The image of an object appearing on the retina of the eye is. Visual system

Impossible figures and ambiguous images are not something that cannot be taken literally: they arise in our brain. Since the process of perceiving such figures follows a strange, unconventional path, the observer comes to understand that something unusual is happening in his head. To better understand the process we call "vision", it is useful to have an understanding of how our senses (eyes and brain) convert light stimuli into useful information.

The eye as an optical device

Figure 1. Anatomy of the eyeball.

The eye (see Fig. 1) works like a camera. The lens (lens) projects an inverted, reduced image from the outside world onto the retina (retina), a network of photosensitive cells located opposite the pupil (pupil) and occupying more than half the area of ​​the inner surface of the eyeball. As an optical instrument, the eye has long been a bit of a mystery. While the camera focuses by moving the lens closer or further from the light-sensitive layer, its ability to refract light is adjusted during accommodation (adaptation of the eye to a certain distance). The shape of the eye lens is changed by the ciliary muscle. When the muscle contracts, the lens becomes rounder, allowing a focused image of closer objects to appear on the retina. The aperture of the human eye is adjusted in the same way as in a camera. The pupil controls the size of the lens opening, expanding or contracting with the help of radial muscles that color the iris of the eye (iris) with its characteristic color. When our eye moves its gaze to the area it wishes to focus on, the focal length and pupil size instantly adjust to the required conditions "automatically".

Figure 2. Sectional view of the retina
Figure 3. Eye with a yellow spot

The structure of the retina (Figure 2), the photosensitive layer inside the eye, is very complex. The optic nerve (along with blood vessels) arises from the back of the eye. This area has no photosensitive cells and is known as the blind spot. Nerve fibers branch and end in three different types of cells that detect the light that enters them. The processes coming from the third, innermost layer of cells contain molecules that temporarily change their structure when processing incoming light, and thereby emit an electrical impulse. Photosensitive cells are called rods and cones based on the shape of their processes. Cones are sensitive to color, while rods are not. On the other hand, the photosensitivity of rods is much higher than that of cones. One eye contains about one hundred million rods and six million cones, distributed unevenly across the retina. Exactly opposite the pupil lies the so-called macula macula (Fig. 3), which consists only of cones in a relatively dense concentration. When we want to see something in focus, we position the eye so that the image falls on the macula. There are many connections between the cells of the retina, and electrical impulses from one hundred million photosensitive cells are sent to the brain along just a million nerve fibers. Thus, the eye can be superficially described as a photographic or television camera loaded with photosensitive film.

Figure 4. Kanizsa figure

From light impulse to information

Figure 5. Illustration from Descartes’ book “Le traité de l’homme”, 1664

But how do we really see? Until recently, this issue was hardly solvable. The best answer to this question was that there is an area in the brain that specializes in vision, in which the image obtained from the retina is formed in the form of brain cells. The more light falls on a retinal cell, the more intensely the corresponding brain cell works, that is, the activity of brain cells in our visual center depends on the distribution of light falling on the retina. In short, the process begins with an image on the retina and ends with a corresponding image on a small “screen” of brain cells. Naturally, this does not explain vision, but simply shifts the problem to a deeper level. Who is meant to see this inner image? This situation is well illustrated by Figure 5, taken from Descartes’ work “Le traité de l’homme". In this case, all the nerve fibers end in a certain gland, which Descartes represented as the seat of the soul, and it is this gland that sees the internal image. But the question remains: How does "vision" actually work?

Figure 6.

The idea of ​​a mini-observer in the brain is not only insufficient to explain vision, but it also ignores three activities that are apparently carried out directly by the visual system itself. For example, let's look at the figure in Figure 4 (by Kanizsa). We see the triangle in the three circular segments by their cutouts. This triangle was not presented to the retina, but it is the result of conjecture by our visual system! Also, it is almost impossible to look at Figure 6 without seeing continuous sequences of circular patterns competing for our attention, as if we were directly experiencing internal visual activity. Many people find that their visual system is completely confused by the Dallenbach figure (Figure 8), as they look for ways to interpret these black and white spots into some form they understand. To save you the trouble, Figure 10 offers an interpretation that your visual system will accept once and for all. In contrast to the previous drawing, you will have no difficulty reconstructing the few ink strokes in Figure 7 into an image of two people talking.

Figure 7. Drawing from the "Mustard Seed Garden Manual of Painting", 1679-1701

For example, a completely different method of vision is illustrated by the research of Werner Reichardt from Tübingen, who spent 14 years studying the vision and flight control system of the housefly. For these studies he was awarded the Heineken Prize in 1985. Like many other insects, the fly has compound eyes, consisting of many hundreds of individual rods, each of which is a separate photosensitive element. The fly's flight control system consists of five independent subsystems that operate extremely quickly (reaction speed is approximately 10 times faster than a human) and efficiently. For example, the landing subsystem works as follows. When the fly's field of view "explodes" (because the surface is close), the fly moves towards the center of the "explosion". If the center is over the fly, it will automatically turn upside down. As soon as the fly's legs touch the surface, the landing "subsystem" is switched off. When flying, a fly extracts only two types of information from its field of view: the point at which a moving spot of a certain size is located (which must coincide with the size of the fly at a distance of 10 centimeters), as well as the direction and speed of movement of this spot across the field of view. Processing this data helps to automatically adjust the flight path. It is highly unlikely that a fly has a complete picture of the world around it. She sees neither surfaces nor objects. Input visual data processed in a certain way is transmitted directly to the motor subsystem. Thus, the visual input is not converted into an internal image, but into a form that allows the fly to respond appropriately to its environment. The same can be said about such an infinitely more complex system as a person.

Figure 8. Dallenbach figure

There are many reasons why scientists have refrained for so long from addressing the fundamental question as one sees it. It turned out that many other issues of vision had to be explained first - the complex structure of the retina, color vision, contrast, afterimages, etc. However, contrary to expectations, discoveries in these areas are not able to shed light on the solution to the main problem. An even more significant problem was the lack of any general concept or scheme that would list all visual phenomena. The relative limitations of conventional areas of research can be gleaned from T.N.'s excellent guide. Comsweet on the topic of visual perception, compiled from his lectures for first and second semester students. In the preface, the author writes: "I seek to describe the fundamental aspects underlying the vast field that we casually call visual perception." However, when examining the contents of this book, these "fundamental topics" turn out to be the absorption of light by the rods and cones of the retina, color vision, the ways in which sensory cells can increase or decrease the limits of mutual influence on each other, the frequency of electrical signals transmitted through sensory cells and etc. Today, research in the field is following entirely new paths, resulting in a bewildering diversity in the professional press. And only a specialist can form a general picture of the developing new science of Vision." There was only one attempt to combine several new ideas and research results in a manner accessible to a layman. And even here the questions "What is Vision?" and "How do we see?" did not become the main ones questions for discussion.

From image to data processing

David Marr of the MIT Artificial Intelligence Laboratory was the first to approach the subject from a completely different angle in his book Vision, published after his death. In it, he sought to examine the main problem and suggest possible ways to solve it. Marr's results are of course not final and are still open to research from different directions, but nevertheless the main advantage of his book is its logic and consistency of conclusions. In any case, Marr's approach provides a very useful basis on which to build studies of impossible objects and dual figures. In the following pages we will try to follow Marr's train of thought.

Marr described the shortcomings of the traditional theory of visual perception as follows:

"Trying to understand visual perception by studying only neurons is like trying to understand the flight of a bird by studying only its feathers. It is simply impossible. To understand the flight of a bird, we need to understand aerodynamics, and only then the structure of the feathers and the different shapes of the bird's wings will make any sense to us." that meaning.” In this context, Marr credits J. J. Gobson as the first to address important issues in this field of vision. According to Marr, Gibson’s most important contribution was that “the most important thing about the senses is that that they are information channels from the outside world to our perception (...) He posed a critical question - How do each of us get the same results in perception in everyday life in constantly changing conditions? This is a very important question, showing that Gibson correctly viewed the problem of visual perception as reconstructing from sensory information the “correct” properties of objects in the external world." And thus we have reached the field of information processing.

There should be no question that Marr wanted to ignore other explanations for the phenomenon of vision. On the contrary, he specifically emphasizes that vision cannot be satisfactorily explained from only one point of view. Explanations must be found for everyday events that are consistent with the results of experimental psychology and all the discoveries in this field made by psychologists and neurologists in the field of the anatomy of the nervous system. When it comes to information processing, computer scientists would like to know how the visual system can be programmed, which algorithms are best suited for a given task. In short, how vision can be programmed. Only a comprehensive theory can be accepted as a satisfactory explanation of the vision process.

Marr worked on this problem from 1973 to 1980. Unfortunately, he was not able to complete his work, but he was able to lay a solid foundation for further research.

From neuroscience to visual mechanism

The belief that many human functions are controlled by the brain has been shared by neurologists since the early 19th century. Opinions differed on whether specific parts of the cerebral cortex were used to perform specific operations or whether the entire brain was used for each operation. Today, the famous experiment of the French neurologist Pierre Paul Broca has led to the general acceptance of the specific location theory. Broca treated a patient who could not speak for 10 years, although his vocal cords were fine. When the man died in 1861, an autopsy revealed that the left side of his brain was deformed. Broca suggested that speech is controlled by this part of the cerebral cortex. His theory was confirmed by subsequent examinations of patients with brain damage, which ultimately made it possible to mark the centers of vital functions of the human brain.

Figure 9. Response of two different brain cells to optical stimuli of different directions

A century later, in the 1950s, scientists D.H. Hubel (D.H. Hubel) and T.N. Wiesel (T.N. Wiesel) conducted experiments in the brains of living monkeys and cats. In the visual center of the cerebral cortex, they found nerve cells that are especially sensitive to horizontal, vertical and diagonal lines in the visual field (Fig. 9). Their sophisticated microsurgery technique was subsequently adopted by other scientists.

Thus, the cerebral cortex not only contains centers for performing various functions, but within each center, as in the visual center, individual nerve cells are activated only when very specific signals are received. These signals coming from the retina of the eye correlate with clearly defined situations in the external world. Today it is assumed that information about the various shapes and spatial arrangements of objects is stored in visual memory, and information from activated nerve cells is compared with this stored information.

This detector theory influenced the direction of visual perception research in the mid-1960s. Scientists associated with “artificial intelligence” followed the same path. Computer simulation of the human vision process, also called "machine vision", was seen as one of the most easily achievable goals in these studies. But everything turned out a little differently. It soon became clear that it was virtually impossible to write programs that would be able to recognize changes in light intensity, shadows, surface structure, and random assemblies of complex objects into meaningful images. Moreover, such pattern recognition required unlimited amounts of memory, since images of countless objects must be stored in memory in countless variations of location and lighting situations.

Any further advances in the field of pattern recognition in real world conditions were not possible. It is doubtful that a computer will ever be able to simulate the human brain. Compared to the human brain, in which each nerve cell has about 10,000 connections with other nerve cells, the equivalent computer ratio of 1:1 hardly seems adequate!

Figure 10. Solution to the Dellenbach figure

Lecture by Elizabeth Warrington

In 1973, Marr attended a lecture by British neurologist Elizabeth Warrington. She noted that a large number of patients with parietal lesions of the right side of the brain whom she examined could perfectly recognize and describe a variety of objects, provided that these objects were observed by them in their usual form. For example, such patients had little difficulty identifying a bucket when viewed from the side, but were unable to recognize the same bucket when viewed from above. In fact, even when they were told that they were looking at the bucket from above, they flatly refused to believe it! Even more surprising was the behavior of patients with damage to the left side of the brain. Such patients typically cannot speak and therefore cannot verbally name the object they are looking at or describe its purpose. However, they can show that they correctly perceive the geometry of an object regardless of viewing angle. This prompted Marr to write the following: "Warrington's lecture led me to the following conclusions. Firstly, the idea of ​​​​an object's shape is stored somewhere else in the brain, which is why ideas about the shape of an object and its purpose are so different. Secondly, vision itself can provide an internal description of the shape of an observed object, even if that object is not recognized in the usual way... Elizabeth Warrington pointed out the most essential fact of human vision - it tells about the shape, space and relative position of objects." If this is indeed the case, then scientists working in the fields of visual perception and artificial intelligence (including those working in computer vision) will have to trade the detector theory of Hubel's experiments for an entirely new set of tactics.

Module theory

Figure 11. Stereograms with random dots by Béla Zhules, floating square

The second starting point in Marr's research (after becoming familiar with Warrington's work) is the assumption that our visual system has a modular structure. In computer parlance, our main Vision program covers a wide range of subroutines, each of which is completely independent of the others, and can operate independently of other subroutines. A prime example of such a routine (or module) is stereoscopic vision, in which depth is perceived as the result of processing images from both eyes that are slightly different images from each other. Previously, it was believed that in order to see in three dimensions, we first recognize entire images, and then decide which objects are closer and which are farther away. In 1960, Bela Julesz, who was awarded the Heineken Prize in 1985, was able to demonstrate that spatial perception in the two eyes occurs solely by comparing small differences between two images obtained from the retinas of both eyes. Thus, one can feel depth even where there are no objects and no objects are supposed to be present. For his experiments, Jules came up with stereograms consisting of randomly located dots (see Fig. 11). The image seen by the right eye is identical to the image seen by the left eye in all respects except for the square central area, which is cropped and offset slightly to one edge and again aligned with the background. The remaining white space was then filled with random dots. If the two images (in which no object is recognized) are viewed through a stereoscope, the square that was previously cut out will appear to be floating above the background. Such stereograms contain spatial data that is automatically processed by our visual system. Thus, stereoscopy is an autonomous module of the visual system. Module theory has proven to be quite effective.

From 2D retinal image to 3D model

Figure 12. During the visual process, the retinal image (left) is converted into a primary sketch in which changes in intensity become apparent (right)

Vision is a multi-step process that transforms two-dimensional representations of the external world (retinal images) into useful information for the observer. It starts with a two-dimensional image taken from the retina of the eye, which, ignoring color vision for now, stores only light intensity levels. In the first step, using just one module, these intensity levels are converted into intensity changes or, in other words, into contours that show abrupt changes in light intensity. Marr established exactly what algorithm is involved in this case (described mathematically, and, by the way, very complex), and how our perception and nerve cells execute this algorithm. The result of the first step is what Marr calls the “primary sketch,” which offers a summary of changes in light intensity, their relationships, and distribution across the visual field (Figure 12). This is an important step because in the world we see, changes in intensity are often associated with the natural contours of objects. The second step brings us to what Marr calls the "2.5-dimensional sketch." The 2.5-dimensional sketch reflects the orientation and depth of visible surfaces in front of the observer. This image is built on the basis of data from not one, but several modules. Marr coined the very broad concept of "2.5-dimensionality" to emphasize that we are working with spatial information that is visible from an observer's point of view. A 2.5-dimensional sketch is characterized by perspective distortions, and at this stage the actual spatial location of objects cannot yet be unambiguously determined. The 2.5-dimensional sketch image shown here (Figure 13) illustrates several information areas when processing such a sketch. However, an image of this type is not formed in our brain.

Figure 13. 2.5D sketch drawing - "centered representation of depth and orientation of visible surfaces"

Until now, the visual system operated using several modules autonomously, automatically and independently of data about the external world stored in the brain. However, during the final stage of the process it is possible to refer to already existing information. This final processing step provides a three-dimensional model—a clear description that is independent of the viewer's viewing angle and suitable for direct comparison with visual information stored in the brain.

According to Marr, the main role in the construction of a three-dimensional model is played by the components of the directing axes of the shapes of objects. Those unfamiliar with this idea may find it far-fetched, but there is actually evidence to support this hypothesis. Firstly, many objects of the surrounding world (in particular, animals and plants) can be quite clearly depicted in the form of tube (or wire) models. Indeed, we can easily recognize what is depicted in the reproduction in the form of components of the guide axes (Fig. 14).

Figure 14. Simple animal models can be identified by their guide axis components.

Secondly, this theory offers a plausible explanation for the fact that we are able to visually disassemble an object into its component parts. This is reflected in our language, which gives different names to each part of an object. Thus, when describing the human body, designations such as “body”, “hand” and “finger” indicate different parts of the body according to their axial components (Fig. 15).

Figure 16. Single axis model (left) broken down into individual axis components (right)

Thirdly, this theory is consistent with our ability to generalize and at the same time differentiate forms. We generalize by grouping together objects with the same principal axes, and differentiate by analyzing the child axes like the branches of a tree. Marr proposed algorithms that convert a 2.5-dimensional model into a three-dimensional one. This process is also largely autonomous. Marr noted that the algorithms he developed only work when pure axes are used. For example, if applied to a crumpled sheet of paper, the possible axes will be very difficult to identify, and the algorithm will not be applicable.

The connection between the three-dimensional model and visual images stored in the brain is activated during the process of object recognition.

There is a big gap in our knowledge here. How are these visual images stored in the brain? How does the recognition process proceed? How is the comparison made between known images and the newly compiled 3D image? This is the last point that Marr touched on (Fig. 16), but a huge amount of scientific data is needed to bring certainty to this issue.

Figure 16. New shape descriptions are related to stored shapes by a comparison that moves from a generalized form (top) to a specific form (bottom)

Although we ourselves are not aware of the different phases of visual processing, there are many clear parallels between the phases and the various ways in which we have conveyed the impression of space on a two-dimensional surface over time.

Thus, pointillists emphasize the contourless image of the retina, while line images correspond to the stage of the primary sketch. Cubist paintings can be compared to the processing of visual data in preparation for the construction of the final three-dimensional model, although this was certainly not the artist's intention.

Man and computer

In his comprehensive approach to the subject, Marr sought to show that we can understand the process of vision without the need to draw on knowledge that is already available to the brain.

Thus, he opened a new path for researchers in the field of visual perception. His ideas can be used to pave a more efficient path to the implementation of a visual machine. When Marr wrote his book, he must have been aware of the effort his readers would have to make to follow his ideas and conclusions. This is evident throughout his work and is most evident in the final chapter, “In Defense of the Approach.” This is a polemical "case" of 25 printed pages in which he takes advantage of the favorable moment to justify his goals. In this chapter he has a conversation with an imaginary opponent who attacks Marr with arguments like the following:

"I am still dissatisfied with the description of this interconnected process and the idea that all the remaining wealth of detail is just a description. It sounds a little too primitive... As we move closer to saying that the brain is a computer, I have to say everything I fear more and more for the preservation of the meaning of human values."

Marr offers an intriguing answer: "The claim that the brain is a computer is correct, but misleading. The brain is indeed a highly specialized information processing device, or rather the largest of them. Viewing our brain as a data processing device does not demean or deny human values. In any case, it only supports them and can, in the end, help us understand what human values ​​are from such an information point of view, why they have selective significance, and how they fit into the social and public norms that our genes have provided us with ".

The eye, the eyeball, is almost spherical in shape, approximately 2.5 cm in diameter. It consists of several shells, of which three are the main ones:

• sclera - outer layer
• choroid - middle,
• retina – internal.

Rice. 1. Schematic representation of the accommodation mechanism on the left - focusing into the distance; on the right - focusing on close objects.

The sclera is white with a milky tint, except for its anterior part, which is transparent and called the cornea. Light enters the eye through the cornea. The choroid, the middle layer, contains blood vessels that carry blood to nourish the eye. Just below the cornea, the choroid becomes the iris, which determines the color of the eyes. In its center is the pupil. The function of this shell is to limit the entry of light into the eye when it is very bright. This is achieved by constricting the pupil in high light conditions and dilating in low light conditions. Behind the iris is a lens, like a biconvex lens, that captures light as it passes through the pupil and focuses it on the retina. Around the lens, the choroid forms the ciliary body, which contains a muscle that regulates the curvature of the lens, which ensures clear and precise vision of objects at different distances. This is achieved as follows (Fig. 1).

Pupil is a hole in the center of the iris through which light rays pass into the eye. In an adult at rest, the diameter of the pupil in daylight is 1.5–2 mm, and in the dark it increases to 7.5 mm. The primary physiological role of the pupil is to regulate the amount of light entering the retina.

Constriction of the pupil (miosis) occurs with increasing illumination (this limits the light flux entering the retina, and, therefore, serves as a protective mechanism), when viewing closely located objects, when accommodation and convergence of the visual axes (convergence) occur, as well as during.

Dilation of the pupil (mydriasis) occurs in low light (which increases the illumination of the retina and thereby increases the sensitivity of the eye), as well as with excitement of any afferent nerves, with emotional reactions of tension associated with an increase in sympathetic tone, with mental arousal, suffocation,.

The size of the pupil is regulated by the annular and radial muscles of the iris. The radial dilator muscle is innervated by the sympathetic nerve coming from the superior cervical ganglion. The annular muscle, which constricts the pupil, is innervated by parasympathetic fibers of the oculomotor nerve.

Fig 2. Diagram of the structure of the visual analyzer

1 - retina, 2 - uncrossed fibers of the optic nerve, 3 - crossed fibers of the optic nerve, 4 - optic tract, 5 - lateral geniculate body, 6 - lateral root, 7 - optic lobes.
The shortest distance from an object to the eye, at which this object is still clearly visible, is called the near point of clear vision, and the greatest distance is called the far point of clear vision. When the object is located at the near point, accommodation is maximum, at the far point there is no accommodation. The difference in the refractive powers of the eye at maximum accommodation and at rest is called the force of accommodation. The unit of optical power is the optical power of a lens with a focal length1 meter. This unit is called diopter. To determine the optical power of a lens in diopters, the unit should be divided by the focal length in meters. The amount of accommodation varies from person to person and varies depending on age from 0 to 14 diopters.

To see an object clearly, it is necessary that the rays of each point of it be focused on the retina. If you look into the distance, then close objects are seen unclearly, blurry, since the rays from nearby points are focused behind the retina. It is impossible to see objects at different distances from the eye with equal clarity at the same time.

Refraction(ray refraction) reflects the ability of the optical system of the eye to focus the image of an object on the retina. The peculiarities of the refractive properties of any eye include the phenomenon spherical aberration . It lies in the fact that rays passing through the peripheral parts of the lens are refracted more strongly than rays passing through its central parts (Fig. 65). Therefore, the central and peripheral rays do not converge at one point. However, this feature of refraction does not interfere with the clear vision of the object, since the iris does not transmit rays and thereby eliminates those that pass through the periphery of the lens. The unequal refraction of rays of different wavelengths is called chromatic aberration .

The refractive power of the optical system (refraction), i.e. the ability of the eye to refract, is measured in conventional units - diopters. Diopter is the refractive power of a lens in which parallel rays, after refraction, converge at a focus at a distance of 1 m.

Rice. 3. The course of rays for various types of clinical refraction of the eye a - emetropia (normal); b - myopia (myopia); c - hypermetropia (farsightedness); d - astigmatism.

We see the world around us clearly when all departments “work” harmoniously and without interference. In order for the image to be sharp, the retina obviously must be in the back focus of the eye's optical system. Various disturbances in the refraction of light rays in the optical system of the eye, leading to defocusing of the image on the retina, are called refractive errors (ametropia). These include myopia, farsightedness, age-related farsightedness and astigmatism (Fig. 3).

With normal vision, which is called emmetropic, visual acuity, i.e. the maximum ability of the eye to distinguish individual details of objects usually reaches one conventional unit. This means that a person is able to consider two separate points visible at an angle of 1 minute.

With refractive error, visual acuity is always below 1. There are three main types of refractive error - astigmatism, myopia (myopia) and farsightedness (hyperopia).

Refractive errors result in nearsightedness or farsightedness. The refraction of the eye changes with age: it is less than normal in newborns, and in old age it can decrease again (the so-called senile farsightedness or presbyopia).

Myopia correction scheme

Astigmatism due to the fact that, due to its innate characteristics, the optical system of the eye (cornea and lens) refracts rays unequally in different directions (along the horizontal or vertical meridian). In other words, the phenomenon of spherical aberration in these people is much more pronounced than usual (and it is not compensated by pupil constriction). Thus, if the curvature of the corneal surface in the vertical section is greater than in the horizontal section, the image on the retina will not be clear, regardless of the distance to the object.

The cornea will have, as it were, two main focuses: one for the vertical section, the other for the horizontal section. Therefore, light rays passing through an astigmatic eye will be focused in different planes: if the horizontal lines of an object are focused on the retina, then the vertical lines will be in front of it. Wearing cylindrical lenses, selected taking into account the actual defect of the optical system, to a certain extent compensates for this refractive error.

Myopia and farsightedness caused by changes in the length of the eyeball. With normal refraction, the distance between the cornea and the fovea (macula) is 24.4 mm. With myopia (myopia), the longitudinal axis of the eye is greater than 24.4 mm, so rays from a distant object are focused not on the retina, but in front of it, in the vitreous body. To see clearly into the distance, it is necessary to place concave glasses in front of myopic eyes, which will push the focused image onto the retina. In the farsighted eye, the longitudinal axis of the eye is shortened, i.e. less than 24.4 mm. Therefore, rays from a distant object are focused not on the retina, but behind it. This lack of refraction can be compensated by accommodative effort, i.e. an increase in the convexity of the lens. Therefore, a farsighted person strains the accommodative muscle, examining not only close, but also distant objects. When viewing close objects, the accommodative efforts of farsighted people are insufficient. Therefore, to read, farsighted people must wear glasses with biconvex lenses that enhance the refraction of light.

Refractive errors, in particular myopia and farsightedness, are also common among animals, for example, horses; Myopia is very often observed in sheep, especially cultivated breeds.

We are used to seeing the world as it is, but in fact, any image appears upside down on the retina. Let's figure out why the human eye sees everything in an altered state and what role other analyzers play in this process.

How do eyes actually work?

In essence, the human eye is a unique camera. Instead of a diaphragm, there is an iris that contracts and constricts the pupil or stretches and dilates it to allow enough light to enter the eye. The lens then acts like a lens: light rays are focused and hit the retina. But since the lens resembles a biconvex lens in characteristics, the rays passing through it are refracted and turned over. Therefore, a smaller, inverted image appears on the retina. However, the eye only perceives the image, and the brain processes it. He flips the picture back, separately for each eye, then combines them into one three-dimensional image, corrects the color and highlights individual objects. Only after this process does a real picture of the world around us appear.

It is believed that a newborn sees the world upside down until the 3rd week of life. Gradually, the child’s brain learns to perceive the world as it is. Moreover, in the process of such training, not only visual functions are important, but also the work of muscles and balance organs. As a result, a true picture of images, phenomena, and objects emerges. Therefore, our habitual ability to reflect reality in exactly this way and not otherwise is considered acquired.

Can a person learn to see the world upside down?

Scientists decided to test whether a person could live in an upside-down world. The experiment involved two volunteers who were fitted with image-reversing glasses. One sat motionless in a chair, not moving either his arms or legs, and the second moved freely and provided assistance to the first. According to the results of the study, the person who was active was able to get used to the new reality, but the second one was not. Only humans have such an ability - the same experiment with a monkey brought the animal into a semi-conscious state, and only a week later it began to gradually react to strong stimuli, remaining motionless.

Fundamentals of psychophysiology., M. INFRA-M, 1998, pp. 57-72, Chapter 2 Responsible editor. Yu.I. Alexandrov

2.1. Structure and functions of the optical apparatus of the eye

The eyeball has a spherical shape, which makes it easier to rotate to point at the object in question and ensures good focusing of the image on the entire light-sensitive membrane of the eye - the retina. On the way to the retina, light rays pass through several transparent media - the cornea, lens and vitreous body. A certain curvature and refractive index of the cornea and, to a lesser extent, the lens determine the refraction of light rays inside the eye. The image obtained on the retina is sharply reduced and turned upside down and from right to left (Fig. 4.1 a). The refractive power of any optical system is expressed in diopters (D). One diopter is equal to the refractive power of a lens with a focal length of 100 cm. The refractive power of a healthy eye is 59D when viewing distant objects and 70.5D when viewing near objects.

Rice. 4.1.

2.2. Accommodation

Accommodation is the adaptation of the eye to clearly seeing objects located at different distances (similar to focusing in photography). To see an object clearly, its image must be focused on the retina (Fig. 4.1 b). The main role in accommodation is played by changes in the curvature of the lens, i.e. its refractive power. When viewing close objects, the lens becomes more convex. The mechanism of accommodation is the contraction of muscles that change the convexity of the lens.

2.3. Refractive errors of the eye

The two main refractive errors of the eye are myopia (myopia) and farsightedness (hyperopia). These anomalies are not caused by insufficiency of the refractive media of the eye, but by a change in the length of the eyeball (Fig. 4.1 c, d). If the longitudinal axis of the eye is too long (Fig. 4.1 c), then rays from a distant object will be focused not on the retina, but in front of it, in the vitreous body. Such an eye is called myopic. To see clearly into the distance, a nearsighted person must place concave glasses in front of their eyes, which will push the focused image onto the retina (Fig. 4.1 e). In contrast, in the farsighted eye (Fig. 4.1 d) the longitudinal axis is shortened, and therefore rays from a distant object are focused behind the retina. This disadvantage can be compensated by increasing the convexity of the lens. However, when viewing close objects, the accommodative efforts of farsighted people are insufficient. That is why, for reading, they must wear glasses with biconvex lenses that enhance the refraction of light (Fig. 4.1 e).

2.4. Pupil and pupillary reflex

The pupil is the hole in the center of the iris through which light passes into the eye. It improves the clarity of the retinal image, increasing the depth of field of the eye and eliminating spherical aberration. The pupil, which dilates during darkening, quickly contracts in the light (the “pupillary reflex”), which regulates the flow of light entering the eye. So, in bright light the pupil has a diameter of 1.8 mm, in average daylight it expands to 2.4 mm, and in the dark - to 7.5 mm. This degrades the quality of the retinal image but increases the absolute sensitivity of vision. The reaction of the pupil to changes in illumination is adaptive in nature, as it stabilizes the illumination of the retina in a small range. In healthy people, the pupils of both eyes have the same diameter. When one eye is illuminated, the pupil of the other also narrows; such a reaction is called friendly.

2.5. Structure and function of the retina

The retina is the inner light-sensitive layer of the eye. It has a complex multilayer structure (Fig. 4.2). There are two types of photoreceptors (rods and cones) and several types of nerve cells. Excitation of photoreceptors activates the first nerve cell of the retina - the bipolar neuron. Excitation of bipolar neurons activates retinal ganglion cells, which transmit their impulses to the subcortical visual centers. Horizontal and amacrine cells are also involved in the processes of transmitting and processing information in the retina. All of the listed retinal neurons with their processes form the nervous apparatus of the eye, which is involved in the analysis and processing of visual information. That is why the retina is called the part of the brain located in the periphery.

2.6. Structure and function of retinal layers

Cells pigment epithelium form the outer layer of the retina, farthest from light. They contain melanosomes, which give them their black color. The pigment absorbs excess light, preventing its reflection and scattering, which contributes to the clarity of the image on the retina. The pigment epithelium plays a critical role in the regeneration of visual purple photoreceptors after its bleaching, in the constant renewal of the outer segments of visual cells, in protecting the receptors from light damage, and in transporting oxygen and nutrients to them.

Photoreceptors. Adjacent to the layer of pigment epithelium from the inside is a layer of visual receptors: rods and cones. Each human retina contains 6-7 million cones and 110-125 million rods. They are distributed unevenly in the retina. The central fovea of ​​the retina, the fovea (fovea centralis), contains only cones. Towards the periphery of the retina, the number of cones decreases and the number of rods increases, so that in the far periphery there are only rods. Cones function in high light conditions; they provide daytime and color vision; the more light-sensitive rods are responsible for twilight vision.

Color is perceived best when light is applied to the fovea of ​​the retina, which contains almost exclusively cones. This is also where visual acuity is greatest. As we move away from the center of the retina, color perception and spatial resolution gradually decrease. The periphery of the retina, which contains only rods, does not perceive color. But the light sensitivity of the cone apparatus of the retina is many times less than that of the rod apparatus. Therefore, at dusk, due to a sharp decrease in cone vision and the predominance of peripheral rod vision, we do not distinguish color (“all cats are gray at night”).

Visual pigments. The rods of the human retina contain the pigment rhodopsin, or visual purple, the maximum absorption spectrum of which is in the region of 500 nanometers (nm). The outer segments of the three types of cones (blue-, green- and red-sensitive) contain three types of visual pigments, the maximum absorption spectra of which are in the blue (420 nm), green (531 nm) and red (558 nm) regions of the spectrum. The red cone pigment is called iodopsin. The visual pigment molecule consists of a protein part (opsin) and a chromophore part (retinal, or vitamin A aldehyde). The source of retinal in the body is carotenoids; if they are deficient, twilight vision is impaired (“night blindness”).

2.7. Retinal neurons

Retinal photoreceptors synapse with bipolar nerve cells (see Fig. 4.2). When exposed to light, the release of the transmitter from the photoreceptor decreases, which hyperpolarizes the membrane of the bipolar cell. From it, the nerve signal is transmitted to ganglion cells, the axons of which are fibers of the optic nerve.

Rice. 4.2. Diagram of the structure of the retina:
1 - sticks; 2 - cones; 3 - horizontal cell; 4 - bipolar cells; 5 - amacrine cells; 6 - ganglion cells; 7 - optic nerve fibers

For 130 million photoreceptor cells there are only 1 million 250 thousand retinal ganglion cells. This means that impulses from many photoreceptors converge (converge) through bipolar neurons to one ganglion cell. Photoreceptors connected to one ganglion cell form its receptive field [Hubel, 1990; Physiol. vision, 1992]. Thus, each ganglion cell summarizes the excitation arising in a large number of photoreceptors. This increases the light sensitivity of the retina, but worsens its spatial resolution. Only in the center of the retina (in the area of ​​the fovea) is each cone connected to one bipolar cell, which, in turn, is connected to one ganglion cell. This provides high spatial resolution of the retinal center, but sharply reduces its light sensitivity.

The interaction of neighboring retinal neurons is ensured by horizontal and amacrine cells, through the processes of which signals propagate that change synaptic transmission between photoreceptors and bipolars (horizontal cells) and between bipolars and ganglion cells (amacrines). Amacrine cells exert lateral inhibition between adjacent ganglion cells. Centrifugal, or efferent, nerve fibers also enter the retina, bringing signals from the brain to it. These impulses regulate the conduction of excitation between bipolar and retinal ganglion cells.

2.8. Neural pathways and connections in the visual system

From the retina, visual information travels along the optic nerve fibers to the brain. The nerves from the two eyes meet at the base of the brain, where some of the fibers cross to the opposite side (optic chiasm, or optic chiasm). This provides each hemisphere of the brain with information from both eyes: the occipital lobe of the right hemisphere receives signals from the right halves of each retina, and the left hemisphere receives signals from the left half of each retina (Fig. 4.3).

Rice. 4.3. Diagram of visual pathways from the retina to the primary visual cortex:
LPZ - left visual field; RPV - right visual field; tf - gaze fixation point; lg - left eye; pg - right eye; zn - optic nerve; x - visual chiasm, or chiasma; from - optical path; tubing - external geniculate body; VK - visual cortex; lp - left hemisphere; pp - right hemisphere

After the chiasm, the optic nerves are called optic tracts and the bulk of their fibers come to the subcortical visual center - the external geniculate body (EC). From here, visual signals enter the primary projection area of ​​the visual cortex (striate cortex, or Brodmann area 17). The visual cortex consists of a number of fields, each of which provides its own specific functions, receiving both direct and indirect signals from the retina and generally maintaining its topology, or retinotopy (signals from neighboring areas of the retina enter neighboring areas of the cortex).

2.9. Electrical activity of the centers of the visual system

When exposed to light, electrical potentials are generated in the receptors, and then in the neurons of the retina, reflecting the parameters of the active stimulus (Fig. 4.4a, a). The total electrical response of the retina to light is called an electroretinogram (ERG).

Rice. 4.4. Electroretinogram (a) and light-evoked potential (EP) of the visual cortex (b):
a,b,c,d in (a) - ERG waves; The arrows indicate the moments when the light is turned on. P 1 - P 5 - positive waves of VP, N 1 - N 5 - negative waves of VP in (b)

It can be recorded from the whole eye: one electrode is placed on the surface of the cornea, and the other on the skin of the face near the eye (or on the earlobe). The ERG clearly reflects the intensity, color, size and duration of the light stimulus. Since the ERG reflects the activity of almost all retinal cells (except ganglion cells), this indicator is widely used to analyze the work and diagnose retinal diseases.

Excitation of retinal ganglion cells causes electrical impulses to be sent along their axons (optic nerve fibers) to the brain. The retinal ganglion cell is the first “classical” type neuron in the retina that generates propagating impulses. Three main types of ganglion cells have been described: those that respond to the light being turned on (on - reaction), turning it off (off - reaction) and to both (on-off - reaction). In the center of the retina, the receptive fields of ganglion cells are small, and in the periphery of the retina they are much larger in diameter. Simultaneous excitation of closely located ganglion cells leads to their mutual inhibition: the responses of each cell become smaller than with a single stimulation. This effect is based on lateral or lateral inhibition (see Chapter 3). Due to their circular shape, the receptive fields of retinal ganglion cells produce what is called a point-by-point description of the retinal image: it is displayed as a very fine, discrete mosaic of excited neurons.

Neurons of the subcortical visual center are excited when impulses arrive from the retina along the fibers of the optic nerve. The receptive fields of these neurons are also round, but smaller than those in the retina. The bursts of impulses they generate in response to a flash of light are shorter than those in the retina. At the level of the NKT, the interaction of afferent signals coming from the retina occurs with efferent signals from the visual cortex, as well as from the reticular formation from the auditory and other sensory systems. This interaction helps to highlight the most significant components of the signal and, possibly, is involved in the organization of selective visual attention (see Chapter 9).

The impulse discharges of the NKT neurons along their axons enter the occipital part of the cerebral hemispheres, in which the primary projection area of ​​the visual cortex (striate cortex) is located. Here, in primates and humans, much more specialized and complex information processing occurs than in the retina and NKT. Neurons of the visual cortex do not have round, but elongated (horizontally, vertically or diagonally) receptive fields (Fig. 4.5) of small size [Hubel, 1990].

Rice. 4.5. The receptive field of a neuron in the visual cortex of a cat’s brain (A) and the responses of this neuron to light strips of different orientations flashing in the receptive field (B). A - pluses indicate the excitatory zone of the receptive field, and minuses indicate two lateral inhibitory zones. B - it is clear that this neuron reacts most strongly to vertical and close to it orientation

Thanks to this, they are able to select from the image individual fragments of lines with one or another orientation and location and react selectively to them (orientation detectors). In each small area of ​​the visual cortex, neurons with the same orientation and localization of receptive fields in the visual field are concentrated along its depth. They form an orientation column neurons, passing vertically through all layers of the cortex. The column is an example of a functional association of cortical neurons that perform a similar function. A group of adjacent orientation columns whose neurons have overlapping receptive fields but different preferred orientations forms a so-called supercolumn. As studies in recent years have shown, the functional unification of distant neurons in the visual cortex can also occur due to the synchrony of their discharges. Recently, neurons with selective sensitivity to cross-shaped and angular figures, related to 2nd order detectors, were found in the visual cortex. Thus, the “niche” between simple orientation detectors describing spatial features of an image and higher-order (face) detectors found in the temporal cortex began to be filled.

In recent years, the so-called “spatial-frequency” tuning of neurons in the visual cortex has been well studied [Glezer, 1985; Physiol. vision, 1992]. It lies in the fact that many neurons selectively react to a grid of light and dark stripes of a certain width that appears in their receptive field. Thus, there are cells that are sensitive to a lattice of small stripes, i.e. to high spatial frequency. Cells with sensitivity to different spatial frequencies have been found. It is believed that this property provides the visual system with the ability to identify areas with different textures from the image [Glezer, 1985].

Many neurons in the visual cortex selectively respond to certain directions of movement (directional detectors) or to a certain color (color-opponent neurons), and some neurons respond best to the relative distance of the object from the eyes. Information about different features of visual objects (shape, color, movement) is processed in parallel in different parts of the visual cortex.

To assess signal transmission at different levels of the visual system, recording of total evoked potentials(VP), which in humans can be simultaneously removed from the retina and from the visual cortex (see Fig. 4.4 b). Comparison of the retinal response (ERG) caused by a light flash and the EP of the cortex makes it possible to evaluate the functioning of the projection visual pathway and establish the localization of the pathological process in the visual system.

2.10. Light sensitivity

Absolute visual sensitivity. For a visual sensation to occur, light must have a certain minimum (threshold) energy. The minimum number of light quanta required to produce the sensation of light in the dark ranges from 8 to 47. One rod can be excited by only 1 light quantum. Thus, the sensitivity of the retinal receptors in the most favorable conditions of light perception is extreme. Single rods and cones of the retina differ slightly in light sensitivity. However, the number of photoreceptors sending signals per ganglion cell differs in the center and periphery of the retina. The number of cones in the receptive field in the center of the retina is approximately 100 times less than the number of rods in the receptive field in the periphery of the retina. Accordingly, the sensitivity of the rod system is 100 times higher than that of the cone system.

2.11. Visual adaptation

When moving from darkness to light, temporary blindness occurs, and then the sensitivity of the eye gradually decreases. This adaptation of the visual system to bright light conditions is called light adaptation. The opposite phenomenon (dark adaptation) is observed when a person moves from a bright room to an almost unlit room. At first, he sees almost nothing due to reduced excitability of photoreceptors and visual neurons. Gradually, the contours of objects begin to emerge, and then their details also differ, as the sensitivity of photoreceptors and visual neurons in the dark gradually increases.

The increase in light sensitivity while in the dark occurs unevenly: in the first 10 minutes it increases tens of times, and then, within an hour, tens of thousands of times. The restoration of visual pigments plays an important role in this process. Since only the rods are sensitive in the dark, a dimly lit object is visible only in peripheral vision. A significant role in adaptation, in addition to visual pigments, is played by switching connections between retinal elements. In the dark, the area of ​​the excitatory center of the receptive field of the ganglion cell increases due to the weakening of circular inhibition, which leads to an increase in light sensitivity. The light sensitivity of the eye also depends on the influences coming from the brain. Illumination of one eye reduces the light sensitivity of the unilluminated eye. In addition, sensitivity to light is also influenced by auditory, olfactory and gustatory signals.

2.12. Differential vision sensitivity

If additional illumination dI falls on an illuminated surface with brightness I, then, according to Weber’s law, a person will notice a difference in illumination only if dI/I = K, where K is a constant equal to 0.01-0.015. The dI/I value is called the differential threshold of light sensitivity. The dI/I ratio is constant under different illumination and means that in order to perceive a difference in the illumination of two surfaces, one of them must be 1 - 1.5% brighter than the other.

2.13. Luminance Contrast

Mutual lateral inhibition of visual neurons (see Chapter 3) underlies the general, or global luminance contrast. Thus, a gray strip of paper lying on a light background appears darker than the same strip lying on a dark background. This is explained by the fact that a light background excites many neurons in the retina, and their excitation inhibits the cells activated by the strip. Lateral inhibition acts most strongly between closely spaced neurons, creating a local contrast effect. There is an apparent increase in the difference in brightness at the border of surfaces of different illumination. This effect is also called edge enhancement, or the Mach effect: at the border of a bright light field and a darker surface, two additional lines can be seen (an even brighter line at the border of the light field and a very dark line at the border of the dark surface).

2.14. Blinding brightness of light

Light that is too bright causes an unpleasant feeling of being blinded. The upper limit of blinding brightness depends on the adaptation of the eye: the longer the dark adaptation, the lower the brightness of the light causes blinding. If very bright (dazzle) objects come into the field of view, they impair the discrimination of signals on a significant part of the retina (for example, on a night road, drivers are blinded by the headlights of oncoming cars). For delicate work that involves eye strain (long reading, working on a computer, assembling small parts), you should use only diffused light that does not dazzle the eye.

2.15. Inertia of vision, merging of flickers, sequential images

The visual sensation does not appear instantly. Before a sensation occurs, multiple transformations and signal transmission must occur in the visual system. The time of “inertia of vision” required for the occurrence of a visual sensation is on average 0.03 - 0.1 s. It should be noted that this sensation also does not disappear immediately after the irritation has stopped - it lasts for some time. If we move a burning match through the air in the dark, we will see a luminous line, since light stimuli quickly following one after another merge into a continuous sensation. The minimum frequency of light stimuli (for example, flashes of light) at which individual sensations are combined is called critical flicker fusion frequency. At average illumination, this frequency is equal to 10-15 flashes per 1 s. Cinema and television are based on this property of vision: we do not see gaps between individual frames (24 frames in 1 s in cinema), since the visual sensation from one frame continues until the next one appears. This provides the illusion of image continuity and movement.

The sensations that continue after the irritation has stopped are called consistent images. If you look at the switched-on lamp and close your eyes, it will still be visible for some time. If, after fixing your gaze on an illuminated object, you turn your gaze to a light background, then for some time you can see a negative image of this object, i.e. its light parts are dark, and its dark parts are light (negative sequential image). This is explained by the fact that excitation from an illuminated object locally inhibits (adapts) certain areas of the retina; If you then turn your gaze to a uniformly illuminated screen, its light will more strongly excite those areas that were not previously excited.

2.16. Color vision

The entire spectrum of electromagnetic radiation that we see lies between short-wavelength (wavelength 400 nm) radiation, which we call violet, and long-wavelength radiation (wavelength 700 nm), called red. The remaining colors of the visible spectrum (blue, green, yellow and orange) have intermediate wavelengths. Mixing the rays of all colors gives white. It can also be obtained by mixing two so-called paired complementary colors: red and blue, yellow and blue. If you mix the three primary colors (red, green and blue), any colors can be obtained.

The three-component theory of G. Helmholtz, according to which color perception is provided by three types of cones with different color sensitivity, enjoys maximum recognition. Some of them are sensitive to red, others to green, and others to blue. Every color affects all three color-sensing elements, but to varying degrees. This theory was directly confirmed in experiments in which the absorption of radiation of different wavelengths in single cones of the human retina was measured.

Partial color blindness was described at the end of the 18th century. D. Dalton, who himself suffered from it. Therefore, the anomaly of color perception was designated by the term “color blindness”. Color blindness occurs in 8% of men; it is associated with the absence of certain genes on the sex-determining unpaired X chromosome in men. To diagnose color blindness, which is important in professional selection, polychromatic tables are used. People suffering from it cannot be full-fledged drivers of transport, since they may not distinguish the color of traffic lights and road signs. There are three types of partial color blindness: protanopia, deuteranopia and tritanopia. Each of them is characterized by a lack of perception of one of the three primary colors. People suffering from protanopia ("red-blind") do not perceive the color red; blue-blue rays seem colorless to them. People suffering from deuteranopia ("green-blind") do not distinguish green colors from dark red and blue. With tritanopia (a rare color vision anomaly), blue and violet light is not perceived. All of the listed types of partial color blindness are well explained by the three-component theory. Each of them is the result of the absence of one of the three cone color-perceiving substances.

2.17. Perception of space

Visual acuity is called the maximum ability to distinguish individual details of objects. It is determined by the shortest distance between two points that the eye can distinguish, i.e. sees separately, not together. The normal eye distinguishes two points, the distance between which is 1 arc minute. The center of the retina, the macula, has maximum visual acuity. To the periphery of it, visual acuity is much less. Visual acuity is measured using special tables, which consist of several rows of letters or open circles of various sizes. Visual acuity, determined from the table, is expressed in relative values, with normal acuity taken as one. There are people who have hyperacuity of vision (visus greater than 2).

Field of view. If you fix your gaze on a small object, its image is projected onto the macula of the retina. In this case, we see the object with central vision. Its angular size in humans is only 1.5-2 angular degrees. Objects whose images fall on the remaining areas of the retina are perceived by peripheral vision. The space visible to the eye when the gaze is fixed at one point is called field of view. The boundary of the field of view is measured along the perimeter. The boundaries of the field of view for colorless objects are 70 degrees downward, 60 degrees upward, 60 degrees inward, and 90 degrees outward. The visual fields of both eyes in humans partially coincide, which is of great importance for the perception of the depth of space. The fields of view for different colors are not the same and are smaller than for black and white objects.

Binocular vision- This is seeing with two eyes. When looking at any object, a person with normal vision does not have the sensation of two objects, although there are two images on two retinas. The image of each point of this object falls on the so-called corresponding, or corresponding areas of the two retinas, and in human perception the two images merge into one. If you press lightly on one eye from the side, you will begin to see double, because the correspondence of the retinas is disrupted. If you look at a close object, then the image of some more distant point falls on non-identical (disparate) points of the two retinas. Disparity plays a big role in judging distance and, therefore, in seeing the depth of space. A person is able to notice a change in depth, creating a shift in the image on the retinas of several arc seconds. Binocular fusion, or the combining of signals from the two retinas into a single neural image, occurs in the primary visual cortex of the brain.

Estimation of the size of an object. The size of a familiar object is estimated as a function of the size of its image on the retina and the distance of the object from the eyes. In cases where it is difficult to estimate the distance to an unfamiliar object, gross errors in determining its size are possible.

Distance estimation. Perception of the depth of space and estimation of the distance to an object are possible both with vision with one eye (monocular vision) and with two eyes (binocular vision). In the second case, the distance estimate is much more accurate. The phenomenon of accommodation is of some importance in assessing close distances with monocular vision. For assessing distance, it is also important that the closer it is, the larger the image of a familiar object on the retina.

The role of eye movements for vision. When looking at any objects, the eyes move. Eye movements are carried out by 6 muscles attached to the eyeball. The movement of the two eyes occurs simultaneously and in a friendly manner. When looking at close objects, it is necessary to bring them together (convergence), and when looking at distant objects, it is necessary to separate the visual axes of the two eyes (divergence). The important role of eye movements for vision is also determined by the fact that for the brain to continuously receive visual information, movement of the image on the retina is necessary. Impulses in the optic nerve occur when the light image is turned on and off. With continued exposure to light on the same photoreceptors, the impulse in the optic nerve fibers quickly stops and the visual sensation with motionless eyes and objects disappears after 1-2 s. If you place a suction cup with a tiny light source on the eye, then a person sees it only at the moment of turning it on or off, since this stimulus moves with the eye and, therefore, is motionless in relation to the retina. To overcome such an adaptation (adaptation) to a still image, the eye, when viewing any object, produces continuous jumps (saccades) that are imperceptible to a person. As a result of each jump, the image on the retina shifts from one photoreceptor to another, again causing impulses in the ganglion cells. The duration of each jump is equal to hundredths of a second, and its amplitude does not exceed 20 angular degrees. The more complex the object in question, the more complex the trajectory of eye movement. They seem to “trace” the contours of the image (Fig. 4.6), lingering on its most informative areas (for example, in the face these are the eyes). In addition to jumping, the eyes continuously tremble and drift (slowly shift from the point of fixation of gaze). These movements are also very important for visual perception.

Rice. 4.6. Trajectory of eye movement (B) when examining the image of Nefertiti (A)

Eye- organ of vision in animals and humans. The human eye consists of the eyeball, connected by the optic nerve to the brain, and the auxiliary apparatus (eyelids, lacrimal organs and muscles that move the eyeball).

The eyeball (Fig. 94) is protected by a dense membrane called the sclera. The anterior (transparent) part of the sclera 1 is called the cornea. The cornea is the most sensitive external part of the human body (even the lightest touch causes an instant reflex closure of the eyelids).

Behind the cornea is the iris 2, which can have different colors in people. Between the cornea and the iris there is a watery fluid. There is a small hole in the iris - pupil 3. The diameter of the pupil can vary from 2 to 8 mm, decreasing in the light and increasing in the dark.

Behind the pupil there is a transparent body resembling a biconvex lens - lens 4. On the outside it is soft and almost gelatinous, on the inside it is harder and more elastic. The lens is surrounded by 5 muscles that attach it to the sclera.

Behind the lens is the vitreous body 6, which is a colorless gelatinous mass. The back part of the sclera - the fundus of the eye - is covered with a retina (retina) 7. It consists of the finest fibers that cover the fundus of the eye and represent the branched endings of the optic nerve.

How do images of various objects appear and are perceived by the eye?

Light, refracted in the optical system of the eye, which is formed by the cornea, lens and vitreous body, gives real, reduced and inverse images of the objects in question on the retina (Fig. 95). Once light reaches the endings of the optic nerve, which make up the retina, it irritates these endings. These irritations are transmitted through nerve fibers to the brain, and a person has a visual sensation: he sees objects.

The image of an object appearing on the retina of the eye is inverted. The first person to prove this by constructing the path of rays in the optical system of the eye was I. Kepler. To test this conclusion, the French scientist R. Descartes (1596-1650) took a bull's eye and, after scraping off the opaque layer from its back wall, placed it in a hole made in a window shutter. And then, on the translucent wall of the fundus, he saw an inverted image of the picture observed from the window.

Why then do we see all objects as they are, that is, not inverted? The fact is that the process of vision is continuously corrected by the brain, which receives information not only through the eyes, but also through other senses. At one time, the English poet William Blake (1757-1827) very correctly noted:

The mind knows how to look at the world.

In 1896, American psychologist J. Stretton conducted an experiment on himself. He put on special glasses, thanks to which the images of surrounding objects on the retina of the eye were not reversed, but direct. So what? The world in Stretton's mind turned upside down. He began to see all objects upside down. Because of this, there was a mismatch in the work of the eyes with other senses. The scientist developed symptoms of seasickness. He felt nauseated for three days. However, on the fourth day the body began to return to normal, and on the fifth day Stretton began to feel the same as before the experiment. The scientist’s brain became accustomed to the new working conditions, and he began to see all objects straight again. But when he took off his glasses, everything turned upside down again. Within an hour and a half, his vision was restored, and he began to see normally again.

It is curious that such adaptability is characteristic only of the human brain. When, in one of the experiments, inverting glasses were put on a monkey, it received such a psychological blow that, after making several wrong movements and falling, it fell into a state reminiscent of a coma. Her reflexes began to fade, her blood pressure dropped, and her breathing became rapid and shallow. Nothing like this is observed in humans.

However, the human brain is not always able to cope with the analysis of the image obtained on the retina. In such cases, visual illusions arise - the observed object does not seem to us as it really is (Fig. 96).

There is one more feature of vision that cannot be ignored. It is known that when the distance from the lens to the object changes, the distance to its image also changes. How does a clear image remain on the retina when we move our gaze from a distant object to a closer one?

It turns out that those muscles that are attached to the lens are capable of changing the curvature of its surfaces and thereby the optical power of the eye. When we look at distant objects, these muscles are in a relaxed state and the curvature of the lens is relatively small. When looking at nearby objects, the eye muscles compress the lens, and its curvature, and therefore the optical power, increases.

The ability of the eye to adapt to vision at both near and far distances is called accommodation(from Latin accomodatio - device). Thanks to accommodation, a person manages to focus images of various objects at the same distance from the lens - on the retina.

However, when the object in question is very close, the tension of the muscles that deform the lens increases, and the work of the eye becomes tiring. The optimal reading and writing distance for a normal eye is about 25 cm. This distance is called the distance of clear (or best) vision.

What is the benefit of seeing with both eyes?

Firstly, it is thanks to the presence of two eyes that we can distinguish which object is closer and which is further from us. The fact is that the retinas of the right and left eyes produce images that differ from each other (corresponding to looking at an object as if from the right and left). The closer the object, the more noticeable this difference. It creates the impression of a difference in distances. The same ability of vision allows you to see an object as three-dimensional, rather than flat.

Secondly, having two eyes increases the field of view. The human field of vision is shown in Figure 97, a. For comparison, the visual fields of a horse (Fig. 97, c) and a hare (Fig. 97, b) are shown next to it. Looking at these pictures, it is easy to understand why it is so difficult for predators to sneak up on these animals without giving themselves away.

Vision allows people to see each other. Is it possible to see yourself, but be invisible to others? The English writer Herbert Wells (1866-1946) first tried to answer this question in his novel The Invisible Man. A person will become invisible after his substance becomes transparent and has the same optical density as the surrounding air. Then there will be no reflection and refraction of light at the border of the human body with air, and it will turn into invisible. For example, crushed glass, which looks like a white powder in air, immediately disappears from view when it is placed in water, a medium that has approximately the same optical density as glass.

In 1911, the German scientist Spalteholtz soaked a preparation of dead animal tissue with a specially prepared liquid, after which he placed it in a vessel with the same liquid. The preparation became invisible.

However, the invisible man must be invisible in air, and not in a specially prepared solution. But this cannot be achieved.

But let’s assume that a person still manages to become transparent. People will stop seeing him. Will he be able to see them himself? No, because all its parts, including the eyes, will stop refracting light rays, and, therefore, no image will appear on the retina of the eye. In addition, in order to form a visible image in a person’s mind, light rays must be absorbed by the retina, transferring their energy to it. This energy is necessary for the generation of signals traveling along the optic nerve to the human brain. If the invisible man's eyes become completely transparent, then this will not happen. And if so, then he will stop seeing altogether. The invisible man will be blind.

H.G. Wells did not take this circumstance into account and therefore endowed his hero with normal vision, allowing him to terrorize an entire city without being noticed.

1. How does the human eye work? Which parts form an optical system? 2. Describe the image appearing on the retina of the eye. 3. How is the image of an object transmitted to the brain? Why do we see objects straight and not upside down? 4. Why, when we move our gaze from a close object to a distant one, do we continue to see its clear image? 5. What is the distance of best vision? 6. What is the advantage of seeing with both eyes? 7. Why must the invisible man be blind?

Accessory apparatus of the visual system and its functions

The visual sensory system is equipped with a complex auxiliary apparatus, which includes the eyeball and three pairs of muscles that provide its movements. Elements of the eyeball carry out the primary transformation of the light signal entering the retina:
the optical system of the eye focuses images on the retina;
the pupil regulates the amount of light falling on the retina;
- the muscles of the eyeball ensure its continuous movement.

Formation of an image on the retina

Natural light reflected from the surface of objects is diffuse, i.e. Light rays from each point on an object come in different directions. Therefore, in the absence of the optical system of the eye, rays from one point of the object ( A) would fall into different parts of the retina ( a1, a2, a3). Such an eye would be able to distinguish the general level of illumination, but not the contours of objects (Fig. 1 A).

In order to see objects in the surrounding world, it is necessary that light rays from each point of the object hit only one point of the retina, i.e. the image needs to be focused. This can be achieved by placing a spherical refractive surface in front of the retina. Light rays emanating from one point ( A), after refraction on such a surface will be collected at one point a1(focus). Thus, a clear inverted image will appear on the retina (Fig. 1 B).

Refraction of light occurs at the interface between two media having different refractive indices. The eyeball contains two spherical lenses: the cornea and the lens. Accordingly, there are 4 refractive surfaces: air/cornea, cornea/aqueous humor of the anterior chamber of the eye, aqueous humor/lens, lens/vitreous.

Accommodation

Accommodation is the adjustment of the refractive power of the optical apparatus of the eye to a certain distance to the object in question. According to the laws of refraction, if a ray of light falls on a refracting surface, it is deflected by an angle depending on the angle of its incidence. When an object approaches, the angle of incidence of the rays emanating from it will change, so the refracted rays will converge at another point, which will be located behind the retina, which will lead to “blurring” of the image (Figure 2 B). In order to focus it again, it is necessary to increase the refractive power of the optical apparatus of the eye (Figure 2 B). This is achieved by increasing the curvature of the lens, which occurs with increasing tone of the ciliary muscle.

Regulating retinal illumination

The amount of light falling on the retina is proportional to the area of ​​the pupil. The diameter of the pupil in an adult varies from 1.5 to 8 mm, which ensures a change in the intensity of light incident on the retina by approximately 30 times. Pupillary reactions are provided by two systems of smooth muscles of the iris: when the circular muscles contract, the pupil narrows, and when the radial muscles contract, the pupil dilates.

As the pupil lumen decreases, the image sharpness increases. This occurs because the constriction of the pupil prevents light from reaching the peripheral areas of the lens and thereby eliminates image distortion caused by spherical aberration.

Eye movements

The human eye is driven by six ocular muscles, which are innervated by three cranial nerves - oculomotor, trochlear and abducens. These muscles provide two types of movements of the eyeball - fast saccadic movements (saccades) and smooth tracking movements.

Jumping eye movements (saccades) arise when viewing stationary objects (Fig. 3). Rapid turns of the eyeball (10 - 80 ms) alternate with periods of motionless gaze fixation at one point (200 - 600 ms). The angle of rotation of the eyeball during one saccade ranges from several arc minutes to 10°, and when moving the gaze from one object to another it can reach 90°. At large displacement angles, saccades are accompanied by head rotation; The displacement of the eyeball usually precedes the movement of the head.

Smooth eye movements accompany objects moving in the field of view. The angular velocity of such movements corresponds to the angular velocity of the object. If the latter exceeds 80°/s, then tracking becomes combined: smooth movements are complemented by saccades and head turns.

Nystagmus - periodic alternation of smooth and jerky movements. When a person traveling on a train looks out the window, his eyes smoothly follow the landscape moving outside the window, and then his gaze abruptly moves to a new point of fixation.

Conversion of light signal in photoreceptors

Types of retinal photoreceptors and their properties

The retina has two types of photoreceptors (rods and cones), which differ in structure and physiological properties.

Table 1. Physiological properties of rods and cones

	Sticks	Cones
Photosensitive pigment	Rhodopsin	Iodopsin
Maximum pigment absorption	Has two maxima - one in the visible part of the spectrum (500 nm), the other in the ultraviolet (350 nm)	There are 3 types of iodopsins that have different absorption maxima: 440 nm (blue), 520 nm (green) and 580 nm (red)
Cell classes		Each cone contains only one pigment. Accordingly, there are 3 classes of cones that are sensitive to light of different wavelengths
Retinal distribution	In the central part of the retina, the density of rods is about 150,000 per mm2, towards the periphery it decreases to 50,000 per mm2. There are no rods in the fovea and the blind spot.	The density of cones in the central fovea reaches 150,000 per mm2, they are absent in the blind spot, and on the entire remaining surface of the retina the density of cones does not exceed 10,000 per mm2.
Sensitivity to light	Rods are about 500 times higher than cones
Function	Provide black and white (scototopic vision)	Provide color (phototopic vision)

Duality of vision theory

The presence of two photoreceptor systems (cones and rods), differing in light sensitivity, provides adjustment to changing levels of external illumination. In low light conditions, the perception of light is provided by rods, while the colors are indistinguishable ( scototopic vision e). In bright light, vision is provided mainly by cones, which makes it possible to distinguish colors well ( phototopic vision ).

The mechanism of light signal conversion in the photoreceptor

In the photoreceptors of the retina, the energy of electromagnetic radiation (light) is converted into the energy of fluctuations in the membrane potential of the cell. The transformation process occurs in several stages (Fig. 4).

At the 1st stage, a photon of visible light, entering a molecule of a light-sensitive pigment, is absorbed by p-electrons of conjugated double bonds 11- cis-retinal, while retinal passes into trance-form. Stereomerization 11- cis-retinal causes conformational changes in the protein part of the rhodopsin molecule.

At the 2nd stage, the transducin protein is activated, which in its inactive state contains tightly bound GDP. After interacting with photoactivated rhodopsin, transducin exchanges a GDP molecule for GTP.

At the 3rd stage, GTP-containing transducin forms a complex with inactive cGMP phosphodiesterase, which leads to activation of the latter.

At the 4th stage, activated cGMP phosphodiesterase hydrolyzes intracellular from GMP to GMP.

At the 5th stage, a drop in cGMP concentration leads to the closure of cation channels and hyperpolarization of the photoreceptor membrane.

During signal transduction along phosphodiesterase mechanism it is strengthened. During the photoreceptor response, one single molecule of excited rhodopsin manages to activate several hundred molecules of transducin. That. At the first stage of signal transduction, an amplification of 100-1000 times occurs. Each activated transducin molecule activates only one phosphodiesterase molecule, but the latter catalyzes the hydrolysis of several thousand molecules with GMP. That. at this stage the signal is amplified another 1,000-10,000 times. Therefore, when transmitting a signal from a photon to cGMP, a more than 100,000-fold amplification can occur.

Information processing in the retina

Elements of the retinal neural network and their functions

The retinal neural network includes 4 types of nerve cells (Fig. 5):

- ganglion cells,
bipolar cells,
- amacrine cells,
- horizontal cells.

Ganglion cells – neurons, the axons of which, as part of the optic nerve, leave the eye and follow to the central nervous system. The function of ganglion cells is to conduct excitation from the retina to the central nervous system.

Bipolar cells connect receptor and ganglion cells. Two branched processes extend from the bipolar cell body: one process forms synaptic contacts with several photoreceptor cells, the other with several ganglion cells. The function of bipolar cells is to conduct excitation from photoreceptors to ganglion cells.

Horizontal cells connect nearby photoreceptors. Several processes extend from the horizontal cell body, which form synaptic contacts with photoreceptors. The main function of horizontal cells is to carry out lateral interactions of photoreceptors.

Amacrine cells are located similar to horizontal ones, but they are formed by contacts not with photoreceptor cells, but with ganglion cells.

Propagation of excitation in the retina

When a photoreceptor is illuminated, a receptor potential develops in it, which represents hyperpolarization. The receptor potential that arises in the photoreceptor cell is transmitted to bipolar and horizontal cells through synaptic contacts with the help of a transmitter.

In a bipolar cell, both depolarization and hyperpolarization can develop (see below for more details), which spreads through synaptic contact to ganglion cells. The latter are spontaneously active, i.e. continuously generate action potentials at a certain frequency. Hyperpolarization of ganglion cells leads to a decrease in the frequency of nerve impulses, depolarization leads to its increase.

Electrical responses of retinal neurons

The receptive field of a bipolar cell is a set of photoreceptor cells with which it forms synaptic contacts. The receptive field of a ganglion cell is understood as a set of photoreceptor cells to which a given ganglion cell is connected through bipolar cells.

The receptive fields of bipolar and ganglion cells are round in shape. In the receptive field, a central and peripheral part can be distinguished (Fig. 6). The boundary between the central and peripheral parts of the receptive field is dynamic and can shift with changes in light levels.

The reactions of retinal nerve cells when illuminated by photoreceptors in the central and peripheral parts of their receptive field are usually opposite. At the same time, there are several classes of ganglion and bipolar cells (ON -, OFF - cells), demonstrating different electrical responses to the action of light (Fig. 6).

Table 2. Classes of ganglion and bipolar cells and their electrical responses

Cell classes	The reaction of nerve cells when illuminated by photoreceptors located
	in the central part of the Republic of Poland	in the peripheral part of the RP
Bipolar cells ON type	Depolarization	Hyperpolarization
Bipolar cells OFF type	Hyperpolarization	Depolarization
Ganglion cells ON type
Ganglion cells OFF type	Hyperpolarization and reduction in AP frequency	Depolarization and increase in AP frequency
Ganglion cells ON- OFF type	They give a short ON response to a stationary light stimulus and a short OFF response to a weakening light.

Processing of visual information in the central nervous system

Sensory pathways of the visual system

The myelinated axons of the retinal ganglion cells are sent to the brain as part of the two optic nerves (Fig. 7). The right and left optic nerves merge at the base of the skull to form the optic chiasm. Here, nerve fibers coming from the medial half of the retina of each eye pass to the contralateral side, and fibers from the lateral halves of the retinas continue ipsilaterally.

After crossing, the axons of ganglion cells in the optic tract follow to the lateral geniculate body (LCC), where they form synaptic contacts with neurons of the central nervous system. Axons of nerve cells of the LCT as part of the so-called. visual radiance reaches the neurons of the primary visual cortex (Brodmann area 17). Further, along intracortical connections, excitation spreads to the secondary visual cortex (fields 18b-19) and associative zones of the cortex.

The sensory pathways of the visual system are organized according to retinotopic principle – excitation from neighboring ganglion cells reaches neighboring points of the LCT and cortex. The surface of the retina is, as it were, projected onto the surface of the LCT and cortex.

Most of the axons of ganglion cells end in the LCT, while some of the fibers follow to the superior colliculus, hypothalamus, pretectal region of the brain stem, and nucleus of the optic tract.

The connection between the retina and the superior colliculus serves to regulate eye movements.

The projection of the retina to the hypothalamus serves to couple endogenous circadian rhythms with daily fluctuations in light levels.

The connection between the retina and the pretectal region of the trunk is extremely important for the regulation of pupillary lumen and accommodation.

Neurons of the optic tract nuclei, which also receive synaptic inputs from ganglion cells, are connected to the vestibular nuclei of the brain stem. This projection allows one to estimate the position of the body in space based on visual signals, and also serves to carry out complex oculomotor reactions (nystagmus).

Processing of visual information in LCT

LCT neurons have round receptive fields. The electrical responses of these cells are similar to those of ganglion cells.

In the LCT there are neurons that are excited when there is a light/dark boundary in their receptive field (contrast neurons) or when this boundary moves within the receptive field (motion detectors).

Processing of visual information in the primary visual cortex

Depending on the response to light stimuli, cortical neurons are divided into several classes.

Neurons with a simple receptive field. The strongest excitation of such a neuron occurs when its receptive field is illuminated by a light strip of a certain orientation. The frequency of nerve impulses generated by such a neuron decreases when the orientation of the light strip changes (Fig. 8 A).

Neurons with a complex receptive field. The maximum degree of neuron excitation is achieved when the light stimulus moves within the ON zone of the receptive field in a certain direction. Moving the light stimulus in a different direction or leaving the light stimulus outside the ON zone causes weaker excitation (Fig. 8 B).

Neurons with a highly complex receptive field. Maximum excitation of such a neuron is achieved under the action of a light stimulus of complex configuration. For example, neurons are known whose strongest excitation develops when crossing two boundaries between light and dark within the ON zone of the receptive field (Fig. 23.8 B).

Despite the huge amount of experimental data on the patterns of cell response to various visual stimuli, to date there is no complete theory explaining the mechanisms of visual information processing in the brain. We cannot explain how the varied electrical responses of retinal, LCT, and cortical neurons enable pattern recognition and other phenomena of visual perception.

Regulation of assistive apparatus functions

Regulation of accommodation. The curvature of the lens changes with the help of the ciliary muscle. When the ciliary muscle contracts, the curvature of the anterior surface of the lens increases and the refractive power increases. The smooth muscle fibers of the ciliary muscle are innervated by postganglionic neurons, the bodies of which are located in the ciliary ganglion.

An adequate stimulus for changing the degree of curvature of the lens is the blurring of the image on the retina, which is registered by the neurons of the primary cortex. Due to the descending connections of the cortex, a change in the degree of excitation of neurons in the pretectal region occurs, which in turn causes activation or inhibition of preganglionic neurons of the oculomotor nucleus (Edinger-Westphal nucleus) and postganglionic neurons of the ciliary ganglion.

Regulation of pupil lumen. Constriction of the pupil occurs with contraction of circular smooth muscle fibers of the cornea, which are innervated by parasympathetic postganglionic neurons of the ciliary ganglion. The latter are excited by high intensity light incident on the retina, which is perceived by neurons in the primary visual cortex.

Pupil dilation is accomplished by contraction of the radial muscles of the cornea, which are innervated by sympathetic neurons of the VSH. The activity of the latter is under the control of the ciliospinal center and the pretectal region. The stimulus for pupil dilation is a decrease in the level of illumination of the retina.

Regulation of eye movements. Some of the fibers of the ganglion cells follow to the neurons of the superior colliculus (midbrain), which are connected to the nuclei of the oculomotor, trochlear and abducens nerves, the neurons of which innervate the striated muscle fibers of the eye muscles. The nerve cells of the superior colliculi will receive synaptic inputs from the vestibular receptors and proprioceptors of the neck muscles, which allows the body to coordinate eye movements with body movements in space.

Phenomena of visual perception

Pattern recognition

The visual system has a remarkable ability to recognize an object in a wide variety of images. We can recognize an image (a familiar face, a letter, etc.) when some of its parts are missing, when it contains unnecessary elements, when it is differently oriented in space, has different angular dimensions, is turned towards us with different sides, etc. p. (Fig. 9). The neurophysiological mechanisms of this phenomenon are currently being intensively studied.

Constancy of shape and size

As a rule, we perceive surrounding objects as unchanged in shape and size. Although in fact their shape and size on the retina are not constant. For example, a cyclist in the field of view always appears the same in size, regardless of the distance from him. Bicycle wheels are perceived as round, although in fact their retinal images may be narrow ellipses. This phenomenon demonstrates the role of experience in seeing the world around us. The neurophysiological mechanisms of this phenomenon are currently unknown.

Perception of spatial depth

The image of the surrounding world on the retina is flat. However, we see the world in volume. There are several mechanisms that ensure the construction of 3-dimensional space based on flat images formed on the retina.

Since the eyes are located at some distance from each other, the images formed on the retina of the left and right eyes are slightly different from each other. The closer the object is to the observer, the more different these images will be.

Overlapping images also helps to evaluate their relative location in space. The image of a close object can overlap the image of a distant one, but not vice versa.

When the observer’s head moves, the images of the observed objects on the retina will also shift (the phenomenon of parallax). For the same head displacement, images of close objects will shift more than images of distant objects

Perception of stillness of space

If, after closing one eye, we press our finger on the second eyeball, we will see that the world around us is shifting to the side. Under normal conditions, the surrounding world is motionless, although the image on the retina constantly “jumps” due to the movement of the eyeballs, turns of the head, and changes in the position of the body in space. The perception of the stillness of the surrounding space is ensured by the fact that when processing visual images, information about eye movements, head movements and body position in space is taken into account. The visual sensory system is able to “subtract” its own eye and body movements from the movement of the image on the retina.

Theories of color vision

Three-component theory

Based on the principle of trichromatic additive mixing. According to this theory, the three types of cones (sensitive to red, green and blue) work as independent receptor systems. By comparing the intensity of the signals from the three types of cones, the visual sensory system produces a “virtual additive bias” and calculates the true color. The authors of the theory are Jung, Maxwell, Helmholtz.

Opponent color theory

It assumes that any color can be unambiguously described by indicating its position on two scales - “blue-yellow”, “red-green”. The colors lying at the poles of these scales are called opponent colors. This theory is supported by the fact that there are neurons in the retina, LCT and cortex that are activated if their receptive field is illuminated with red light and inhibited if the light is green. Other neurons are excited when exposed to yellow and inhibited when exposed to blue. It is assumed that by comparing the degree of excitation of neurons in the “red-green” and “yellow-blue” systems, the visual sensory system can calculate the color characteristics of light. The authors of the theory are Mach, Goering.

Thus, there is experimental evidence for both theories of color vision. Currently considered. That the three-component theory adequately describes the mechanisms of color perception at the level of retinal photoreceptors, and the theory of opposing colors - the mechanisms of color perception at the level of neural networks.

Through the eye, not with the eye
The mind knows how to look at the world.
William Blake

Lesson objectives:

Educational:

• reveal the structure and significance of the visual analyzer, visual sensations and perception;
• deepen knowledge about the structure and function of the eye as an optical system;
• explain how images are formed on the retina,
• give an idea of ​​myopia and farsightedness, and types of vision correction.

Educational:

• develop the ability to observe, compare and draw conclusions;
• continue to develop logical thinking;
• continue to form an idea of ​​the unity of concepts of the surrounding world.

Educational:

• to cultivate a caring attitude towards one’s health, to address issues of visual hygiene;
• continue to develop a responsible attitude towards learning.

Equipment:

• table "Visual analyzer",
• collapsible eye model,
• wet preparation "Mammalian Eye"
• handouts with illustrations.

Lesson progress

1. Organizational moment.

2. Updating knowledge. Repetition of the topic "Structure of the eye."

3. Explanation of new material:

Optical system of the eye.

Retina. Formation of images on the retina.

Optical illusions.

Accommodation of the eye.

The advantage of seeing with both eyes.

Eye movement.

Visual defects and their correction.

Visual hygiene.

4. Consolidation.

5. Lesson summary. Setting homework.

Repetition of the topic "Structure of the eye."

Biology teacher:

In the last lesson we studied the topic “Structure of the eye”. Let's remember the material of this lesson. Continue the sentence:

1) The visual zone of the cerebral hemispheres is located in ...

2) Gives color to the eye...

3) The analyzer consists of...

4) The auxiliary organs of the eye are...

5) The eyeball has... membranes

6) The convex - concave lens of the eyeball is ...

Using the drawing, tell about the structure and purpose of the constituent parts of the eye.

Explanation of new material.

Biology teacher:

The eye is the organ of vision in animals and humans. This is a self-adjusting device. It allows you to see near and distant objects. The lens either shrinks almost into a ball, or stretches, thereby changing the focal length.

The optical system of the eye consists of the cornea, lens, and vitreous body.

The retina (the mesh covering the fundus of the eye) has a thickness of 0.15 -0.20 mm and consists of several layers of nerve cells. The first layer is adjacent to the black pigment cells. It is formed by visual receptors - rods and cones. In the human retina there are hundreds of times more rods than cones. The rods are excited very quickly by weak twilight light, but cannot perceive color. Cones are excited slowly and only by bright light - they are able to perceive color. The rods are evenly distributed across the retina. Directly opposite the pupil in the retina is the yellow spot, which consists exclusively of cones. When examining an object, the gaze moves so that the image falls on the yellow spot.

Processes extend from nerve cells. In one place of the retina they gather in a bundle and form the optic nerve. More than a million fibers transmit visual information to the brain in the form of nerve impulses. This place, devoid of receptors, is called a blind spot. The analysis of the color, shape, illumination of an object, and its details, which began in the retina, ends in the cortex. Here all the information is collected, deciphered and summarized. As a result, an idea of ​​the subject is formed. It is the brain that “sees,” not the eye.

So, vision is a subcortical process. It depends on the quality of information coming from the eyes to the cerebral cortex (occipital region).

Physics teacher:

We found out that the optical system of the eye consists of the cornea, lens and vitreous body. Light, refracted in the optical system, gives real, reduced, inverse images of the objects in question on the retina.

The first to prove that the image on the retina is inverted by plotting the path of rays in the optical system of the eye was Johannes Kepler (1571 - 1630). To test this conclusion, the French scientist René Descartes (1596 - 1650) took a bull's eye and, after scraping off the opaque layer from its back wall, placed it in a hole made in a window shutter. And then, on the translucent wall of the fundus, he saw an inverted image of the picture observed from the window.

Why then do we see all objects as they are, i.e. not upside down?

The fact is that the process of vision is continuously corrected by the brain, which receives information not only through the eyes, but also through other senses.

In 1896, American psychologist J. Stretton conducted an experiment on himself. He put on special glasses, thanks to which the images of surrounding objects on the retina of the eye were not reversed, but forward. So what? The world in Stretton's mind turned upside down. He began to see all objects upside down. Because of this, there was a mismatch in the work of the eyes with other senses. The scientist developed symptoms of seasickness. For three days he felt nauseous. However, on the fourth day the body began to return to normal, and on the fifth day Stretton began to feel the same as before the experiment. The scientist’s brain became accustomed to the new working conditions, and he began to see all objects straight again. But when he took off his glasses, everything turned upside down again. Within an hour and a half, his vision was restored, and he began to see normally again.

It is curious that such an adaptation is characteristic only of the human brain. When, in one of the experiments, inverting glasses were put on a monkey, it received such a psychological blow that, after making several wrong movements and falling, it fell into a state reminiscent of a coma. Her reflexes began to fade, her blood pressure dropped, and her breathing became rapid and shallow. Nothing like this is observed in humans. However, the human brain is not always able to cope with the analysis of the image obtained on the retina. In such cases, visual illusions arise - the observed object does not seem to us as it really is.

Our eyes cannot perceive the nature of objects. Therefore, do not impose delusions of reason on them. (Lucretius)

Visual self-deceptions

We often talk about “deception of the eye”, “deception of hearing”, but these expressions are incorrect. There are no deceptions of feelings. The philosopher Kant aptly said about this: “The senses do not deceive us, not because they always judge correctly, but because they do not judge at all.”

What then deceives us in the so-called “deceptions” of the senses? Of course, what in this case “judges”, i.e. our own brain. Indeed, most of the optical illusions depend solely on the fact that we not only see, but also unconsciously reason, and unwittingly mislead ourselves. These are deceptions of judgment, not feelings.

Gallery of images, or what you see

Daughter, mother and mustachioed father?

An Indian proudly looking at the sun and an Eskimo in a hood with his back turned...

Young and old men

Young and old women

Are the lines parallel?

Is a quadrilateral a square?

Which ellipse is larger - the lower one or the inner upper one?

What is greater in this figure - height or width?

Which line is a continuation of the first?

Do you notice the circle "shaking"?

There is one more feature of vision that cannot be ignored. It is known that when the distance from the lens to the object changes, the distance to its image also changes. How does a clear image remain on the retina when we move our gaze from a distant object to a closer one?

As you know, the muscles that are attached to the lens are capable of changing the curvature of its surfaces and thereby the optical power of the eye. When we look at distant objects, these muscles are in a relaxed state and the curvature of the lens is relatively small. When looking at nearby objects, the eye muscles compress the lens, and its curvature, and, consequently, optical power, increases.

The ability of the eye to adapt to vision, both at close and further distances, is called accommodation(from Latin accomodatio - device).

Thanks to accommodation, a person manages to focus images of various objects at the same distance from the lens - on the retina.

However, when the object in question is very close, the tension of the muscles that deform the lens increases, and the work of the eye becomes tiring. The optimal distance for reading and writing for a normal eye is about 25 cm. This distance is called the distance of best vision.

Biology teacher:

What advantage does seeing with both eyes give?

1. The human field of vision increases.

2. It is thanks to the presence of two eyes that we can distinguish which object is closer and which is further from us.

The fact is that the retina of the right and left eyes produces images that differ from each other (corresponding to looking at objects as if on the right and left). The closer the object, the more noticeable this difference. It creates the impression of a difference in distances. This same ability of the eye allows you to see an object as three-dimensional and not flat. This ability is called stereoscopic vision. The joint work of both cerebral hemispheres ensures the distinction of objects, their shape, size, location, and movement. The effect of volumetric space can occur in cases where we consider a flat picture.

For several minutes, look at the picture at a distance of 20 - 25 cm from your eyes.

For 30 seconds, look at the witch on the broom without looking away.

Quickly shift your gaze to the drawing of the castle and look, counting to 10, into the gate opening. In the opening you will see a white witch on a gray background.

When you look at your eyes in the mirror, you probably notice that both eyes make large and subtle movements strictly simultaneously, in the same direction.

Do the eyes always look at everything like this? How do we behave in an already familiar room? Why do we need eye movements? They are needed for the initial inspection. By examining, we form a holistic image, and all this is transferred to storage in memory. Therefore, eye movement is not necessary to recognize well-known objects.

Physics teacher:

One of the main characteristics of vision is acuity. People's vision changes with age, because... the lens loses elasticity and the ability to change its curvature. Farsightedness or nearsightedness appears.

Myopia is a vision deficiency in which parallel rays, after refraction in the eye, are collected not on the retina, but closer to the lens. Images of distant objects therefore appear fuzzy and blurry on the retina. In order to get a sharp image on the retina, the object in question must be brought closer to the eye.

The distance of best vision for a myopic person is less than 25 cm. Therefore, people with a similar deficiency of rhenium are forced to read the text, placing it close to the eyes. Myopia may be due to the following reasons:

• excessive optical power of the eye;
• elongation of the eye along its optical axis.

It usually develops during school years and is usually associated with prolonged reading or writing, especially in insufficient lighting and improper placement of light sources.

Farsightedness is a defect of vision in which parallel rays, after refraction in the eye, converge at such an angle that the focus is located not on the retina, but behind it. Images of distant objects on the retina again turn out to be fuzzy and blurry.

Biology teacher:

To prevent visual fatigue, there are a number of exercises. We offer you some of them:

Option 1 (duration 3-5 minutes).

1. Starting position - sitting in a comfortable position: the spine is straight, the eyes are open, the gaze is directed straight. It’s very easy to do, without stress.

Direct your gaze to the left - straight, to the right - straight, up - straight, down - straight, without delay in the abducted position. Repeat 1-10 times.

2. Shift your gaze diagonally: left - down - straight, right - up - straight, right - down - straight, left - up - straight. And gradually increase the delays in the abducted position, breathing is voluntary, but make sure that there is no delay. Repeat 1-10 times.

3. Circular eye movements: from 1 to 10 circles left and right. Faster at first, then gradually reduce the pace.

4. Look at the tip of a finger or pencil held at a distance of 30 cm from the eyes, and then into the distance. Repeat several times.

5. Look straight ahead intently and motionlessly, trying to see more clearly, then blink several times. Squeeze your eyelids, then blink several times.

6. Changing the focal length: look at the tip of the nose, then into the distance. Repeat several times.

7. Massage the eyelids, gently stroking them with the index and middle fingers in the direction from the nose to the temples. Or: close your eyes and, touching very gently, move the pads of your palms along the upper eyelids from the temples to the bridge of the nose and back, a total of 10 times at an average pace.

8. Rub your palms together and easily, without effort, cover your previously closed eyes with them to completely block them from the light for 1 minute. Imagine being plunged into complete darkness. Open your eyes.

Option 2 (duration 1-2 minutes).

1. When counting 1-2, the eyes fixate on a close (distance 15-20 cm) object; when counting 3-7, the gaze is transferred to a distant object. At the count of 8, the gaze is again transferred to the nearest object.

2. With the head motionless, on the count of 1, turn the eyes vertically up, on the count of 2, down, then up again. Repeat 10-15 times.

3. Close your eyes for 10-15 seconds, open and move your eyes to the right and left, then up and down (5 times). Freely, without tension, direct your gaze into the distance.

Option 3 (duration 2-3 minutes).

The exercises are performed in a sitting position, leaning back in a chair.

1. Look straight ahead for 2-3 seconds, then lower your eyes down for 3-4 seconds. Repeat the exercise for 30 seconds.

2. Raise your eyes up, lower them down, look to the right, then to the left. Repeat 3-4 times. Duration 6 seconds.

3. Raise your eyes up, make circular movements with them counterclockwise, then clockwise. Repeat 3-4 times.

4. Close your eyes tightly for 3-5 seconds, open for 3-5 seconds. Repeat 4-5 times. Duration 30-50 seconds.

Consolidation.

Non-standard situations are offered.

1. A myopic student perceives the letters written on the board as blurry and indistinct. He has to strain his eyesight in order to accommodate his eyes either on the board or on the notebook, which is harmful for both the visual and nervous systems. Suggest a design for such glasses for schoolchildren to avoid stress when reading text from the board.

2. When a person's eye lens becomes cloudy (for example, with cataracts), it is usually removed and replaced with a plastic lens. Such a replacement deprives the eyes of the ability to accommodate and the patient has to use glasses. More recently, Germany began producing an artificial lens that can self-focus. Guess what design feature was invented for the accommodation of the eye?

3. H.G. Wells wrote the novel "The Invisible Man". An aggressive invisible personality wanted to subjugate the whole world. Think about what is wrong with this idea? When is an object in the environment invisible? How can the eye of an invisible man see?

Lesson summary. Setting homework.

• § 57, 58 (biology),
• § 37.38 (physics), offer non-standard problems on the topic studied (optional).

It is important to know the structure of the retina and how we receive visual information, at least in the most general form.

1. Look at the structure of the eyes. After the light rays pass through the lens, they penetrate the vitreous body and enter the inner, very thin layer of the eye - the retina. It is she who plays the main role in capturing the image. The retina is the central link of our visual analyzer.

The retina is adjacent to the choroid, but in many areas it is loose. Here it tends to flake off due to various diseases. In diseases of the retina, the choroid is very often involved in the pathological process. There are no nerve endings in the choroid, so when it is diseased, there is no pain, which usually signals some kind of problem.

The light-receiving retina can be functionally divided into central (the macula area) and peripheral (the entire remaining surface of the retina). Accordingly, a distinction is made between central vision, which makes it possible to clearly examine small details of objects, and peripheral vision, in which the shape of an object is perceived less clearly, but with its help orientation in space occurs.

2. The retina has a complex multilayer structure. It consists of photoreceptors (specialized neuroepithelium) and nerve cells. Photoreceptors located in the retina of the eye are divided into two types, called according to their shape: cones and rods. Rods (there are about 130 million of them in the retina) are highly photosensitivity and allow you to see in poor lighting; they are also responsible for peripheral vision. Cones (there are about 7 million of them in the retina), on the contrary, require more light for their excitation, but they are the ones that allow you to see small details (responsible for central vision) and make it possible to distinguish colors. The largest concentration of cones is in the area of ​​the retina known as the macula or macula, which occupies approximately 1% of the retina's area.

The rods contain visual purple, due to which they are excited very quickly and by weak light. Vitamin A is involved in the formation of visual purple, a deficiency of which develops so-called night blindness. Cones do not contain visual purple, so they are slowly excited only by bright light, but they are capable of perceiving color: the outer segments of the three types of cones (blue-, green- and red-sensitive) contain three types of visual pigments, the maximum absorption spectra of which are in blue, green and red regions of the spectrum.

3 . In the rods and cones, located in the outer layers of the retina, light energy is converted into electrical energy in the nervous tissue. Impulses arising in the outer layers of the retina reach intermediate neurons located in its inner layers, and then nerve cells. The processes of these nerve cells converge radially to one area of ​​the retina and form the optic disc, visible when examining the fundus.

The optic nerve consists of processes of nerve cells of the retina and exits the eyeball near its posterior pole. It transmits signals from nerve endings to the brain.

As it leaves the eye, the optic nerve divides into two halves. The inner half intersects with the same half of the other eye. The right side of the retina of each eye transmits the right part of the image to the right side of the brain through the optic nerve, and the left side of the retina, respectively, transmits the left part of the image to the left side of the brain. The overall picture of what we see is recreated directly by the brain.

Thus, visual perception begins with the projection of an image onto the retina and excitation of photoreceptors, and then the received information is sequentially processed in the subcortical and cortical visual centers. As a result, a visual image arises, which, thanks to the interaction of the visual analyzer with other analyzers and accumulated experience (visual memory), correctly reflects objective reality. The retina produces a reduced and inverted image of an object, but we see the image upright and in real size. This also happens because, along with visual images, nerve impulses from the extraocular muscles also enter the brain, for example, when we look up, the muscles rotate the eyes upward. The eye muscles work continuously, describing the contours of an object, and these movements are also recorded by the brain.

The structure of the eye.

The human eye is a visual analyzer; we receive 95% of information about the world around us through our eyes. Modern people have to work with nearby objects all day long: look at a computer screen, read, etc. Our eyes are under enormous strain, as a result of which many people suffer from eye diseases and vision defects. Everyone should know how the eye works and what its functions are.

The eye is an optical system; it has an almost spherical shape. The eye is a spherical body with a diameter of about 25 mm and a mass of 8 g. The walls of the eyeball are formed by three membranes. The outer tunica albuginea consists of dense, opaque connective tissue. It allows the eye to maintain its shape. The next layer of the eye is vascular; it contains all the blood vessels that nourish the tissues of the eye. The choroid is black because its cells contain a black pigment that absorbs light rays, preventing them from scattering around the eye. The choroid passes into the iris 2; in different people it has a different color, which determines the color of the eyes. The iris is a circular muscular diaphragm with a small hole in the center - pupil 3. It is black because the place from which light rays do not come is perceived by us as black. Through the pupil, light rays penetrate into the eye, but do not come back out, as if they are trapped. The pupil regulates the flow of light into the eye, reflexively narrowing or dilating; the pupil can have a size from 2 to 8 mm depending on the lighting.

Between the cornea and the iris there is a watery fluid, behind which - lens 4. The lens is a biconvex lens, it is elastic, and can change its curvature with the help of the ciliary muscle 5, therefore, precise focusing of light rays is ensured. . The refractive index of the lens is 1.45. Behind the lens is vitreous 6, which fills the main part of the eye. The vitreous body and aqueous humor have a refractive index almost the same as that of water - 1.33. The back wall of the sclera is covered with very thin fibers that line the bottom of the eye, and are called retina 7. These fibers are branching of the optic nerve. It is on the retina of the eye that the image appears. The place of the best image, which is located above the exit of the optic nerve, is called yellow spot 8, and the area of ​​the retina where the optic nerve exits the eye, which does not produce an image, is called blind spot 9.

Image in the eye.

Now let's look at the eye as an optical system. It includes the cornea, lens, and vitreous body. The main role in creating an image belongs to the lens. It focuses the rays on the retina, resulting in a truly reduced, inverted image of objects, which the brain corrects into an upright one. The rays are focused on the retina, on the back wall of the eye.

The "Experiments" section gives an example of how you can obtain an image of a light source on the pupil created by rays reflected from the eye.