30-09-2011, 10:29
Description
The corpus callosum is a powerful bundle of myelinated fibers connecting the two hemispheres of the brain. Stereoscopic vision (stereopsis) is the ability to perceive the depth of space and assess the distance of objects from the eyes. These two things are not particularly closely related, but it is known that a small part of the fibers of the corpus callosum does play some role in stereopsis. It turned out to be convenient to include both of these topics in one chapter, since when considering them we will have to take into account the same feature of the structure of the visual system, namely, that in the chiasm there are both crossed and uncrossed fibers of the optic nerve.
Corpus callosum
The corpus callosum (corpus callosum in Latin) is the largest bundle of nerve fibers in the entire nervous system. According to a rough estimate, there are about 200 million axons in it. The true number of fibers is likely even higher, since the estimate given is based on conventional light microscopy rather than electron microscopy.
This number is incomparable to the number of fibers in each optic nerve (1.5 million) and in the auditory nerve (32,000). The cross-sectional area of the corpus callosum is about 700 mm square, whereas that of the optic nerve does not exceed a few square millimeters. The corpus callosum, together with a thin bundle of fibers called anterior commissure, connects the two hemispheres of the brain (Fig. 98 and 99).
Term commissary means a set of fibers connecting two homologous nerve structures located in the left and right halves of the brain or spinal cord. The corpus callosum is also sometimes called the greater commissure of the brain.
Until about 1950, the role of the corpus callosum was completely unknown. In rare cases, there is a congenital absence ( aplasia) corpus callosum. This formation can also be partially or completely cut during a neurosurgical operation, which is done deliberately - in some cases in the treatment of epilepsy (so that a convulsive discharge occurring in one hemisphere of the brain cannot spread to the other hemisphere), in other cases in order to get from above to a deep-lying tumor (if, for example, the tumor is located in the pituitary gland). According to the observations of neurologists and psychiatrists, after this type of operation no mental disorders occur. Some have even suggested (though hardly seriously) that the sole function of the corpus callosum is to hold the two hemispheres of the brain together. Until the 1950s, little was known about the details of the distribution of connections in the corpus callosum. It was obvious that the corpus callosum connects the two hemispheres, and on the basis of data obtained by rather crude neurophysiological methods, it was believed that in the striate cortex the fibers of the corpus callosum connect exactly symmetrical areas of the two hemispheres.
In 1955, Ronald Myers, a graduate student of psychologist Roger Sperry at the University of Chicago, conducted the first experiment that revealed some of the functions of this huge fiber tract. Myers trained cats by placing them in a box with two side-by-side screens on which different images could be projected, such as a circle on one screen and a square on the other. The cat was trained to rest its nose on the screen that showed a circle and ignore the other screen that showed a square. Correct answers were reinforced with food, and for incorrect answers the cats were slightly punished - a loud bell was turned on, and the cat was not rudely, but decisively pulled away from the screen. With this method, over several thousand repetitions, the cat can be brought to the level of reliable discrimination of figures. (Cats learn slowly; for example, pigeons need from several tens to several hundred repetitions to learn a similar task, but a person can generally be taught immediately by giving him verbal instructions. This difference seems somewhat strange - after all, a cat has a brain many times larger, than that of a pigeon.)
It is not surprising that Myers's cats learned to solve this problem just as well when one of the animal's eyes was covered with a mask. It is also not surprising that if training in such a task as choosing a triangle or a square was carried out with only one eye open - the left one, and during testing the left eye was closed and the right one was opened, then the accuracy of discrimination remained the same. This does not surprise us because we ourselves can easily solve a similar problem. The ease of solving such problems is understandable if we take into account the anatomy of the visual system. Each hemisphere receives input from both eyes. As we already said in the article, most of the cells in field 17 also have inputs from both eyes. Myers created a more interesting situation by performing a longitudinal section of the chiasm along the midline. Thus, he cut the intersecting fibers and kept the non-intersecting ones intact (this operation requires a certain skill from the surgeon). As a result of such a transection, the animal's left eye was connected only to the left hemisphere, and the right eye - only to the right.
Experiment idea was to train the cat using the left eye, and at the "exam" address the stimulus to the right eye. If the cat can solve the problem correctly, this will mean that the necessary information is transmitted from the left hemisphere to the right along the only known path - through the corpus callosum. So Myers cut the chiasm longitudinally, trained the cat with one eye open, and then tested it by opening the other eye and closing the first. Under these conditions, the cats still successfully solved the problem. Finally, Myers repeated the experiment on animals in which both the chiasm and the corpus callosum had previously been cut. This time the cats did not solve the problem. Thus, Myers experimentally established that the corpus callosum actually performs some functions (although one could hardly think that it exists only so that individual people or animals with a cut optic chiasm can solve certain problems using one eye after learning using another).
Study of the physiology of the corpus callosum
One of the first neurophysiological studies in this area was carried out several years after Myers' experiments by D. Whitteridge, then working in Edinburgh. Whitteridge reasoned that there was little reason for bundles of nerve fibers to connect homologous mirror-symmetrical areas of fields 17. Indeed, there seems to be no reason for a nerve cell in the left hemisphere connected with some points in the right half of the visual field , connected to a cell in the right hemisphere associated with a symmetrical area of the left half of the visual field. To test his assumptions, Whitteridge cut the optic tract on the right side of the brain behind the chiasm, thereby blocking the path of input signals to the right occipital lobe; but this, of course, did not exclude the transmission of signals there from the left occipital lobe through the corpus callosum (Fig. 100).
Whitteridge then began to turn on the light stimulus and use a metal electrode to record electrical activity from the surface of the cortex. He did get responses in his experiment, but they occurred only at the inner edge of area 17, that is, in the area receiving input signals from a long, narrow vertical strip in the middle of the visual field: when stimulated with small spots of light, responses appeared only when the light flashed at or near the vertical midline. If the cortex of the opposite hemisphere was cooled, thereby temporarily suppressing its function, the responses stopped; This was also caused by cooling of the corpus callosum. Then it became clear that the corpus callosum cannot connect the entire field 17 of the left hemisphere with the entire field 17 of the right hemisphere, but connects only small areas of these fields, where the projections of the vertical line are located in the middle of the visual field.
A similar result could have been predicted based on a number of anatomical data. Only one portion of area 17, very close to the border with area 18, sends axons through the corpus callosum to the other hemisphere, and most of them seem to terminate in area 18 near the border with area 17. If we assume that the inputs to cortex from the NKT exactly correspond to the contralateral parts of the visual field (namely, the left hemifield is displayed in the cortex of the right hemisphere, and the right - in the cortex of the left), then the presence of connections between the hemispheres through the corpus callosum should ultimately lead to the fact that each hemisphere will receive signals from an area slightly larger than half the field of view. In other words, due to connections through the corpus callosum, there will be an overlap of hemifields projected into the two hemispheres. This is exactly what we found. Using two electrodes inserted into the cortex at the border of fields 17 and 18 in each hemisphere, we were often able to record the activity of cells whose receptive fields overlapped by several angular degrees.
T. Wiesel and I soon made microelectrode leads directly from the area of the corpus callosum (in its very posterior part) where there are fibers associated with the visual system. We found that almost all the fibers that we could activate with visual stimuli responded exactly like ordinary neurons in area 17, that is, they exhibited the properties of both simple and complex cells, selectively sensitive to the orientation of the stimulus and usually responding to stimulate both eyes. In all these cases, the receptive fields were located very close to the mid-vertical below or above (or at the level of) the fixation point, as shown in Fig. 101.
Perhaps the most elegant neurophysiological demonstration of the role of the corpus callosum was the work of G. Berlucchi and G. Rizzolatti from Pisa, performed in 1968. Having cut the optic chiasm along the midline, they recorded responses in area 17 near the border with area 18, looking for those cells that could be activated binocularly. It is clear that any binocular cell in this area in the right hemisphere must receive input signals both directly from the right eye (via the NKT) and from the left eye and left hemisphere through the corpus callosum. As it turned out, the receptive field of each binocular cell captured the middle vertical of the retina, with that part of it that belongs to the left half of the visual field delivering information from the right eye, and the part that goes into the right half from the left eye. Other cell properties studied in this experiment, including orientation selectivity, turned out to be identical (Fig. 102).
The results clearly showed that the corpus callosum connects cells to each other in such a way that their receptive fields can extend both to the right and to the left of the middle vertical. Thus, it seems to glue two halves of the image of the surrounding world. To better imagine this, let us assume that initially the cortex of our brain formed as one whole, not divided into two hemispheres. In this case, field 17 would have the appearance of one continuous layer onto which the entire visual field would be mapped. Then neighboring cells, in order to realize such properties as, for example, sensitivity to movement and orientation selectivity, would, of course, have to have a complex system of mutual connections. Now let’s imagine that the “designer” (be it God, or, say, natural selection) decided that it cannot be left like this any longer - from now on, half of all cells should form one hemisphere, and the other half - the other hemisphere.
What then must be done with all the multitude of intercellular connections if two sets of cells must now move away from each other?
Apparently, you can simply stretch these connections, forming part of the corpus callosum from them. In order to eliminate the delay in transmitting signals along such a long path (about 12-15 centimeters in humans), it is necessary to increase the transmission speed by providing the fibers with a myelin sheath. Of course, nothing of this sort actually happened during evolution; long before the cortex arose, the brain already had two separate hemispheres.
The experiment of Berlucchi and Rizzolatti, in my opinion, provided one of the most striking confirmations of the amazing specificity of neural connections. The cell shown in Fig. 108 (near the tip of the electrode) and probably a million other similar cells connected through the corpus callosum acquire their orientation selectivity both due to local connections with neighboring cells and due to connections going through the corpus callosum from the other hemisphere from cells with such the same orientation sensitivity and similar arrangement of receptive fields (the above also applies to other properties of cells, such as directional specificity, the ability to respond to the ends of lines, as well as complexity).
Each of the cells in the visual cortex that have connections through the corpus callosum must receive input signals from cells in the other hemisphere with exactly the same properties. We know many facts indicating the selectivity of compounds in the nervous system, but I think that this example is the most striking and convincing.
The axons discussed above cells of the visual cortex make up only a small proportion of all fibers of the corpus callosum. Experiments using axonal transport were carried out on the somatosensory cortex, similar to the experiments described in previous chapters with the injection of a radioactive amino acid into the eye. Their results indicate that the corpus callosum similarly connects those areas of the cortex that are activated by cutaneous and joint receptors located near the midline of the body on the trunk and head, but does not connect cortical projections of the limbs.
Each cortical area connects to several or even many other cortical areas of the same hemisphere. For example, the primary visual cortex is connected to area 18 (visual area 2), medial temporal area (area MT), visual area 4, and one or two other areas. Many areas of the cortex also have connections with several areas of the other hemisphere, through the corpus callosum, and in some cases through the anterior commissure.
Therefore we can consider these commissural connections are simply a special type of cortico-cortical connections. It is easy to imagine that this is evidenced by such a simple example: if I tell you that my left hand feels cold or that I saw something to the left, then I formulate words using my cortical speech areas located in the left hemisphere (what is said may be, and not entirely true, since I am left-handed); information coming from the left half of the visual field or from the left hand is transmitted to my right hemisphere; then the corresponding signals must be transmitted through the corpus callosum to the speech zone of the cortex of the other hemisphere so that I can say something about my sensations. In a series of studies beginning in the early 1960s, R. Sperry (now at the California Institute of Technology) and his associates showed that a person whose corpus callosum is cut (to treat epilepsy) loses the ability to talk about events about which information enters the right hemisphere. Work with such subjects has become a valuable source of new information about various functions of the cortex, including thinking and consciousness. The first articles about this appeared in the journal Brain; they are extremely interesting, and can be easily understood by anyone who has read the real book.
Stereoscopic vision
The distance estimation mechanism, based on the comparison of two retinal images, is so reliable that many people (unless they are psychologists or specialists in visual physiology) are not even aware of its existence. To see the importance of this mechanism, try driving a car or bicycle, playing tennis or skiing for a few minutes with one eye closed. Stereoscopes have fallen out of fashion and you can only find them in antique stores. However, most readers watched stereoscopic films (when the viewer has to wear special glasses). The operating principle of both a stereoscope and stereoscopic glasses is based on the use of the stereopsis mechanism.
Retinal images are two-dimensional, and yet we see the world in three dimensions. Obviously, the ability to determine the distance to objects is important for both humans and animals. Similarly, perceiving the three-dimensional shape of objects means judging relative depth. Let's take a round object as a simple example. If it is located obliquely relative to the line of sight, its image on the retinas will be elliptical, but usually we easily perceive such an object as round. This requires the ability to perceive depth.
Humans have many mechanisms for judging depth. Some of them are so obvious that they hardly deserve mention. Nevertheless, I will mention them. If the size of an object is approximately known, for example in the case of objects such as a person, a tree or a cat, then we can estimate the distance to it (although there is a risk of error if we encounter a dwarf, a dwarf tree or a lion). If one object is located in front of another and partially obscures it, then we perceive the front object as being closer. If you take a projection of parallel lines, for example, railway rails, going into the distance, then in the projection they will come closer. This is an example of perspective, a very effective indicator of depth.
A convex section of a wall appears lighter in its upper part if the light source is located higher (usually light sources are located at the top), and a recess in its surface, if illuminated from above, appears darker in the upper part. If the light source is placed at the bottom, then the convexity will look like a recess, and the recess will look like a convexity. An important sign of distance is motion parallax - the apparent relative displacement of close and more distant objects if the observer moves his head left and right or up and down. If a solid object is rotated, even at a small angle, its three-dimensional shape is immediately revealed. If we focus the lens of our eye on a nearby object, then a more distant object will be out of focus; Thus, by changing the shape of the lens, i.e., changing the accommodation of the eye, we are able to assess the distance of objects.
If you change the relative direction of the axes of both eyes, bringing them together or spreading them apart(carrying out convergence or divergence), then you can bring together two images of an object and hold them in this position. Thus, by controlling either the lens or the position of the eyes, it is possible to estimate the distance of an object. The designs of a number of rangefinders are based on these principles. With the exception of convergence and divergence, all other distance measures listed so far are monocular. The most important mechanism of depth perception, stereopsis, depends on the joint use of the two eyes.
When viewing any three-dimensional scene, the two eyes form slightly different images on the retina. You can easily verify this if you look straight ahead and quickly move your head from side to side by about 10 cm, or quickly close one eye or the other. If you have a flat object in front of you, you won't notice much of a difference. However, if the scene includes objects at different distances from you, you will notice significant changes in the picture. During stereopsis, the brain compares images of the same scene on two retinas and estimates relative depth with great accuracy.
Suppose the observer fixes with his gaze a certain point P. This statement is equivalent to if we say: the eyes are directed in such a way that the images of the point appear in the central fossa of both eyes (F in Fig. 103).
Let us now assume that Q is another point in space that appears to the observer to be located at the same depth as P. Let Qlh Qr be the images of point Q on the retinas of the left and right eyes. In this case, points QL and QR are called corresponding points of the two retinas. Obviously, two points coinciding with the central fovea of the retina will be corresponding. From geometric considerations it is also clear that the point Q", assessed by the observer as located closer than Q, will give two projections on the retinas - and Q"R - at non-corresponding points located further from each other than if these the points were corresponding (this situation is depicted on the right side of the figure). In the same way, if we consider a point located further from the observer, it turns out that its projections on the retinas will be located closer to each other than the corresponding points.
What is said above about the corresponding points is partly definitions, and partly statements arising from geometric considerations. When considering this issue, the psychophysiology of perception is also taken into account, since the observer subjectively evaluates whether the object is located further or closer to point P. Let's introduce one more definition. All points which, like point Q (and, of course, point P), are perceived as equidistant, lie on the horopter - a surface passing through points P and Q, the shape of which differs from both a plane and a sphere and depends on our ability assess distance, i.e. from our brain. The distances from the central fovea F to the projections of point Q (QL and QR) are close, but not equal. If they were always equal, then the line of intersection of the horopter with the horizontal plane would be a circle.
Let us now assume that we fix with our gaze a certain point in space and that in this space there are two point sources of light that give a projection on each retina in the form of a light point, and these points are not corresponding: the distance between them is slightly greater than between the corresponding points . We will call any such deviation from the position of the corresponding points disparity. If this deviation in the horizontal direction does not exceed 2° (0.6 mm on the retina), and in the vertical direction no more than several arc minutes, then we will visually perceive a single point in space located closer than the one we are fixing. If the distances between the projections of a point are not greater, but smaller, than between the corresponding points, then this point will seem to be located further than the fixation point. Finally, if the vertical deviation exceeds several minutes of arc or the horizontal deviation exceeds 2°, then we will see two separate points that may appear to be located further or closer to the fixation point. These experimental results illustrate the basic principle of stereo perception first formulated in 1838 by Sir C. Wheatstone (who also invented the device known in electrical engineering as the “Wheatstone bridge”).
It seems almost incredible that, until this discovery, no one seemed to realize that the presence of subtle differences in the images projected on the retinas of the two eyes could give rise to a distinct impression of depth. This stereo effect can demonstrated in a few minutes by any person who can arbitrarily move the axes of their eyes together or apart, or by someone who has a pencil, a piece of paper and several small mirrors or prisms. It is unclear how Euclid, Archimedes and Newton missed this discovery. In his article, Wheatstone notes that Leonardo da Vinci was very close to discovering this principle. Leonardo pointed out that a ball located in front of any spatial scene is seen differently by each eye - with the left eye we see its left side a little further, and with the right eye we see the right side. Wheatstone further notes that if Leonardo had chosen a cube instead of a ball, he would certainly have noticed that its projections were different for different eyes. After this, he might, like Wheatstone, become interested in what would happen if two similar images were specially projected onto the retinas of two eyes.
An important physiological fact is that the sensation of depth (i.e., the ability to “directly” see whether a particular object is located further or closer than the point of fixation) occurs in cases where two retinal images are slightly displaced relative to each other in the horizontal direction - moved apart or, conversely, , are close together (unless this displacement exceeds about 2°, and the vertical displacement is close to zero). This, of course, corresponds to geometric relationships: if, relative to a certain distance reference point, an object is located closer or further, then its projections on the retinas will be moved apart or brought closer together horizontally, while no significant vertical displacement of the images will occur.
This is the basis of the action of the stereoscope invented by Wheatstone. The stereoscope was so popular for about half a century that it was found in almost every home. The same principle underlies the stereo cinema that we now watch using special Polaroid glasses. In the original design of the stereoscope, the observer viewed two images placed in a box using two mirrors that were positioned so that each eye saw only one image. For convenience, prisms and focusing lenses are now often used. The two images are identical in every way except for slight horizontal offsets, which create the impression of depth. Anyone can produce a photograph suitable for use in a stereoscope by selecting a stationary object (or scene), taking a photograph, and then moving the camera 5 centimeters to the right or left and taking a second photograph.
Not everyone has the ability to perceive depth using a stereoscope. You can easily check your stereopsis yourself if you use the stereo pairs shown in Fig. 105 and 106.
If you have a stereoscope, you can make copies of the stereo pairs shown here and paste them into the stereoscope. You can also place a thin piece of cardboard perpendicularly between two images from the same stereo pair and try to look at your image with each eye, setting your eyes parallel, as if you were looking into the distance. You can also learn to move your eyes together and apart with your finger, placing it between your eyes and the stereo pair and moving it forward or back until the images merge, after which (this is the most difficult) you can examine the merged image, trying not to split it into two. If you can do this, the apparent depth relationships will be the opposite of those perceived when using a stereoscope.
Even if you fail to repeat the experience with depth perception- whether because you do not have a stereoscope, or because you cannot arbitrarily move the axes of your eyes together, you will still be able to understand the essence of the matter, although you will not get pleasure from the stereo effect.
In the top stereo pair in Fig. 105 in two square frames there is a small circle, one of which is shifted slightly to the left of the center, and the other slightly to the right. If you examine this stereopair with both eyes, using a stereoscope or another method of combining images, you will see a circle not in the plane of the sheet, but in front of it at a distance of about 2.5 cm. If you also examine the lower stereopair in Fig. 105, then the circle will be visible behind the plane of the sheet. You perceive the position of the circle in this way because the retinas of your eyes receive exactly the same information as if the circle were actually in front or behind the plane of the frame.
In 1960 Bela Jules from Bell Telephone Laboratories came up with a very useful and elegant technique for demonstrating the stereo effect. The image shown in Fig. 107, at first glance appears to be a homogeneous random mosaic of small triangles.
This is true, except that there is a larger hidden triangle in the central part. If you view this image with two pieces of colored cellophane placed in front of your eyes - red in front of one eye and green in front of the other, then you should see a triangle in the center protruding forward from the plane of the sheet, as in the previous case with a small circle on stereo pairs . (You may have to watch for a minute or so the first time until the stereo effect occurs.) If you swap the pieces of cellophane, a depth inversion will occur. The value of these Yulesz stereo pairs is that if you have impaired stereo perception, you will not see the triangle in front of or behind the surrounding background.
To summarize, we can say that our ability to perceive the stereo effect depends on five conditions:
1. There are many indirect signs of depth - partial obscuring of some objects by others, motion parallax, rotation of an object, relative sizes, casting shadows, perspective. However, the most powerful mechanism is stereopsis.
2. If we fix our gaze on some point in space, then the projections of this point fall into the central fossa of both retinas. Any point that is judged to be located at the same distance from the eyes as the point of fixation forms two projections at corresponding points on the retinas.
3. The stereo effect is determined by a simple geometric fact - if some object is closer to the point of fixation, then its two projections on the retinas are farther from each other than the corresponding points.
4. The main conclusion, based on the results of experiments with subjects, is the following: an object whose projections on the retinas of the right and left eyes fall on the corresponding points is perceived as located at the same distance from the eyes as the fixation point; if the projections of this object are moved apart compared to the corresponding points, the object appears to be located closer to the fixation point; if, on the contrary, they are close, the object appears to be located further than the point of fixation.
5. When the horizontal displacement of projections is more than 2° or the vertical displacement is more than several arc minutes, double vision occurs.
Physiology of stereoscopic vision
If we want to know what the brain mechanisms of stereopsis are, the easiest place to start is by asking: Are there neurons whose responses are specifically determined by the relative horizontal displacement of the images on the retinas of the two eyes? Let's first look at how the cells of the lower levels of the visual system respond when both eyes are simultaneously stimulated. We must start with neurons in area 17 or higher, since the retinal ganglion cells are clearly monocular, and the cells of the lateral geniculate body, in which the inputs from the right and left eyes are distributed in different layers, can also be considered monocular - they respond to stimulation of either one eye , or the other, but not both at the same time. In area 17, approximately half of the neurons are binocular cells that respond to stimulation of both eyes.
Upon careful testing, it turns out that the responses of these cells seem to depend little on the relative position of the stimulus projections on the retinas of the two eyes. Consider a typical complex cell that responds with a continuous discharge to the movement of a stimulus strip through its receptive field in one eye or the other. When both eyes are simultaneously stimulated, the frequency of discharges of this cell is higher than when one eye is stimulated, but it is usually not important for the response of such a cell whether at any moment the stimulus projections fall into exactly the same parts of the two receptive fields.
The best response is recorded when these projections enter and exit the respective receptive fields of the two eyes at approximately the same time; however, it is not so important which projection is slightly ahead of the other. In Fig. 108 shows a characteristic curve of the response (for example, the total number of impulses in the response during one passage of the stimulus through the receptive field) on the difference in the position of the stimulus on both retinas. This curve is very close to a horizontal straight line, which makes it clear that the relative position of the stimuli on the two retinas is not very significant.
A cell of this type will respond well to a line of proper orientation regardless of its distance - the distance to the line may be greater than, equal to, or less than the distance to the point fixed by the gaze.
Compared to this cell, the neurons whose responses are presented in Fig. 109 and 110 are very sensitive to the relative position of the two stimuli on the two retinas, i.e., they are depth sensitive.
The first neuron (Fig. 109) responds best if the stimuli fall exactly on the corresponding areas of the two retinas. The amount of horizontal misalignment of stimuli (i.e., disparity) at which the cell stops responding is a certain fraction of the width of its receptive field. Therefore, the cell responds if and only if the object is approximately the same distance from the eyes as the fixation point. The second neuron (Fig. 110) responds only when the object is located further than the fixation point. There are also cells that respond only when the stimulus is located closer to this point. When the degree of disparity changes, neurons of the last two types, called distant cells And nearby cells, very sharply change the intensity of their responses at or near the point of zero disparity. Neurons of all three types (cells, tuned to disparity) were discovered in field 17 monkeys.
It is not yet entirely clear how often they occur there, whether they are located in certain layers of the cortex, and whether they are in certain spatial relationships to the ocular dominance columns. These cells are highly sensitive to the distance of an object from the eyes, which is encoded as the relative position of the corresponding stimuli on the two retinas. Another feature of these cells is that they do not respond to stimulation of only one eye or respond, but very weakly. All these cells have the common property of orientation selectivity; as far as we know, they are similar to ordinary complex cells of the upper layers of the cortex, but they have an additional property - sensitivity to depth. In addition, these cells respond well to moving stimuli and sometimes to the ends of lines.
J. Poggio of Johns Hopkins Medical School recorded the responses of such cells in field 17 of an awake monkey with implanted electrodes, which had previously been trained to fixate with its gaze a specific object. In anesthetized monkeys, such cells were also detected in the cortex, but were rarely found in area 17 and very often in area 18. I would be extremely surprised if it turned out that animals and humans can stereoscopically estimate distances to objects using only the three described above types of cells - configured to zero disparity, “near” and “far”. I would rather expect to find a complete set of cells for all possible depths. In awake monkeys, Poggio also encountered narrowly tuned cells that responded best not to zero disparity, but to small deviations from it; Apparently, there may be specific neurons in the cortex for all levels of disparity. Although we still don't know exactly how the brain "reconstructs" a scene involving many widely spaced objects (whatever we mean by "reconstruction"), cells like those described above are likely involved in the early stages of this process.
Some problems associated with stereoscopic vision
During the study of stereopsis psychophysicists faced a number of problems. It turned out that the processing of some binocular stimuli occurs in the visual system in completely unclear ways. I could give many examples of this kind, but I will limit myself to just two.
Using the example of stereo pairs shown in Fig. 105, we saw that moving two identical images (in this case circles) towards each other leads to a feeling of greater proximity, and towards each other - to a feeling of greater distance. Let us now assume that we perform both of these operations simultaneously, for which we place two circles in each frame, located next to each other (Fig. 111).
Obviously, considering this stereo pairs could lead to the perception of two circles - one closer and the other further away from the plane of fixation. However, another option can be assumed: we will simply see two circles lying side by side in the plane of fixation. The fact is that these two spatial situations correspond to the same images on the retinas. In reality, this pair of stimuli can be perceived only as two circles in the plane of fixation, which can be easily verified if the square frames in Fig. 1 are merged in any way. 111.
In exactly the same way, we can imagine a situation where we consider two chains of x signs, say, six characters per chain. If we look at them through a stereoscope, then in principle one can perceive any of a number of possible configurations depending on which sign x from the left chain merges with a certain sign x in the right chain. In fact, if we examine such a stereopair through a stereoscope (or in another way that creates a stereo effect), we will always see six x signs in the plane of fixation. We still don't know how the brain resolves this ambiguity and chooses the simplest possible combination. Because of this kind of ambiguity, it is difficult to even imagine how we manage to perceive a three-dimensional scene that includes many branches of different sizes located at different distances from us. True, physiological evidence suggests that the task may not be so difficult, since different branches are likely to have different orientations, and we already know that cells involved in stereopsis are always orientation-selective.
A second example of the unpredictability of binocular effects, related to stereopsis is the so-called struggle of visual fields, which we also mention in the section on strabismus (Chapter 9). If very different images are created on the retinas of the right and left eyes, then often one of them ceases to be perceived. If you look with your left eye at a grid of vertical lines, and with your right eye at a grid of horizontal lines (Fig. 112; you can use a stereoscope or eye convergence), you would expect to see a grid of intersecting lines.
However, in reality it is almost impossible to see both sets of lines at the same time. Either one or the other is visible, each of them only for a few seconds, after which it disappears and the other appears. Sometimes you can also see a kind of mosaic of these two images, in which individual more homogeneous sections will move, merge or separate, and the orientation of the lines in them will change (see Fig. 112, below). For some reason, the nervous system cannot perceive so many different stimuli simultaneously in the same part of the visual field, and it suppresses the processing of one of them.
Word " suppress" we use here simply as another description of the same phenomenon: in fact, we do not know how such suppression is carried out and at what level of the central nervous system it occurs. I think the mosaic nature of the perceived image when the visual fields compete suggests that the "decision making" in this process occurs quite early in the processing of visual information, perhaps in field 17 or 18. (I'm glad I don't have to defend this assumption .)
The phenomenon of visual field struggle means that in cases where the visual system cannot combine the images on the two retinas (into a flat scene if the images are the same, or into a three-dimensional scene if there is only slight horizontal disparity), it simply rejects one of the images - either completely when, for example, we look through a microscope while keeping the other eye open, either partially or temporarily, as in the example described above. In the microscope situation, attention plays a significant role, but the neural mechanisms underlying this shift in attention are also unknown.
You can observe another example of the struggle between visual fields if you simply look at some multicolor scene or picture through glasses with red and green filters. The impressions of different observers in this case may vary greatly, but most people (myself included) notice a transition from an overall reddish tone to a greenish tone and back again, but without the yellow color that is obtained when red light is usually mixed with green.
Stereo blindness
If a person is blind in one eye, then it is obvious that he will not have stereoscopic vision. However, it is also absent in some people whose vision is otherwise normal. The surprising thing is that the proportion of such people is not too small. So, if you show stereo pairs like those shown in Fig. 105 and 106, with one hundred student subjects (using Polaroids and polarized light), it is usually found that four or five of them cannot achieve the stereo effect.
This often surprises them, since in everyday conditions they do not experience any inconvenience. The latter may seem strange to anyone who, for the sake of experiment, tried to drive a car with one eye closed. Apparently, the lack of stereopsis is fairly well compensated by the use of other depth cues, such as motion parallax, perspective, partial occlusion of some objects by others, etc. In Chapter 9 we will look at cases of congenital strabismus, when the eyes work uncoordinated for a long time. This can lead to disruption of connections in the cortex that provide binocular interaction, and as a result, to the loss of stereopsis. Strabismus is not very rare, and even a mild degree of it, which may go unnoticed, is likely to cause stereoblindness in some cases. In other cases, stereopsis disorder, like color blindness, can be hereditary.
Since this chapter has dealt with both the corpus callosum and stereoscopic vision, I will take this opportunity to say something about the connection between these two things. Try asking yourself the question: what kind of stereopsis disturbances can be expected in a person with a cut corpus callosum? The answer to this question is clear from the diagram shown in Fig. 113.
If a person fixes point P with his gaze, then the projections of point Q, located closer to the eyes within the acute angle FPF - QL and QR - will appear in the left and right eyes on opposite sides of the fovea. Accordingly, the Ql projection transmits information to the left hemisphere, and the Qr projection - to the right hemisphere. In order to see that point Q is closer than P (i.e., to obtain a stereo effect), you need to combine information from the left and right hemispheres. But the only way to do this is to transmit information along the corpus callosum. If the path through the corpus callosum is destroyed, the person will be stereoblind in the area shaded in the figure. In 1970, D. Mitchell and K. Blakemore of the University of California, Berkeley, studied stereoscopic vision in one person with a transected corpus callosum and obtained exactly the result predicted above.
The second question, closely related to the first, is what disruption of stereopsis will occur if the optic chiasm is cut along the midline (as R. Myers did on cats). The result here will be in a certain sense the opposite. From Fig. 114 it should be clear that in this case each eye will become blind to stimuli falling on the nasal region of the retina, that is, emanating from the temporal part of the visual field.
Therefore, there will be no stereopsis in the lighter-colored area of space, where it is normally present. The lateral zones outside this area are generally accessible only to one eye, so there is no stereopsis here even under normal conditions, and after cutting the chiasm they will be zones of blindness (this is shown in a darker color in the figure). In the area behind the fixation point, where the temporal parts of the visual fields overlap, now invisible, blindness will also occur.
However, in the area closer to the fixation point, the remaining hemifields of both eyes overlap, so stereopsis should be preserved here, unless the corpus callosum is damaged. K. Blakemore nevertheless found a patient with complete cutting of the chiasm in the midline (this patient, as a child, received a skull fracture while riding a bicycle, which apparently led to a longitudinal rupture of the chiasm). During the examination, he was found to have exactly the combination of vision defects that we have just hypothetically described.
Article from the book: .
Binocular (stereoscopic) vision is a person’s vision of the surrounding world with two eyes. This ability is due to a complex mechanism in the brain that merges images received from each eye.
Thanks to stereoscopic vision, a person is able to perceive surrounding objects in a three-dimensional image (i.e., in relief and three-dimensional). Monocular vision limits a person professionally, i.e. he cannot engage in activities related to precise actions near an object (for example, hitting a needle with a thread).
The formation of a single visual image is possible provided that the images fall on identical areas of the retina.
Formation of three-dimensional vision
Every newborn has monocular vision and cannot fix his gaze on surrounding objects. However, after 1.5-2 months, the baby begins to develop the ability to see with both eyes, which makes it possible to fix objects with his gaze.
At 4-6 months, the child develops many reflexes, both unconditioned and conditioned (for example, the reaction of the pupils to light, coordinated movements of both eyes, etc.).
However, full-fledged binocular vision, which includes the ability to determine not only the shape and volume of objects, but also their spatial arrangement, finally develops after the child begins to crawl and walk.
Stereoscopic vision conditions
Full binocular vision is possible under the following conditions:
- visual acuity of both eyes is at least 0.5;
- normal tone of the extraocular muscles;
- the absence of injuries, inflammatory diseases and tumors of the orbit, which can determine the asymmetric location of the eyeballs;
- absence of pathologies of the retina, pathways, and cortex.
Research methods
There are several ways to determine a person's stereoscopic vision.
Test with knitting needles. The doctor holds the knitting needle at arm's length in a vertical position, the patient is positioned opposite and must touch the doctor's knitting needle with the tip of his knitting needle so that a straight line of two knitting needles is obtained. The subject's eyes are open. The doctor puts slight pressure on the eyeball in the eyelid area, and the patient experiences double vision (in the case of stereoscopic vision).
Experience with a “hole” in the palm. The patient looks through the tube with one eye, and places his palm towards the end of the tube from the side of the other eye. Normally, the examinee should see a hole in the palm, and in this hole - the image that he sees through the tube with his first eye.
Pathology of stereoscopic vision
Binocular vision can be impaired when the visual axis of one eye deviates outward, inward, upward, or downward. This phenomenon is called heterophoria (hidden strabismus).
To learn more about eye diseases and their treatment, use the convenient site search or ask a specialist a question.
Vision is vital for most living organisms. It helps to correctly navigate and react to the environment. It is the eyes that transmit about 90 percent of information to the brain. But the structure and placement of the eyes differs among different representatives of the living world.
What kind of vision is there?
The following types of vision are distinguished:
- panoramic (monocular);
- stereoscopic (binocular).
The surrounding world is perceived, as a rule, with one eye. This is typical mainly for birds and herbivores. This feature allows you to notice and respond to impending danger in time.
Stereoscopic vision is inferior to panoramic vision with less visibility. But it also has a number of advantages, one of which is a three-dimensional image.
stereoscopic vision
Stereoscopic vision is the ability to see the world around us with two eyes. In other words, the overall picture consists of a fusion of images entering the brain from each eye simultaneously.
With this type of vision, you can correctly estimate not only the distance to a visible object, but also its approximate size and shape.
Besides this, stereoscopic vision has another significant advantage - the ability to see through objects. So, if you place, for example, a fountain pen in a vertical position in front of your eyes and look alternately with each eye, then a certain area will be closed in both the first and second cases. But if you look with both eyes at the same time, then the pen ceases to be a hindrance. But this ability to “look through objects” loses its power when the width of such an object is greater than the distance between the eyes.
The peculiarities of this type of vision in various representatives of the globe are presented below.
Features of insects
Their vision has a unique insect-like appearance that resembles a mosaic (for example, the eyes of a wasp). Moreover, the number of these mosaics (facets) differs among different representatives of a given representative of the living world and ranges from 6 to 30,000. Each facet perceives only part of the information, but in total they provide a complete picture of the surrounding world.
And insects perceive colors differently than people. For example, a red flower that a person sees is perceived by a wasp as black.
Birds
Stereoscopic vision in birds is the exception rather than the rule. The fact is that most birds have eyes located on the sides, which provides a wider viewing angle.
This type of vision is characteristic mainly of birds of prey. This helps them correctly calculate the distance to moving prey.
But birds have much less visibility than, for example, people. If a person is able to see at 150°, then birds only from 10° (sparrows and bullfinches) to 60° (owls and nightjars).
But there is no need to rush into asserting that the feathered representatives of the living world are deprived of the ability to fully see. Not at all. The point is that they have other unique capabilities.
For example, owls have eyes closer to their beaks. However, as already noted, their viewing angle is only 60°. Therefore, owls are able to see only what is directly in front of them, and not the environment to the sides and behind. These birds have another distinctive feature - their eyes are motionless. But at the same time they are endowed with another unique ability. Due to their structure, they are able to rotate their heads 270°.
Fish
As you know, the vast majority of fish species have eyes located on both sides of the head. They have monocular vision. The exception is predatory fish, especially hammerhead sharks. For many centuries, people have been interested in the question of why this fish needs such a head shape. American scientists have found a possible solution. They put forward the version that the hammerhead fish sees a three-dimensional image, i.e. she is endowed with stereoscopic vision.
To confirm their theory, scientists conducted an experiment. To do this, sensors were placed on the heads of several species of sharks, with the help of which activity activity was measured when exposed to bright light. The subjects were then placed in an aquarium. As a result of this experiment, it became known that the hammerhead fish is endowed with stereoscopic vision. Moreover, the greater the distance between the eyes of this type of shark, the greater the accuracy of determining the distance to an object.
In addition, it became known that the hammerhead fish's eyes rotate, which allows it to fully see its surroundings. This gives it a significant advantage over other predators.
Animals
Animals, depending on the species and habitat, are endowed with both monocular and stereoscopic vision. For example, herbivores that live in open spaces, in order to preserve their lives and quickly respond to impending danger, must see as much space around them as possible. Therefore, they are endowed with monocular vision.
Stereoscopic vision in animals is characteristic of predators and inhabitants of forests and jungles. First, it helps to correctly calculate the distance to its victim. For the second, such vision allows them to better focus their gaze among many obstacles.
For example, this type of vision helps wolves during long pursuit of prey. For cats - during a lightning attack. By the way, it is in cats that, thanks to their parallel visual axes, the visual angle reaches 120°. But some dog breeds have developed both monocular and stereoscopic vision. Their eyes are located on the sides. Therefore, to view an object at a great distance, they use frontal stereoscopic vision. And to view nearby objects, dogs are forced to turn their heads.
For the inhabitants of treetops (primates, squirrels, etc.), stereoscopic vision helps in searching for food and in calculating the trajectory of a jump.
People
Stereoscopic vision in humans is not developed from birth. At birth, babies cannot focus their eyes on a specific object. They begin to form only at 2 months of life. However, children begin to fully navigate correctly in space only when they begin to crawl and walk.
Despite the apparent identity, human eyes are different. One of them is the leader, the other is the follower. To recognize it, it is enough to conduct an experiment. Place a sheet with a small hole at a distance of about 30 cm and look through it at a distant object. Then alternately do the same, covering either the left or the right eye. The position of the head should remain constant. The eye for which the image does not change position will be the leading one. This definition is important for photographers, videographers, hunters and some other professions.
The role of binocular vision for humans
This type of vision arose in humans, as well as in some other representatives of the living world, as a result of evolution.
Of course, modern humans do not need to hunt prey. But at the same time, stereoscopic vision plays a significant role in their lives. It is especially important for athletes. Thus, without an accurate calculation of the distance, biathletes will not hit the target, and gymnasts will not be able to perform on the balance beam.
This type of vision is very important for professions that require instant reaction (drivers, hunters, pilots).
And in everyday life you cannot do without stereoscopic vision. For example, it is quite difficult, seeing with one eye, to insert a thread into the eye of a needle. Partial loss of vision is very dangerous for a person. Seeing with only one eye, he will not be able to correctly navigate in space. And the multifaceted world will turn into a flat image.
Obviously, stereoscopic vision is the result of evolution. And only a select few are endowed with it.
What is binocular vision? Binocular vision is the ability to see images clearly with both eyes at once. Two images received by both eyes are formed into one three-dimensional image in the cerebral cortex.
Binocular vision or stereoscopic vision allows you to see three-dimensional features and check the distance between objects. This type of vision is mandatory for many professions - drivers, pilots, sailors, hunters.
In addition to binocular vision, there is also monocular vision, this is vision with only one eye, the brain of the head selects only one picture for perception and blocks the second. This type of vision allows you to determine the parameters of an object - its shape, width and height, but does not provide information about the location of objects in space.
Although monocular vision gives good results in general, binocular vision has significant advantages - visual acuity, three-dimensional objects, and an excellent eye.
Mechanism and conditions
The main mechanism of binocular vision is the fusion reflex, that is, the ability to merge two images into one stereoscopic picture in the cerebral cortex. In order for the pictures to become one, the images received from both retinas must have equal formats - shape and size, in addition, they must fall on identical corresponding points of the retina.
Each point on the surface of one retina has its corresponding point on the retina of the other eye. Non-identical points are disparate or asymmetrical areas. When the image hits disparate points, merging will not occur; on the contrary, doubling of the picture will occur.
What conditions are needed for normal binocular vision:
- ability to fusion - bifoveal fusion;
- consistency in the work of the oculomotor muscles, which allows for the parallel position of the eyeballs when looking at a distance and the corresponding convergence of the visual axes when looking close; joint work helps to obtain the correct eye movements in the direction of the object in question;
- location of the eyeballs in the same horizontal and frontal plane;
- visual acuity of both visual organs is at least 0.3-0.4;
- obtaining images of equal size on the retinas of both eyes;
- transparency of the cornea, vitreous body, lens;
- absence of pathological changes in the retina, optic nerve and other parts of the organ of vision, as well as subcortical centers and cerebral cortex.
How to determine
To determine whether you have binocular vision, use one or more of the following methods:
- “Hole in the palm” or Sokolov’s method - put a tube to your eye (you can use a folded sheet of paper) and look into the distance. Then place your palm on the side of the other eye. With normal binocular vision, a person will have the impression that there is a hole in the center of the palm that allows one to see, but in fact the image is viewed through a tube.
- Kalfa method or test with misses - take two knitting needles or 2 pencils, their ends must be sharp. Hold one knitting needle vertically in front of you and the other horizontally. Then connect the ends of the knitting needles (pencils). If you have binocular vision, you will easily complete the task; if you have monocular vision, you will miss the connection.
- Reading test with a pencil - while reading a book, place a pencil a few centimeters from your nose, which will cover part of the text. With binocular vision, you can still read it because the images from both eyes are superimposed in the brain of the head without changing the position of the head;
- Four-point color test - this test is based on the separation of the visual fields of the two eyes, which can be achieved using colored glasses - filters. Place two green, one red and one white objects in front of you. Wear glasses with green and red lenses. With binocular vision, you will see green and red objects, and white objects will turn green-red. With monocular vision, a white object will be colored the same color as the lens of the dominant eye.
Binocular vision can be developed at any age. However, this type of vision is not possible with strabismus, since in this case one eye deviates to the side, which does not allow the visual axes to converge.
Important facts about the development of strabismus in children
Strabismus is a condition of the eyes in which the visual axes do not converge on the object in question. Outwardly, this is manifested by the fact that the eye deviates in one direction or another (to the right or to the left, more rarely up or down, there are also various combined options).
If the eye is brought to the nose, the strabismus is called convergent (more common), and if it is brought to the temple - divergent. It can be squinting in one eye or both. Most often, parents turn to a pediatric ophthalmologist after noticing that the child’s eyes are looking “wrong.”
Strabismus is not just a problem of appearance. The effect of strabismus is a consequence of disturbances in perception and the conduction of visual information throughout the child’s visual system. With strabismus, visual acuity decreases, connections between the right and left eyes, and the correct balance between the muscles that move the eyes in different directions are disrupted. Apart from this, the ability for three-dimensional visual perception is impaired.
Strabismus can be congenital, but more often it appears in early childhood. If the disease manifests itself before 1 year of age, then it is called early acquired. The pathology is likely to appear at 6 years of age. However, strabismus most often develops between the ages of 1 and 3 years.
At birth, the child cannot yet see with “two eyes”; the ability for binocular vision develops gradually until the age of 4 years. In this case, each deviation of the visual axis from the point of immobilization must be qualified as strabismus and under no circumstances be considered as a variant of the norm. This applies even to similar, seemingly cosmetically insignificant cases, such as small-angle strabismus and unstable strabismus.
Most often, strabismus develops in children with farsightedness - when the baby has trouble seeing objects that are close. Strabismus can also develop in children with astigmatism. With astigmatism, certain areas of the image may be focused on the retina, others behind or in front of it (there are more complex cases).
As a result, a person sees a distorted image. You can get an idea of this by looking at your reflection in an oval teaspoon. The same distorted image is formed with astigmatism on the retina. However, the picture itself with astigmatism may turn out to be indistinct and blurry; a person, as a rule, is not aware of this distortion, since the brain’s central nervous system “corrects” his perception.
Strabismus can also occur with myopia - when a child has difficulty seeing objects located far away. With strabismus, a gradual decrease in visual acuity occurs in the always squinting eye - amblyopia. This complication is due to the fact that the visual system, in order to avoid chaos, blocks the transmission to the central nervous system of the image of the object that the squinting eye perceives. This position leads to an even greater deviation of this eye, i.e. strabismus intensifies.
The process of vision loss depends on the age of onset of the disease. If this happened in early childhood, in the first year of life, then the decrease in visual acuity can be very, very rapid.
The causes of strabismus may be:
- hereditary tendency, when close relatives (parents, uncles, aunts, etc.) have the disease;
- the presence of any optical defect (defocusing) of the child’s organ of vision, for example, with farsightedness in children;
- various poisonings of the fetus during pregnancy;
- severe infectious diseases of the child (for example, scarlet fever, mumps, etc.);
- neurological pathologies.
In addition, the impetus for the occurrence of strabismus (against the background of prerequisites) can be high temperature (over 38°C), mental or physical damage.
Treatment of strabismus in children
There are more than 20 different types of strabismus. Outwardly, all of them are manifested by deviation of the visual axis from the point of immobilization, but in terms of their own causal factors and development mechanism, and in the depth of the disorders, they are very different from each other.
Any type of strabismus requires an individual approach. Unfortunately, even among medical professionals, there is a widespread assumption that until the age of 6, a child with strabismus does not need to do anything and everything will go away on its own.
This is the greatest misconception. Every deviation of the eye at any age should be considered the beginning of pathology. If no measures are taken, loss of visual acuity may occur, and then treatment will require more effort and time, and in some situations the changes become irreversible.
From time to time, strabismus is imaginary: due to the wide bridge of the baby’s nose, parents suspect the presence of this visual defect, but in reality it does not exist - it is just an illusion. In newborns, the eyes are set very close, and the bridge of the nose, due to the peculiarity of their facial skeleton, is wide.
As the facial skeleton forms, the distance between the eyes increases and the width of the bridge of the nose decreases. It is then that everything actually goes away with age and nothing needs to be corrected, however, only a doctor can determine whether it is an imaginary strabismus or a real one.
Any suspicion of a deviation from the norm should alert parents and prompt them to visit a pediatric ophthalmologist as soon as possible. Timing of preventive visits to an ophthalmologist in the first year of a child’s life.
The first examination is advisable immediately after childbirth. It must be stated that in maternity hospitals, all babies without exception are not examined by an ophthalmologist. A neonatologist at the maternity hospital or a local pediatrician may classify the baby as a danger group, and then he will be prescribed a consultation with an ophthalmologist already in the maternity hospital or immediately after discharge.
The danger group includes children with a family history of eye diseases (if their parents have them), premature newborns, children born during pathological births, and children whose parents have bad habits (alcohol addiction, smoking). Further examination by an ophthalmologist is necessary for the baby at the age of 2 months, at six months and at the age of one year.
During this time, all children are referred to an ophthalmologist. The specialist will detect the absence or presence of farsightedness (myopia) in the child, the acuity and nature of vision, the angle of strabismus, and, if necessary, will refer you for consultation to other experts, for example, to a neurologist. Only after a thorough examination can complex treatment of strabismus be started, including conservative therapy and surgical treatment.
The conservative part of treatment includes methods aimed at increasing visual acuity. If there is farsightedness or nearsightedness, according to indications, the child needs glasses. From time to time they completely correct strabismus. Although just wearing glasses is not enough. It is very important to teach your child to combine images from the right and left eyes into 1 image.
This is achieved through a set of therapeutic measures, carried out in courses several times a year. The treatment is conservative and takes place in a playful way. Apart from this, the occlusion method is used - covering the healthy eye with a bandage for a certain time every day, so that the child learns to rely more on the weak eye.
It should be especially emphasized that the success of strabismus treatment depends on correctly selected individual treatment tactics. The treatment complex often involves the use of both conservative and, in most cases, surgical treatment. In this case, the procedure does not need to be treated as an alternative to conservative treatment.
Surgery is one of the stages of treatment, the place and time of which depend on the type of strabismus and the depth of damage to the visual system.
Before and after surgical treatment, it is necessary to carry out conservative therapeutic measures aimed at increasing visual acuity, to restore the connection between the eyes and stereoscopic volumetric visual perception - this is achieved with the help of special exercises.
They use techniques that make it possible to increase the functional position of the visual part of the cerebral cortex of the central nervous system, to force the visual cells of the cortex to work in a normal mode and thereby ensure clear and correct visual perception.
These techniques are stimulating in nature. Classes are conducted using special devices on an outpatient basis in courses of 2-3 weeks. several times a year. During the treatment at a certain stage, in the presence of high visual acuity, restoration of the ability to merge 2 images from the left and right eyes into a single visual image, in the presence of eye deviation, surgical intervention is performed on the eye muscles. The procedure is aimed at restoring the correct balance of the muscles that move the eyeballs (oculomotor muscles).
It is important to understand that the procedure does not replace therapeutic techniques, but solves a specific problem that cannot be solved conservatively. To decide the timing of surgical intervention, it is important that the patient has sufficient visual acuity. The earlier you put your eyes in a symmetrical state with direct gaze, the better. There are no special age restrictions.
In case of congenital strabismus, it is important to complete the surgical stage no later than 3 years, in case of acquired strabismus, depending on the timing of achieving good visual acuity at the conservative stage of treatment and restoring the potential ability to merge images from 2 eyes into a single visual image. Surgical treatment tactics are developed depending on the type of strabismus.
From a surgical standpoint, treating a permanent form of strabismus with a huge squint angle, when the eye is seriously deviated, does not pose a huge difficulty. The effect of these operations is obvious to the patient. And for surgeons with certain qualifications it will not be an effort. It is difficult to operate on strabismus with unstable and small angles.
Technologies have now been developed for making an incision without the use of a cutting structure (scissors, scalpel, laser beams). The tissues are not cut, but rather moved apart by a high-frequency stream of radio waves, providing bloodless exposure of the surgical field.
The technique of operations for strabismus is microsurgical, general anesthesia with specific anesthesia is used, which allows you to completely relax the oculomotor muscles. Depending on the volume of the operation, its duration ranges from 20 minutes. before 1.5 hours.
The child is discharged home on the second day after surgery. In the absence of a vertical component (when the eye is not displaced upward or downward), usually 1 or 2 operations are performed on one and the second eye, depending on the size of the eyeball and the type of strabismus.
The earlier the symmetrical position of the eye is achieved, the more favorable the prospect of cure. By school, a child with strabismus should be rehabilitated to the maximum extent possible. If you deal with the problem of strabismus comprehensively, then cure occurs in 97 percent of cases.
Thanks to a timely treated disease, the child can study normally, get rid of psychological difficulties due to visual defects, and subsequently do what he loves.
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The binocular function formed in patients with concomitant strabismus in the process of orthoptic and diploptic treatment may be more or less perfect. The fusion of images of one and the second eye can occur only in one plane - this is planar binocular vision, determined by the color test, synoptophore and Bagolini test.
The binocular function is considered complete only in cases where the fusion of images of both eyes is accompanied by the perception of depth, volume, and stereoscopicity. This is the highest form of binocular function - stereoscopic vision.
The perception of depth and stereoscopicity arises due to the disparity of images on the retina of both eyes. The right and left eyes are at some distance from each other. The images of each point of the fixed object on the retina of one and the second eye are slightly shifted in the horizontal direction relative to the central fovea. The consequence of this displacement, disparity, is the feeling of depth, stereoscopicity.
The formation of full-fledged stereoscopic vision, according to R. Sachsenweger (1956), is completed by the 8th year of a child’s life.
R. Sachsenweger introduces the term "stereoamaurosis"- complete absence of stereoscopic vision (similar to the term “amaurosis” - complete blindness) and “stereoamblyopia” - functional inferiority of stereoscopic vision (similar to the term “amblyopia” - functional decrease in central vision).
The quality of depth vision is determined by a threshold. The threshold of depth vision is taken to be the maximum difference in depth that the subject is no longer able to perceive. The higher the threshold, the worse the depth vision. Depth vision thresholds are not the same when examined with different instruments and at different distances. They are expressed in millimeters or arcseconds.
The appearance of strabismus in a child destroys his binocular and stereoscopic vision.
Restoration of stereoscopic vision is carried out at the final stage of strabismus treatment, when planar binocular vision has already been formed and normal fusion reserves have been developed. When restoring deep vision in children with strabismus, T.P. Kashchenko (1973) noted the dependence of the results on the level of visual acuity of both eyes, the magnitude of the strabismus angle and fusion ability. V.A. Khenkin (1986) additionally noted the dependence of depth vision thresholds on the timing of strabismus, the final visual acuity of the squinting eye, the difference in visual acuity of both eyes and the magnitude of aniseikonia.
Depth, stereoscopic vision is better the later the strabismus appeared, the higher the final visual acuity of both eyes, the better the fusion and the lower the degree of aniseikonia. With aniseikonia of 5%, depth perception is possible only in individual patients and its quality is very low.
It should be noted that stereo vision can be restored only in that part of children with concomitant strabismus in whom it was formed to some extent before the onset of strabismus. With congenital and early developed strabismus, it is not possible to develop stereoscopic vision.
There are special devices for diagnosing, forming and training stereoscopic vision.
1) The classic device for assessing real depth vision remains the three-spoke Howard-Dolman device (Fig. 47).
It consists of a 50 cm long rod on which three knitting needles are placed. Two of them are fixed on the sides of the rod, and the third, the middle one, is movable. For eyes, horizontal slits are made at one end of the rod. A diaphragm in the form of a horizontal slit is installed between the eyes and the spokes, which does not allow the patient to see the tops and bases of the spokes. The middle spoke moves back and forth.
The patient must determine whether it is in front of the two spokes or behind them and, finally, install all three spokes in the frontal plane, catching the moment when the displaced spoke becomes equal to the stationary ones. This distance between the moving and fixed spokes determines the threshold for depth vision.
R. Sachsenweger's monograph “Anomalies of stereoscopic vision in strabismus and their treatment” (1963) describes many devices used for diagnosing and training stereoscopic vision. Let us introduce readers to some of them.
Rice. 47. A device with three spokes, a) with the diaphragm removed, b) with the diaphragm installed.
2) (Fig. 48) consists of a body 1, inside of which two glass plates 3 and 4 are placed. They are illuminated by an electric lamp 2 placed behind them. There are small round dots glued on both plates. On plate 3 they are arranged in no particular order, and on plate 4 they form the outline of a figure. When the plates stand directly next to each other, the figure cannot be distinguished. As the distance between them increases, the figure, depending on the spatial threshold, begins to differ earlier or later.
Rice. 48 Parallax Visoscope
3) (Fig. 49) has drawers 1,2,3, equipped with light bulbs. The drawers can be moved forward and backward on the rails. There are slots in the front wall of the drawers into which any templates, as well as color and neutral density filters, can be inserted.
The study is carried out in the dark, and the size of the light object, its brightness and color are often changed. The patient must determine which of the objects is closer and which is farther away, place the objects in the same frontal plane, arrange them evenly in depth, etc.
4) (Fig. 50). The basis of the device is a wire circuit standing vertically in the middle plane, inside which the patient must pass a metal pencil without touching the wire. Touching the wire with a pencil causes the current circuit to close and a buzzer to sound. The patient's view is limited in such a way that he cannot examine the wire frame from the side.
The difficulty of the task depends on the distance between the wires forming the contour. This distance can be changed using a set screw. The device develops the acuity of deep vision, as visual stimuli are combined with proprioceptive ones. Without deep visual acuity, for example, when using one eye, the exercise cannot be performed even after long training.
Rice. 50 Stereo buzzer
5) Binarimeter(Fig. 51) is a new generation device that uses diploptics methods aimed at developing binocular and stereoscopic vision. In the binarimeter, spatial visual effects are formed that arise when identical images are doubled based on physiological double vision in free haploscopy without optics and separation of visual fields.
Treatment with a binarimeter is carried out after the patient has achieved the ability to bifixate. The design of the device provides the possibility of carrying out treatment not only with a symmetrical position of the eyes, but also with slight deviations horizontally and vertically.
Fig.51. Binarimeter "Binar"
Exercises on the device activate sensory-motor interactions, helping to restore binocular and stereoscopic vision.
We used the binarimeter in combination with other methods for restoring binocular and stereoscopic vision in school-age children and adolescents, since treatment with it requires a certain intelligence.
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