Rods and cones are the light-sensitive receptors of the eye. Photosensitive receptors found in the eye are rods and cones.

The main light-sensitive elements (receptors) are two types of cells: one in the form of a stalk - sticks 110-123 million. (height 30 microns, thickness 2 microns), others are shorter and thicker - cones 6-7 million. (height 10 microns, thickness 6-7 microns). They are distributed unevenly in the retina. The central fovea of the retina (fovea centralis) contains only cones (up to 140 thousand per 1 mm). Towards the periphery of the retina, their number decreases, and the number of rods increases.

Each photoreceptor - rod or cone - consists of a light-sensitive outer segment containing visual pigment and an inner segment that contains the nucleus and mitochondria that provide energy processes in the photoreceptor cell

The outer segment is a photosensitive region where light energy is converted into receptor potential. Electron microscopic studies have revealed that the outer segment is filled with membranous disks formed by the plasma membrane. In sticks, in each outer segment, contains 600-1000 disks, which are flattened membrane sacs arranged like a column of coins. Cones have fewer membrane disks. This partly explains higher sensitivity of the rod to light(a wand can excite anything one quantum of light, A it takes more than a hundred quanta to activate a cone).

Each disc is a double membrane consisting of a double layer phospholipid molecules , between which there are protein molecules. Retinal, which is part of the visual pigment rhodopsin, is associated with protein molecules.

The outer and inner segments of the photoreceptor cell are separated by membranes through which a beam of 16-18 thin fibrils. Internal segment passes into a process with the help of which the photoreceptor cell transmits excitation through the synapse to the bipolar nerve cell in contact with it

The outer segments of the receptors face the pigment epithelium, so that light initially passes through 2 layers nerve cells and internal segments of receptors, and then reaches the pigment layer.

Cones operate in high light conditions - provide day and color vision, and the sticks- are responsible for twilight vision.

visible to us range electromagnetic radiation enclosed between shortwave (wavelengthfrom 400nm) radiation that we call violet light and long wave radiation (wavelengthup to 700 nm ) called red. The rods contain a special pigment - rhodopsin, (consists of vitamin A aldehyde or retinal and protein) or visual purple, the maximum of the spectrum, the absorption of which is in the region of 500 nanometers. It is resynthesized in the dark and fades in light. With a lack of vitamin A, twilight vision is impaired - " night blindness".

In the outer segments of the three types of cones ( blue-, green- and red-sensitive) contains three types of visual pigments, the maximum absorption spectra of which are in blue (420 nm), green(531 nm) And red(558 nm) parts of the spectrum. Red cone pigment got the name - "iodopsin". The structure of iodopsin is close to rhodopsin.

Let's look at the sequence of changes:

Molecular physiology of photoreception: Intracellular recordings from cone and rod animals show that in the dark, a dark current flows along the photoreceptor, leaving the inner segment and entering the outer segment. Lighting leads to a blockade of this current. The receptor potential modulates the release of the transmitter ( glutamate) at the photoreceptor synapse. It has been shown that in the dark the photoreceptor continuously releases a transmitter that acts depolarizing way onto the membranes of postsynaptic processes of horizontal and bipolar cells.

Rods and cones have unique electrical activity among all receptors; their receptor potentials when exposed to light are hyperpolarizing, action potentials do not arise under their influence.

(When light is absorbed by a molecule of the visual pigment - rhodopsin, an instantaneous isomerization its chromophore group: 11-cis-retinal is converted to trans-retinal. Following the photoisomerization of retinal, spatial changes occur in the protein part of the molecule: it becomes discolored and passes into the state methorodopsin II As a result of this, the visual pigment molecule acquires the ability to interact with another near-membrane proteinG uanosine triphosphate (GTP) -binding protein - transducin (T) .

In complex with metarhodopsin, transducin enters an active state and exchanges ganosite diphosphate (GDP) bound to it in the dark for (GTP). Transfducin+ GTP activates a molecule of another near-membrane protein - the enzyme phosphodiesterase (PDE). Activated PDE destroys several thousand cGMP molecules .

As a result, the concentration of cGMP in the cytoplasm of the outer segment of the receptor decreases. This leads to the closure of ion channels in plasma membrane outer segment that have been opened In the dark and through which inside the cell included Na + and Ca. Ion channels close due to the concentration of cGMP, which kept the channels open, drops. It has now been found that the pores in the receptor open due to cGMP to cyclic guanosine monophosphate .

Mechanism for restoring the original dark state of the photoreceptor associated with increased concentrations of cGMP. (in the dark phase with the participation of alkaldehydrogenase + NADP)

Thus, the absorption of light by photopigment molecules leads to a decrease in permeability for Na, which is accompanied by hyperpolarization, i.e. the emergence of receptor potential. The hyperpolarizing receptor potential that arises on the membrane of the outer segment then spreads along the cell to its presynaptic end and leads to a decrease in the rate of transmitter release - glutamate . In addition to glutamate, retinal neurons can synthesize other neurotransmitters, such as acetylcholine, dopamine, glycine GABA.

Photoreceptors are connected to each other by electrical (slot) contacts. This connection is selective: sticks are connected to sticks, etc.

These responses from photoreceptors converge on horizontal cells, which lead to depolarization in neighboring cones, creating negative feedback that increases light contrast.

At the receptor level, inhibition occurs and the cone signal no longer reflects the number of absorbed photons, but carries information about the color, distribution and intensity of light incident on the retina in the vicinity of the receptor.

There are 3 types of retinal neurons - bipolar, horizontal and amacrine cells. Bipolar cells directly connect photoreceptors with ganglion cells, i.e. transmit information through the retina in the vertical direction. Horizontal and amacrine cells transmit information horizontally.

Bipolar cells occupy in the retina strategic position since all signals arising in the receptors arriving at the ganglion cells must pass through them.

It has been experimentally proven that bipolar cells have receptive fields in which they highlight center and periphery (John Dowling- et al. Harvard Medical School).

A receptive field is a set of receptors that send signals to a given neuron through one or more synapses.

Receptive field size: d=10 µm or 0.01 mm - outside the central fossa.

In the hole itselfd=2.5µm (due to this, we are able to distinguish between 2 points at visible distance between them there are only 0.5 arc minutes - 2.5 microns - if you compare, this is a 5-kopeck coin at a distance of about 150 meters)

Starting from the level of bipolar cells, neurons of the visual system differentiate into two groups that react in opposite ways to lighting and darkening:

1 - cells, excited when illuminated and inhibited when darkened "on" - neurons And

Cells excited when darkened and inhibited when illuminated - " off" - neurons. An on-center cell discharges at a markedly increased frequency.

If you listen to the discharges of such a cell through a loudspeaker, then first you will hear spontaneous impulses, individual random clicks, and then after turning on the light, a volley of impulses appears, reminiscent of a machine gun burst. On the contrary, in cells with an off-reaction (when the light is turned off - a volley of impulses) This separation is preserved at all levels of the visual system, up to and including the cortex.

Within the retina itself, information is transmitted in a non-pulse way (propagation and transsynaptic transmission of gradual potentials).

In horizontal, bipolar and amocrine cells, signal processing occurs through slow changes in membrane potentials (tonic response). PD is not generated.

The responses of rods, cones, and horizontal cells are hyperpolarizing, and the responses of bipolar cells can be either hyperpolarizing or depolarizing. Amacrine cells create depolarizing potentials.

To understand why this is so, we need to imagine the effect of a small bright spot. The receptors are active in the dark, and light, causing hyperpolarization, reduces their activity. If excitatory synapse, the bipolar will activate in the dark, A inactivate in the light; if the synapse is inhibitory, the bipolar cell is inhibited in the dark, and in the light, turning off the receptor, it removes this inhibition, i.e. the bipolar cell is activated. That. whether the receptor-bipolar synapse is excitatory or inhibitory depends on the transmitter released by the receptor.

Horizontal cells participate in the transmission of signals from bipolar cells to ganglion cells, which transmit information from photoreceptors to bipolar cells and further to ganglion cells.

Horizontal cells respond to light with hyperpolarization with pronounced spatial summation.

Horizontal cells do not generate nerve impulses, but the membrane has nonlinear properties that ensure impulse-free signal transmission without attenuation.

Cells are divided into two types: B and C. B-type, or luminance, cells always respond with hyperpolarization, regardless of the wavelength of light. C-type cells, or chromatic cells, are divided into two- and three-phase. Chromatic cells respond with either hyper or depolarization depending on the length of the stimulating light.

Biphasic cells are either red-green (depolarized by red light, hyperpolarized by green) or green-blue (depolarized by green, hyperpolarized by blue). Triphasic cells are depolarized by green light, while blue and red light cause hyperpolarization of the membrane. Amacrine cells regulate synaptic transmission at the next stage from bipolar to ganglion cells.

The dendrites of amacrine cells branch in the inner layer, where they contact the processes of bipolars and the dendrites of ganglion cells. Centrifugal fibers coming from the brain end on amacrine cells.

Amacrine cells generate gradual and pulsed potentials (phasic response). These cells respond with rapid depolarization to light on and off and exhibit weak

spatial antagonism between center and periphery.

Cones and rods belong to the receptor apparatus of the eyeball. They are responsible for transmitting light energy by transforming it into nerve impulse. The latter passes through the fibers optic nerve V central structures brain. Rods provide vision in low light conditions; they are capable of perceiving only light and dark, that is, black and white images. Cones are capable of perceiving different colors and are also an indicator of visual acuity. Each photoreceptor has a structure that allows it to perform its functions.

Structure of rods and cones

The sticks are shaped like a cylinder, which is why they got their name. They are divided into four segments:

Basal, connecting nerve cells;
A binder that provides connection with eyelashes;
Outer;
Internal, containing mitochondria that produce energy.

The energy of one photon is quite enough to excite the rod. This is perceived by a person as light, which allows him to see even in very low light conditions.

The rods contain a special pigment (rhodopsin), which absorbs light waves in two ranges.
Cones by appearance They look like flasks, which is why they have their name. They contain four segments. Inside the cones is another pigment (iodopsin), which provides the perception of red and green colors. Pigment responsible for recognition of blue color still not installed.

Physiological role of rods and cones

Cones and rods perform the main function of perceiving light waves and transforming them into a visual image (photoreception). Each receptor has its own characteristics. For example, rods are needed to see at dusk. If for some reason they stop performing their function, a person cannot see in low light conditions. Cones are responsible for clear color vision in normal lighting.

In another way, we can say that rods belong to the light-perceiving system, and cones belong to the color-perceiving system. This is the basis for differential diagnosis.

Video about the structure of rods and cones

Symptoms of damage to rods and cones

In diseases accompanied by damage to rods and cones, the following symptoms occur:

Decreased visual acuity;
The appearance of flashes or glare before the eyes;
Decreased twilight vision;
Inability to distinguish colors;
Narrowing of visual fields (in as a last resort formation of tubular vision).

Some diseases are very specific symptoms, which easily allow you to diagnose pathology. This applies to hemeralopia or. Other symptoms may be present when various pathologies, in connection with which it is necessary to conduct additional diagnostic examination.

Diagnostic methods for rod and cone lesions

To diagnose diseases in which there is damage to rods or cones, it is necessary to perform the following examinations:

with state definition;
(study of visual fields);
Diagnosis of color perception using Ishihara tables or the 100-shade test;
Ultrasonography;
Fluorescent hagiography, providing visualization of blood vessels;
Computer refractometry.

It is worth recalling once again that photoreceptors are responsible for color perception and light perception. Due to the work, a person can perceive an object, the image of which is formed in visual analyzer. For pathologies

Rods and cones are the light-sensitive receptors of the eye, also called photoreceptors. Their main task is to convert light stimulation into nervous stimulation. That is, they are the ones who transform light rays into electrical impulses that enter the brain via , which, after certain processing, become images that we perceive. Each type of photoreceptor has its own task. The rods are responsible for light perception in low light conditions (night vision). Cones are responsible for visual acuity, as well as color perception (daytime vision).

Retinal rods

These photoreceptors are cylindrical in shape, with a length of approximately 0.06 mm and a diameter of approximately 0.002 mm. Thus, such a cylinder is really quite similar to a stick. Eye healthy person contains approximately 115-120 million rods.

The human eye rod can be divided into 4 segmental zones:

1 - Outer segmental zone (includes membranous discs containing rhodopsin),
2 - Connecting segmental zone (cilium),

4 - Basal segmental zone (nerve connection).

Rods are highly photosensitive. So, for their reaction, the energy of 1 photon (the smallest, elementary particle of light) is enough. This fact is very important for night vision, which allows you to see in low light.

Rods cannot distinguish colors; this is primarily due to the presence of only one pigment in them - rhodopsin. The pigment rhodopsin, otherwise called visual purple, due to the included protein groups (chromophores and opsins), has 2 light absorption maxima. True, one of the maxima exists beyond the range of light visible to the human eye (278 nm - the region of ultraviolet radiation), therefore, it is probably worth calling it the wave absorption maximum. But the second maximum is visible to the eye - it exists at around 498 nm, located on the border of the green and blue color spectrum.

It is reliably known that rhodopsin, present in rods, reacts to light much more slowly than iodopsin, contained in cones. Therefore, rods are characterized by a weak reaction to the dynamics of light fluxes, and in addition, they poorly distinguish the movements of objects. And visual acuity is not their prerogative.

Cones of the retina

These photoreceptors also get their name from characteristic form, similar to the shape of laboratory flasks. The length of the cone is approximately 0.05 mm, its diameter at the narrowest point is approximately 0.001 mm, and at the widest point it is 0.004. The retina of a healthy adult contains about 7 million cones.

Cones have less sensitivity to light. That is, to excite their activity, a luminous flux will be required, which is tens of times more intense than to excite the work of rods. But cones process light fluxes much more intensely than rods, so they perceive their changes better (for example, they better distinguish light when objects move, in dynamics relative to the eye). They also define images more clearly.

Cones human eye, also include 4 segmental zones:

1 - Outer segmental zone (includes membranous discs containing iodopsin),
2 - Connecting segmental zone (constriction),
3 - Inner segmental zone (includes mitochondria),
4 - Synaptic connection zone or basal segment.

The reason for the above-described properties of cones is the content of the specific pigment iodopsin in them. Today, 2 types of this pigment have been isolated and proven: erythrolab (iodopsin, sensitive to the red spectrum and long L-waves), and chlorolab (iodopsin, sensitive to the green spectrum and medium M-waves). A pigment that is sensitive to the blue spectrum and short S-waves has not yet been found, although the name has already been assigned to it - cyanolab.

The division of cones according to the type of dominance of color pigment in them (erythrolab, chlorolab, cyanolab) is due to the three-component vision hypothesis. There is, however, another theory of vision - nonlinear two-component. Its adherents believe that all cones contain erythrolab and chlorolab at the same time, and therefore are able to perceive colors in both the red and green spectrum. The role of cyanolabe, in this case, is played by the faded rhodopsin of the rods. This theory is also confirmed by examples of people suffering from the inability to distinguish the blue part of the spectrum (tritanopia). They also have difficulty with twilight vision (

The sticks have the shape of a cylinder with an uneven, but approximately equal diameter of the circumference along the length. In addition, the length (equal to 0.000006 m or 0.06 mm) is 30 times greater than their diameter (0.000002 m or 0.002 mm), which is why the elongated cylinder really looks very much like a stick. There are about 115-120 million rods in the eye of a healthy person.

The human eye rod consists of 4 segments:

1 - Outer segment (contains membrane disks),

2 - Connecting segment (cilium),

4 - Basal segment (nerve connection)

The rods are extremely photosensitive. The energy of one photon (the smallest, elementary particle of light) is enough for the rods to react. This fact helps with so-called night vision, allowing you to see at dusk.

Rods are not able to distinguish colors, first of all, this is due to the presence of only one pigment, rhodopsin, in rods. Rhodopsin, or otherwise called visual purple, due to the inclusion of two groups of proteins (chromophore and opsin), has two light absorption maxima, although, given that one of these maxima is beyond the light visible to the human eye (278 nm is the ultraviolet region, invisible to the eye), we should call them wave absorption maxima. However, the second absorption maximum is still visible to the eye - it is located at around 498 nm, which is, as it were, on the border between green color spectrum and blue.

It is reliably known that rhodopsin contained in rods reacts to light more slowly than iodopsin in cones. Therefore, the rods react weaker to the dynamics of the light flux and poorly distinguish objects in motion. For the same reason, visual acuity is also not a specialization of rods.

Cones of the retina

Cones get their name due to their shape, similar to laboratory flasks. The length of a cone is 0.00005 meters, or 0.05 mm. Its diameter at its narrowest point is about 0.000001 meters, or 0.001 mm, and 0.004 mm at its widest. There are about 7 million cones per healthy adult.

Cones are less sensitive to light; in other words, to excite them, a light flux that is tens of times more intense will be required than to excite rods. However, cones are able to process light more intensely than rods, which is why they better perceive changes in light flux (for example, they are better than rods at distinguishing light in dynamics when objects move relative to the eye), and also determine a clearer image.

The cone of the human eye consists of 4 segments:

1 - Outer segment (contains membrane disks with iodopsin),

2 - Connecting segment (constriction),

3 - Inner segment (contains mitochondria),

4 - Area of synaptic connection (basal segment).

The reason for the above-described properties of cones is the content of the biological pigment iodopsin in them. At the time of writing this article, two types of iodopsin were found (isolated and proven): erythrolab (a pigment sensitive to the red part of the spectrum, to long L-waves), chlorolab (a pigment sensitive to the green part of the spectrum, to medium M-waves). To date, a pigment that is sensitive to the blue part of the spectrum, to short S-waves, has not been found, although it has already been given a name - cyanolab.

The division of cones into 3 types (based on the dominance of color pigments in them: erythrolab, chlorolaba, cyanolabe) is called the three-component vision hypothesis. However, there is also a nonlinear two-component theory vision, adherents of which believe that each cone simultaneously contains both erythrolab and chlorolab, and therefore is capable of perceiving the colors of the red and green spectrum. In this case, the role of cyanolabe is taken over by faded rhodopsin from the rods. This theory is supported by the fact that people suffering, namely in the blue part of the spectrum (tritanopia), also experience difficulties with twilight vision (night blindness), which is a sign of abnormal functioning of the retinal rods.

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

Visual adaptation

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

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

Differential sensitivity vision

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

Luminance Contrast

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

Blinding brightness of light

Too bright light causes unpleasant feeling blindness. Upper limit blinding brightness depends on the adaptation of the eye: the longer the dark adaptation, the lower the brightness of the light causes blinding. If very bright (dazzle) objects come into the field of view, they impair the discrimination of signals on a significant part of the retina (for example, on a night road, drivers are blinded by the headlights of oncoming cars). At fine work associated with eye strain (long reading, working on a computer, assembling small parts), should only be used diffused light without dazzling eyes.

The inertia of vision, the fusion of flickering, successive images

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

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

color vision

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

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

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

Perception of space

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

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

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

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

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

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

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

AUDITORY SYSTEM

In connection with the emergence of speech as a means interpersonal communication, a person’s hearing plays special role. Acoustic (sound) signals are air vibrations with different frequency and strength. They excite auditory receptors located in the cochlea inner ear. Receptors activate the first auditory neurons, after which sensory information is transmitted to the auditory area of the cerebral cortex through a number of successive sections, which are especially numerous in the auditory system.