Латеральное коленчатое тело: ключевой элемент зрительной системы мозга.

Component of the visual system in the brain's thalamus

In neuroanatomy, the lateral geniculate nucleus (LGN; also called the lateral geniculate body or lateral geniculate complex) is a structure in the thalamus and a key component of the mammalian visual pathway. It is a small, ovoid, ventral projection of the thalamus where the thalamus connects with the optic nerve. 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 neurons (grey matter) alternating with optic fibers (white matter). The LGN receives information directly from the ascending retinal ganglion cells via the optic tract and from the reticular activating system. Neurons of the LGN send their axons through the optic radiation, a direct pathway to the primary visual cortex. In addition, the LGN receives many strong feedback connections from the primary visual cortex. In humans as well as other mammals, 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.

Both the left and right hemisphere of the brain have a lateral geniculate nucleus, named after its resemblance to a bent knee (genu is Latin for "knee"). In humans as well as in many other primates, 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.

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 (known as the superior colliculus in mammals). These non retinal inputs can be excitatory, inhibitory, or modulatory. Axons from layer 6 of visual cortex 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.

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 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, particularly other relay nuclei, the LGN likely helps the visual system focus its attention on the most important information. That is, if you hear a sound slightly to your left, the auditory system likely "tells" the visual system, 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. 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.

The LGN is integral in the early steps of color processing, where opponent channels are created that compare signals between the different Photoreceptor cell types. The output of P cells comprises red green opponent signals. The output of M cells does not include much color opponency, rather a sum of the red green signal that evokes luminance. The output of K cells comprises mostly blue yellow opponent signals.

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.

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.

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.

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.