
s as well as in many other
primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter includin ...
s, the LGN has layers of magnocellular cells and parvocellular cells that are interleaved with layers of koniocellular cells. In humans the LGN is normally described as having six distinctive layers. The inner two layers, (1 and 2) are magnocellular layers, while the outer four layers, (3,4,5 and 6), are parvocellular layers. An additional set of neurons, known as the koniocellular layers, are found ventral to each of the magnocellular and parvocellular layers. This layering is variable between primate species, and extra leafleting is variable within species.


M, P, K cells

*Size relates to cell body, dendritic tree and receptive field The magnocellular, parvocellular, and koniocellular layers of the LGN correspond with the similarly named types of
retinal ganglion cell A retinal ganglion cell (RGC) is a type of neuron located near the inner surface (the ganglion cell layer) of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and reti ...
s. Retinal P ganglion cells send axons to a parvocellular layer, M ganglion cells send axons to a magnocellular layer, and K ganglion cells send axons to a koniocellular layer. Koniocellular cells are functionally and neurochemically distinct from M and P cells and provide a third channel to the visual cortex. They project their axons between the layers of the lateral geniculate nucleus where M and P cells project. Their role in visual perception is presently unclear; however, the koniocellular system has been linked with the integration of somatosensory system-proprioceptive information with visual perception, and it may also be involved in color perception. The parvo- and magnocellular fibers were previously thought to dominate the Ungerleider–Mishkin ventral stream and dorsal stream, respectively. However, new evidence has accumulated showing that the two streams appear to feed on a more even mixture of different types of nerve fibers. The other major retino–cortical visual pathway is the tectopulvinar pathway, routing primarily through the
superior colliculus In neuroanatomy, the superior colliculus () is a structure lying on the roof of the mammalian midbrain. In non-mammalian vertebrates, the homologous structure is known as the optic tectum, or optic lobe. The adjective form '' tectal'' is common ...
and thalamic pulvinar nucleus onto posterior parietal cortex and visual area MT.


Ipsilateral and contralateral layers

Layer 1, 2 * Large cells, called magnocellular pathways * Input from Y-ganglion cells * Very rapid conduction * Colour blind system Layer 3–6 * Parvocellular * Input from X- ganglion cells * Colour vision * Moderate velocity. Both the LGN in the right hemisphere and the LGN in the left hemisphere receive input from each eye. However, each LGN only receives information from one half of the visual field. This occurs due to axons of the ganglion cells from the inner halves of the retina (the nasal sides) decussating (crossing to the other side of the brain) through the
optic chiasma In neuroanatomy, the optic chiasm, or optic chiasma (; , ), is the part of the brain where the optic nerves cross. It is located at the bottom of the brain immediately inferior to the hypothalamus. The optic chiasm is found in all vertebr ...
(''khiasma'' means "cross-shaped"). The axons of the ganglion cells from the outer half of the retina (the temporal sides) remain on the same side of the brain. Therefore, the right hemisphere receives visual information from the left visual field, and the left hemisphere receives visual information from the right visual field. Within one LGN, the visual information is divided among the various layers as follows: * the eye on the same side (the ''ipsilateral'' eye) sends information to layers 2, 3 and 5 * the eye on the opposite side (the ''contralateral'' eye) sends information to layers 1, 4 and 6. This description applies to the LGN of many primates, but not all. The sequence of layers receiving information from the ipsilateral and contralateral (opposite side of the head) eyes is different in the
tarsier Tarsiers ( ) are haplorhine primates of the family Tarsiidae, which is itself the lone extant family within the infraorder Tarsiiformes. Although the group was once more widespread, all of its species living today are found in Maritime Southeas ...
. Some neuroscientists suggested that "this apparent difference distinguishes tarsiers from all other primates, reinforcing the view that they arose in an early, independent line of primate evolution". In
visual perception Visual perception is the ability to interpret the surrounding environment through photopic vision (daytime vision), color vision, scotopic vision (night vision), and mesopic vision (twilight vision), using light in the visible spectrum ref ...
, the right eye gets information from the right side of the world (the right
visual field The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments". Or simply, visual field can be defined as the entire area that can be seen when an eye is fixed straight at a poin ...
), as well as the left side of the world (the left
visual field The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments". Or simply, visual field can be defined as the entire area that can be seen when an eye is fixed straight at a poin ...
). You can confirm this by covering your left eye: the right eye still sees to your left and right, although on the left side your field of view may be partially blocked by your nose.


Input

The LGN receives input from the retina and many other brain structures, especially visual cortex. The principal neurons in the LGN receive strong inputs from the retina. However, the retina only accounts for a small percentage of LGN input. As much as 95% of input in the LGN comes from the visual cortex, superior colliculus, pretectum, thalamic reticular nuclei, and local LGN interneurons. Regions in the brainstem that are not involved in visual perception also project to the LGN, such as the mesencephalic reticular formation, dorsal raphe nucleus, periaqueuctal grey matter, and the locus coeruleus. The LGN also receives some inputs from the optic tectum (also known as the
superior colliculus In neuroanatomy, the superior colliculus () is a structure lying on the roof of the mammalian midbrain. In non-mammalian vertebrates, the homologous structure is known as the optic tectum, or optic lobe. The adjective form '' tectal'' is common ...
). These non-retinal inputs can be excitatory, inhibitory, or modulatory.


Output

Information leaving the LGN travels out on the
optic radiation In neuroanatomy, the optic radiation (also known as the geniculocalcarine tract, the geniculostriate pathway, and posterior thalamic radiation) are axons from the neurons in the lateral geniculate nucleus to the primary visual cortex. The op ...
s, which form part of the ''retrolenticular portion'' of the
internal capsule The internal capsule is a white matter structure situated in the inferomedial part of each cerebral hemisphere of the brain. It carries information past the basal ganglia, separating the caudate nucleus and the thalamus from the putamen and the ...
. The
axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...
s that leave the LGN go to V1
visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus ...
. Both the magnocellular layers 1–2 and the parvocellular layers 3–6 send their axons to layer 4 in V1. Within layer 4 of V1, layer 4cβ receives parvocellular input, and layer 4cα receives magnocellular input. However, the koniocellular layers, intercalated between LGN layers 1–6 send their axons primarily to the cytochrome-oxidase rich blobs of layers 2 and 3 in V1.
Axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...
s from layer 6 of
visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus ...
send information back to the LGN. Studies involving blindsight have suggested that projections from the LGN travel not only to the primary visual cortex but also to higher cortical areas V2 and V3. Patients with blindsight are phenomenally blind in certain areas of the visual field corresponding to a contralateral lesion in the primary visual cortex; however, these patients are able to perform certain motor tasks accurately in their blind field, such as grasping. This suggests that neurons travel from the LGN to both the primary visual cortex and higher cortex regions.


Function in visual perception

The output of the LGN serves several functions. Computations are achieved to determine the position of every major element in object space relative to the principal plane. Through subsequent motion of the eyes, a larger stereoscopic mapping of the visual field is achieved. It has been shown that while the retina accomplishes spatial
decorrelation Decorrelation is a general term for any process that is used to reduce autocorrelation within a signal, or cross-correlation within a set of signals, while preserving other aspects of the signal. A frequently used method of decorrelation is the us ...
through center surround inhibition, the LGN accomplishes temporal decorrelation. This spatial–temporal decorrelation makes for much more efficient coding. However, there is almost certainly much more going on. Like other areas of the
thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all direct ...
, particularly other ''relay nuclei'', the LGN likely helps the
visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (th ...
focus its attention on the most important information. That is, if you hear a sound slightly to your left, the
auditory system The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs (the ears) and the auditory parts of the sensory system. System overview The outer ear funnels sound vibrations to the eardrum, increasin ...
likely "tells" the
visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (th ...
, through the LGN via its surrounding peri-reticular nucleus, to direct visual attention to that part of space. The LGN is also a station that refines certain
receptive fields The receptive field, or sensory space, is a delimited medium where some physiological stimuli can evoke a sensory neuronal response in specific organisms. Complexity of the receptive field ranges from the unidimensional chemical structure of od ...
. Axiomatically determined functional models of LGN cells have been determined by Lindeberg in terms of Laplacian of Gaussian kernels over the spatial domain in combination with temporal derivatives of either non-causal or time-causal scale-space kernels over the temporal domain. It has been shown that this theory both leads to predictions about receptive fields with good qualitative agreement with the biological receptive field measurements performed by DeAngelis et al. and guarantees good theoretical properties of the mathematical receptive field model, including covariance and invariance properties under natural image transformations. Specifically according to this theory, non-lagged LGN cells correspond to first-order temporal derivatives, whereas lagged LGN cells correspond to second-order temporal derivatives. For an extensive overview of the function of the LGN in visual perception, see Ghodrati et al.M. Ghodrati, S.-M. Khaligh-Razavi, S.R. Lehky, Towards building a more complex view of the lateral geniculate nucleus: recent advances in understanding its role, Prog. Neurobiol. 156:214–255, 2017.
/ref>


Rodents

In rodents, the lateral geniculate nucleus contains the dorsal lateral geniculate nucleus (dLGN), the ventral lateral geniculate nucleus (vLGN), and the region in between called the intergeniculate leaflet (IGL). These are distinct subcortical nuclei with differences in function.


dLGN

The dorsolateral geniculate nucleus is the main division of the lateral geniculate body. The majority of input to the dLGN comes from the retina. It is laminated and shows retinotopic organization.


vLGN

The ventrolateral geniculate nucleus has been found to be relatively large in several species such as lizards, rodents, cows, cats, and primates. An initial cytoarchitectural scheme, which has been confirmed in several studies, suggests that the vLGN is divided into two parts. The external and internal divisions are separated by a group of fine fibers and a zone of thinly dispersed neurons. Additionally, several studies have suggested further subdivisions of the vLGN in other species. For example, studies indicate that the cytoarchitecture of the vLGN in the cat differs from rodents. Although five subdivisions of the vLGN in the cat have been identified by some, the scheme that divides the vLGN into three regions (medial, intermediate, and lateral) has been more widely accepted.


IGL

The intergeniculate leaflet is a relatively small area found dorsal to the vLGN. Earlier studies had referred to the IGL as the internal dorsal division of the vLGN. Several studies have described homologous regions in several species, including humans. The vLGN and IGL appear to be closely related based on similarities in neurochemicals, inputs and outputs, and physiological properties. The vLGN and IGL have been reported to share many neurochemicals that are found concentrated in the cells, including neuropeptide Y, GABA, encephalin, and nitric oxide synthase. The neurochemicals serotonin, acetylcholine, histamine, dopamine, and noradrenaline have been found in the fibers of these nuclei. Both the vLGN and IGL receive input from the retina, locus coreuleus, and raphe. Other connections that have been found to be reciprocal include the superior colliculus, pretectum, and hypothalamus, as well as other thalamic nuclei. Physiological and behavioral studies have shown spectral-sensitive and motion-sensitive responses that vary with species. The vLGN and IGL seem to play an important role in mediating phases of the circadian rhythms that are not involved with light, as well as phase shifts that are light-dependent.


Additional images

File:Constudthal.gif, Thalamus File:Gray683.png, Dissection of brain-stem. Lateral view. File:Gray722.svg, Scheme showing central connections of the
optic nerve In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived fro ...
s and optic tracts. File:ThalamicNuclei.svg, Thalamic nuclei File:ERP_-_optic_cabling.jpg, 3D schematic representation of optic tracts File:Slide2BRA.JPG, Brainstem. Posterior view.


References


External links

* Malpeli J

Retrieved September 1, 2004. * * – "The Visual Pathway from Below" * * {{DEFAULTSORT:Lateral Geniculate Nucleus Thalamic nuclei Visual system