Transformation of visual signals in the lateral (outer) geniculate body. Outer geniculate body

The optic nerve fibers start from each eye and end on the cells of the right and left lateral geniculate body (LCB) (Fig. 1), which has a clearly distinguishable layered structure (“geniculate” - geniculate - means “curved like a knee”). In the LCT of a cat, three distinct, well-defined cell layers (A, A 1 , C) can be seen, one of which (A 1) has a complex structure and is further subdivided. In monkeys and other primates, including

Rice. 1. Lateral geniculate body (LCB). (A) Cat LCT has three cell layers: A, A, and C. (B) Monkey LCT has 6 major layers, including small cell (parvocellular), or C (3, 4, 5, 6), large cell (magnocellular ), or M (1, 2) separated by koniocellular layers (K). In both animals, each layer receives signals from only one eye and contains cells that have specialized physiological properties.

human, LKT has six layers of cells. Cells in deeper layers 1 and 2 are larger than in layers 3, 4, 5 and 6, which is why these layers are called large-celled (M, magnocellular) and small-celled (P, parvocellular), respectively. The classification also correlates with large (M) and small (P) retinal ganglion cells, which send their outgrowths to the LCT. Between each M and P layers lies a zone of very small cells: the intralaminar, or koniocellular (K, koniocellular) layer. Layer K cells differ from M and P cells in their functional and neurochemical properties, forming a third channel of information to the visual cortex.

In both the cat and the monkey, each layer of the LCT receives signals from either one eye or the other. In monkeys, layers 6, 4, and 1 receive information from the contralateral eye, and layers 5, 3, and 2 from the ipsilateral eye. The separation of the course of nerve endings from each eye into different layers has been shown using electrophysiological and a number of anatomical methods. Particularly surprising is the type of branching of an individual fiber of the optic nerve when horseradish peroxidase is injected into it (Fig. 2).

The formation of terminals is limited to the layers of the LCT for this eye, without going beyond the boundaries of these layers. Due to the systematic and specific division of the optic nerve fibers in the region of the chiasm, all the receptive fields of the LCT cells are located in the visual field of the opposite side.

Rice. 2. Endings of the optic nerve fibers in the LCT of a cat. Horseradish peroxidase was injected into one of the axons from the zone with the "on" center of the contralateral eye. Axon branches end on cells of layers A and C, but not A1.

Rice. 3. Receptive fields of ST cells. The concentric receptive fields of the LCT cells resemble the fields of ganglion cells in the retina, dividing into fields with "on" and "off" centers. The responses of the cell with the "on" center of the LCT of a cat are shown. The bar above the signal shows the duration of illumination. Central and peripheral the zones offset each other's effects, so diffuse illumination of the entire receptive field gives only weak responses (bottom notation), even less pronounced than in retinal ganglion cells.

This is the subcortical center, which ensures the transmission of information already in the visual cortex.

In humans, this structure has six layers of cells, as in the visual cortex. Fibers from the retina come crossed and uncrossed to the chiasma opticus. 1st, 4th, 6th layers receive crossed fibers. 2nd, 3rd, 5th layers are received uncrossed.

All information coming to the lateral geniculate body from the retina is ordered and the retinotopic projection is preserved. Since the fibers enter the lateral geniculate body in a comb-like manner, 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 NKT neurons. Fibers from M-cells and P-cells enter the tubing. 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 the 3rd, 4th, 5th, 6th layers. In the 1st and 2nd layers of the tubing, the receptive fields are highly sensitive to motion and do not distinguish spectral characteristics (color). Such receptive fields are also present in small amounts 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. In the 5th and 6th layers, neurons with ON centers are mainly red-green. The receptive fields of the cells of the lateral geniculate body have the same receptive fields as the ganglion cells.

The difference between these receptive fields and ganglion cells:

1. In the sizes of receptive fields. The cells of the lateral geniculate body are smaller.

2. Some neurons of the NKT 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 the neurons of the NKT. These layers are the basis for the survival of a particular species. Humans have six layers, predators have four.

Detector theory appeared in the late 1950s. In the frog 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 made it possible to create the concept, according to which the image presented on the retina is processed by ganglion cells specifically tuned to image elements. Such ganglion cells have a specific dendritic branching that corresponds to a certain structure of the receptive field. Several types of such ganglion cells have been found. Subsequently, neurons with this property were called detector neurons. Thus, a detector is a neuron that reacts to a certain 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. A moving small contrast detector - its excitation led to an attempt to capture this object; in contrast corresponds to the captured objects; 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 highlight certain elements of the environment.

A group of researchers working on this topic: Letvin, Maturano, Mokkalo, Pitz.

Neurons of other sensory systems also have detector properties. Most detectors in the visual system are associated with motion detection. Neurons have increased reactions with an increase in the speed of movement of objects. Detectors have been found in both birds and mammals. The detectors of other animals are directly connected to the surrounding space. The birds were found to have horizontal surface detectors, which is associated with the need to land on horizontal objects. Detectors of vertical surfaces have also been found, which provide the birds' own movements towards these objects. It turned out that the higher the animal in the evolutionary hierarchy, the higher the detectors are, i.e. these neurons can 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, the detectors are located in the visual cortex. This is important because the specific way that provides responses to elements of the external environment is transferred to the higher levels of the brain, and at the same time, each animal species has its own specific types of detectors. Later it turned out that in ontogenesis the detector properties of sensory systems are formed under the influence of the environment. To demonstrate this property, experiments were done by researchers, Nobel laureates, Hubel and Wiesel. Experiments were carried out that proved that the formation of detector properties occurs in the earliest ontogeny. 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 was placed in conditions where there were mostly horizontal lines. The researchers tested which neurons formed in the cortex of each group of kittens. In the cortex of these animals, there were 50% of neurons that were activated both horizontally and 50% vertically. Animals brought up in a horizontal environment had a significant number of neurons in the cortex that were activated by horizontal objects, there were practically no neurons that were activated when perceiving vertical objects. In the second experimental group there was a similar situation with horizontal objects. The kittens of both horizontal groups had certain defects. Kittens in a horizontal environment could perfectly jump on steps and horizontal surfaces, but did not perform well relative to vertical objects (table leg). The kittens of the second experimental group had a corresponding situation for vertical objects. This experiment proved:

1) formation of neurons in early ontogenesis;

2) the animal cannot adequately interact.

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 go to 4 layers of the visual cortex. The bulk of the axons of the lateral geniculate body (NKT) is 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 LNT. All information from the retina goes to the neurons of the visual cortex. The neurons of the visual cortex, like the neurons of the underlying 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 the receptive fields of neurons in the visual cortex (RPNZrK). H. and V. found that RPNZrK are not concentric, but rectangular in shape. They can be oriented at different angles, 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 allocate the same objects as simple neurons, but these objects can be located anywhere in the retina;

3. supercomplex fields- can select objects that have gaps, borders or changes in the shape of the object, i.e. highly complex receptive fields can highlight geometric shapes.

Gestalts are neurons that highlight subimages.

The 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 various color characteristics. The combination of opponent colors allows you to highlight different shades. Lateral inhibition is required.

Receptive fields have antagonistic zones. The neurons of the visual cortex are able to fire peripherally to green, while the middle is fired 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 in addition to the layers, the visual cortex also has vertically oriented columns. Improvement in registration techniques led to more subtle experiments. The neurons of the visual cortex, in addition to the layers, also have a horizontal organization. A microelectrode was passed strictly perpendicular to the surface of the cortex. All major visual fields are in the medial occipital cortex. Since the receptive fields have a rectangular organization, dots, spots, any concentric object does not cause any reaction in the cortex.

Column - 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 studies have shown that columns are located nearby, in which the angle changes with an equal step. About 20-22 adjacent columns will highlight all slopes from 0 to 180 degrees. The set of columns capable of highlighting all the gradations of this feature 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 trait. In further studies, it was shown that next to the macrocolumns that fix the angle, there are macrocolumns that can highlight other image properties: colors, direction of movement, speed of movement, as well as macrocolumns associated with the right or left retina (columns of eye dominance). Thus, all macrocolumns are compactly located on the surface of the cortex. It was proposed to call sets of macrocolumns hypercolumns. Hypercolumns can analyze a feature set of images located in a local area of the retina. Hypercolumns is a module that highlights a set of features in a local area of the retina (1 and 2 are 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 in the processing of visual information.

Properties of binocular vision (stereo vision)

These properties make it easier for both animals and humans to perceive the remoteness 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, there is a separation (divergence) of the optical axes and convergence for closely spaced ones (convergence). Such a system of binocular vision is presented in different animal species. This system is most perfect in those animals in which the eyes are located on the frontal surface of the head: in many predatory animals, birds, primates, most predatory monkeys.

In another part of the 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, the 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.

There are also monocular signs of the depth of space.

Properties of binocular perception:

1. Fusion - the fusion of completely identical images of two retinas. In this case, the object is perceived as two-dimensional, planar.

2. Fusion of two non-identical retinal images. In this case, the object is perceived three-dimensionally.

3. Rivalry of visual fields. Two different images come from the right and left retinas. The brain cannot combine two different images, and therefore they are perceived alternately.

The rest of the retinal points are disparate. The degree of disparity will determine whether the object is perceived three-dimensionally or whether it will be perceived with rivalry of fields of view. If the disparity is low, 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.

What is the difference between the receptive fields of such cells: 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 retinas. 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 constancy is absent in all areas of the visual cortex.

Similar information.

Outer geniculate body

Outer geniculate body (corpus geniculatum laterale) is the location of the so-called "second neuron" of the visual pathway. About 70% of the fibers of the optic tract pass through the lateral geniculate body. The external geniculate body is a hill corresponding to the location of one of the nuclei of the thalamus opticus (Fig. 4.2.26-4.2.28). It contains about 1,800,000 neurons, on the dendrites of which the axons of the ganglion cells of the retina end.

Previously, it was assumed that the lateral geniculate body is only a "relay station", transmitting information from retinal neurons through the optic radiation to the cerebral cortex. It has now been shown that quite significant and diverse processing of visual information takes place at the level of the lateral geniculate body. The neurophysiological significance of this formation will be discussed below. Initially, you need

Rice. 4.2.26. Model of the left external geniculate body (according to Wolff, 1951):

a- rear and inside view; b - rear and outside view (/ - optic tract; 2 - saddle; 3 - visual radiance; 4 - head; 5 - body; 6 - isthmus)

dimo to dwell on its anatomical features.

The nucleus of the lateral geniculate body is one of the nuclei of the optic tubercle. It is located between the ventroposterior lateral nucleus of the thalamus and the thalamus pad (Fig. 4.2.27).

The external geniculate nucleus consists of the dorsal and phylogenetically older ventral nuclei. The ventral nucleus in humans is preserved as a rudiment and consists of a group of neurons located rostral to the dorsal nucleus. In lower mammals, this nucleus provides the most primitive photostatic reactions. The fibers of the optic tract do not fit into this nucleus.

The dorsal nucleus makes up the main part of the nucleus of the lateral geniculate body. It is a multilayer structure in the form of a saddle or an asymmetric cone with a rounded top (Fig. 4.2.25-4.2.28). The horizontal section shows that the lateral geniculate body is connected anteriorly with the optic tract, laterally with the retrolenticular part of the internal capsule, medially with the middle geniculate body, posteriorly with the hippocampal gyrus, and posteriolaterally with the inferior horn of the lateral ventricle. The cushion of the thalamus is adjacent to the nucleus of the lateral geniculate body from above, anterio-laterally - temporopontine fibers and the posterior part of the internal capsule, laterally - Wernicke's area, and on the inside - the medial nucleus (Fig. 4.2.27). Wernicke's area is the innermost part of the internal capsule. It is in it that visual radiance begins. The fibers of the optic radiation are located on the dorsolateral side of the nucleus of the external geniculate body, while the fibers of the auditory tract are located on the dorsomedial side.

■ .■. ■>

Rice. 4.2.27. External geniculate body and its relation to brain structures:

a- horizontal section of the brain (/ - external geniculate body; 2 - internal capsule; 3 -cushion of the thalamus); b - sagittal section of the brain (histological section stained with hematoxylin and eosin) (NKT- external geniculate body)

The lateral geniculate body is connected to the superior quadrigemina by a ligament called the anterior humerus.

Even with a macroscopic examination of the external geniculate body, it is revealed that this formation has a layered structure. In monkeys and humans, six bands of “gray matter” are clearly distinguished and “white” layers located between them, consisting of axons and dendrites (Fig. 4.2.28). The first layer denotes the layer located on the ventral side. The two inner layers are composed of large cells (magnocellular layers 1 and 2). They got this name

Rice. 4.2.28. Outer geniculate body:

/ - hippocampus; 2 - subarachnoid space; 3 - leg of the brain; 4 - layer 1; 5 - layer 2; 6 - lower horn of the lateral ventricle; 7 - layer 3; 8 - layer 4; 9 - layer 5; 10 - layer 6. The lateral geniculate body is the core of the thalamus. The presence of six dark layers of clusters of neurons is clearly visible, separated by light layers consisting of nerve fibers. Layers 1 and 2 are made up of large neurons (magnocellular), and layers 3-6 are made up of small cells (parvocellular)

for the reason that they consist of large neurons with an eccentrically located nucleus and a large amount of Nissl substance in the cytoplasm. The axons of the neurons of the magnocellular layer form not only visual radiation, but also go to the superior tubercles of the quadrigemina. The four outer layers are composed of small to medium sized cells (parvocellular layers, 3-6). They contain neurons that receive information from the retina and transmit it only to the visual cortex of the brain (they form visual radiation). Neurons are also found that provide communication between the neurons of the lateral geniculate body. These are the so-called "intercalary neurons" (interneurons). It is believed that two layers consisting of small neurons (parvocellular layers) appear in connection with the development of central vision.

It is important to note that the fibers coming from different parts of the retina of both eyes are projected onto the listed layers of neurons. So, the crossed fibers of the optic tract end in the 1st, 4th and 6th layers, and the non-crossed ones - in the 2nd, 3rd and 5th (Fig. 4.2.29). This happens in such a way that the fibers from the corresponding parts of the two halves of the retina (for example, the right temporal and left nasal halves of the retina) terminate in adjacent layers. The given features of the projection on the lateral geniculate body are established based on the use of various methods.

Chapter 4 BRAIN AND EYE

Rice. 4.2.29. Representation of the retina in the lateral geniculate body:

Impulses from Corresponding Points (a, b) two retinas pass into the optic tract. Non-crossed fibers (a") end in the 2nd, 3rd and 5th layers of the lateral geniculate body. Crossed fibers (b") end in layers 1, 4 and 6. Pulses after passing NKT(c") are projected onto the cerebral cortex

research. Thus, in cases of destruction of the contralateral optic nerve or prior removal of the eyeball, degeneration of neurons in the 1st, 4th, and 6th layers of the lateral geniculate body develops (Fig. 4.2.30). With the destruction of the homolateral fibers of the optic nerve, degeneration of neurons of the 2nd, 3rd and 5th layers occurs. This phenomenon is called transsynaptic degeneration. It has also been established that if the eyelids of one eye are sewn together at birth, then in three months 25-40% of the neurons of the lateral geniculate body will degenerate. A similar form of transsynaptic degeneration can explain some of the mechanisms of development of amblyopia that develops with congenital strabismus.

Experimental studies also testify to the different projection on the lateral geniculate body of crossed and non-crossed fibers. In these studies, a radioactive amino acid is injected into one of the eyeballs, spreading transaxonally in the direction of the lateral geniculate body and accumulating in its neurons (Fig. 4.2.31).

Rice. 4.2.31. The distribution of the radioactive label in the external geniculate bodies after the introduction of a radioactive amino acid into the left eyeball of a monkey:

a- left external geniculate body; b - right external geniculate body. (The amino acid is taken up by retinal ganglion cells and transported along the axons via the optic nerve, optic chiasm, and optic tract to the lateral geniculate body. The illustration indicates that layers 2, 3, and 5 receive information from the ipsilateral eye, while layers 1, 4 and 6 - from the contralateral eye)

Rice. 4.2.30. Changes in the microscopic structure of the lateral geniculate body on both sides when one eyeball is removed (according to Alvord, Spence, 1997):

a- lateral geniculate body (NKT), located ipsilaterally relative to the enucleated eye; b- tubing located contralateral to the enucleated eye. (After the death of a patient whose eyeball was removed long before death, the external geniculate bodies were examined microscopically. After a violation of the normal projection of the ganglionic cells of the retina on the neurons of the NKT, atrophy of the latter occurs. At the same time, the intensity of staining of the layers decreases. The figure shows that 3- the th and 5th layers of the NKT, located ipsilateral to the removed eye, are much weaker stained with hematoxylin and eosin.At the same time, layers 3 and 5 of the NKT, located contralateral to the removed eye, are stained more intensely than layers 4 and 6. It can also be noted that layers 1 and 2 are least affected)

Functional anatomy of the visual system

Features of the projection of the retina on the external geniculate body. Recently, features of the projection of the retina on the external geniculate body have been revealed. They come down to the fact that each point of the half of the retina is accurately projected onto a certain point of the nucleus of the external geniculate body (“point to point”). Thus, spatial excitation in the layer of retinal ganglion cells is "mapped" by the spatial distribution of neuronal excitation in different layers of the lateral geniculate body. A strict topographic order of connections is also observed between cells of different layers. The projections of each point of the field of view in all layers are directly below one another, so that a columnar area can be distinguished that intersects all layers of the lateral geniculate body and corresponds to the projection of the local area of the field of view.

The given pattern of projection was revealed on the basis of experimental studies. Thus, it has been shown that local point damage to the retina leads to the development of transneuronal degeneration of small, but clearly defined clusters of cells in three layers of the lateral geniculate body on both sides. Focal damage to the visual cortex or the introduction of a radioactive tracer into it results in "marking" cells or fibers located in a line extending across all layers of the lateral geniculate body at the same level. These areas correspond to the "receptive fields" of the lateral geniculate body and are called the "projection column" (Fig. 4.2.32).

At this point in the presentation of the material, it is advisable to dwell on the features of the receptive fields of the lateral geniculate body. The receptive fields of the lateral geniculate body resemble those of the ganglion cells of the retina. There are several basic types of receptive fields. The first type is characterized by the presence of an ON-response upon excitation of the center and an OFF-response upon excitation of the periphery (ON/OFF-type). The second type of receptive fields is characterized by an inverse relationship - OFF/ON-type. The lateral geniculate body is also characterized by the fact that in layers 1 and 2 a mixture of receptive fields of the first and second types is found. At the same time, only one type of receptive fields is found in layers 3-6 (fields of the first type in two layers, and fields of the second type in the other two). Linear receptive fields with different ratios of ON and OFF centers are also found (Fig. 4.2.33). The use of electrophysiological methods made it possible to reveal that the receptive fields of the lateral geniculate body have a more pronounced opponent reaction than the receptive fields of the ganglion cells of the network.

Lateral

Rice. 4.2.32. Schematic representation of the parasagittal-

tal cut of the outer geniculate body. Projection

visual signal with the formation of a receptive

1t*- Rear

* * *Z* x

Rice. 4.2.33. The structure of the receptive fields of the lateral geniculate body (a, b) and primary visual cortex (c-g) (according to Hubel, Weisel, 1962):

a- ON-center receptive field of the lateral geniculate body; b- OFF-center receptive field of the lateral geniculate body; in-and- various variants of the structure of simple receptive fields. (The crosses mark the fields corresponding to the ON-reaction, and the triangles to the OFF-reaction. The axis of the receptive field is marked by a solid line passing through the center of the receptive field)

chats. This is what predetermines the great importance of the external geniculate body in enhancing the contrast. The phenomena of spatiotemporal summation of incoming signals, analysis of the spectral characteristics of the signal, etc., have also been revealed.

Chapter 4. BRAIN AND EYE

The cells are "red-green" and "blue-yellow". Like retinal ganglion cells, they are characterized by a linear summation of cone signals over the area of the retina. Magnocellular layers also consist of opponent neurons with inputs from cones of different types spatially distributed in receptive fields. It should be noted that the anatomical segregation of neurons with different functional properties is already observed in the retina, where the processes of bipolar and ganglion cells of ON- and OFF-types are localized in different sublayers of the inner plexiform layer. Such an "anatomical isolation" of neuronal systems that form different channels of information transmission is a general principle in the construction of analyzer structures and is most pronounced in the columnar structure of the cortex, which we will discuss below.

Retina

the outer part of the outer geniculate body (Fig. 4.2.29). The macular area of the retina is projected onto the wedge-shaped sector located in the posterior two-thirds or three-quarters of the lateral geniculate body (Fig. 4.2.34, 4.2.35).

It is noted that the representation of the visual hemifields in the optic tract, as it were, "turns" at the level of the lateral geniculate body in such a way that the vertical section becomes horizontal. In this case, the upper part of the retina is projected onto the medial part, and the lower part onto the lateral part of the lateral geniculate body. This rotation reverses in visual radiation such that when the fibers reach the visual cortex, the upper quadrant of the retina is at the top of the tract and the lower quadrant is at the bottom.

Outer geniculate body

Rice. 4.2.34. Projection of the retina on the lateral geniculate body: / - macula; 2 - monocular crescent

h p Yaa

Continuing the description of the features of the projection of the retina onto the lateral geniculate body, it should be noted that the peripheral temporal areas of the retina of the opposite eye are projected onto layers 2, 3, and 5 and are called the monocular crescent.

The most complete data on the retinotopic organization of the fibers of the optic nerve, optic chiasm, and nuclei of the lateral geniculate body in humans and monkeys were obtained by Brouewer, Zeeman, Polyak, Hoyt, Luis. We will first describe the projection of non-macular fibers. Non-crossing fibers coming from the superior temporal quadrant of the retina are located dorso-medially in the optic chiasm and project onto the medial part of the nucleus of the lateral geniculate body. Non-crossing fibers coming from the lower temporal quadrant of the retina are located in the optic chiasm from below and laterally. They are projected onto

Rice. 4.2.35. Schematic representation of the coronary

slice through the lateral geniculate body (posterior view)

(according to Miller, 1985):

noteworthy is the large representation in the lateral geniculate body of the macular region (1-6-numbers of layers of the NKT)

Functional anatomy of the visual system

Synaptic interactions of neurons of the lateral geniculate body. Previously, it was assumed that the ganglion cell axon contacts only one neuron of the lateral geniculate body. Thanks to electron microscopy, it was found that afferent fibers form synapses with several neurons (Fig. 4.2.36). At the same time, each neuron of the lateral geniculate body receives information from several retinal ganglion cells. Based on ultrastructural studies, various synaptic contacts between them have also been revealed. Axons of ganglion cells can end both on the body of neurons of the lateral geniculate body, and on their primary or secondary dendrites. In this case, the so-called "glomerular" endings are formed (Fig. 4.2.37, see col. on). In cats, the "glomeruli" are separated from the surrounding formations by a thin capsule made up of outgrowths of glial cells. Such isolation of "glomeruli" is absent in monkeys.

Synaptic "glomerules" contain synapses of axons of retinal ganglion cells, synapses of neurons of the lateral geniculate body and interneurons ("interneurons"). These synaptic formations resemble the "triads" of the retina.

Each "glomerulus" consists of a zone of densely packed neurons and their terminals. In the center of this zone is the axon of the ganglionic

Rice. 4.2.36. Schematic representation of the interaction of axon terminals of retinal ganglion cells with neurons of the lateral geniculate body in a monkey (according to Glees, Le Gros, Clark, 1941):

bundle of optic nerve fibers (a) enters the cell layer (b) of the lateral geniculate body (NKT) on the right. Some fibers give off 5-6 branches, approach the body of NKT neurons and form a synapse. The axons of the NKT cells (c) leave the cellular layer of the NKT, pass through the fibrous layer and form visual radiation.

retinal cells, which is presynaptic. It forms synapses with the lateral geniculate neuron and interneurons. The dendrites of the neurons of the lateral geniculate body enter the "balls" in the form of a spike, which directly forms a synapse with the retinal axon. The dendrite of interneurons (interneurons) forms a synapse with an adjacent "glomerulus", forming successive synapses between them.

Lieberman identifies pre- and postsynaptic "inhibitory" and "excitatory" dendritic and "glomerular" synapses. They are a complex cluster of synapses between axons and dendrites. It is these synapses that structurally provide the phenomenon of inhibition and excitation of the receptive fields of the lateral geniculate body.

Functions of external crankshaft body. It is assumed that the functions of the lateral geniculate body include: enhancing image contrast, organizing visual information (color, movement, shape), modulating the level of visual information processing with their activation (through the reticular formation). Possesses lateral geniculate body and binocular receptive fields. It is important to note that the functions of the lateral geniculate body are also influenced by higher located centers of the brain. Confirmation of the role of the lateral geniculate body in the processing of information coming from the higher parts of the brain is the detection of a projection onto it efferent fibers, coming from the cerebral cortex. They arise in the VI layer of the visual cortex and are projected onto all layers of the external geniculate body. For this reason, minor damage to the visual cortex causes neuronal atrophy in all six layers of the lateral geniculate body. The terminals of these fibers are small and contain numerous synaptic vesicles. They end both on the dendrites of the neurons of the external geniculate body, and on the intercalary neurons ("interneurons"). It is assumed that through these fibers the cerebral cortex modulates the activity of the lateral geniculate body. On the other hand, changes in the activity of neurons in the lateral geniculate body have been shown to selectively activate or inhibit neurons in the visual cortex.

There are other connections of the lateral geniculate nucleus. It is a connection with the thalamus thalamus, ventral and lateral thalamus nuclei.

Blood supply to the lateral geniculate body carried out by the posterior cerebral and posterior villous arteries (Fig. 4.2.38). The main vessel supplying the external geniculate body, especially its posterior-not-internal surface, is the posterior

Chapter 4. BRAIN AND EYE

90 80 70 60150 40 30 20-10

Rice. 4.2.38. Arterial blood supply to the surface of the lateral geniculate body:

/ - anterior villous (choroidal) artery; 2 - villous plexus; 3 - leg of the brain; 4 - the gate of the outer cranked body; 5 - outer cranked body; 6 - medial geniculate body; 7 - oculomotor nerve; 8 - the nucleus of the oculomotor nerve; 9 - posterior cerebral artery; 10 - posterior villous artery; // - black substance

nyaya cerebral artery. In some cases, a branch departs from this artery - the posterior villous (choroidal) artery. In violation of blood circulation in this artery, violations of the field of the upper homonymous quadrant of the retina are detected.

The anterior villous (choroidal) artery almost completely supplies the anterior and lateral surfaces of the lateral geniculate body. For this reason, a violation of blood circulation in it leads to damage to the fibers emanating from the lower quadrant of the retina (Fig. 4.2.39). This artery arises from the internal carotid artery (sometimes from the middle cerebral artery) just distal to the exit of the posterior communicating artery. Upon reaching the anterior part of the lateral geniculate body, the anterior villous artery gives off a different number of branches before entering the inferior horn of the lateral ventricle.

The part of the lateral geniculate body, onto which fibers emanating from the macula are projected, is supplied by both the anterior and posterior villous arteries. In addition, from a well-developed system of anastomoses located in the pia mater and arachnoid, numerous arterioles extend into the lateral geniculate body. There they form a dense network of capillaries in all its layers.

^--^--^ Horizontal meridian of the visual field - - - - - Inferior oblique meridian of the visual field

I I Territory of the anterior choroidal artery VIV Territory of the external choroidal artery

Rice. 4.2.39. Scheme of the blood supply to the right external geniculate body and features of visual field loss (homonymous visual field defect), which occurs as a result of circulatory disorders in the basin of the villous (choroidal) artery (after Frisen et al., 1978):

a- retina; b- external geniculate body (/- anterior villous artery; 2 - medial surface; 3 - lateral surface; 4 - posterior villous artery; 5 - posterior artery of the brain)

Functional anatomy of the visual system

4.2.6. Visual radiance

Visual radiance (radiatio optica; gra-ciole, gratiolet) is analogous to other rays of the thalamus, such as auditory, occipital, parietal, and frontal. All of the listed radiances pass through the internal capsule connecting the hemispheres of the brain and

brain stem, spinal cord. The internal capsule is located lateral to the thalamus and lateral ventricles of the brain and medial to the lenticular nucleus (Fig. 4.2.40, 4.2.41). The posteriormost part of the internal capsule contains fibers of the auditory and visual radiation and descending fibers running from the occipital cortex to the superior tubercles of the quadrigemina.

and

Rice. 4.2.41. Horizontal section of the brain at the level of the location of the visual radiation:

/ - spur furrow; 2 - visual radiance; 3 - internal capsule; 4 - outer capsule; 5 - fourth ventricle;

6 - a plate of a transparent partition;

7 - anterior horn of the lateral ventricle; 8 -
longitudinal fissure of the brain; 9 - mozo knee
sheet body; 10 - transparent pe cavity
partitions; // - head of the caudate nucleus;
12 - fence; 13 - shell; 14 - pale
ball; 15 - visual tubercle; 16 - hippo-
camp; 17 - posterior knee of the lateral jelly
daughter

Chapter 4 BRAIN AND EYE

The optic radiation connects the lateral geniculate body to the occipital cortex. At the same time, the course of the fibers emanating from various sections of the external geniculate body differs quite significantly. So, the fibers coming from the neurons of the lateral part of the external geniculate body go around the lower horn of the lateral ventricle, located in the temporal lobe, and then, heading backwards, pass under the posterior horn of this ventricle, reaching the lower parts of the visual cortex, near the spur groove (Fig. 4.2 .40, 4.2.41). Fibers from the medial lateral geniculate body take a somewhat more direct route to the primary visual cortex (Brodmann area 17) located in the medial occipital lobe. The fibers of this path deviate laterally, passing directly anterior to the entrance to the lateral ventricle, and then turn posteriorly, go in a caudal direction, bending around the posterior horn of this ventricle from above and terminate in the cortex located along the upper edge of the spur groove.

The superior fibers leaving the lateral geniculate body go directly to the visual cortex. The inferior fibers make a loop around the ventricles of the brain (Meyer's loop) and travel to the temporal lobe. The lower fibers are closely adjacent to the sensory and motor fibers of the internal capsule. Even a small stroke that occurs in this area leads to upper hemianopic visual field defects and hemiparesis (contralateral).

The most anterior fibers are found approximately 5 h posterior to the apex of the temporal lobe. It is noted that lobectomy, in which the brain tissue is excised 4 cm from top of the temporal lobe, does not lead to visual field defects. If a larger area is damaged (deeply located tumors, temporal decompression due to injury or infectious disease), homonymous upper quadrant hemianopsia develops. The most typical forms of a visual field defect in case of damage to the visual radiation are shown in fig. 4.2.19, 4.2.43.

As stated above, optic radiation contains 3 main groups of fibers. The upper part contains fibers serving the lower fields of vision, the lower part - the upper fields. The central part contains macular fibers.

The retinotopic organization of the fibers of the lateral geniculate body also extends to the optic radiation, but with some changes in the position of the fibers (Fig. 4.2.42). The dorsal bundle of fibers, representing the upper peripheral quadrant of the retina, originates from the medial part of the lateral geniculate body and passes to the dorsal lip of the bird.

whose spurs. The ventral bundle of fibers represents the periphery of the lower quadrant of the retina. It passes in the lateral part of the external geniculate body and approaches the ventral lip of the bird's spur. It is assumed that these projections of the periphery of the retina lie in the visual radiation of the medial projection of the macular fibers. Macular fibers spread forward, occupying a large central part of the visual radiation in the form of a wedge. Then they go backwards and converge in the region of the upper and lower lips of the bird's spur.

As a result of the separation of peripheral and central projections, damage to the visual radiation can lead to quadrant dropouts in the visual field with a clear horizontal border.

The most peripherally located nasal projections of the retina, which are a "monocular crescent", are collected near the upper and lower borders of the dorsal and ventral optic radiation beams.

Violations in the field of visual radiation lead to a number of specific visual field disorders, some of which are shown in Fig. 4.2.43. The nature of the visual field loss is largely determined by the level of damage. The reason for such violations can be

Outer geniculate body

(3(3

Rice. 4.2.43. Scheme of distribution of fibers in the optic tract, lateral geniculate body and optic radiation. Violation of the visual field in case of damage to areas located after the optic chiasm:

/ - compression of the optic tract - homonymous hemianopsia with a fuzzy edge; 2 - compression of the proximal part of the optic tract, the lateral geniculate body or the lower part of the optic radiation - homonymous hemianopia without preserving the macular field with a clear edge; 3 - compression of the anterior loop of optic radiation - upper quadrant anopia with fuzzy edges; 4 - compression of the upper part of the visual radiation - lower quadrant anopia with fuzzy edges;

5 - compression of the middle part of the visual radiation - homonym
hemianopia with indistinct margins and central prolapse
th vision; 6 - compression of the back of the visual radiation -
congruent homonymous hemianopia with preservation of central
leg vision; 7 - compression of the anterior part of the cortex in the area of the spo
ry - temporal loss of the visual field from the opposite
sides; 8 - compression of the middle part of the cortex in the region of the spur -
homonymous hemianopia with central vision preserved
side of the lesion and the preservation of the temporal field of view with
opposite side; 9 - compression of the back of the cortex behind
back region - congruent homonymous hemianopsis

kaya scotoma

various diseases of the brain. Most often, this is a circulatory disorder (thrombosis, embolism in hypertension, stroke) and tumor development (glioma).

Due to the fact that a violation of the structure and function of visual radiation is often associated with impaired blood circulation, it is important to know

06 features of the blood supply to this area.
Blood supply of optic radiation

carried out at 3 levels (Fig. 4.2.24):

1. Part of the visual radiance, passing
cabbage soup laterally and above the lower horn of the lateral
ventricle, supplied by a branch of the anterior
villous (choroidal) artery.

2. Part of the visual radiance, located
vein behind and lateral to the horn of the stomach
ka, blood supply by the deep ophthalmic branch
middle cerebral artery. The last penetration

flows into this area through the anterior perforated substance together with the lateral striated arteries.

3. When visual radiation approaches the cerebral cortex, blood supply is carried out by perforating cortical arteries, mainly branches of the avian spur artery. The avian spur artery arises from the posterior cerebral artery, and sometimes from the middle cerebral artery.

All perforating arteries belong to the so-called terminal arteries.

visual cortex

As mentioned above, the neuronal systems of the retina and lateral geniculate body analyze visual stimuli, evaluating their color characteristics, spatial contrast, and average illumination in different parts of the visual field. The next step in the analysis of afferent signals is performed by a system of neurons in the primary visual cortex (visul cortex).

The identification of areas of the cerebral cortex responsible for the processing of visual information has its own rather long history. As early as 1782, medical student Francesco German described a white stripe running through the gray matter of the occipital lobe. It was he who first suggested that the cortex may contain anatomically different areas. Prior to the discovery of Gennari, anatomists assumed that the cortex was a homogeneous sheet of tissue. Gennari had no idea that he had stumbled upon the primary visual cortex. It took more than a century for Henschen to prove that the Gennari strip corresponded to the primary visual cortex.

Retinal ganglion cells project their processes into the lateral geniculate body, where they form a retinotopic map. In mammals, the lateral geniculate body consists of 6 layers, each of which is innervated by either one or the other eye and receives a signal from different subtypes of ganglion cells, forming layers of large cell (magnocellular), small cell (parvocellular) and koniocellular (koniocellular) neurons. The neurons of the lateral geniculate body have center-background receptive fields, similar to retinal ganglion cells.

Neurons of the lateral geniculate body project and form a retinotopic map in the primary visual cortex V 1 , also called "area 17" or striate cortex (striatecortex). The receptive fields of cortical cells, instead of the already familiar organization of receptive fields according to the “center-background” type, consist of lines, or edges, which is a fundamentally new step in the analysis of visual information. The six layers of V 1 have structural features: afferent fibers from the geniculate body terminate mainly in layer 4 (and some in layer 6); cells in layers 2, 3, and 5 receive signals from cortical neurons. The cells of layers 5 and b project processes into the subcortical regions, and the cells of layers 2 and 3 project into other cortical zones. Each vertical column of cells functions as a module, receiving the initial visual signal from a certain place in space and sending the processed visual information to the secondary visual zones. The columnar organization of the visual cortex is obvious, since the localization of receptive fields remains the same throughout the entire depth of the cortex, and visual information from each eye (right or left) is always processed in strictly defined columns.

Two classes of neurons in region V 1 have been described that differ in their physiological properties. The receptive fields of simple cells are elongated and contain conjugated "on" and "off" "zones. Therefore, the most optimal stimulus for a simple cell is specially oriented beams of light or shadow. A complex cell responds to a certain oriented strip of light; this strip can be located in any area of the receptive field.The inhibition of simple or complex cells resulting from image recognition carries even more detailed information about the properties of the signal, such as the presence of a line of a certain length or a certain angle within a given receptive field.

The receptive fields of a simple cell are formed as a result of the convergence of a significant number of afferents from the geniculate body. The centers of several receptive fields adjacent to each other form one cortical receptive zone. The field of a complex cell depends on the signals of a simple cell and other cortical cells. The successive change in the organization of receptive fields from the retina to the lateral geniculate body and then to simple and complex cortical cells speaks of a hierarchy in information processing, whereby a number of neural structures of one level are integrated into the next, where an even more abstract concept is formed based on the initial information. At all levels of the visual analyzer, special attention is paid to contrast and definition of image boundaries, and not to the general illumination of the eye. Thus, the complex cells of the visual cortex can "see" the lines that are the boundaries of the rectangle, and they care little about the absolute intensity of the light inside this rectangle. A series of clear and continuous research into the mechanisms of perception of visual information, begun by the pioneering work of Kuffler with the retina, was continued at the level of the visual cortex by Hubel and Wiesel. Hubel gave a vivid description of early experiments on the visual cortex in Stephen Kuffler's laboratory at Johns Hopkins University (USA) in the 1950s. Since then, our understanding of the physiology and anatomy of the cerebral cortex has evolved significantly due to the experiments of Hubel and Wiesel, and also due to a large number of works for which their research was a starting point or source of inspiration. Our goal is to provide a concise, narrative description of signal coding and cortical architecture from a perceptual perspective, based on the classic work of Hubel and Wiesel, as well as more recent experiments by them, their colleagues, and many others. In this chapter, we will only give a schematic sketch of the functional architecture of the lateral geniculate body and the visual cortex, and their role in providing the first steps in the analysis of visual siena: the definition of lines and shapes based on the center-background signal coming from the retina.

When moving from the retina to the lateral geniculate body, and then to the cortex of the hemispheres, questions arise that are beyond the scope of technology. For a long time, it was generally accepted that to understand the functioning of any part of the nervous system, knowledge of the properties of its constituent neurons is necessary: how they conduct signals and carry information, how they transmit the received information from one cell to another through synapses. However, monitoring the activity of only one individual cell can hardly be an effective method for studying higher functions, where a large number of neurons are involved. The argument that has been used here and continues to be used from time to time is that the brain contains about 10 10 or more cells. Even the simplest task or event involves hundreds of thousands of nerve cells located in various parts of the nervous system. What are the chances of a physiologist to be able to penetrate into the essence of the mechanism of formation of a complex action in the brain, if he can simultaneously examine only one or a few nerve cells, a hopelessly small fraction of the total?

Upon closer examination, the logic of such arguments regarding the main complexity of the study associated with a large number of cells and complex higher functions no longer seems so flawless. As is often the case, a simplifying principle emerges that opens up a new and clearer view of the problem. The situation in the visual cortex is simplified by the fact that the main cell types are located separately from each other, in the form of well-organized and repetitive units. This repetitive pattern of neural tissue is closely intertwined with the retinotopic map of the visual cortex. Thus, neighboring points of the retina are projected onto neighboring points on the surface of the visual cortex. This means that the visual cortex is organized in such a way that for each smallest segment of the visual field there is a set of neurons for analyzing information and transmitting it. In addition, using methods that make it possible to isolate functionally related cellular ensembles, patterns of cortical organization of a higher level were identified. Indeed, the architecture of the cortex determines the structural basis of cortical function, so new anatomical approaches inspire new analytical research. Thus, before we describe the functional connections of the visual neurons, it is useful to briefly summarize the general structure of the central visual pathways that originate from the nuclei of the lateral geniculate body.

Lateral geniculate body

The optic nerve fibers start from each eye and end on the cells of the right and left lateral geniculate body (LCT) (Fig. 1), which has a clearly distinguishable layered structure (“geniculate” - geniculate - means “curved like a knee”). In the LCT of a cat, three distinct, well-defined cell layers (A, A 1 , C) can be seen, one of which (A 1) has a complex structure and is further subdivided. In monkeys and other primates, including

Maps of visual fields in the lateral geniculate body

An important topographic feature is the high orderliness in the organization of receptive fields within each layer of the LKT. Neighboring regions of the retina form connections with neighboring cells of the LC, so that the receptive fields of nearby LC neurons overlap over a large area. Cells in the central zone of the cat's retina (the region where the cat's retina has small receptive fields with small centers) as well as in the fovea of the monkey form connections with a relatively large number of cells within each layer of the LCT. A similar distribution of bonds was found in humans using NMR. The number of cells associated with the peripheral regions of the retina is relatively small. This overrepresentation of the optic fossa reflects the high density of photoreceptors in the zone that is necessary for vision with maximum acuity. Although the number of optic nerve fibers and the number of LC cells are probably approximately equal, each LC neuron nevertheless receives convergent signals from several optic nerve fibers. Each fiber of the optic nerve in turn forms divergent synaptic connections with several LC neurons.

However, each layer is not only topographically ordered, but also the cells of different layers are in retinotopic relation to each other. That is, if the electrode is advanced strictly perpendicular to the surface of the LKT, then the activity of cells receiving information from the corresponding zones of one and then the other eye will be recorded first, as the microelectrode crosses one layer of the LKT after another. The location of the receptive fields is in strictly corresponding positions on both retinas, i.e. they represent the same area of the visual field. There is no significant mixing of information from the right and left eyes and interaction between them in the cells of the LKT, only a small number of neurons (which have receptive fields in both eyes) are excited exclusively binocularly.

Surprisingly, the responses of LCT cells do not differ dramatically from those of ganglion cells (Fig. 3). LCT neurons also have concentrically organized antagonistic receptive fields, either with an "off" or "on" center, but the contrast mechanism is finer tuned due to the greater correspondence between

inhibitory and excitatory zones. Thus, similarly to retinal ganglion cells, contrast is the optimal stimulus for LC neurons, but they respond even weaker to general illumination. The study of the receptive fields of LC neurons has not yet been completed. For example, neurons were found in the LCT, whose contribution to the work of the LCT has not been established, as well as pathways leading from the cortex down to the LCT. Cortical feedback is necessary for the synchronized activity of LC neurons.

Functional layers of LCT

Why does the LCT have more than one layer per eye? It has now been found that neurons in different layers have different functional properties. For example, cells found in the fourth dorsal small cell layer of the monkey LC, like P ganglion cells, are able to respond to light of different colors, showing good color discrimination. Conversely, layers 1 and 2 (large cell layers) contain M-like cells that give fast ("alive") responses and are color insensitive, while K layers receive signals from "blue-on" retinal ganglion cells and can play a special role in color vision. In cats, X and Y fibers (see section "Classification of ganglion cells" end in different sublayers A, C and A 1, therefore, specific inactivation of layer A, but not C, sharply reduces the accuracy of eye movements. Cells with "on" - and "off" "-center is also divided into different layers in the LCT of mink and ferret, and, to some extent, in monkeys. In summary, the LCT is a staging station in which ganglion cell axons are sorted in such a way that neighboring cells receive signals from identical regions of the visual fields, and neurons that process information are organized in clusters.Thus, in the LCT, the anatomical basis for parallel processing of visual information is obvious.

Cytoarchitectonics of the visual cortex

Visual information enters the cortex and LCT through optical radiation. In monkeys, optical radiation ends at a folded plate about 2 mm thick (Fig. 4). This region of the brain - known as the primary visual cortex, visual area 1 or V 1 - is also called the striated cortex, or "area 17". Older terminology was based on anatomical criteria developed at the beginning of the 20th century. V 1 lies behind, in the region of the occipital lobe, and can be recognized in a transverse section by its special appearance. The bundles of fibers in this area form a strip that is clearly visible to the naked eye (which is why the zone is called “striped”, Fig. 4B). Neighboring zones outside the banding zone are also associated with vision. The area immediately surrounding zone V is called zone V 2 (or "zone 18") and receives signals from zone V, (see Figure 4C). The clear boundaries of the so-called extrastriate visual cortex (V 2 -V 5) cannot be established using visual examination of the brain, although a number of criteria have been developed for this. For example, in V 2 the striation disappears, large cells are located superficially, and coarse, oblique myelin fibers are visible in deeper layers.

Each zone has its own representation of the visual field of the retina, projected in a strictly defined, retinotopic manner. Projection maps were compiled back in an era when it was not possible to analyze the activity of individual cells. Therefore, for mapping, illumination of small areas of the retina with light beams and registration of cortical activity using a large electrode were used. These maps, as well as their modern counterparts recently compiled using brain imaging techniques such as positron emission tomography and functional nuclear magnetic resonance, have shown that the area of the cortex devoted to representing the fovea is much larger than the area assigned to the rest of the retina. These findings, in principle, met expectations, since pattern recognition by the cortex is carried out mainly due to the processing of information from photoreceptors densely located in the fovea zone. This representation is analogous to the extended representation of the hand and face in the region of the primary somatosensory cortex. The retinal fossa projects into the occipital pole of the cerebral cortex. The retinal periphery map extends anteriorly along the medial surface of the occipital lobe (Fig. 5). Due to the inverted picture formed on the retina with the help of the lens, the upper visual field is projected onto the lower region of the retina and is transmitted to the region V 1 located below the spur groove; the lower visual field is projected over the spur groove.

On sections of the cortex, neurons can be classified according to their shape. The two main groups of neurons form stellate and pyramidal cells. Examples of these cells are shown in Fig. 6B. The main differences between them are the length of the axons and the shape of the cell bodies. Axons of pyramidal cells are longer, descend into the white matter, leaving the cortex; the processes of stellate cells end in the nearest zones. These two groups of cells may have other differences, such as the presence or absence of spines on the dendrites, which provide their functional properties. There are other bizarrely named neurons (two-flower cells, chandelier cells, basket cells, crescent cells), as well as neuroglial cells. Their characteristic feature is that the processes of these cells are directed mainly in the radial direction: up and down through the thickness of the cortex (at an appropriate angle to the surface). Conversely, many (but not all) of their lateral processes are short. Connections between the primary visual cortex and the higher order cortex are carried out by axons, which pass in the form of bundles through the white matter located under the cell layers.

Rice. 7. Connections of the visual cortex. (A) Layers of cells with different incoming and outgoing processes. Note that the original processes from the LKT are mostly interrupted in the 4th layer. The outgrowths from the LCT coming from the large cell layers are predominantly interrupted in the 4C and 4B layers, while the outgrowths from the small cell ones are interrupted in the 4A and 4C. Simple cells are located mainly in layers 4 and 6, complex cells - in layers 2, 3, 5 and 6. Cells in layers 2, 3 and 4B send axons to other cortical zones; cells in layers 5 and 6 send axons to the superior colliculus and LC. (B) Typical branching of axons of the LCT and cortical neurons in a cat. In addition to these vertical connections, many cells have long horizontal connections that run within one layer to distant regions of the cortex.

Incoming, outgoing pathways and layered organization of the cortex

The main feature of the mammalian cortex is that the cells are arranged in 6 layers within the gray matter (Fig. 6A). The layers vary greatly in appearance, depending on the density of the cells, as well as the thickness of each of the zones of the cortex. Incoming paths are shown in fig. 7A on the left side. Based on the LCT, the fibers mostly terminate in layer 4 with few connections also formed in layer 6. The superficial layers receive signals from the pulvinarzone or other areas of the thalamus. A large number of cortical cells, especially in the region of layer 2, as well as in the upper parts of layers 3 and 5, receive signals from neurons also located within the cortex. The bulk of the fibers coming from the LCT to layer 4 is then divided between the various sublayers.

Fibers outgoing from layers 6, 5, 4, 3 and 2 are shown on the right in Fig. 7A. Cells that send efferent signals from the cortex can also manage intracortical connections between different layers. For example, the axons of a cell from layer 6, in addition to the LCT, can also be directed to one of the other cortical layers, depending on the type of response of this cell 34) . Based on this structure of the visual pathways, the following pathway of the visual signal can be imagined: information from the retina is transmitted to the cortical cells (mainly in layer 4) by the axons of the LCT cells; information is transmitted from layer to layer, from neuron to neuron throughout the thickness of the cortex; processed information is sent to other areas of the cortex with the help of fibers that go deep into the white matter and return back to the area of the cortex. Thus, the radial or vertical organization of the cortex gives us reason to believe that the columns of neurons work as separate computing units, processing various details of visual scenes and forwarding the received information further to other regions of the cortex.

Separation of incoming fibers from LQT in layer 4

LCT afferent fibers terminate in layer 4 of the primary visual cortex, which has a complex organization and can be studied both physiologically and anatomically. The first feature we want to demonstrate is the separation of incoming fibers coming from different eyes. In adult cats and monkeys, cells within one layer of the LCT, receiving signals from one eye, send processes to strictly defined clusters of cortical cells in layer 4C, which are responsible for this particular eye. Accumulations of cells are grouped in the form of alternating strips or bundles of cortical cells that receive information exclusively from the right or left eye. In the more superficial and deeper layers, neurons are controlled by both eyes, although usually with a predominance of one of them. Hubel and Wiesel made an original demonstration of the separation of information from different eyes and the dominance of one of them in the primary visual cortex using electrophysiological methods. They used the term "ocular dominance columns" to describe their observations, following Mountcastle's concept of cortical columns for the somatosensory cortex. A series of experimental techniques were developed to demonstrate alternating groups of cells in layer 4 receiving information from the right or left eye. Initially, it was proposed to inflict a small amount of damage within only one layer of the LKT (recall that each layer receives information from only one eye). If this is done, then the degenerating terminals appear in layer 4, forming a certain pattern of alternating spots, which correspond to zones controlled by the eye, sending information to the damaged area of the LCT. Later, a startling demonstration of the existence of a particular pattern of ocular dominance was made using the transport of radioactive amino acids from one eye. The experiment consists in injecting an amino acid (proline or lecithin) containing atoms of radioactive tritium into the eye. The injection is carried out in the vitreous body of the eye, from which the amino acid is captured by the bodies of retinal nerve cells and included in the protein. Over time, the protein labeled in this way is transported to the ganglion cells and along the optic nerve fibers to their terminals within the LCT. The remarkable feature is that this radioactive label is also transmitted from neuron to neuron through chemical synapses. The label eventually ends up at the end of the LCT fibers within the visual cortex.

On fig. 8 shows the location within layer 4 of the radioactive terminals formed by the axons of the LCT cells associated with the eye into which the label was injected.

Rice. Fig. 8. Eye-dominant columns in the monkey cortex obtained by injecting radioactive proline into one eye. Autoradiograms taken under dark-field illumination showing silver grains in white. (A) At the top of the figure, the slice passes through layer 4 of the visual cortex at an angle to the surface, forming a perpendicular slice of the columns. In the center, layer 4 has been cut horizontally, showing that the column consists of elongated plates. (B) Reconstruction from multiple horizontal sections of layer 4C in another monkey that was injected into the ilsilateral eye. (Any horizontal cut may reveal

only part of layer 4, due to the curvature of the cortex.) In both A and B, the visual dominance columns look like stripes of equal width, receiving information from either one or the other eye.

located directly above the visual cortex, so such areas look like white spots on the dark background of the photograph). Marker spots are interspersed with unmarked areas that receive information from the contralateral eye where the mark was not applied. The distance from center to center between the spots, which correspond to the eye-dominant columns, is approximately 1 mm.

At the cellular level, a similar structure was revealed in layer 4 by injecting horseradish peroxidase into individual cortical-bound axons of LC neurons. The axon shown in Fig. 9 comes from the LCT neuron with an "off" center that responds with short signals to shadows and moving spots. The axon terminates in two different groups of processes in layer 4. The groups of labeled processes are separated by an empty unlabeled zone corresponding in size to the territory responsible for the other eye. This kind of morphological study expands the boundaries and allows a deeper understanding of the original description of the columns of ocular dominance, compiled by Hubel and Wiesel in 1962.

Literature

1. o Hubel, D. H. 1988. Eye, Brain and Vision. Scientific American Library. new york.

2.o Ferster, D., Chung, S., and Wheat, H. 1996. Orientation selectivity of thalamic input to simple cells of cat visual cortex. Nature 380: 249-252.

3. o Hubel, D. H., and Wiesel, T. N. 1959. Receptive fields of single neurones in the cat's striate cortex. /. Physiol. 148: 574-591.

4. About Hubel, D.H., and Wiesel, T.N. 1961. Integrative action in the cat's lateral geniculate body. /. Physiol. 155: 385-398.

5. O Hubel, D. H., and Wiesel, T. N. 1962. Receptive fields, binocular interaction and functional architecture in the cat's visual cortex. /. Physiol. 160: 106-154.

represents a small oblong elevation at the posterior-lower end of the visual mound on the side of the pulvinar. At the ganglion cells of the external geniculate body, the fibers of the optic tract end and the fibers of the Graziole bundle originate from them. Thus, the peripheral neuron ends here and the central neuron of the optic pathway originates.

It has been established that although most of the fibers of the optic tract end in the lateral geniculate body, still a small part of them goes to the pulvinar and anterior quadrigemina. These anatomical data provided the basis for the long held view that both the lateral geniculate body and the pulvinar and anterior quadrigemina were considered primary visual centers.

At present, a lot of data has accumulated that does not allow us to consider the pulvinar and the anterior quadrigemina as primary visual centers.

A comparison of clinical and pathoanatomical data, as well as embryological and comparative anatomy data, does not allow us to attribute the role of the primary visual center to pulvinar. So, according to Genshen's observations, in the presence of pathological changes in the pulvinar field of view remains normal. Brouwer notes that with an altered lateral geniculate body and an unchanged pulvinar, homonymous hemianopsia is observed; with changes in the pulvinar and unchanged lateral geniculate body, the visual field remains normal.

The same is true with anterior quadrigemina. 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 all of the 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, the following should be noted. Monakov in general denied the presence of any projection of the retina 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 entire external geniculate body. Genshen back in the 90s of the last century proved the fallacy of this view. In 2 patients with homonymous lower quadrant hemianopsia, a post-mortem examination revealed limited changes in the dorsal part of the lateral geniculate body.

Ronne (Ronne) with atrophy of the optic nerves with central scotomas due to alcohol intoxication found limited changes in ganglion cells in the lateral geniculate body, indicating that the area of the macula is projected onto the dorsal part of the geniculate body.

The above observations unequivocally prove the presence of a certain projection of the retina in the external geniculate body. But the clinical and anatomical observations available in this regard are too few and do not yet give accurate ideas about the nature of this projection. The experimental studies of Brouwer and Zeman on monkeys, which we have mentioned, made it possible to study to some extent the projection of the retina in the lateral geniculate body. They found that most of the lateral geniculate body is occupied by the projection of the retinal regions 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 a large area in the dorsal part. The upper quadrants of the retina project onto the lateral geniculate body ventro-medially; lower quadrants - ventro-laterally. The projection of the retina in the lateral geniculate body in a monkey is shown in Fig. eight.

In the outer geniculate body (Fig. 9)

Rice. 9. The structure of the external geniculate body (according to Pfeifer).

there is also a separate projection of crossed and non-crossed fibers. The studies of M. Minkowski make a significant contribution to the clarification of this issue. He established that in a number of animals after enucleation of one eye, as well as in humans with prolonged unilateral blindness, there are observed in the external geniculate body optic nerve fiber atrophy and ganglion cell atrophy. At the same time, Minkowski discovered a characteristic feature: in both geniculate bodies, atrophy with a certain regularity spreads to different layers of ganglion cells. In the lateral geniculate body of each side, layers with atrophied ganglion cells alternate with layers in which the cells remain normal. Atrophic layers on the side of enucleation correspond to identical layers on the opposite side, which remain normal. At the same time, similar layers, which remain normal on the side of enucleation, atrophy on the opposite side. Thus, the atrophy of the cell layers in the lateral geniculate body that occurs after the enucleation of one eye is definitely alternating in nature. Based on his observations, Minkowski concluded that each eye has a separate representation in the lateral geniculate body. Crossed and non-crossed fibers thus terminate at different ganglion cell layers, as is well illustrated in Le Gros Clark's diagram (Fig. 10).

Rice. ten. Scheme of the end of the fibers of the optic tract and the beginning of the fibers of the Graziola bundle in the lateral geniculate body (according to Le Gros Clark).
Solid lines are crossed fibers, dashed lines are non-crossed fibers. 1 - visual tract; 2 - external geniculate body 3 - Graziola bundle; 4 - cortex of the occipital lobe.

Minkowski's data were later confirmed by experimental and clinical and anatomical studies by other authors. L. Ya. Pines and I. E. Prigonnikov examined the lateral geniculate body 3.5 months after enucleation of one eye. At the same time, degenerative changes were noted in the ganglion cells of the central layers in the lateral geniculate body on the side of enucleation, while the peripheral layers remained normal. In the opposite side of the lateral geniculate body, inverse relationships were observed: the central layers remained normal, while degenerative changes were noted in the peripheral layers.

Interesting observations related to the case unilateral blindness long ago, was recently published by the Czechoslovak scientist F. Vrabeg. A 50-year-old patient had one eye removed at the age of ten. Postmortem examination of the lateral geniculate bodies confirmed the presence of alternating degeneration of ganglion cells.

Based on the data presented, it can be considered established that both eyes have a separate representation in the lateral geniculate body and, therefore, crossed and non-crossed fibers end in different layers of ganglion cells.

Portal for the student. Self-training

Outer geniculate body

Lateral geniculate body

Maps of visual fields in the lateral geniculate body

Functional layers of LCT

Cytoarchitectonics of the visual cortex

Incoming, outgoing pathways and layered organization of the cortex

Separation of incoming fibers from LQT in layer 4

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