A reduced image appears on the retina. Image of objects on the retina of the eye, what is the retina

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 distinct 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.

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. And 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 distance for reading and writing 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. This 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 benefit 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 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 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.

Jerky 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 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 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 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

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 device 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 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 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.

During the classes

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 us 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 over 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. And 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 nauseated. 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"?

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.

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 deficiency of vision 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 use the pads of your palms, touching very gently, to move 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 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 takes up approximately 1% of the retina.

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 leads to the development of 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 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.

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 visual 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 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 emanate 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.

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.

During the classes

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 us about the structure and purpose of the constituent parts of the eye.

Explanation of new material.

Biology teacher:

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

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:

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. And 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 nauseated. 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.

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

Visual self-deceptions

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"?

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.

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.

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.

Physics teacher:

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.

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.

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.

Option 2 (duration 1-2 minutes).

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.

Lesson summary. Setting homework.

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

A beam of light reaches the retina, passing through a number of refractive surfaces and media: the cornea, the aqueous humor of the anterior chamber, the lens and the 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. The entire space visible to the eye with a motionless gaze directed forward is called the visual field.

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 is of great importance for the perception of the outside world. For drivers of various types of transport, violating it 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 the white dot with the other in the center of the perimeter arc in front of him. 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 of 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 terms, this phenomenon can be observed when entering a darkened cinema hall. To study the progress of adaptation, there are special devices - adaptomers.

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. Rays of light, passing through the intraocular structures, fall on the retina - the thinnest membrane of 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 by 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 has difficulty changing 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 strength 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.