What image falls on the retina of the human eye. Image on the human retina. Construction of an image on the retina

The eye consists of eyeball with a diameter of 22-24 mm, covered with an opaque shell, sclera, and the front is transparent cornea(or cornea). The sclera and cornea protect the eye and serve as anchorage for the oculomotor muscles.

Iris- a thin vascular plate that limits the passing beam of rays. Light enters the eye through pupil. Depending on the lighting, the diameter of the pupil can vary from 1 to 8 mm.

Lens is an elastic lens that is attached to the muscles ciliary body. The ciliary body changes the shape of the lens. The lens divides the inner surface of the eye into an anterior chamber filled with aqueous humor and a posterior chamber filled with vitreous body.

The inner surface of the rear camera is covered with a photosensitive layer - retina. From the retina, the light signal is transmitted to the brain by optic nerve. Between the retina and sclera is choroid, consisting of a network blood vessels, nourishing the eye.

The retina has yellow spot- the area of ​​the clearest vision. The line passing through the center of the macula and the center of the lens is called visual axis. It is tilted upward from the optical axis of the eye by an angle of about 5 degrees. The diameter of the macula is about 1 mm, and the corresponding field of vision of the eye is 6-8 degrees.

The retina is covered with light-sensitive elements: with chopsticks And cones. Rods are more sensitive to light, but do not distinguish colors and are used for twilight vision. Cones are sensitive to colors but less sensitive to light and therefore serve for daytime vision. In the macula area, cones predominate and rods are few; To the periphery of the retina, on the contrary, the number of cones quickly decreases, and only rods remain.

In the middle of the macula is central fossa. The bottom of the pit is lined only with cones. The diameter of the fovea is 0.4 mm, the field of view is 1 degree.

In the macula, individual fibers of the optic nerve approach most of the cones. Outside the macula, one optic nerve fiber serves a group of cones or rods. Therefore, in the area of ​​the fovea and the macula, the eye can distinguish fine details, and the image falling on the rest of the retina becomes less clear. The peripheral part of the retina serves mainly for orientation in space.

The sticks contain pigment rhodopsin, gathering in them in the dark and fading in the light. The perception of light by rods is due to chemical reactions under the influence of light on rhodopsin. Cones react to light through a reaction iodopsin.

In addition to rhodopsin and iodopsin, back surface The retina has a black pigment. When exposed to light, this pigment penetrates the layers of the retina and, absorbing a significant part of the light energy, protects the rods and cones from strong light exposure.

At the site of the optic nerve trunk is located blind spot. This part of the retina is not sensitive to light. The diameter of the blind spot is 1.88 mm, which corresponds to a field of view of 6 degrees. This means that a person from a distance of 1 m may not see an object with a diameter of 10 cm if its image is projected onto a blind spot.

The optical system of the eye consists of the cornea, aqueous humor, lens and vitreous body. Refraction of light in the eye occurs mainly at the cornea and lens surfaces.

Light from the observed object passes through the optical system of the eye and is focused on the retina, forming a reverse and reduced image on it (the brain “inverts” the reverse image, and it is perceived as direct).

The refractive index of the vitreous body is greater than one, so the focal lengths of the eye in the outer space (anterior focal length) and inside the eye (posterior focal length) are not the same.

The optical power of the eye (in diopters) is calculated as the inverse of the back focal length of the eye, expressed in meters. The optical power of the eye depends on whether it is at rest (58 diopters for a normal eye) or in the state of greatest accommodation (70 diopters).

Accommodation is the ability of the eye to clearly distinguish objects located at different distances. Accommodation occurs due to changes in the curvature of the lens when the muscles of the ciliary body are tense or relaxed. When the ciliary body is taut, the lens stretches and its radii of curvature increase. As muscle tension decreases, the curvature of the lens increases under the influence of elastic forces.

In a free, relaxed state of a normal eye, clear images of infinitely distant objects are obtained on the retina, and with the greatest accommodation, the closest objects are visible.

The position of an object at which a sharp image is created on the retina for an unstrained eye is called farthest point of the eye.

The position of the object at which a sharp image is created on the retina with the greatest possible eye strain is called the closest point of the eye.

When the eye accommodates to infinity, the back focus coincides with the retina. At the highest voltage on the retina, an image of an object located at a distance of about 9 cm is obtained.

The difference between the reciprocals of the distances between the near and far points is called range of accommodation of the eye(measured in diopters).

With age, the eye's ability to accommodate decreases. At the age of 20, for the average eye, the nearest point is at a distance of about 10 cm (accommodation range of 10 diopters), at 50 years of age, the nearest point is already at a distance of about 40 cm (accommodation range of 2.5 diopters), and by the age of 60 it goes to infinity , that is, accommodation stops. This phenomenon is called age-related farsightedness or presbyopia.

Distance best vision - this is the distance at which the normal eye experiences the least strain when examining the details of an object. With normal vision, it averages 25-30 cm.

Adaptation of the eye to changing lighting conditions is called adaptation. Adaptation occurs due to changes in the diameter of the pupil opening, movement of black pigment in the layers of the retina and different reactions to light of rods and cones. The pupil contracts in 5 seconds, and its full dilation occurs in 5 minutes.

Dark adaptation occurs during the transition from high to low brightnesses. In bright light, the cones work, but the rods are “blinded”, the rhodopsin has faded, the black pigment has penetrated into the retina, shielding the cones from the light. At sharp decline brightness, the opening of the pupil opens, allowing more light to pass through. Then the black pigment leaves the retina, rhodopsin is restored, and when there is enough of it, the rods begin to function. Since cones are not sensitive to low brightness, at first the eye does not distinguish anything. The sensitivity of the eye reaches its maximum value after 50-60 minutes of being in the dark.

Light adaptation- this is the process of adaptation of the eye during the transition from low brightness to high brightness. At first, the rods are greatly irritated, “blinded” due to the rapid decomposition of rhodopsin. The cones, not yet protected by grains of black pigment, are also too irritated. After 8-10 minutes the feeling of blindness stops and the eye sees again.

line of sight the eyes are quite wide (125 degrees vertically and 150 degrees horizontally), but only a small part of it is used for clear discrimination. The field of the most perfect vision (corresponding to the fovea) is about 1-1.5°, satisfactory (in the area of ​​the entire macula) is about 8° horizontally and 6° vertically. The rest of the visual field serves for rough orientation in space. To view the surrounding space, the eye has to make a continuous rotational movement in its orbit within 45-50°. This rotation brings the images various items on the central fovea and makes it possible to examine them in detail. Eye movements occur without the participation of consciousness and, as a rule, are not noticed by a person.

Angular limit of eye resolution- this is the minimum angle at which the eye observes two luminous points separately. The angular resolution limit of the eye is about 1 minute and depends on the contrast of objects, illumination, pupil diameter and wavelength of light. In addition, the resolution limit increases as the image moves away from the fovea and in the presence of visual defects.

Visual defects and their correction

With normal vision, the far point of the eye is infinitely distant. This means that the focal length of the relaxed eye is equal to the length of the eye axis, and the image falls exactly on the retina in the area of ​​the fovea.

Such an eye can distinguish objects well in the distance, and with sufficient accommodation, also near objects.

Myopia

With myopia, rays from an infinitely distant object are focused in front of the retina, so a blurry image is formed on the retina.

Most often this occurs due to elongation (deformation) of the eyeball. Less commonly, myopia occurs with a normal eye length (about 24 mm) due to too much optical power of the optical system of the eye (more than 60 diopters).

In both cases, the image from distant objects is inside the eye and not on the retina. The retina receives only the focus from objects close to the eye, that is, the far point of the eye is at a finite distance in front of it.

Far point of the eye

Myopia is corrected using negative lenses, which create an image of an infinitely distant point at the farthest point of the eye.

Far point of the eye

Myopia most often appears in childhood and adolescence, and as the eyeball grows in length, myopia increases. True myopia, as a rule, is preceded by the so-called false myopia - a consequence of a spasm of accommodation. In this case, normal vision can be restored with the help of means that dilate the pupil and relieve tension in the ciliary muscle.

Farsightedness

With farsightedness, rays from an infinitely distant object are focused behind the retina.

Farsightedness is caused by weak optical power of the eye for a given length of the eyeball: either a short eye with normal optical power, or low optical power of the eye with normal length.

To focus the image on the retina, you have to constantly strain the muscles of the ciliary body. The closer objects are to the eye, the further their image goes beyond the retina and the more effort the eye muscles require.

The far point of the farsighted eye is behind the retina, i.e., in a relaxed state, it can clearly see only an object that is behind it.

Far point of the eye

Of course, you cannot place an object behind the eye, but you can project its image there using positive lenses.

Far point of the eye

With slight farsightedness, distance and near vision is good, but there may be complaints about fatigue and headache at work. With moderate farsightedness, distance vision remains good, but near vision is difficult. With high farsightedness, both distance and near vision becomes poor, since all the eye’s ability to focus images on the retina even of distant objects has been exhausted.

In a newborn, the eye is slightly compressed in the horizontal direction, so the eye has a slight farsightedness, which disappears as the eyeball grows.

Ametropia

Ametropia (nearsightedness or farsightedness) of the eye is expressed in diopters as the reciprocal of the distance from the surface of the eye to the far point, expressed in meters.

The optical power of the lens required to correct myopia or hyperopia depends on the distance from the glasses to the eye. Contact lenses are placed close to the eye, so their optical power is equal to ametropia.

For example, if, with myopia, the far point is located in front of the eye at a distance of 50 cm, then to correct it you need contact lenses with an optical power of −2 diopters.

A weak degree of ametropia is considered up to 3 diopters, an average degree - from 3 to 6 diopters and high degree- above 6 diopters.

Astigmatism

With astigmatism, the focal lengths of the eye are different in different sections passing through its optical axis. With astigmatism in one eye, the effects of nearsightedness, farsightedness and normal vision are combined. For example, an eye may be nearsighted in a horizontal section and farsighted in a vertical section. Then at infinity he will not be able to clearly see horizontal lines, but he will clearly distinguish vertical ones. At a close distance, on the contrary, such an eye sees vertical lines well, but horizontal ones will be blurry.

The cause of astigmatism is either the irregular shape of the cornea or the deviation of the lens from the optical axis of the eye. Astigmatism is most often congenital, but can result from surgery or eye trauma. In addition to defects in visual perception, astigmatism is usually accompanied by fatigue eyes and headaches. Astigmatism is corrected using cylindrical (converging or diverging) lenses in combination with spherical lenses.

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 through vitreous and fall on the inner, very thin shell 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 choroid, but in many areas it is loose. Here it tends to flake off when various diseases. For diseases retina very often involved in pathological process and choroid. Not in the choroid nerve endings, therefore, when she is ill, 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 takes up approximately 1% of the retina.

The rods contain visual purple, thanks to which they are excited very quickly and by weak light. Vitamin A is involved in the formation of visual purpura, with a deficiency of which the 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 . Rods and cones, located in the outer layers of the retina, convert light energy into electrical energy nerve 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 side of the image to the right side brain, and the left side of the retina, respectively, the left side of the image - to the left side of the brain. The big picture 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 of the eye 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.

Since ancient times, the eye has been a symbol of omniscience, secret knowledge, wisdom and vigilance. And this is not surprising. After all, it is through vision that we receive most of the information about the world around us. With the help of our eyes, we evaluate the size, shape, distance and relative position of objects, enjoy the variety of colors and observe movement.

How does the inquisitive eye work?

The human eye is often compared to a camera. The cornea, the clear and convex part of the outer shell, is like an objective lens. The second membrane, the choroid, is represented in front by the iris, the pigment content of which determines the color of the eyes. The hole in the center of the iris - the pupil - narrows in bright light and widens in dim light, regulating the amount of light entering the eye, similar to a diaphragm. The second lens is a movable and flexible lens surrounded by the ciliary muscle, which changes the degree of its curvature. Behind the lens is the vitreous body, a transparent gelatinous substance that maintains the elasticity and spherical shape of the eyeball. Light rays passing through the intraocular structures fall on the retina - the thinnest shell from the nervous tissue lining the inside of the eye. Photoreceptors are light-sensitive cells in the retina that, like photographic film, record images.

Why do they say that we “see” with our brains?

And yet the organ of vision is much more complex than the most modern photographic equipment. After all, we don’t just record what we see, but evaluate the situation and react with words, actions and emotions.

The right and left eyes see objects from different angles. The brain connects both images together, as a result of which we can estimate the volume of objects and their relative positions.

Thus, the picture of visual perception is formed in the brain.

Why, when trying to look at something, do we turn our gaze in this direction?

The clearest image is formed when light rays hit the central zone of the retina - the macula. Therefore, when trying to look at something more closely, we turn our gaze in the appropriate direction. The free movement of each eye in all directions is ensured by the work of six muscles.

Eyelids, eyelashes and eyebrows - not only a beautiful frame?

The eyeball is protected from external influences the bony walls of the orbit, the soft fatty tissue lining its cavity, and the eyelids.

We squint, trying to protect our eyes from the blinding light, drying wind and dust. Thick eyelashes close together, forming a protective barrier. And eyebrows are designed to trap beads of sweat flowing from the forehead.

The conjunctiva is a thin mucous membrane covering the eyeball and the inner surface of the eyelids, containing hundreds of tiny glands. They produce a “lubricant” that allows the eyelids to move freely when closed and protects the cornea from drying out.

Accommodation of the eye

How is the image formed on the retina?

In order to understand how an image is formed on the retina, it is necessary to remember that when passing from one transparent medium to another, light rays are refracted (i.e., deviated from rectilinear propagation).

The transparent media in the eye are the cornea with its tear film, aqueous humor, lens and vitreous body. The cornea has the greatest refractive power, the second most powerful lens is the lens. The tear film, aqueous humor and vitreous humor have negligible refractive power.

Passing through the intraocular media, light rays are refracted and converge on the retina, forming a clear image.

What is accommodation?

Any attempt to shift your gaze leads to defocusing of the image and requires additional adjustment of the optical system of the eye. It is carried out due to accommodation - a change in the refractive power of the lens.

The mobile and flexible lens is attached to the ciliary muscle by fibers of the ligament of Zinn. During distance vision, the muscle is relaxed, the fibers of the ligament of zinn are in a tense state, preventing the lens from taking a convex shape. When trying to look at objects close up, the ciliary muscle contracts, the muscle circle narrows, the ligament of Zinn relaxes and the lens takes on a convex shape. Thus, its refractive power increases, and objects located at a close distance are focused on the retina. This process is called accommodation.

Why do we think that “arms get shorter with age”?

With age, the lens loses its elastic properties, becomes dense and hardly changes its refractive power. As a result, we gradually lose the ability to accommodate, which makes it difficult to work at close range. When reading, we try to move the newspaper or book further away from our eyes, but soon our arms are not long enough to ensure clear vision.

To correct presbyopia, converging lenses are used, the power of which increases with age.

Visual impairment

38% of residents of our country have visual impairments that require glasses correction.

Normally, the eye's optical system is able to refract light rays so that they converge precisely on the retina, providing clear vision. An eye with refractive error requires an additional lens to focus the image on the retina.

What are the types of visual impairments?

The refractive power of the eye is determined by two main anatomical factors: the length of the anteroposterior axis of the eye and the curvature of the cornea.

Myopia or myopia. If the length of the eye's axis is increased or the cornea has greater refractive power, the image is formed in front of the retina. This visual impairment is called myopia or myopia. Myopic people see well at close range but poorly at distance. Correction is achieved by wearing glasses with diverging (minus) lenses.

Farsightedness or hypermetropia. If the length of the eye axis is reduced or the refractive power of the cornea is small, the image is formed at an imaginary point behind the retina. This visual impairment is called farsightedness or hyperopia. There is a misconception that farsighted people see well into the distance. They have difficulty working at close range and often have difficulty seeing into the distance. Correction is achieved by wearing glasses with converging (plus) lenses.

Astigmatism. When the sphericity of the cornea is violated, there is a difference in the refractive power along the two main meridians. The image of objects on the retina is distorted: some lines are clear, others are blurry. This visual impairment is called astigmatism and requires wearing glasses with cylindrical lenses.

A ray of light reaches the retina, passing through a number of refractive surfaces and media: the cornea, aqueous humor anterior chamber, lens and vitreous body. Rays emanating from one point in external space must be focused to one point on the retina, only then is clear vision possible.

The image on the retina is real, inverted and reduced. Despite the fact that the image is upside down, we perceive objects upright. This happens because the activity of some sense organs is checked by others. For us, “bottom” is where the force of gravity is directed.

Rice. 2. Construction of an image in the eye, a, b - an object: a, b - its inverted and reduced image on the retina; C is the nodal point through which the rays pass without refraction, and α is the angle of view

Visual acuity.

Visual acuity is the ability of the eye to see two points separately. This is accessible to a normal eye if the size of their image on the retina is 4 microns and the visual angle is 1 minute. At a smaller viewing angle, clear vision is not obtained; the dots merge.

Visual acuity is determined using special tables that depict 12 rows of letters. On the left side of each line it is written from what distance it should be visible to a person with normal vision. The subject is placed at a certain distance from the table and a line is found that he reads without errors.

Visual acuity increases in bright light and is very low in low light.

line of sight. All space visible to the eye with a motionless gaze directed forward, it is called the field of vision.

There are central (in the macula area) and peripheral vision. The greatest visual acuity is in the area of ​​the central fovea. There are only cones, their diameter is small, they are closely adjacent to each other. Each cone is connected to one bipolar neuron, which in turn is connected to one ganglion neuron, from which a separate nerve fiber departs, transmitting impulses to the brain.

Peripheral vision is less sharp. This is explained by the fact that at the periphery of the retina, the cones are surrounded by rods and each no longer has a separate path to the brain. A group of cones ends on one bipolar cell, and many such cells send their impulses to one ganglion cell. There are approximately 1 million fibers in the optic nerve, and there are about 140 million receptors in the eye.

The periphery of the retina poorly distinguishes the details of an object, but perceives their movements well. Lateral vision has great importance to perceive the outside world. For drivers various types transport violation is unacceptable.

The field of view is determined using a special device - the perimeter (Fig. 133), consisting of a semicircle divided into degrees and a chin rest.

Rice. 3. Determination of the field of view using the Forstner perimeter

The subject, closing one eye, fixes with the other white dot in the center of the perimeter arc in front of you. To determine the boundaries of the field of view along the perimeter arc, starting from its end, slowly advance the white mark and determine the angle at which it is visible with a fixed eye.

The field of view is greatest outward, to the temple - 90°, to the nose and up and down - about 70°. You can determine the boundaries of color vision and at the same time be convinced of amazing facts: the peripheral parts of the retina do not perceive colors; The color fields of vision are not the same for different colors, the narrowest being green.

Accommodation. The eye is often compared to a camera. It has a light-sensitive screen - the retina, on which, with the help of the cornea and lens, a clear image of the outside world is obtained. The eye is capable of clearly seeing equidistant objects. This ability of his is called accommodation.

The refractive power of the cornea remains constant; fine, precise focusing occurs due to changes in the curvature of the lens. He performs this function passively. The fact is that the lens is located in a capsule, or bag, which is attached to the ciliary muscle through the ciliary ligament. When the muscle is relaxed and the ligament is tense, it pulls on the capsule, which flattens the lens. When accommodation is strained for viewing close objects, reading, writing, the ciliary muscle contracts, the ligament that tensions the capsule relaxes and the lens, due to its elasticity, becomes more round, and its refractive power increases.

With age, the elasticity of the lens decreases, it hardens and loses the ability to change its curvature when the ciliary muscle contracts. This makes it difficult to see clearly at close range. Senile farsightedness (presbyopia) develops after 40 years of age. It is corrected with the help of glasses - biconvex lenses that are worn when reading.

Vision anomaly. The anomaly that occurs in young people is most often a consequence of improper development of the eye, namely its incorrect length. When the eyeball lengthens, nearsightedness (myopia) occurs and the image is focused in front of the retina. Distant objects are not clearly visible. Biconcave lenses are used to correct myopia. When the eyeball is shortened, farsightedness (hyperopia) is observed. The image is focused behind the retina. Correction requires biconvex lenses (Fig. 134).

Rice. 4. Refraction with normal vision (a), with myopia (b) and farsightedness (d). Optical correction myopia (c) and farsightedness (d) (diagram) [Kositsky G.I., 1985]

A visual impairment called astigmatism occurs when the curvature of the cornea or lens is abnormal. In this case, the image in the eye is distorted. To fix it, you need cylindrical glass, which is not always easy to find.

Eye adaptation.

When leaving a dark room into bright light, we are initially blinded and may even experience pain in our eyes. These phenomena pass very quickly, the eyes get used to the bright lighting.

A decrease in the sensitivity of the eye receptors to light is called adaptation. This causes fading of visual purple. Light adaptation ends in the first 4 - 6 minutes.

When moving from a light room to a dark one, dark adaptation occurs, lasting more than 45 minutes. The sensitivity of the rods increases by 200,000 - 400,000 times. IN general outline this phenomenon can be observed when entering a darkened cinema hall. To study the progress of adaptation, there are special devices - adaptomers.

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. There are 2 in the eyeball spherical lenses: cornea and lens. Accordingly, there are 4 refractive surfaces: air/cornea, cornea/aqueous humor of the anterior chamber of the eye, aqueous humor/lens, lens/vitreous body.

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 refractive 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 a “blur” 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 eye 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

Duality 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 ).

Mechanism of light signal conversion in the photoreceptor

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

At the 1st stage, the photon visible light, entering the light-sensitive pigment molecule, 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 specific 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. The receptive field can be divided into a central and peripheral part (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 the photoreceptors of 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

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). Right and left optic nerves They merge at the base of the skull, forming 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 you 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 ganglion cells follow to the neurons of the superior colliculus ( midbrain), which are associated with 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 reality 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 world motionless, although the image on the retina constantly “jumps” due to movement eyeballs, turning the head, changing 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, three types of cones (sensitive to red, green and blue color) 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 by action yellow color and are inhibited by the action of 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.