Binocular and stereoscopic vision. Stereoscopic vision: what is it, how does it work, how is it measured? Definition of stereoscopic vision

Full-fledged two-way vision is necessary for surgeons, jewelers, pilots. Sometimes a violation of binocularity becomes the cause of strabismus. Deviations in the work of the eyes can be detected independently in several ways.

Mechanism and conditions for binocular vision

Monocular vision is seeing an object with one eye. It evaluates object parameters such as shape, width, and height. However, it will not be possible to get an idea of ​​the relative position of objects.

Looking with two eyes provides a full perception of things. Stereoscopic vision determines the distance between objects. It also expands the field of vision and increases visual acuity.

The formation of binocular vision is based on the fusion reflex. He is physiological phenomenon- combining two reflections of an object from the retinas into one picture in the cerebral cortex. In this way, a stereoscopic image is formed. If the pictures do not merge, they say that binocular vision is impaired. .

For the correct formation of the vision of objects, the following conditions are necessary:

• objects on the retina match in shape and size;
• the picture appears on equivalent areas of the retina, if the images are on asymmetrical points, then doubling appears;
• good degree of transparency of the lens, vitreous body and cornea;
• synchronized movement of the visual muscles;
• the position of the eyeballs in the same horizontal and frontal plane;
• visual acuity in the range of 0.3–0.4.

Violation of binocular vision leads to a distortion of the real vision of objects. This negatively affects people who are associated with precise professions.

How to check?

For any eye pathology, it is necessary to visit an ophthalmologist. With the help of precise equipment, a specialist will conduct a study of binocular vision. For self-checking at home, there are several tests.

Kalf test

Binocular vision is determined using two long pencils or knitting needles. One pencil is placed in a horizontal plane, and the other is held vertically. You need to connect them at an angle of 90 degrees or hit the tip of a pencil.

Normal stereoscopic vision will make it easy to complete the task. With monocular vision, a person will not cope with it and will miss.

Sokolov's experience

To perform the test, you will need a folded piece of paper or a paper towel roll. The man looks straight through the round hole. The hand is placed in front of the second eye near the end of the tube. If stereoscopic vision is in order, a hole in the palm is visible, and an object is viewed in the distance.

If the point is not in the center of the hand, then they speak of simultaneous vision. In this case, the pictures in the brain do not merge. When conducting a test for stereoscopic vision, it must be taken into account that the objects in question should be 4-5 meters from the eyes.

Reading with a pencil

The reader places an object such as a pencil or pen between the book and the eyes. The distance from the nose should be 15 cm. If stereoscopic vision is normal, the pencil does not interfere with reading. The brain superimposes two images from both eyes and gives the big picture.

With monocular vision, the subject cannot read the closed part of the newspaper. The reason for the deviation of stereo vision is that the brain receives information from only one eye.

Four-point color test

Best of all, binocular vision is determined by the four-point color test. Diagnosis is carried out using the apparatus in the ophthalmology department. The operation of the device is based on the separation of the fields of vision of the eyes using color filters. In special glasses, green glass is installed in front of the left pupil, and red glass in front of the right one.

Deviation is set depending on which color is perceived. With binocular vision, a red and green filter is visible, and a colorless one takes on a mixed tint. Simultaneous vision is characterized by seeing five points. In monocular vision, the color of the filter in each eye is determined.

Binocular vision and strabismus

The problem arises when the axis of the eye deviates from the point of fixation with the second organ. With this position of one or two eyeballs in the brain, the two images do not merge. One of the pictures is excluded. Outwardly, the disorder is manifested by the incorrect position of the eyeball in the orbit.

There are several types of strabismus associated with binocularity:

• Explicit secondary form . Occurs with clouding of the lens, diseases of the retina or optic nerve.
• Imaginary strabismus . It develops due to an anomaly in the structure of eye tissues. The study of binocular vision does not reveal pathology. The patient sees well with both eyes.
• Latent deviation of the eyeball . Associated with a violation of the symmetry of the eye muscles. It manifests itself when a person looks at an object without fixing his gaze. Although the organ is sometimes deviated, visual function is not impaired.

May be intermittent. The provoking factor is nervous strain, fright, excessive physical effort.

Treatment

imaginary and hidden form do not need correction. Clarity of vision of the deviated eye with obvious secondary form decreases over time, so treatment should be started as early as possible.

If the study of binocular vision confirmed a clear strabismus, several types of restoration of eye functions are used:

• binocular stimulation;
• usage , ;
• hardware treatment (diploptika and orthoptika) to improve visual acuity;
• exercises for the eyes under the supervision of a methodologist;
• surgical intervention.

The operation is done to get rid of a cosmetic defect. As a result, one of the oculomotor muscles. Restoration of binocularity in this case is impossible.

To maintain a three-dimensional perception of the world with a large visual load, you need to do exercises for the eyes. It is important to eat right, to visit often fresh air. If there are problems with the organs of vision, you should not postpone a visit to the ophthalmologist.

Useful video about binocular vision

The image of objects on the retinas of the eyes is two-dimensional, but meanwhile a person sees the world in three dimensions, i.e. he has the ability to perceive the depth of space, or stereoscopic (stereo - from the Greek stereos - solid, spatial) vision.

A person has many mechanisms for estimating depth. Some of them are quite obvious. For example, if the approximate value of an object (a person, a tree, etc.) is known, then it is possible to estimate the distance to it or understand which of the objects is closer by comparing the angular value of the object. If one object is located in front of the other and partially obscures it, then the person perceives the front object as being closer. If we take a projection of parallel lines, for example, railroad tracks going into the distance, then in the projection they will converge. This is an example of perspective - a very effective indicator of the depth of space.

The convex section of the wall appears lighter in its upper part if the light source is located higher, and the recess in its surface appears darker in the upper part. An important sign of remoteness is motion parallax - the apparent relative displacement of near and more distant objects if the observer moves his head left and right or up and down. The “railway effect” is known when viewed from the window of a moving train: the apparent speed of movement of closely spaced objects is higher than those located at a great distance.

It is also possible to estimate the distance of objects by the size of the accommodation of the eye, i.e. by the tension of the ciliary body and the zinn ligaments that control the lens. By strengthening convergence or divergence, one can also judge the remoteness of the object of observation. With the exception of the latter, all of the above distance indicators are monocular. The most important depth perception mechanism, stereopsis, depends on the sharing of two eyes. When viewing any three-dimensional scene, the two eyes form slightly different images on the retinas.

During stereopsis, the brain compares images of the same scene on two retinas and estimates relative depth with great accuracy. The fusion of two monocular images, seen separately by the right and left eyes when viewing objects simultaneously with two eyes, into one three-dimensional image is called fusion.

Assume that the observer fixes his gaze on some point R, (Fig. 1) in this case, the images of the point are in the central fovea (fovea) F both eyes. Let Q be another point in space that appears to the observer to be at the same depth as the point R, while Q L and Q R are images of the Q point on the retinas of the left and right eyes. In this case, the points Q L and Q R are called corresponding points of two retinas.

Fig 1. Geometric scheme for explaining the stereo effect

It is obvious that two points coinciding with the central pits of the retinas are also corresponding. From geometric considerations, it is clear that the point Q′, estimated by observers as located closer than the Q point, will give two images on the retinas - Q′ L and Q′ R - at non-corresponding (disparate) points located farther apart than at if these points were corresponding.

In the same way, if we consider a point located farther from the observer, then it turns out that its projections on the retinas will be located closer to each other than the corresponding points. All points which, like the points Q and R, are perceived as equidistant, lie on horoptera- the surface passing through the points R and Q, which has a different shape than a sphere and depends on a person's ability to judge distance. Distances from the fovea F up to the projections Q R and Q L for the right and left eyes 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.

Angles α and α′ in stereoscopy are called parallactic angles. Their value will change from zero, when the fixation point lies at infinity, and up to 15°, when the fixation point is at a distance of 250 mm.

Suppose now that we are fixing a certain point in space with our eyes and that there are two point sources of light in this space, one of which is projected only on the retina of the left, and the other - on the right eye in the form of points of light, and these points are non-corresponding: the distance between them slightly more than between the corresponding points. Any such deviation from the position of the corresponding points is called 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 a few arc minutes, then we will visually perceive a single point in space located closer than the fixation point.

If the distances between the projections of a point are no more, but less than between the corresponding points, then this point will seem to be located further than the fixation point. Finally, if the vertical deviation exceeds a few arc minutes, or the horizontal deviation is greater than 2°, then we will see two separate points, which may appear to be located further or closer to the fixation point. Such an experiment illustrates the basic principle of stereo perception, first formulated by C. Wheatstone in 1838 and underlying the creation of a whole series of stereoscopic instruments, starting with the Wheatstone stereoscope up to stereo rangefinders and stereo television.

Not everyone has the ability to perceive depth with a stereoscope. You can easily check your stereopsis yourself if you use Fig.2. 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 sheet of cardboard perpendicularly between two images from the same stereopair and try to look at your image with each eye, setting the eyes parallel as if you were looking into the distance.

Fig 2. Examples of stereopairs

In 1960, Bela Yulesh (Bell Telephone Laboratories, USA) proposed an original method for demonstrating the stereo effect, excluding monocular observation of an object.

Based on this principle, by the way, a whole series of entertaining books has been published, which, at the same time, can also be used to train stereopsis. Figure 3 shows one of the drawings from this book in black and white. By setting the visual lines of your eyes in parallel (for this you need to look into the distance, as if through a drawing), you can see a stereoscopic picture. Such patterns are called autostereograms. Based on the method of Bel Yulesh, the Novosibirsk State Medical Institute, together with the Novosibirsk State Technical University, created a device for studying the threshold of stereoscopic vision, and we proposed its modification, which makes it possible to increase the accuracy of determining the threshold of stereoscopic vision. The measurement of the threshold of stereoscopic vision is based on the presentation of test objects to each eye of the observer against the so-called randomized background. Each of these test objects is a set of points on the plane, located according to an individual probabilistic law. Moreover, each test object has identical areas of points, which can be a figure of arbitrary shape.

If identical points of figures on the test object have zero values ​​of parallax angles, then the observer sees the total picture in the generalized image in the form of a random distribution of points, in other words, the observer is not able to distinguish the figure against a randomized background. Thus, monocular vision of the figure is excluded. If one of the test objects is displaced perpendicular to the optical axis of the system, then the parallax angle between the figures will change, and at a certain value of it, the observer will see a figure that, as it were, breaks away from the background and begins to approach or move away from it. The parallax angle is changed using an optical compensator inserted into one of the branches of the instrument. The moment the figure appears in the field of view is fixed by the observer, and the corresponding value of the stereoscopic vision threshold appears on the indicator.

Fig 3. Autostereogram

Studies of the last decades in the field of neurophysiology of stereoscopic vision have made it possible to identify specific cells tuned to disparity in the primary visual cortex of the brain. Cells were found that react only if the stimuli hit exactly the corresponding areas of the two retinas. Cells of the second type respond if and only if the object is located further than the fixation point. There are also cells that respond only when the stimulus is closer to the fixation point. Apparently, in the primary visual cortex there may be specific neurons for different degrees of disparity. All these cells also have the property of orientational selectivity and respond well to moving stimuli and to the ends of lines. According to D. Hubel, “although we still do not know exactly how the brain “reconstructs” a scene that includes many objects at different distances, cells that are sensitive to disparity are involved in the first stages of this process.”

When studying stereopsis, researchers faced a number of problems. It turned out that the processing of some binocular stimuli occurs in the visual system in a completely incomprehensible way. For example, if we turn again to the stereopairs shown in Fig. 37a and 37b, then we get the feeling that in one case the circle is located closer, in the other - further than the plane of the frame. If two stereopairs are combined, i.e. in each frame, place two circles located next to each other, then it would seem that we should see one circle closer, the other farther. However, in reality this will not work: both circles are visible at the same distance as the frame.

The second example of the unpredictability of binocular effects is the so-called struggle of the visual fields. 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 (for example, through a stereoscope), it is impossible to see both sets of lines at the same time. Either one or the other is visible, and each of them is only for a few seconds; sometimes you can see a mosaic of these images. The phenomenon of visual field struggle means that in cases where the visual system cannot combine images on two retinas, it simply rejects one of the images, either completely or partially.

So, for normal stereoscopic vision, the following conditions are necessary: ​​normal functioning of the oculomotor system of the eyes; sufficient visual acuity and not a very big difference in the acuity of the right and left eyes; strong connection between accommodation, convergence and fusion; small difference in image scales in the left and right eyes.

Size inequality or different scale of images obtained on the retinas of the right and left eyes when viewing the same object is called aniseikonia. Aniseikonia is one of the reasons for the instability or lack of stereoscopic vision. Aniseikonia is most often based on a difference in the refraction of the eyes, i.e. anisometronia. If aniseikonia does not exceed 2-2.5%, then it can be corrected with conventional stigmatic lenses, otherwise aniseikonic glasses are used.

Disruption of the connection between accommodation and convergence is one of the reasons for the appearance of various types of strabismus. Explicit strabismus, in addition to being a cosmetic defect, as a rule, leads to a decrease in visual acuity of the squinting eye until it is turned off from the process of vision. Hidden strabismus, or heterophoria, does not create a cosmetic defect, but may prevent stereopsis. So, persons with heterophoria more than 3 ° cannot work with binocular devices.

Threshold of stereoscopic vision characterize the minimum difference of parallactic angles Δα, which is still perceived by the observer. Relationship between Δα (in seconds) and minimum distance Δ l between objects that are perceived by the observer as being at different distances, the following:

,

where b is the distance between the pupils of the observer's eyes;
l is the distance from the eye to the nearest object under consideration.

The threshold of stereoscopic vision depends on various factors: the brightness of the background (the greatest sharpness is observed at a background brightness of about 300 cd / m 2), the contrast of objects (with an increase in contrast, the depth vision threshold decreases), the duration of observation (Fig. 4).

Figure 4. Dependence of the threshold of stereoscopic vision on the duration of observation

The depth perception threshold under optimal observation conditions ranges from 10 - 12 to 5″ (for some observers it reaches 2 - 5″).

Taking the value Δα =10″ as the threshold, we can calculate the maximum distance at which the eye still perceives depth. This distance l= 1400 m (radius of stereoscopic vision).

There are several ways to assess, define and study stereoscopic vision:

1) using a stereoscope according to the Pulfrich tables (the minimum threshold for stereoscopic perception determined by this method is 15″);
2) using different kind stereoscopes with a set of more accurate tables with a measurement range of 10 - 90 ";
3) using the device mentioned above, using a randomized background, which excludes monocular observation of objects, the measurement error is 1 - 2″.

Binocular vision provides three-dimensional perception of the surrounding world in three-dimensional space. With the help of this visual function, a person can cover with attention not only the objects in front of him, but also those located on the sides. Binocular vision is also called stereoscopic. What is fraught with a violation of the stereoscopic perception of the world, and how to improve the visual function? Consider the questions in the article.

What is binocular vision? Its function is to provide a monolithic visual picture as a result of combining the images of both eyes into a single image. A feature of binocular perception is the formation of a three-dimensional picture of the world with the determination of the location of objects in perspective and the distance between them.

Monocular vision is able to determine the height and volume of an object, but does not give an idea of ​​the mutual position of objects on a plane. Binocularity is a spatial perception of the world, giving a complete 3D picture of the surrounding reality.

Note! Binocularity improves visual acuity, providing a clear perception of visual images.

Volumetric perception begins to form at the age of two years: the child is able to perceive the world in a three-dimensional image. Immediately after birth, this ability is absent due to inconsistency in the movement of the eyeballs - the eyes “float”. By the age of two months, the baby can already fix the object with its eyes. At three months, the baby tracks objects in motion, located in the immediate vicinity of the eyes - hanging bright toys. That is, a binocular fixation and a fusion reflex are formed.

At the age of six months, babies are already able to see objects at different distances. By the age of 12-16, the fundus of the eye is completely stabilized, which indicates the completion of the process of formation of binocularity.

Why is binocular vision impaired? For the perfect development of a stereoscopic image, certain conditions are necessary:

• lack of strabismus;
• coordinated work of the muscles of the eye;
• coordinated movements of the eyeballs;
• visual acuity from 0.4;
• equal visual acuity in both eyes;
• proper functioning of the peripheral and central nervous systems;
• no pathology of the structure of the lens, retina and cornea.

Same for normal operation visual centers requires the symmetry of the location of the eyeballs, the absence of pathology ophthalmic nerves, coincidence of the degree of refraction of the corneas of both eyes and same vision both eyes. In the absence of these parameters, binocular vision is impaired. Also, stereoscopic vision is impossible in the absence of one eye.

Stereoscopic vision depends on the correct functioning of the visual centers of the brain, which coordinates the fusion reflex of merging two images into one.

stereoscopic vision disorder

To obtain a clear three-dimensional image, coordinated work of both eyes is necessary. If the functioning of the eyes is not coordinated, we are talking about the pathology of visual function.

Violation of binocular vision can occur for the following reasons:

• pathology of muscle coordination - motor disorder;
• pathology of the mechanism of synchronization of images into one whole - sensory disorder;
• a combination of sensory and motor impairment.

Determination of binocular vision is carried out using orthooptic devices. The first check is carried out at the age of three: babies are tested for the work of the sensory and motor components of the visual function. With strabismus, an additional test of the sensory component of binocular vision is performed. An ophthalmologist specializes in problems of stereoscopic vision.

Timely examination of the child by an ophthalmologist prevents the development of strabismus and serious problems with a vision for the future.

What causes a violation of stereoscopic vision? These include:

• mismatched refraction of the eyes;
• eye muscle defects
• deformation of the cranial bones;
• pathological processes of the tissues of the orbit;
• brain pathology;
• toxic poisoning;
• neoplasms in the brain;
• tumors of the visual organs.

Strabismus is the most common pathology of the visual system.

Strabismus

Strabismus is always the absence of binocular vision, since the visual axes of both eyeballs do not converge. There are several forms of pathology:

• valid;
• false;
• hidden.

With a false form of strabismus, a stereoscopic perception of the world is present - this makes it possible to distinguish it from real strabismus. False strabismus does not require treatment.

Heterophoria (hidden strabismus) is detected by the following method. If the patient closes one eye with a sheet of paper, then he deviates to the side. If the sheet of paper is removed, the eyeball is in the correct position. This feature is not a defect and does not require treatment.

Violation of visual function in strabismus is expressed in the following symptoms:

• bifurcation of the resulting picture of the world;
• frequent dizziness with nausea;
• head tilt towards the affected eye muscle;
• blockage of the eye muscle.

The reasons for the development of strabismus are as follows:

• hereditary factor;
• head injury;
• severe infections;
• mental disorder;
• pathology of the central nervous system.

Strabismus can be corrected, especially in early age. Various methods are used to treat the disease:

• the use of physiotherapy;
• physiotherapy;
• eye lenses and glasses;
• laser correction.

With heterophoria, eye fatigue and double vision are possible. In this case, prismatic glasses are used for permanent wear. With a severe degree of heterophoria, surgical correction, as in obvious strabismus.

With paralytic strabismus, the cause that caused the visual defect is first removed. Congenital paralytic strabismus in children should be treated as early as possible. Acquired paralytic strabismus is characteristic of adult patients who have had severe infections or illnesses. internal organs. Treatment to eliminate the cause of strabismus is usually long-term.

Post-traumatic strabismus is not corrected immediately: 6 months must pass from the moment of injury. In this case, surgical intervention is indicated.

How to diagnose binocular vision

Binocular vision is determined using the following devices:

• autorefractometer;
• ophthalmoscope;
• slit lamp;
• monobinoscope.

How to determine binocular vision yourself? For this, simple methods have been developed. Let's consider them.

Sokolov's technique

Hold a hollow object resembling binoculars, such as rolled paper, to one eye. Focus your eyes through the pipe on one distant object. Now bring your palm to your open eye: it is located near the end of the pipe. If binocularity is not out of balance, you will find a hole in your palm through which you can observe a distant object.

Calf Method

Take a couple of felt-tip pens / pencils: keep one in horizontal position, the other is vertical. Now try to aim and connect the vertical pencil with the horizontal one. If binocularity is not impaired, you can easily do this, because the orientation in space is well developed.

Read method

Hold a pen or pencil in front of the tip of your nose (2-3 cm) and try to read the printed text. If you can fully grasp the text and read it, then the motor and sensory functions are not impaired. A foreign object (a pen in front of the nose) should not interfere with the perception of the text.

Prevention of binocular defects

Binocular vision in adults can be impaired for several reasons. Correction consists in exercises to strengthen the eye muscles. At the same time, the healthy eye is closed, and the patient is loaded.

An exercise

This exercise for the development of stereoscopic vision can be performed at home. The algorithm of actions is as follows:

1. Attach the visual object to the wall.
2. Move away from the wall at a distance of two meters.
3. Stretch your hand forward with your index finger raised.
4. Move the focus of attention to the visual object and look at it through the tip of your finger - the tip of the finger should split in two.
5. Move the focus of attention from the finger to the visual object - now it should split in two.

The purpose of this exercise is to alternately switch the focus of attention from the finger to the object. An important indicator The correctness of the development of stereoscopic vision is the clarity of the perceived picture. If the image is blurred, this indicates the presence of monocular vision.

Important! Any eye exercises should be discussed with an ophthalmologist in advance.

Prevention of visual impairment in children and adults:

• you can not read books lying down;
• the workplace should be well lit;
• take vitamin C regularly to prevent senile vision loss;
• regularly replenish the body with a complex of essential minerals;
• should be unloaded regularly eye muscles from tension - look into the distance, close and open your eyes, rotate your eyeballs.

You should also be regularly examined by an ophthalmologist, adhere to healthy lifestyle life, unload the eyes and not let them get tired, perform exercises for the eyes, treat eye diseases in a timely manner.

Outcome

Binocular vision is the ability to perceive the picture of the world with both eyes, determine the shape and parameters of objects, navigate in space and determine the location of objects relative to each other. The absence of binocularity is always a decrease in the quality of life due to the limited perception of the picture of the world, as well as a violation of health. Strabismus is one of the consequences of impaired binocular vision, which can be congenital or acquired. Modern medicine can easily cope with the restoration of visual functions. The sooner you start vision correction, the more successful the result will be.

Vision is vital to most living organisms. It helps to correctly navigate and respond to the environment. It is the eyes that transmit about 90 percent of the information to the brain. But the structure and placement of the eyes of various representatives of the living world is different.

What vision is

There are the following types of vision:

• panoramic (monocular);
• stereoscopic (binocular).

When 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 an 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 with two eyes. In other words, the overall picture is made up of a merger of images entering the brain from each eye at the same time.

With the help of this type of vision, it is possible to correctly estimate not only the distance to the visible object, but also its approximate size and shape.

In addition, 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 two eyes at the same time, then the pen ceases to be a hindrance. But this ability to "look through objects" loses its power if the width of such an object is greater than the distance between the eyes.

The peculiarity of this type of vision in various representatives of the globe is presented below.

Features in insects

Their eyesight has a unique insect-like appearance that resembles a mosaic (such as the eyes of a wasp). Moreover, the number of these mosaics (facets) in different representatives of this representative of the living world differs 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 humans. For example, a red flower that a person sees is perceived by the eyes of a wasp as black.

Birds

Stereoscopic vision in birds is the exception rather than the rule. The fact is that in most birds the eyes are located on the sides, which provides a wider viewing angle.

This type of vision is inherent mainly in birds of prey. This helps them correctly calculate the distance to moving prey.

But visibility in birds is much less than, for example, in humans. If a person is able to see at 150 °, then birds are only from 10 ° (sparrows and bullfinches) to 60 ° (owls and nightjars).

But do not rush, arguing that the feathered representatives of the living world are deprived of the ability to fully see. Not at all. The fact is that they have other unique features.

For example, in owls, the eyes are closer to the beak. In this case, 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 situation to the side and behind. These birds have another distinguishing feature their eyes are fixed. But at the same time they are endowed with another unique ability. Due to their structure, they are able to turn their heads 270 °.

Fish

As you know, in the vast majority of fish species, the eyes are 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 has such a head shape. A possible solution was found by American scientists. They put forward the version that the hammerhead fish sees a three-dimensional image, i.e. She has 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 the activity of activity was measured when exposed to bright light. The subjects were then placed in an aquarium. As a result of this experience, it became known that the hammerhead fish is endowed with stereoscopic vision. Moreover, the accuracy of determining the distance to the object is the more accurate, the greater the distance between the eyes of this species of shark.

In addition, it became known that the eyes of the hammerhead fish rotate, which allows it to fully see the environment. This gives her 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. The second such vision allows you to better focus your eyes among many obstacles.

So, for example, this type of vision helps wolves with long-term pursuit of prey. Cats - with a lightning attack. By the way, it is in cats that, thanks to parallel visual axes, the viewing angle reaches 120 °. But some breeds of dogs have developed both monocular and stereoscopic vision. Their eyes are located on the sides. Therefore, in order 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.

The inhabitants of the treetops (primates, squirrels, etc.) are helped by stereoscopic vision in search of food and in calculating the trajectory of the jump.

People

Stereoscopic vision in humans is not developed from the very birth. At birth, babies cannot focus on a particular object. they begin to form only at 2 months of age. However, in full, children begin to correctly orient themselves in space only when they begin to crawl and walk.

Despite the apparent identity, human eyes are different. One is the leader, the other is the follower. For recognition, 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 must 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 in some other representatives of the living world, as a result of evolution.

Of course, modern humans do not need to hunt for prey. But at the same time, stereoscopic vision plays a significant role in their lives. It is especially important for athletes. So, 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 an instant reaction (drivers, hunters, pilots).

And in everyday life, stereoscopic vision is indispensable. For example, it is quite difficult, seeing with one eye, to put 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 given it.

30-09-2011, 10:29

Description

The corpus callosum is a powerful bundle of myelinated fibers that connects 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 to each other, but it is known that a small part of the fibers of the corpus callosum still play some role in stereopsis. It turned out to be convenient to include both of these topics in one chapter, since when considering them, one and the same feature of the structure of the visual system will have to be taken into account, namely, that there are both crossed and uncrossed optic nerve fibers in the chiasm.

corpus callosum

The corpus callosum (in Latin corpus callosum) 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 probably even higher, since the estimate given is based on conventional light microscopy, not electron microscopy.

This number is incomparable with 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, while 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 commissure means a collection of fibers connecting two homologous nerve structures located in the left and right halves of the head 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 intentionally - in some cases in the treatment of epilepsy (so that a convulsive discharge that occurs in one hemisphere of the brain cannot spread to the other hemisphere), in other cases in order to get from above to a deeply located tumor (if, for example, the tumor is located in the pituitary gland). According to the observations of neuropathologists and psychiatrists, after such operations, no mental disorders occur. Someone has 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 connected the two hemispheres, and on the basis of data obtained by rather crude neurophysiological methods, it was believed that in the striatal cortex, the fibers of the corpus callosum connected exactly symmetrical regions of the two hemispheres.

In 1955 Ronald Myers, a graduate student of psychologist Roger Sperry of the University of Chicago, conducted the first experiment that revealed some of the functions of this huge fibrous tract. Myers trained cats placed in a box with two screens placed side by side, onto which various images could be projected, such as a circle on one screen and a square on another. The cat was trained to put its nose on the screen with the image of a circle, and ignore the other - with the image of a square. Correct answers were reinforced with food, and cats were slightly punished for erroneous answers - a loud bell was turned on, and the cat was not rudely, but decisively pulled away from the screen. With this method, in 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 in a similar task, and 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 dove.)

There is nothing surprising in the fact that Myers' cats learned to solve this problem just as well in the case when one eye of the animal was covered with a mask. It is also not surprising that if the training of such a task as choosing a triangle or a square was carried out with only one open eye- left, and when checking the left eye was closed and the right eye 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, given the anatomy of the visual system. Each hemisphere receives input from both eyes. As we said in the article, most of the cells in field 17 also have inputs from both eyes. Myers created a more interesting situation by making a longitudinal transection of the chiasma in the midline. Thus, he cut the criss-crossing fibers and kept the non-crossing fibers intact (this operation requires a certain skill from the surgeon). As a result of such a transection, the left eye of the animal turned out to be 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 on the "exam" to address the stimulus to the right eye. If the cat can solve the problem correctly, then 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 lengthwise, trained the cat with one eye open, and then made a test 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 empirically established that the corpus callosum does perform some function (although one could hardly think that it exists only so that individual people or animals with a cut optic chiasm can perform certain tasks 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 a few years after the experiments of Myers by D. Witteridge, who was then working in Edinburgh. Whitteridge reasoned that it did not make much sense for bundles of nerve fibers to connect homologous mirror-symmetric sections of fields 17. Indeed, there is no reason to nerve cell in the left hemisphere, associated with some points in the right half of the visual field, connected with a cell in the right hemisphere, associated with a symmetrical section of the left half of the visual field. To test his assumptions, Whitteridge cut the optic tract at right side brain behind the chiasm and thereby blocked the input signals from the path 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).

Then Whitteridge began to turn on the light stimulus and register with a metal electrode electrical activity from the surface of the bark. He did get answers in his experience, however, they only appeared at the inner border of field 17, i.e., in the area receiving input signals from a long, narrow vertical strip in the middle of the field of view: when stimulated with small spots of light, answers appeared only when the light flashed on or near the vertical midline. If the cortex of the opposite hemisphere was cooled, thereby temporarily suppressing its function, the responses stopped; cooling of the corpus callosum also led to this. 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 only connects small areas of these fields, where there are projections of a vertical line in the middle of the field of view.

A similar result could be expected based on a number of anatomical data. Only one section of field 17, very close to the border with field 18, sends axons through the corpus callosum to the other hemisphere, and most of them seem to terminate in field 18 near the border with field 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 hemisphere 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 eventually lead to the fact that each hemisphere will receive signals from areas slightly larger than half of the field of view. In other words, due to connections through the corpus callosum, there will be an overlap of the hemifields projected into the two hemispheres. This is exactly what we found. With the help of two electrodes inserted into the cortical region at the border of fields 17 and 18 in each of the hemispheres, we were often able to register the activity of cells whose receptive fields mutually overlapped by several angular degrees.

T. Wiesel and I soon made microelectrode leads directly from that zone of the corpus callosum (in its most 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 in exactly the same way as ordinary field 17 neurons, i.e., 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 middle 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. By cutting the optic chiasm along the midline, they recorded responses in field 17 near the border with field 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 (through the LNT) 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, and that part of it that belongs to the left half of the visual field delivered information from the right eye, and the one that goes into the right half - from the left eye. Other cell properties studied in this experiment, including orientational selectivity, were found to be identical (Fig. 102).

The results obtained clearly showed that the corpus callosum connects cells to each other in such a way that their receptive fields can go both to the right and to the left of the middle vertical. Thus, it seems to stick together the two halves of the image of the surrounding world. To better imagine this, suppose that initially the cortex of our brain was formed as a whole, not divided into two hemispheres. In this case, field 17 would have the form of one continuous layer onto which the entire visual field would be mapped. Then neighboring cells, in order to implement such properties as, for example, sensitivity to movement and orientation selectivity, would, of course, have to have complex system mutual connections. Now imagine that the "constructor" (be it a god, or, say, natural selection) decided that it was impossible to leave it like that - from now on, half of all cells should form one hemisphere, and the other half - the other hemisphere.

What then needs to be done with the whole multitude of intercellular connections, if the two sets of cells must now move away from each other?

Apparently, one can simply stretch these connections, forming part of the corpus callosum from them. In order to eliminate the delay in the transmission of signals over such long way(in a person about 12-15 centimeters), you need to increase the transmission rate by providing the fibers with a myelin sheath. Of course, in fact, nothing like this happened in the process of evolution; long before the cortex arose, the brain already had two separate hemispheres.

The experiment of Berlucca and Rizzolatti, in my opinion, gave one of the most striking confirmations of the amazing specificity 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 orientational selectivity both through local connections with neighboring cells and through connections going through the corpus callosum from the other hemisphere from cells with such the same orientational sensitivity and a similar arrangement of receptive fields (this also applies to other properties of cells, such as directional specificity, the ability to respond to the ends of lines, and also complexity).

Each of the cells in the visual cortex that have connections through the corpus callosum must receive input from cells in the other hemisphere with exactly the same properties. We know a lot of facts that indicate 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. In the somatosensory cortex, experiments were carried out using axon transport, similar to those described in previous chapters with the injection of a radioactive amino acid into the eye. Their results show that the corpus callosum similarly binds those areas of the cortex that are activated by skin and articular receptors located near the midline of the body on the trunk and head, but does not bind the cortical projections of the extremities.

Each area of ​​the cortex is connected to several or even many other areas of the cortex of the same hemisphere. For example, the primary visual cortex is connected to area 18 (visual area 2), to the medial temporal area (MT area), to visual area 4, and to 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 simply as a special kind of cortico-cortical connections. It is easy to see that this is evidenced by such a simple example: if I tell you that my left hand feels cold or that I saw something on the left, then I formulate words using my cortical speech zones located in the left hemisphere (this may not be 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 appropriate signals must be transmitted through the corpus callosum to the speech cortex of the other hemisphere so that I can say something about my sensations. In a series of works begun in the early 1960s, R. Sperry (now working at the California Institute of Technology) and his colleagues showed that a person with a cut corpus callosum (for the treatment of epilepsy) loses the ability to talk about those events, information about which enters the right hemisphere. Working with such subjects has become a valuable source of new information about various functions cortex, including thinking and consciousness. The first articles about this appeared in Brain magazine; they are extremely interesting, and anyone who has read a real book can easily understand them.

stereoscopic vision

The distance estimation mechanism based on the comparison of two retinal images is so reliable that many people (unless they are psychologists and visual physiologists) are not even aware of its existence. To see the importance of this mechanism, try driving a car or bicycle, playing tennis, or skiing with one eye closed for a few minutes. Stereoscopes have gone out of fashion and you can only find them in antique shops. However, most readers have watched stereoscopic films (where the viewer has to wear special glasses). The principle of operation of both a stereoscope and stereoscopic glasses is based on the use of the stereopsis mechanism.

Images on the retinas are two-dimensional while we see the world in three dimensions. It is obvious that 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. Consider as a simple example round object. If it is oblique with respect 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.

A person has many mechanisms for estimating depth. Some of them are so obvious that they hardly deserve mention. However, I will mention them. If the approximate size of an object is 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 making a mistake if we encounter a dwarf, bonsai or lion). If one object is located in front of the other and partially obscures it, then we perceive the front object as being closer. If we take a projection of parallel lines, for example, railroad tracks going into the distance, then in the projection they will converge. This is an example of perspective - a very effective measure of depth.

The convex section of the wall appears lighter in its upper part if the light source is located higher (usually the light sources are at the top), and the recess in its surface, if it is illuminated from above, appears darker in the upper part. If the light source is placed below, then the bulge will look like a recess, and the recess will look like a bulge. An important sign of remoteness is motion parallax - the apparent relative displacement of near and more distant objects if the observer moves his head left and right or up and down. If some solid object is rotated, even at a small angle, then its three-dimensional shape is immediately revealed. If we focus the lens of our eye on a nearby object, then the more distant object will be out of focus; thus, by changing the shape of the lens, i.e., by changing the accommodation of the eye, we are able to estimate the distance of objects.

If you change the relative direction of the axes of both eyes, bringing them together or spreading(performing convergence or divergence), then two images of an object can be brought together and held in this position. Thus, by controlling either the lens or the position of the eyes, one can 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 depth perception mechanism, stereopsis, depends on the sharing of two eyes.

When viewing any three-dimensional scene, the two eyes form slightly different images on the retina. You can easily be convinced of 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 in turn. 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 a certain point P with his gaze. This statement is equivalent to saying: the eyes are directed in such a way that the images of the point are in the central pits of both eyes (F in Fig. 103).

Suppose now that Q is another point in space, which seems to the observer located at the same depth as P. Let Qlh Qr be the images of the point Q on the retinas of the left and right eyes. In this case, the points QL and QR are called the corresponding points of the two retinas. It is obvious that two points coinciding with the central pits of the retinas will be corresponding. From geometrical considerations it is also clear that the point Q ", estimated by the observer as located closer than Q, will give two projections on the retinas - and Q" R - at non-corresponding points located farther apart 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 farther from the observer, then it turns out that its projections on the retinas will be located closer to each other than the corresponding points.

What has been said above about the corresponding points is partly definitions and partly statements following from geometrical 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 the 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 a 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 estimate distance, i.e. from our brain. The distances from the fovea F to the projections of the Q point (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 a certain point in space with our eyes and that in this space there are two point sources of light that give a projection on each retina in the form of a point of light, and these points are not corresponding: the distance between them is somewhat greater than between the corresponding points . Any such deviation from the position of the corresponding points we will call disparity. If this deviation in the horizontal direction does not exceed 2° (0.6 mm on the retina), and vertically does not exceed a few minutes of arc, then we will visually perceive a single point in space located closer than the one we fix. If the distances between the projections of a point are no more, but less than between the corresponding points, then this point will seem to be located farther than the fixation point. Finally, if the vertical deviation exceeds a few arc minutes, or the horizontal deviation is greater than 2°, then we will see two separate points, which 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 unbelievable that prior to this discovery no one seemed to have realized that subtle differences in the images projected on the retinas of the two eyes could lead to a distinct impression of depth. This stereo effect demonstrated in a few minutes by any person who can arbitrarily reduce or separate the axes of his eyes, or whoever has a pencil, a piece of paper and several small mirrors or prisms. It is not clear how Euclid, Archimedes and Newton missed this discovery. In his article, Wheatstone notes that Leonardo da Vinci came very close to discovering this principle. Leonardo pointed out that a ball located in front of a 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 - the right. Wheatstone further notes that if Leonardo had chosen a cube instead of a sphere, he would certainly have noticed that its projections are different for different eyes. After that, he might, like Wheatstone, be interested in what would happen if two similar images were specifically 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 this or that object is located further or closer to the fixation point) occurs when two retinal images are slightly shifted relative to each other in the horizontal direction - moved apart or vice versa , are close together (unless this offset is greater than about 2° and the vertical offset is close to zero). This, of course, corresponds to geometric relationships: if an object is located closer or farther with respect to a certain distance reference point, then its projections on the retinas will be moved apart or brought together horizontally, while there will be no significant vertical displacement of images.

This is the basis of the action of the stereoscope invented by Wheatstone. The stereoscope was so popular for about half a century that almost every home had one. The same principle underlies the stereo movies that we now watch using special polaroid glasses for this. 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. Prisms and focusing lenses are now often used for convenience. The two images are identical in every way, except for small horizontal offsets, which give the impression of depth. Anyone can make a photograph suitable for use in a stereoscope if they select a fixed object (or scene), take a picture, then move the camera 5 centimeters to the right or left and take a second picture.

Not everyone has the ability to perceive depth with a stereoscope. You can easily check your stereopsis yourself if you use the stereopairs 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 stereopair 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 in and out with your finger, placing it between the eyes and the stereo pair and moving it forward or backward until the images merge, after which (this is the most difficult) you can look at the merged image, trying not to split it into two. If you succeed, then 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 it's because you don't have a stereoscope, or because you can't arbitrarily move the axes of your eyes in and out, you can still get the gist of the matter, although you won't enjoy the stereo effect.

In the upper stereopair 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 is slightly to the right. If you look at this stereopair with two eyes, using a stereoscope or another method of image alignment, then 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 consider the lower stereopair in fig. 105, the circle will be visible behind the sheet plane. You perceive the position of the circle in this way because exactly the same information is received on the retinas of your eyes as if the circle were actually in front of or behind the plane of the frame.

In 1960 Bela Yulesh 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, seems to be a homogeneous random mosaic of small triangles.

So it is, except that in the central part there is a hidden triangle of a larger size. If you look at 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 stereopairs . (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 Yulesh stereo pairs lies in the fact that if your stereo perception is disturbed, then you will not see a triangle in front of or behind the surrounding background.

Summing up, we can say that our ability to perceive the stereo effect depends on five conditions:

1. There are many indirect signs of depth - partial obscuration of some objects by others, motion parallax, object rotation, relative dimensions, shadow casting, perspective. However, stereopsis is the most powerful mechanism.

2. If we fix a point in space with our eyes, then the projections of this point fall into the central pits of both retinas. Any point judged to be at the same distance from the eyes as the fixation point forms two projections at the corresponding points on the retinas.

3. The stereo effect is determined by a simple geometric fact - if an object is closer than the fixation point, then its two projections on the retinas are farther apart than the corresponding points.

4. The main conclusion based on the results of experiments with the subjects is as follows: 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 point of fixation; if the projections of this object are moved apart in comparison with the corresponding points, the object seems to be located closer to the fixation point; if, on the contrary, they are close, the object seems to be located further than the fixation point.

5. With a horizontal projection shift of more than 2° or a vertical shift of more than a few minutes of arc, doubling occurs.

Physiology of stereoscopic vision

If we want to know what are the brain mechanisms of stereopsis, then the easiest way to start is with the question: are there neurons whose responses are specifically determined by the relative horizontal displacement of images on the retinas of the two eyes? Let us first see how the cells of the lower levels of the visual system respond when both eyes are stimulated simultaneously. We must start with field 17 neurons or more high level, since the ganglion cells of the retina are clearly monocular, and the cells of the lateral geniculate body, in which inputs from the right and left eyes are distributed over 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 field 17, approximately half of the neurons are binocular cells that respond to stimulation from both eyes.

Upon careful testing, it turns out that the responses of these cells, apparently, 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 or the other eye. With simultaneous stimulation of both eyes, the frequency of discharges of this cell is higher than with stimulation of one eye, but usually for the response of such a cell it is immaterial whether at some point the projections of the stimulus hit exactly the same areas 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 of the projections is slightly ahead of the other. On fig. 108 shows a characteristic response curve (for example, total number impulses in response to one passage of the stimulus through the receptive field) from the difference in the position of the stimulus on both retinas. This curve is very close to a horizontal straight line, from which it is 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 can be greater than, equal to, or less than the distance to the point> fixed by the eye.

Compared to this cell, the neurons whose responses are shown in Fig. 109 and 110 are very sensitive to the relative position of the two stimuli on the two retinas, i.e., sensitive to depth.

The first neuron (Fig. 109) responds best if the stimuli hit exactly the corresponding areas of the two retinas. The amount of horizontal misalignment of stimuli (i.e., disparity), at which the cell already 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 point of fixation. 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 closer than this point. When the degree of disparity changes, neurons of the last two types, called distant cells and near cells, very sharply change the intensity of their responses at the point of zero disparity or close to it. Neurons of all three types (cells, tuned to disparity) were found 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 columns of oculodominance. 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 they respond, but very weakly. All these cells have common property orientation selectivity; as far as we know, they are similar to the usual complex cells upper layers of the cortex, but 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 School of Medicine recorded the responses of such cells in field 17 of an awake monkey with electrodes implanted, which had previously been trained to fix the gaze of a certain object. In anesthetized monkeys, such cells were also found in the cortex, but they were rare in field 17 and very common in field 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. cell types - tuned 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 found narrowly tuned cells that responded best not to zero disparity, but to small deviations from it; Apparently, the cortex may contain specific neurons for all levels of disparity. Although we still don't know exactly how the brain "reconstructs" a scene involving many objects at different distances (whatever we mean by "reconstruction"), cells like those described above are probably involved in the first stages of this process.

Some problems associated with stereoscopic vision

During the study of stereopsis psychophysicists are faced with a number of problems. It turned out that the processing of some binocular stimuli occurs in the visual system in completely incomprehensible ways. I could give many examples of this kind, but I will confine myself to two.

On the example of stereopairs shown in Fig. 105, we have seen that moving two identical images (in this case circles) towards each other results in a feeling of greater proximity, and moving away from each other leads to a feeling of greater distance. Suppose now that we are doing both of these operations simultaneously, for which we place two circles in each frame, located next to each other (Fig. 111).

Obviously, considering such stereo pairs could lead to the perception of two circles - one closer and the other farther than the plane of fixation. However, we can assume another option: we will see just 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 fact, this pair of stimuli can be perceived only as two circles in the plane of fixation, which can be easily seen if the square frames in Fig. 2 are merged by any means. 111.

In the same way, we can imagine a situation where we consider two strings of characters x, say, six characters in a string. When viewed through a stereoscope, one can in principle perceive any of a number of possible configurations, depending on which x sign from the left chain merges with a certain x sign in the right chain. In fact, if we consider such a stereopair through a stereoscope (or in another way that creates a stereo effect), we will always see six x signs in the fixation plane. We still don't know how the brain resolves this ambiguity and chooses the simplest of all possible combinations. Because of this kind of ambiguity, it is difficult even to imagine how we manage to perceive a three-dimensional scene, which includes many branches of different sizes, located at different distances from us. True, physiological data suggest that the task may not be so difficult, since different branches are likely to have different orientations, and we already know that the cells involved in stereopsis are always orientation-selective.

The second example of the unpredictability of binocular effects, related to stereopsis is the so-called struggle of the visual fields, which we also mention in the section on strabismus (chap. 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 convergence of the eyes), then one would expect that you will 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, and each of them is only for a few seconds, after which it disappears and another appears. Sometimes you can also see, as it were, a mosaic of these two images, in which separate homogeneous areas 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 such different stimuli at the same time 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 occurs and at what level of the central nervous system it occurs. It seems to me that the mosaic nature of the perceived image during the struggle of visual fields suggests that “decision-making” in this process takes place for quite a long time. early stages processing 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 picture if the images are the same, or into a three-dimensional scene if there is only a slight horizontal disparity), it simply rejects one of the images - either completely when, for example, we look through a microscope with the other eye open, either partially or temporarily, as in the example above. Attention plays a significant role in the microscope situation, but the neural mechanisms underlying this shift in attention are also unknown.

You can observe another example of the struggle of visual fields if you simply look at some multi-colored scene or picture through glasses with red and green filters. The impressions of different observers in this case can be very different, but most people (including myself) notice transitions from a general reddish tone to greenish and vice versa, but without yellow color, which is obtained by the usual mixing of red light 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 a certain proportion of people whose vision is otherwise normal. Surprisingly, the proportion of such people is not too small. So, if we show stereopairs like those shown in Fig. 105 and 106 to a hundred student subjects (using polaroids and polarized light), it usually turns out that four or five of them cannot achieve the stereo effect.

Often this surprises them themselves, 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 absence of stereopsis is quite 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 consider cases of congenital strabismus, when the eyes long time work inconsistently. 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 uncommon, and even a mild degree, which may go unnoticed, in some cases is probably the cause of stereo blindness. In other cases, a violation of stereopsis, like color blindness, may be hereditary.

Since this chapter has dealt with both the corpus callosum and stereoscopic vision, I will take the 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 of the FPF, - QL and QR - will be in the left and right eyes on opposite sides of the fovea. Accordingly, the projection Ql transmits information to left hemisphere, and the Qr projection - to the right hemisphere. In order to see that the Q point is closer than P (i.e., to get a stereo effect), you need to combine the information of 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 from the University of California at Berkeley investigated stereoscopic vision in one person with a cut corpus callosum and obtained exactly the result predicted above.

The second question, closely related to the first, is what kind of stereopsis disorder will occur if the optic chiasm is cut along the midline (which 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 in relation to stimuli falling on the nasal region of the retina, i.e., coming from the temporal part of the visual field.

Therefore, there will be no stereopsis in the area of ​​space, colored lighter, where it is normally present. Lateral zones outside this region are generally accessible only to one eye, so there is no stereopsis here in normal conditions, and after transection of the chiasm, they will be zones of blindness (in the figure this is shown more dark color). In the area behind the point of fixation, where the temporal parts of the visual fields overlap, now invisible, blindness will also set in.

However, in the area closer to the point of fixation, the remaining half-fields of both eyes overlap, so stereopsis should be preserved here, unless the corpus callosum is damaged. K. Blakemore nevertheless found a patient with a complete cutting of the chiasm along the midline (this patient, as a child, received a skull fracture while riding a bicycle, which, apparently, led to longitudinal tear chiasma). When tested, he was found to have exactly the combination of visual defects that we have just hypothetically described.

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