home All hormones Three-dimensional vision in humans. Binocular vision: mechanism of operation, deviations and methods of correction. Definition of stereoscopic vision

Three-dimensional vision in humans. Binocular vision: mechanism of operation, deviations and methods of correction. Definition of stereoscopic vision

Human vision is the body’s amazing ability to perceive the world around us in all its colors.

Thanks to the special structure of the visual system, every person is able to evaluate the environment in terms of volume, distance, shape, width and height.

Also, the eyes are able to perceive all available colors and shades, to perceive color in all its gradations.

But it happens that a failure occurs in the system and those affected will not be able to appreciate all the depths of the external environment.

What is binocular and stereoscopic vision?

The eyes are a paired organ that works harmoniously with each other and with the brain. When a person looks at one object, he sees one object, not two objects. In addition, when looking at an object, a person is automatically and instantly able to determine its size, volume, shape and other parameters and features. This is binocular vision.

Stereoscopic vision - the ability to see three-dimensionally - is the quality of binocular vision, thanks to which a person sees relief, depth, that is, perceives the world three-dimensionally.

It was stereoscopic vision that formed the basis of the once innovation - 3D technology, which conquered the world. With binocular vision, the field of vision expands and visual acuity increases.

How to determine binocular vision?

Many techniques are used for this. The most popular technique is the Sokolova test.

To carry out the test you will need: take any notebook, which you will need to roll into a tube and place it on your right eye. At this time, extend your left hand forward, mentally resting your palm into the distance. The distance from the palm to the left eye should be about 15 cm.

This way, two “pictures” are obtained - a palm and a “tunnel”. Looking at them at the same time, these pictures overlap each other. As a result, a “hole in the palm” is formed. This indicates that vision is binocular.

What is necessary to develop binocular vision?

Binocular vision is possible when:

Visual acuity of at least 0.4 DPT, which ensures clear imprinting of objects on the retina.
There is free mobility of both eyeballs. This indicates that all muscles are toned. And this is a prerequisite for binocular vision.

It is the muscles that ensure the necessary parallel alignment of the visual axes, which guarantees the refraction of light rays precisely on the retina of the eye.

Causes of binocular vision impairment

Stereoscopic vision (binocular) is the norm for humans. But there are a number of reasons that can disrupt the normal course of vital activity of the organ of vision.

These reasons are:

Note that impaired binocular vision requires prompt diagnosis by an ophthalmologist, as it poses a threat to its owner. Having minimal impairment of binocularity, a person becomes unprofessional and his activity becomes limited.

What causes monocular vision?

Monocular vision is seeing through one eye. That is, with monocular vision, the environment is perceived indirectly. That is, everything is perceived based on the size and shape of objects. With monocular vision, three-dimensional vision is not possible. For example, a person who can see with one eye will have great difficulty pouring water into a glass, much less threading a thread through an eye.

This significantly limits a person’s capabilities, both socially and professionally.

The causes of monocular vision are the causes that impair binocular vision. We wrote about these reasons earlier.

To check whether binocular vision is impaired, that is, whether monocular vision occurs, you can do this:

Take one sharpened pencil in both hands.
Now extend your arm a little, close one eye and connect your hands with the pencils, trying to connect the sharp leads of the pencils.
The more difficult it is to do this, the more signs of monocular vision there are.

Color vision: what it is and what disorders there are

Color vision is provided by cones - color receptors that were formed as a result of mutation. Today, this mutation determines the usefulness of vision, which is considered to be vision capable of perceiving, distinguishing and feeling colors of all spectrums.

Color vision is an advantage of the higher primate - humans, which distinguishes its retina from the retinas of other representatives of this order.

How does color vision work?

Normally, the iris of the eye contains, in addition to other receptors, three different types of cones. Each cone absorbs rays of different lengths. Rays of different lengths make up the color characteristic.

Color is characterized by: hue, color saturation and brightness. Saturation, in turn, reflects the depth, purity and brightness of the color and its shade. And the brightness of the color depends on the intensity of the light flux.

Color vision disorders

Color vision disorders can be congenital or acquired. As a rule, innate color perception is more typical for men.

The main cause of loss of color perception is the loss of cones. Depending on which cone is missing, the eye loses the ability to perceive the color spectrum that this cone “reads.”

Loss of the ability to perceive colors is popularly known as color blindness. This pathology is named after Dalton, who himself suffered from color vision impairment and was involved in the study of this disorder and color vision in general.

Nowadays, a distinction is made between normal and abnormal trichromasia. Let us recall that everyone who distinguishes all three color spectrums is a trichromat. Accordingly, those who distinguish only two color spectrums are dichromats. We wrote earlier about what is typical for each group and what other color vision disorders there are.

Thus, it is worth paying attention once again to how unique the human visual system is, how important it is to protect and constantly care for it. As a result, pathologies of various kinds will simply not be scary for you.

Video

Stereoscopic vision serves as the most reliable and sensitive indicator of the ability to analyze spatial relationships. According to E.M. Belostotsky (1959), the ability of the visual analyzer to correctly assess the third spatial dimension, i.e. depth vision is one of the components of the complex process of binocular perception of space.

Thanks to the ability to merge images falling on identical or slightly disparate areas of the retinas of both eyes (within the Panum zone), a person is able to freely navigate the surrounding space and evaluate it in three dimensions.

Due to the fact that both eyes are located in the frontal plane and at a certain distance from each other, not quite identical, somewhat shifted images of the object of fixation fall on the retinas of both eyes.

This displacement, or the so-called transverse dispersion, is the main condition for stereoscopic (depth) perception of objects in the external world or the primary factor in depth perception. However, there are differences between stereoscopic and depth vision. Stereoscopic vision can only be reproduced under artificial conditions using stereoscopic devices. It is carried out only with two eyes open, while deep vision, i.e. the ability to estimate the third spatial dimension in natural conditions can occur in both binocular and monocular vision.

The smallest perceived difference in the relative distance of two objects from each other is called acuity, or the threshold of depth vision. Determining the acuity or threshold of depth vision makes it possible to judge the presence or absence of a given subject’s ability to perceive depth and give it a quantitative assessment (in disparity angles or binocular parallax angles).

Stereo perception is also facilitated by secondary factors assessing depth, which also operate in monolateral vision: distribution of light and shade, relative sizes of objects, linear perspective, and other factors that help in assessing the third spatial dimension. There is evidence that the stereoscopic effect persists at a distance of 0.1-100 m. For normal depth vision, the following are necessary: high visual acuity in each eye, the correct structure of both eyes, and the absence of gross disturbances in the function of the oculomotor system.

In clinical practice, special methods for studying stereoscopic vision are used. Some of the methods are based on the use of real depth differences with different locations of test objects in depth: for example, Litinsky’s depth-eye apparatus (1940), three-rod devices of various designs. Other methods are based on the creation of artificial transverse (horizontal) disparity, which is provided by shifting the left and right images of the test object when paired pictures are presented (for example, in a lens stereoscope), or by demonstrating disparate images on the display screen, which are viewed through color, Polaroid or liquid crystal lenses. glasses that allow you to separate the fields of vision of the right and left eyes.

Frubise and Jeansch found that as the observation distance increases, the transverse disparity is better determined. They found that for the same subject, when observed from a distance of 26 m, the depth threshold is 3.2", and when observed from a distance of 6 m - 5.5" (cited from: Sachsenweger R., 1963).

Adams W.E. et al. conducted a study of stereo vision using the FD2 test in children aged 3 to 6 years and found that when the test object was located at a distance of 3 m, the stereo vision threshold was 92", and at a distance of 6 m - 29.6". Thus, they argue that stereo visual acuity at distance is much better than near.

Garnham L. and Sloper J.J. studied stereo visual acuity using four tests - TNO, Titmus, Frisby (near), Frisby-Davis (distance) - in 60 healthy subjects aged 17-83 years.

The TNO test uses random dots, dividing the visual fields of the two eyes using red-green glasses, the Titmus test uses black circles and Polaroid glasses, and the Frisby test uses real objects. The study of stereoscopic and depth vision using these tests is carried out at close range. For distance, the Frisby-Davis test is used with real objects, the angular dimensions of which correspond to the angular dimensions of objects for near.

The figure shows the values of stereo visual acuity using various tests according to Garnham L. and Sloper J.J. . The figure shows that there are significant differences in the acuity of stereo vision in people of different ages, as well as when different tests are used. Thus, when examining persons 17-29 years old, the acuity of stereo vision according to histogram A was 15-240", according to histogram B - 40-60", and according to histogram C - 20-55". For distance, their stereo visual acuity was 4-20", those. The highest acuity of stereo vision is revealed when using real objects, and with distance vision it is higher than with near vision. A similar trend was noted in other age groups.

Kolosova S.A. determined the acuity of deep vision in persons selected for the cosmonaut corps and found that the average thresholds of deep vision with a background illumination of 700 lux at a distance of 30 cm are equal to 10.8", at a distance of 5 m - 4.4", at a distance of 10 m - 2.1", and for some subjects the depth discrimination threshold was below 1". As professional experience accumulates, the acuity of depth vision increases, and when the intensity of background lighting increases to maximum values, it decreases.

Thus, the acuity of stereo vision largely depends on the tests used and the distance to them, the intensity of background lighting, the age of the patients, the degree of their training, the state of their visual functions, the method of processing the data obtained and other factors.

Researchers' opinions about the age norm of stereovision thresholds in children are divided: some believe that children reach the level of the “adult” norm by 7 years, while others note an improvement in indicators by 11-12 years.

High accuracy in measuring stereoscopic vision up to 1" is provided by the Stereopsis computer program. It uses stereo pairs as test objects, consisting of vertical sinusoidal gratings located one above the other with the same spatial frequency (IF) and different disparities, shown on the monitor screen.

In this case, the measurement of stereoscopic vision thresholds can be carried out in a wide range of spatial frequencies from 0.35 to 32 cycles/deg. When measuring the stereo vision threshold, the division of visual fields is carried out using glasses with color (red-green) filters. For each of the studied frequencies, the stereovision threshold is determined as the minimum difference in the disparities of the upper and lower halves of the stereo pair, at which the patient can still distinguish their relative positions in depth.

Vasilyeva N.N., Rozhkova G.I., Belozerov A.E. studied the acuity of stereo vision using the Stereopsis program in 178 schoolchildren aged 7 to 17 years at a distance of 2.27 m. In all age groups, the lowest thresholds were recorded at frequencies of 1.0-2.0 cycle/deg. In the age group of 7-10 years, there were 12% of children with thresholds from 4 to 8"; in the age group of 11-14 years - 42% with thresholds of 1-8"; in the age group 15-17 years - 49% with thresholds of 3-8".

According to Rozhkova G.I. (1992) at least two subsystems of binocular vision—pure binocular and postmonocular—can contribute to the perception and analysis of stimuli. When using a random point image, only the binocular vision subsystem works; when using spatial-frequency stereovisometry, the binocular and postmonocular subsystems work.

In our work, the Stereopsis computer program was used to study stereoscopic vision. Study of stereo vision acuity at distances 5; 2.5; 1; 0.5; 0.33 m from the object was carried out at low spatial frequencies of the observed grating (0.7-1.0 cycles/deg). The initial disparity value for 2.25 m was 1.8", when applying geometric calculations it becomes clear that for a distance of 5 m the given disparity will correspond to 0.8", when approaching a distance of 1 m it will be 4", at a distance of 0 .5 m - 8", and at 0.33 m - 12.2". If the patient sees the minimum specified disparity at different distances, then as they approach the screen, the stereo visual acuity indicators will decrease.

When comparing the data we obtained for a distance of 2.5 m (with emmetropia - 2.1±0.1", with hypermetropia - 1.6±0.2", with myopia - 5.3±0.3"), we do not found great disagreement with the data obtained by Vasilyeva N.N. et al., who used the “Stereopsis” program: in slightly less than half of the cases, stereo vision thresholds for a distance of 2.27 m in children 11-14 years old were 1-8". At the same time, it is necessary to take into account the fact that they examined children with the glasses that they had, and not with a full correction that eliminated ametropia, and some children, as the authors themselves note, did not use the correction at all, being embarrassed to wear glasses. In our case, we selected children only with weak and moderate degrees of ametropia, without astigmatism, and when studying stereo vision, ametropia was completely corrected. Therefore, certain differences in results may be observed. It would be incorrect to compare the obtained stereo vision thresholds with the results of other methods based on the use of tests fundamentally different from those used by us. Assessing the effect of distance on stereoscopic visual acuity undoubtedly depends on the sensitivity of the technique used.

Conclusion

Analysis of literary data confirms the known fact of the dependence of binocular, stereoscopic and depth vision on the methods used, research conditions, the nature and degree of the haploscopic effect of the test objects used.

We do not present the data we obtained, published in the journal “Ophthalmosurgery” (2012, No. 1, pp. 13-19) in the article “State of stereoscopic vision in children with different types of refraction,” as criteria for stereovision thresholds in children; they should be regarded as the thresholds of stereoscopic vision, determined using the Stereopsis computer program, adapted for various research distances, with the same angular magnitude of objects corresponding to a spatial frequency of 0.7-1.0 cycle/deg, in children 10-15 years old with emmetropia and corrected ametropia of weak and moderate degree.

We express our deep gratitude to Professor A.A. Shpak, who showed interest in our work, which once again indicates the relevance of this problem and the need for further study and development of methods for studying such a complex function as stereoscopic vision.

21.06.2015

Stereoscopic vision is widely used when processing aerial photography materials, interpreting aerial photographs and aerial taxation of forests. It significantly increases the accuracy of measurements, so let’s briefly look at its main properties.
To better understand the essence of stereoscopic vision, consider the structure of the human eye. The human eye is a spherical body consisting of three shells; sclera, choroid and retina (Fig. 53).
The sclera is the outer hard protein shell. Adjacent to it is the choroid, which turns into a thickened and opaque iris, which houses the pupil of the eye. It can change its diameter, being a diaphragm that regulates the amount of light entering the eye.

The distance between the centers of the pupils of the eye is called the ocular basis. It varies from 58 to 72 mm for different people. On average it is 65 mm. The lens is located behind the pupil. It is a biconvex lens and can be considered as the lens of the eye, which serves to construct images of observed objects on the retina. In order for images of objects at different distances from us to be sharp, the shape of the lens changes with the help of muscles, and therefore its focal length also changes (from 12 to 16 mm). The ability of the eye to change the curvature of the surfaces of the lens is called accommodation. The membrane lines the inner surface of the eye and is called the retina. Its sensitive elements consist of rods and cones, which are the ends of the branches of the optic nerve and transmit their irritation through the nervous system to the brain of the observer.
Rods and cones are distributed unevenly on the retina. An important part of the retina is the macula macula. It is the place of the clearest vision, located in the middle of the retina, opposite the pupil and slightly offset from the axis of symmetry of the eye. The macula is composed primarily of cones.
The image of objects provided by the lens is built within the macula. The part of the macula that is most sensitive to light is the depression located in the macula. It is called the fovea centralis. Its diameter is 0.4 mm. The straight line passing through the fovea and the center of the lens is called the visual axis of the eye.
In order for a normal eye to see objects without much strain, the distance to them should be about 250 mm. This is called the distance of best vision.

Vision in one eye is called monocular. It allows you to determine the position of an object in a plane and has a certain resolution. Resolution (acuity) of vision is the minimum angle at which the eye can still distinguish two points separately. The resolution of the eye is about 30-40". It depends on the characteristics of the eye and observation conditions.
The depth of space is felt with binocular vision (seeing with two eyes). It has two remarkable properties. Its first property is the merging in the visual impression of two images obtained on the retinas of the eyes into one spatial image.
The second property is an assessment of depth, i.e., the distance of observed objects. Only at large distances does the binocular sense of depth of space do not differ from monocular vision. When moving to closer objects, it turns into stereoscopic vision, remaining binocular. Consequently, stereoscopic vision is a special case of binocular vision, in which the depth of space, the relief of terrain objects and their spatial location are most clearly perceived.
Let's look at some properties of stereoscopic vision.
With binocular vision, the observer positions the eyes so that their visual axes intersect on the object that we are looking at. The point of intersection of the visual axes is called the fixation point M (Fig. 54). When attention is fixed on any point, a field of clear visibility appears. It is limited by the size of the central fossae of the eyes. Within the field of clear visibility, stereoscopic vision occurs with the greatest clarity. With stereoscopic vision on the retina, images of variously distant points are obtained at different distances from the centers of the yellow spots.
The difference between these distances is called physiological parallax

The farther in depth point K is from point M, the greater c will be.
The angle of intersection of the visual axes of the eyes is called the angle of convergence γc. The closer the point is from the observer, the larger the angle γс and, conversely, as the point moves away, the angle γс decreases. The extremely small difference in parallax angles γc-γ"c (see Fig. 54), perceived by the observer, is called stereoscopic visual acuity. Its value is about 20-30" for individual points, and for vertical lines - 10-15".
From the isosceles triangle MSS" it follows that br/2: L = tan γc/2, where L is the distance (distance) of point M from the eye base.
If the angle γc/2 is small, then

where γc is expressed in radians.
This formula allows one to judge the distance L of objects or terrain objects from the observer.
When moving from point M to another point K (Fig. 55) in a field of clear visibility and with a corresponding change in the parallactic angle γ"с, transforming formula (42), we obtain

Formulas (42) and (43) are the basic formulas for stereoscopic vision.
If we take γc = 30", bg = 65 mm, then from formula (42) it follows that

In this case, the angle γc is equal to the acuity of stereoscopic vision, therefore Lg = 450 m is the radius of naked stereoscopic vision. At a distance greater than 450 m, the observer does not receive spatial perception of objects and the terrain should seem flat to him.
The radius of stereoscopic vision can be increased by increasing the basis and acuity of stereoscopic vision. For this purpose, special devices are used, in which the basis is increased by introducing mirrors or prisms, and by introducing lenses, the acuity of stereoscopic vision is increased. Such devices are called stereoscopic.
Stereoscopic perception can be obtained not only by examining the terrain objects themselves, but also their perspective images - aerial photographs.
During a routine aerial survey, each subsequent aerial photograph overlaps the previous aerial photograph by 60%.

Let's place adjacent aerial photographs - a stereo pair in front of the eyes so that there are overlapping parts in the field of view and the shooting basis is parallel to the eye basis (Fig. 56).
By moving these aerial photographs along the basis line of the aerial photograph by the appropriate amount and examining the same image in places of overlap with the left and right eyes, we obtain instead of two one spatial image of the area, giving a clear idea of the relationship of height between different objects. A stereoscopic image of a captured area is called a stereoscopic terrain model.
The stereoscopic effect occurs because the difference in longitudinal parallaxes Δp of the points of aerial photographs, when viewed, is converted into a difference in physiological parallaxes.
To obtain a stereo effect, special devices are used - stereoscopes. A stereoscope allows you to see one image with one eye and another with the other.
If the left eye sees the left aerial photograph, and the right eye sees the right one, then a direct stereo effect occurs (mountains are depicted as mountains, hollows as hollows), Fig. 56, a.
If the left eye sees the right aerial photograph, and the right eye sees the left, a reverse stereo effect occurs (mountains are depicted as ravines, and ravines as mountains) - see fig. 56.6, If aerial photographs prepared for direct stereo effect are rotated by 90°, then zero stereo effect occurs. In this case, all objects will seem to lie in the same plane (see Fig. 56,a).
Let's consider the device of a mirror stereoscope. It consists of four mirrors, parallel in pairs (Fig. 57).

When working with a mirror stereoscope, rays o1m1 and o2m2, which initially go vertically from the aerial photograph, after reflection will go horizontally, then from the second mirrors they will again go vertically and hit the observer’s eyes.
Distance o1m1k1S1 = o2m2k2S2 = fc, where is the main distance of the stereoscope, measured from the center of the mirror along the beam to the aerial photograph.
It should be noted that when viewing aerial photographs under a stereoscope, an imaginary model (stereomodel) is obtained, since the actual intersection of the rays does not occur.
The magnification of the visible image on aerial photographs viewed under a stereoscope is equal to the ratio of the distance of best vision ρ0 to the main distance of the stereoscope Vc = ρ0/fc. A mirror stereoscope has fc = 250, so Vc = 1X.
If lenses are installed between the mirrors, then fc is measured from the center of the lens along the main ray to the plane of the aerial photograph.
To determine the minimum height difference hmin (point elevations) that we see on aerial photographs, we transform the second of the basic formulas for stereo vision ΔL = L2v/bg, in which ΔL is replaced by hmin (or Δh), L - by the photographing height H, bg - by the photographing basis B .
Then we get

Taking into account the relative magnification of the stereoscope, the formula for hmin will take the following form:

But the basis b on the aerial photograph scale is b = B f/H. Then hmin = H2fc/bH v, or hmin = Hfc/b v. This formula determines the minimum difference in the height of objects, estimated using a stereoscope.
When visually assessing height using a stereoscope, it should be taken into account that there is a difference in the vertical and horizontal scales of the stereo model, as a result of which the vertical dimensions of terrain objects and its relief are exaggerated.
To derive the vertical scale formula, we will use the following stereophotogrammetry formulas:
formula used to determine the height of an object observed through a stereoscope hc,

From this formula (47) it follows:

If we take into account the magnification vc with a stereoscope, the formula will take the following form:

This formula shows that the vertical scale will be larger than the horizontal scale as many times as f is less than ρ0 (250 mm) (assuming that for 60% longitudinal overlap of 18x18 cm format aerial photographs b≈bg) and increases in proportion to the value of vc. For example, when taking aerial photographs with aerial cameras with focal lengths of 70 and 100 mm and at a distance in a stereoscope from the eye to the aerial photograph ρ0 = 250 mm, the relief visible in the stereoscope will be exaggerated, i.e. elongated upward by 3.5 and 2.5 times compared to with real.
The properties of the stereo model outlined above must be carefully taken into account when interpreting forest aerial photographs and especially when using the eye-stereoscopic method of measuring the height of trees and plantings.

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: .

The ability to see the world three-dimensionally gives a person binocular vision. If it is impaired, visual acuity deteriorates and problems arise with orientation in space. This happens for various reasons. Binocularity can be restored using hardware and surgical methods. The doctor also prescribes eye exercises.

What is binocular vision and how does it work?

Binocular vision is seeing with both eyes. It is also called stereoscopic and spatial, because it allows you to see in 3D projection. Thanks to this function, a person sees objects, recognizing their sizes by width and height, shape, and distance between them. Both eyes of a person receive one image, which they transmit to the brain. It combines these images into one picture.

If binocular vision is absent, the brain will receive two different visual images that cannot be combined into one. As a result, diplopia occurs - double image. This happens with anisometropia (a strong difference between the refraction of the right and left eyes), diseases of the lens, cornea and retina, damage to the nervous system and for other reasons. Binocular vision is impossible if one eye is not involved in the process of visual perception, as is the case with strabismus.

The development of binocular vision begins in childhood. From the very first months, the prerequisites for its emergence and development begin to form. First, the child develops light sensitivity, color perception, and central vision. Over time, visual acuity improves and the field of vision expands. All this contributes to the formation of binocularity. This process is completed by approximately 12-14 years. Disorders can occur at any age. A variety of factors can provoke them.

Causes of binocular vision impairment

The main reason for the lack of binocular vision is uncoordinated movements of the eyeballs. This occurs due to weakening of the eye muscles or damage to the extraocular muscles. The eyes begin to look in different directions, the visual axis shifts, which leads to a deterioration in the visual functions of one eye. In some cases, complete loss of vision occurs in one of them. This pathology most often occurs in childhood and manifests itself in strabismus - the most common form of binocular vision impairment.

Other reasons also lead to loss of binocularity. In fact, there are a lot of them. Hemorrhages in the retina, cataracts, and rupture of the retina cause a severe deterioration in the visual abilities of the eye, and one of the conditions for the existence of stereoscopic vision is the absence of pathologies of the retina and cornea.

Thus, the loss of binocular vision is caused by various pathologies of the body in general and the eyes in particular. Any disease that negatively affects eye health and vision can become a factor provoking disturbances in spatial perception.

Restoring binocular vision

Restoring binocularity begins with treating the pathology that led to visual impairment. Only after eliminating the causes can stereoscopic vision be restored.

The most common pathology in which binocular vision is absent is strabismus. This ophthalmological disease is treated with surgery, hardware methods and eye gymnastics. Surgical intervention is necessary only in extreme cases, when the eye is greatly displaced from its normal position and is not involved in the process of vision.

Restoring and training binocular vision at home

Daily training of spatial vision is the key to its rapid recovery. There are various exercises that you can do yourself right at home. The simplest exercise is with a sheet of paper.

Worksheet exercise

You will need a sheet of paper on which you need to draw a vertical line 10 cm long and 1 cm wide with a felt-tip pen. Fasten the sheet to the wall at eye level and move 1 meter away from it. Look at the line and gradually tilt your head down, continuing to look at the line until it begins to double. Next time, move your head up and then to the sides. You need to do these exercises three times a day for five minutes. A prerequisite for fulfillment is good lighting in the room.

This exercise is the simplest in technique. There are other techniques related to focusing. They also help train and restore binocular vision.

Exercise "Training"

Place some object (a sheet with an image) on the wall and move away from it at a distance of 2-3 meters. Next, you should clench your fist, but your index finger should be extended upward. The hand is positioned at a distance of 40 cm from the face, and the tip of the index finger should be on the same visual axis with the object on the wall. Look at the object through your fingertip. It will immediately begin to split into two. After this, you need to shift the focus from the wall to your finger. At this moment, the visual object will begin to double. This way you can train both eyes alternately. It is the weak eye that should be loaded more. The workout will take you approximately 3-5 minutes. It is advisable to perform it several times a day. Over time, you will notice that your visual acuity has improved.

Exercise "Focus"

It will require a colored object (any picture). First you need to look at the whole picture, then at its individual details (the image should be complex, multi-colored). An even smaller object is then selected. So, if the object is a butterfly, then first you examine it as a whole, then outline its outline with your eyes, then examine the wing or its half. The last object to focus your gaze on should be no larger than 0.5 cm in size. This way you will gradually learn to focus faster and more accurately without putting much strain on your eyes.

Exercise "Stereogram"

The stereogram drawing can be downloaded from the Internet and printed. It represents encrypted drawings in which you can see some figures. The stereogram should be located at a distance of 30-40 cm from the face. The gaze must be focused as if behind the image. After some time, the hidden picture will begin to appear. After this has happened, you need to increase the distance between the stereogram and the eyes, but try not to lose the found picture. The next steps are turning your head up and down and left and right while holding the image you see. This may not work the first time. However, over time, the eyes will get used to it and the visible object will be recognized from different angles. Stereograms are very useful for training binocularity, as well as for relieving stress on the visual apparatus. This exercise will be especially useful for people who work at a computer. Stereograms do not need to be printed, but viewed directly from the monitor. You just need to set its optimal brightness.

In addition to these exercises, you can perform general eye exercises, which help with fatigue and improve visual acuity. There are also quite a lot of such techniques. Before performing them, consult an ophthalmologist.

A person with binocular (stereoscopic) vision can fully navigate in space. It is possible to distinguish objects by shape even if you have monocular vision. However, it is possible to determine the distance between objects only with developed stereoscopic perception. Any pathologies that lead to impaired binocularity must be treated promptly, especially if they occur in childhood, when vision is just developing.