Anatomically, the LCT belongs to the metathalamus, its dimensions are 8.5 x 5 mm. The cytoarchitecture of the LCT is determined by its six-layer structure, which is found only in higher mammals, primates and humans.
Each LCT contains two main nuclei: dorsal (upper) and ventral (lower). There are six layers of nerve cells in the LCT, four layers in the dorsal nucleus and two in the ventral nucleus. In the ventral part of the LCT, the nerve cells are larger and respond in a special way to visual stimuli. The nerve cells of the dorsal nucleus of the LCT are smaller and similar to each other histologically and in electrophysiological properties. In this regard, the ventral layers of the LCT are called large cell (magnocellular), and the dorsal layers are called small cell (parvocellular).
The parvocellular structures of the LCT are represented by layers 3, 4, 5, 6 (P-cells); magnocellular layers - 1 and 2 (M cells). The axon endings of magno- and parvocellular ganglion cells of the retina are morphologically different, and therefore in different layers of LCT nerve cells there are synapses that differ from each other. Magnaxon terminals are radially symmetrical, have thick dendrites and large ovoid endings. Parvoaxon terminals are elongated, have thin dendrites and medium-sized round terminal endings.
The LCT also contains axon terminals with a different morphology, belonging to other classes of retinal ganglion cells, in particular the blue-sensitive cone system. These axon terminals create synapses in a heterogeneous group of LCT layers collectively called “koniocellular” or K layers.
Due to the intersection of optic nerve fibers in the chiasm from the right and left eyes, nerve fibers from the retinas of both eyes enter the LCT on each side. The endings of the nerve fibers in each of the layers of the LCT are distributed in accordance with the principle of retinotopic projection and form a projection of the retina onto the layers of nerve cells of the LCT. This is facilitated by the fact that 1.5 million LCT neurons with their dendrites provide a very reliable connection of synaptic impulse transmission from 1 million axons of retinal ganglion cells.
The projection of the central fossa is most fully represented in the geniculate body macular spot. The projection of the visual pathway into the LCT contributes to the recognition of objects, their color, movement and stereoscopic depth perception (primary center of vision).
(module direct4)
Functionally, the receptive fields of LCT neurons have concentric shape and are similar to similar fields of retinal ganglion cells, for example, the central zone is excitatory, and the peripheral, ring zone is inhibitory. LCT neurons are divided into two classes: with an on-center and with an off-center (darkening of the center activates the neuron). LCT neurons perform different function.
Pathological processes localized in the area of the chiasm, optic tract and LCT are characterized by symmetrical binocular loss of the visual field.
These are true hemianopsias, which, depending on the location of the lesion, can be:
Visual acuity in such neurological patients decreases depending on the degree of damage to the papillomacular bundle of the optic pathway. Even with unilateral damage to the visual pathway in the LCT area (right or left), the central vision of both eyes is affected. In this case, one feature is noted that has important differential diagnostic significance. Pathological foci located peripherally from the LCT give positive scotomas in the field of view and are felt by patients as a darkening of vision or a vision of a gray spot. In contrast to these lesions, lesions located above the LCT, including lesions in the occipital lobe of the brain, usually produce negative scotomas, i.e., are not perceived by patients as visual impairment.
Foreign country or metathalamus
Metathalamus (lat. Metathalamus) is part of the thalamic region of the mammalian brain. Formed by paired medial and lateral geniculate bodies, lying behind each thalamus.
The medial geniculate body is located behind the thalamic cushion; it, along with the lower colliculi of the midbrain roof plate (quadrigeminal), is the subcortical center of the auditory analyzer. The lateral geniculate body is located inferior to the pillow. Together with the superior colliculi of the roof plate, it is the subcortical center of the visual analyzer. The nuclei of the geniculate bodies are connected by pathways to the cortical centers of the visual and auditory analyzers.
In the medial part of the thalamus there is a mediodorsal nucleus and a group of nuclei midline.
The mediodorsal nucleus has bilateral connections with the olfactory cortex of the frontal lobe and the cingulate gyrus of the cerebral hemispheres, the amygdala and the anteromedial nucleus of the thalamus. Functionally, it is also closely connected with the limbic system and has bilateral connections with the parietal, temporal and insular cortex of the brain.
The mediodorsal nucleus is involved in the implementation of higher mental processes. Its destruction leads to a decrease in anxiety, anxiety, tension, aggressiveness, and the elimination of obsessive thoughts.
The midline nuclei are numerous and occupy the most medial position in the thalamus. They receive afferent (i.e., ascending) fibers from the hypothalamus, from the raphe nuclei, the locus coeruleus of the reticular formation of the brain stem, and partially from the spinothalamic tracts as part of the medial lemniscus. Efferent fibers from the midline nuclei are sent to the hippocampus, amygdala and cingulate gyrus of the cerebral hemispheres, which are part of the limbic system. Connections with the cerebral cortex are bilateral.
The midline nuclei play an important role in the processes of awakening and activation of the cerebral cortex, as well as in supporting memory processes.
In the lateral (i.e. lateral) part of the thalamus there are dorsolateral, ventrolateral, ventral posteromedial and posterior groups of nuclei.
The nuclei of the dorsolateral group have been studied relatively little. They are known to be involved in the pain perception system.
The nuclei of the ventrolateral group are anatomically and functionally different from each other. The posterior nuclei of the ventrolateral group are often considered as one ventrolateral nucleus of the thalamus. This group receives fibers from the ascending tract of general sensitivity as part of the medial lemniscus. Fibers of taste sensitivity and fibers from the vestibular nuclei also come here. Efferent fibers starting from the nuclei of the ventrolateral group are sent to the cortex of the parietal lobe of the cerebral hemispheres, where they carry somatosensory information from the whole body.
Afferent fibers from the superior colliculus and fibers in the optic tracts go to the nuclei of the posterior group (nucleus of the thalamic cushion). Efferent fibers are widely distributed in the cortex of the frontal, parietal, occipital, temporal and limbic lobes of the cerebral hemispheres.
The nuclear centers of the thalamic cushion are involved in comprehensive analysis various sensory stimuli. They play a significant role in the perceptual (related to perception) and cognitive (cognitive, thinking) activity of the brain, as well as in memory processes - storing and reproducing information.
The intralaminar group of thalamic nuclei lies deep in the vertical Y-shaped layer of white matter. The intralaminar nuclei are interconnected with the basal ganglia, the dentate nucleus of the cerebellum and the cerebral cortex.
These nuclei play an important role in the activation system of the brain. Damage to the intralaminar nuclei in both thalami leads to sharp decline motor activity, as well as apathy and destruction of the motivational structure of the individual.
The cerebral cortex, thanks to bilateral connections with the nuclei of the thalamus, is capable of exerting a regulatory effect on their functional activity.
Thus, the main functions of the thalamus are:
processing of sensory information from receptors and subcortical switching centers with its subsequent transfer to the cortex;
participation in the regulation of movements;
ensuring communication and integration of different parts of the brain
This is a subcortical center that ensures the transmission of information to the visual cortex.
In humans, this structure has six layers of cells, as in the visual cortex. Fibers from the retina enter the chiasma opticus, crossed and uncrossed. The 1st, 4th, 6th layers receive crossed fibers. The 2nd, 3rd, 5th layers are uncrossed.
All information coming to the lateral geniculate body from the retina is ordered and the retinotopic projection is maintained. Since the fibers enter the lateral geniculate body like a comb, there are no neurons in the NKT that receive information from two retinas simultaneously. It follows from this that there is no binocular interaction in the NKT neurons. The tubing receives fibers from M-cells and P-cells. The M-path, which communicates information from large cells, transmits information about the movements of objects and ends in the 1st and 2nd layers. The P-path is associated with color information and the fibers terminate in layers 3, 4, 5, 6. In the 1st and 2nd layers of the NKT, the receptive fields are highly sensitive to movement and do not distinguish spectral characteristics (color). Such receptive fields are also present in small quantities in other layers of the tubing. In the 3rd and 4th layers, neurons with an OFF center predominate. It is blue-yellow or blue-red + green. The 5th and 6th layers contain neurons with ON centers, mainly red-green. Receptive fields of cells of the outer geniculate body have the same receptive fields as ganglion cells.
The difference between these receptive fields and ganglion cells is:
1. In the size of the receptive fields. The cells of the external geniculate body are smaller.
2. Some NKT neurons have an additional inhibitory zone surrounding the periphery.
For cells with an ON center, such an additional zone will have a reaction sign coinciding with the center. These zones are formed only in some neurons due to increased lateral inhibition between NKT neurons. These layers are the basis of survival specific type. Humans have six layers, predators have four.
Detector theory appeared in the late 1950s. In the frog's retina (in ganglion cells), reactions were found that were directly related to behavioral responses. Excitation of certain retinal ganglion cells led to behavioral responses. This fact allowed us to create the concept that the image presented on the retina is processed by ganglion cells specifically tuned to the elements of the image. Such ganglion cells have specific dendritic branching, which corresponds to a certain structure of the receptive field. Several types of such ganglion cells have been discovered. Subsequently, neurons with this property were called detector neurons. Thus, a detector is a neuron that responds to a specific image or part of it. It turned out that other, more highly developed animals also have the ability to highlight a specific symbol.
1. Convex edge detectors - the cell was activated when a large object appeared in the field of view;
2. Detector of moving fine contrast - its activation led to an attempt to capture this object; corresponds in contrast to the objects being captured; these reactions are associated with food reactions;
3. Blackout detector - causes a defensive reaction (the appearance of large enemies).
These retinal ganglion cells are tuned to secrete certain elements environment.
Group of researchers who worked on this topic: Letvin, Maturano, Moccalo, Pitz.
Neurons of other types also have detector properties. sensory systems. Most detectors in the visual system are concerned with motion detection. Neurons increase their reactions when the speed of movement of objects increases. Detectors have been found in both birds and mammals. Detectors of other animals are directly connected to the surrounding space. Birds have been found to have horizontal surface detectors, due to the need to land on horizontal objects. Detectors were also found vertical surfaces, which ensure the birds’ own movements towards these objects. It turned out that the higher the animal is in the evolutionary hierarchy, the higher the detectors are, i.e. these neurons may already be located not only in the retina, but also in the higher parts of the visual system. In higher mammals: in monkeys and humans, detectors are located in the visual cortex. This is important because the specific way that the elements react external environment, is transferred to higher levels of the brain, and each animal species has its own specific types of detectors. Later it turned out that during ontogenesis the detector properties of sensory systems are formed under the influence of the environment. To demonstrate this property, experiments were carried out by Nobel laureate researchers Hubel and Wiesel. Experiments were carried out that proved that the formation of detector properties occurs in the earliest ontogenesis. For example, three groups of kittens were used: one control and two experimental. The first experimental one was placed in conditions where mainly horizontally oriented lines were present. The second experimental one was placed in conditions where there were mainly horizontal lines. The researchers checked which neurons formed in the cortex of the kittens in each group. In the cortex of these animals, it turned out that 50% of neurons were activated both horizontally and 50% vertically. Animals raised in a horizontal environment had a significant number of neurons in the cortex that were activated by horizontal objects; there were practically no neurons activated when perceiving vertical objects. In the second experimental group there was a similar situation with horizontal objects. Kittens of both horizontal groups developed certain defects. Kittens in a horizontal environment could jump perfectly on steps and horizontal surfaces, but were poorly able to carry out movements relative to vertical objects (table leg). The kittens of the second experimental group had the corresponding situation for vertical objects. This experiment proved:
1) formation of neurons in early ontogenesis;
2) the animal cannot interact adequately.
Changing animal behavior in a changing environment. Each generation has its own set of external stimuli that produce a new set of neurons.
Specific features of the visual cortex
From the cells of the external geniculate body (has a 6-layer structure), axons enter the 4 layers of the visual cortex. The bulk of the axons of the external geniculate body (ECC) are distributed in the fourth layer and its sublayers. From the fourth layer, information flows to other layers of the cortex. The visual cortex retains the principle of retinotopic projection in the same way as the NKT. All information from the retina goes to the neurons of the visual cortex. Neurons in the visual cortex, like neurons at lower levels, have receptive fields. The structure of the receptive fields of neurons in the visual cortex differs from the receptive fields of the NKT and retinal cells. Hubel and Wiesel also studied the visual cortex. Their work made it possible to create a classification of receptive fields of neurons in the visual cortex (RPNFrK). H. and V. Found that RPNZrK have not concentric, but rectangular shape. They can be oriented at different angles and have 2 or 3 antagonistic zones.
Such a receptive field can highlight:
1. change in illumination, contrast - such fields were called simple receptive fields;
2. neurons with complex receptive fields– can select the same objects as simple neurons, but these objects can be located anywhere in the retina;
3. super complex fields- can highlight objects that have breaks, boundaries or changes in the shape of the object, i.e. super complex receptive fields can highlight geometric shapes.
Gestalts are neurons that highlight sub-images.
Cells of the visual cortex can only form certain elements of the image. Where does constancy come from, where does the visual image appear? The answer was found in association neurons, which are also associated with vision.
The visual system can distinguish different color characteristics. The combination of opposing colors allows you to highlight various shades. Lateral inhibition is necessarily involved.
Receptive fields have antagonistic zones. Neurons of the visual cortex are able to be excited peripherally to green while the middle is excited to the action of a red source. The action of green will cause an inhibitory reaction, the action of red will cause an excitatory reaction.
The visual system perceives not only pure spectral colors, but also any combination of shades. Many areas of the cerebral cortex have not only a horizontal, but also a vertical structure. This was discovered in the mid-1970s. This has been shown for the somatosensory system. Vertical or columnar organization. It turned out that the visual cortex, in addition to layers, also has vertically oriented columns. Improvements in recording techniques have led to more sophisticated experiments. Neurons of the visual cortex, in addition to layers, also have a horizontal organization. A microelectrode was placed strictly perpendicular to the surface of the cortex. All major visual fields are in the medial occipital cortex. Since receptive fields have a rectangular organization, dots, spots, or any concentric objects do not cause any reaction in the cortex.
The column is the type of reaction, the adjacent column also highlights the slope of the line, but it differs from the previous one by 7-10 degrees. Further research showed that there are columns located nearby in which the angle changes in equal increments. About 20-22 adjacent columns will highlight all tilts from 0 to 180 degrees. The set of columns capable of highlighting all gradations of this characteristic is called a macrocolumn. These were the first studies that showed that the visual cortex can highlight not only a single property, but also a complex - all possible changes in a feature. In further studies, it was shown that next to the macrocolumns that fix the angle, there are macrocolumns that can highlight other properties of the image: colors, direction of movement, speed of movement, as well as macrocolumns associated with the right or left retina (ocular dominance columns). Thus, all macrocolumns are compactly located on the surface of the cortex. It was proposed to call collections of macrocolumns hypercolumns. Hypercolumns can analyze a set of image features located in a local region of the retina. Hypercolumns are a module that highlights a set of features in a local area of the retina (1 and 2 identical concepts).
Thus, the visual cortex consists of a set of modules that analyze the properties of images and create subimages. The visual cortex is not the final stage of processing visual information.
Properties of binocular vision (stereo vision)
These properties make it easier for both animals and humans to perceive the distance of objects and the depth of space. In order for this ability to manifest itself, eye movements (convergent-divergent) to the central fovea of the retina are required. When considering a distant object, the optical axes move apart (divergence) and converge for nearby ones (convergence). This binocular vision system is represented by different types animals. This system is most perfect in those animals whose eyes are located on the front surface of the head: in many predatory animals, birds, primates, most predatory monkeys.
In other animals, the eyes are located laterally (ungulates, mammals, etc.). It is very important for them to have a large volume of perception of space.
This is due to the habitat and their place in the food chain (predator - prey).
With this method of perception, perception thresholds are reduced by 10-15%, i.e. Organisms with this property have an advantage in the accuracy of their own movements and their correlation with the movements of the target.
Monocular cues to spatial depth also exist.
Properties of binocular perception:
1. Fusion - fusion of completely identical images of two retinas. In this case, the object is perceived as two-dimensional, flat.
2. Fusion of two non-identical retinal images. In this case, the object is perceived three-dimensionally, three-dimensionally.
3. Rivalry of visual fields. There are two different images coming from the right and left retinas. The brain cannot combine two different images, and therefore they are perceived alternately.
The remaining points of the retina are disparate. The degree of disparity will determine whether the object is perceived three-dimensionally or whether it will be perceived with competing visual fields. If the disparity is small, then the image is perceived three-dimensionally. If the disparity is very high, then the object is not perceived.
Such neurons were found not in the 17th, but in the 18th and 19th fields.
How do the receptive fields of such cells differ: for such neurons in the visual cortex, the receptive fields are either simple or complex. In these neurons, there is a difference in receptive fields from the right and left retina. The disparity of the receptive fields of such neurons can be either vertical or horizontal (see next page):
This property allows for better adaptation.
(+) The visual cortex does not allow us to say that a visual image is formed in it, then there is no constancy in all areas of the visual cortex.
Related information.
It has been established that although most of the fibers of the optic tract end in the external geniculate body, a small part of them goes to the pulvinar and the anterior quadrigeminal. These anatomical data served as the basis for the opinion, widespread for a long time, according to which both the external geniculate body and the pulvinar and anterior quadrigemale were considered primary visual centers.
At present, a lot of data has accumulated that does not allow us to consider the pulvinar and anterior quadrigemina as the primary visual centers.
A comparison of clinical and pathological data, as well as embryological and comparative anatomy data, does not allow us to attribute the role of the primary visual center to the pulvinar. Thus, according to Genshen’s observations, in the presence of pathological changes in the pulvinar, the visual field remains normal. Brouwer notes that with a changed lateral geniculate body and an unchanged pulvinar, homonymous hemianopsia is observed; with changes in the pulvinar and unchanged external geniculate body, the visual field remains normal.
The situation is similar with anterior quadrigeminal. The fibers of the optic tract form the visual layer in it and end in cell groups located near this layer. However, Pribytkov's experiments showed that enucleation of one eye in animals is not accompanied by degeneration of these fibers.
Based on everything stated above, there is currently reason to believe that only the lateral geniculate body is the primary visual center.
Turning to the question of the projection of the retina in the lateral geniculate body, it is necessary to note the following. Monakov in general denied the presence of any retinal projection in the lateral geniculate body. He believed that all fibers coming from different parts of the retina, including papillomacular ones, are evenly distributed throughout the external geniculate body. Genshen proved the fallacy of this view back in the 90s of the last century. In 2 patients with homonymous lower quadrant hemianopia, during postmortem examination, he found limited changes in the dorsal part of the lateral geniculate body.
Ronne, in atrophy of the optic nerves with central scotomas due to alcohol intoxication, found limited changes in ganglion cells in the external geniculate body, indicating that the area of the macula is projected onto the dorsal part of the geniculate body.
The above observations undoubtedly prove the presence of a certain projection of the retina in the lateral geniculate body. But the clinical and anatomical observations available in this regard are too few in number and do not yet provide accurate ideas about the nature of this projection. Mentioned by us experimental studies Brouwer and Zeman's experiments in monkeys made it possible to study to some extent the projection of the retina in the lateral geniculate body. They found that most of the external geniculate body is occupied by the projection of the retinal sections involved in the binocular act of vision. The extreme periphery of the nasal half of the retina, corresponding to the monocularly perceived temporal crescent, is projected onto a narrow zone in the ventral part of the lateral geniculate body. The projection of the macula occupies large plot in the dorsal part. The superior quadrants of the retina project ventromedially to the lateral geniculate body; lower quadrants - ventro-lateral. The projection of the retina in the external geniculate body in a monkey is shown in Fig. 8.
In the external geniculate body (Fig. 9)
Rice. 9. The structure of the external geniculate body (according to Pfeiffer).
there is also a separate projection of crossed and uncrossed fibers. The research of M. Minkowski makes a significant contribution to clarifying this issue. He found that in a number of animals after enucleation of one eye, as well as in humans with prolonged one-sided blindness, there are observed in the external geniculate body atrophy of optic nerve fibers and ganglion cell atrophy. Minkowski discovered at the same time characteristic feature: in both geniculate bodies, atrophy spreads with a certain pattern to various layers of ganglion cells. In the external geniculate body of each side, layers with atrophied ganglion cells alternate with layers in which the cells remain normal. The atrophic layers on the enucleation side correspond to identical layers on the opposite side, which remain normal. At the same time, similar layers that remain normal on the side of enucleation atrophy on the opposite side. Thus, the atrophy of the cell layers in the external geniculate body that occurs after enucleation of one eye is definitely alternating in nature. Based on his observations, Minkowski came to the conclusion that each eye has a separate representation in the external geniculate body. The crossed and uncrossed fibers thus terminate at different layers of ganglion cells, as is well depicted in Le Gros Clark's diagram (Fig. 10).
Rice. 10. Diagram of the end of the fibers of the optic tract and the beginning of the fibers of the Graziole bundle in the external geniculate body (according to Le Gros Clark).
Solid lines are crossed fibers, broken lines are uncrossed fibers. 1 - visual tract; 2 - external geniculate body 3 - Graziole bundle; 4 - occipital lobe cortex.
Minkowski's data were subsequently confirmed by experimental and clinical-anatomical works of other authors. L. Ya. Pines and I. E. Prigonnikov examined the external geniculate body 3.5 months after enucleation of one eye. At the same time, in the external geniculate body on the side of enucleation, degenerative changes were noted in the ganglion cells of the central layers, while the peripheral layers remained normal. On the opposite side of the lateral geniculate body, the opposite relationships were observed: the central layers remained normal, while degenerative changes were noted in the peripheral layers.
Interesting observations related to the case one-sided blindness a long time ago, was recently published by the Czechoslovakian scientist F. Vrabeg. A 50-year-old patient had one eye removed at the age of ten. Pathological examination of the external geniculate bodies confirmed the presence of alternating degeneration of ganglion cells.
Based on the above data, it can be considered established that both eyes have separate representation in the external geniculate body and, therefore, crossed and uncrossed fibers end in different layers of ganglion cells.
Table of contents of the topic "Receptor potential of rods and cones. Receptive fields of retinal cells. Pathways and centers of the visual system. Visual perception.":Ganglion cell axons form topographically organized connections with neurons of the lateral geniculate body, which are represented by six layers of cells. The first two layers, located ventrally, consist of magnocellular cells that have synapses with M cells of the retina, with the first layer receiving signals from the nasal half of the retina of the contralateral eye, and the second from the temporal half of the ipsilateral eye. The remaining four layers of cells, located more dorsally, receive signals from P-cells of the retina: the fourth and sixth - from the nasal half of the retina of the contralateral eye, and the third and fifth - from the temporal half of the retina of the ipsilateral eye. As a result of this organization of afferent inputs in each lateral geniculate body, i.e. left and right, six neural maps of the opposite half of the visual field are formed, located exactly one above the other. Neuronal maps are organized retinotopically, in each of them about 25% of cells receive information from the photoreceptors of the fovea.
Receptive fields of neurons in the lateral geniculate body have a rounded shape with on- or off-type centers and a periphery antagonistic to the center. Converges to each neuron a small amount of axons of ganglion cells, and therefore the nature of the information transmitted to the visual cortex here almost does not change. Signals from the parvocellular and magnocellular cells of the retina are processed independently of each other and transmitted to the visual cortex along parallel pathways. Neurons lateral geniculate body receive no more than 20% of afferent inputs from the retina, and the remaining afferents are formed mainly by neurons of the reticular formation and cortex. These entrances to lateral geniculate body regulate the transmission of signals from the retina to the cortex.
