Light adaptation and mechanisms providing it. Light and dark adaptation of the eye Light adaptation of the eyes by the body is faster

3-11-2012, 22:44

Description

The range of brightness perceived by the eye

adaptation is called the restructuring of the visual system for the best adaptation to a given level of brightness. The eye has to work at brightnesses varying over an extremely wide range, approximately from 104 to 10-6 cd/m2, i.e., within ten orders of magnitude. When the brightness level of the field of view changes, a number of mechanisms automatically turn on, which provide adaptive restructuring of vision. If the brightness level does not change significantly for a long time, the adaptation state comes in line with this level. In such cases, we can no longer speak about the process of adaptation, but about the state: adaptation of the eye to such and such a brightness L.

When there is a sudden change in brightness, gap between brightness and state of the visual system, a gap, which serves as a signal for the inclusion of adaptive mechanisms.

Depending on the sign of the change in brightness, light adaptation is distinguished - tuning to a higher brightness and dark - tuning to a lower brightness.

Light adaptation

Light adaptation proceeds much faster than the dark one. Leaving a dark room into bright daylight, a person is blinded and in the first seconds he sees almost nothing. Figuratively speaking, the visual device rolls over. But if a millivoltmeter burns out when trying to measure a voltage of tens of volts with it, then the eye refuses to work only for a short time. Its sensitivity automatically and quickly falls. First of all, the pupil narrows. In addition, under the direct action of light, the visual purple of the rods fades, as a result of which their sensitivity drops sharply. Cones begin to act, which, apparently, have an inhibitory effect on the rod apparatus and turn it off. Finally, there is a restructuring of the nerve connections in the retina and a decrease in the excitability of the brain centers. As a result, after a few seconds, a person begins to see in general terms the surrounding picture, and after about five minutes, the light sensitivity of his vision comes into full compliance with the surrounding brightness, which ensures the normal functioning of the eye in new conditions.

Dark adaptation. Adaptometer

Dark adaptation studied much better than light, which is largely due to the practical importance of this process. In many cases, when a person enters low light conditions, it is important to know in advance how long and what he will be able to see. In addition, the normal course of dark adaptation is disturbed in some diseases, and therefore its study is of diagnostic value. Therefore, special devices have been created to study dark adaptation - adaptometers. In the Soviet Union, the ADM adaptometer is mass-produced. Let's describe its device and method of working with it. The optical scheme of the device is shown in fig. 22.

Rice. 22. Scheme of ADM adaptometer

The patient presses his face against the rubber half-mask 2 and looks with both eyes into the ball 1, coated from the inside with white barium oxide. Through the opening 12, the doctor can see the eyes of the patient. Using lamp 3 and filters 4, the walls of the ball can be given brightness Lc, which creates a preliminary light adaptation, during which the holes of the ball are closed with shutters 6 and 33, white on the inside.

When measuring the light sensitivity, the lamp 3 is turned off and the dampers 6 and 33 are opened. The lamp 22 is turned on and the centering of its thread is checked from the image on the plate 20. Lamp 22 illuminates milk glass 25 through condenser 23 and daylight filter 24, which serves as a secondary light source for milk glass plate 16. Part of this plate, visible to the patient through one of the cutouts in disk 15, serves as a test object when measuring threshold brightness. The brightness of the test object is adjusted in steps using filters 27-31 and smoothly using diaphragm 26, the area of ​​​​which changes when the drum 17 rotates. Filter 31 has an optical density of 2, i.e., a transmission of 1%, and the remaining filters have a density of 1, 3, i.e. 5% transmission. The illuminator 7-11 is used for lateral illumination of the eyes through the hole 5 in the study of visual acuity in conditions of blindness. When the adaptation curve is removed, lamp 7 is off.

A small hole in plate 14 covered with a red light filter, illuminated by lamp 22 with a matte plate 18 and mirror 19, serves as a fixation point, which the patient sees through hole 13.

The basic procedure for measuring the course of dark adaptation is as follows.. In a darkened room, the patient sits down in front of the adaptometer and looks into the ball, pressing his face tightly against the half mask. The doctor turns on lamp 3, setting the brightness Lc to 38 cd/m2 using filters 4. The patient adapts to this brightness within 10 minutes. By turning the disk 15 to set a circular diaphragm visible to the patient at an angle of 10°, after 10 minutes the doctor extinguishes lamp 3, turns on lamp 22, filter 31 and opens hole 32. With the diaphragm fully open and filter 31, the brightness L1 of glass 16 is 0.07 cd /m2. The patient is instructed to look at the fixation point 14 and say “I see” as soon as he sees a bright spot at the place of the plate 16. The doctor notes this time t1 reduces the brightness of the plate 16 to the value L2, waits for the patient to say “I see” again, notes the time t2 and decrease the brightness again. The measurement lasts 1 hour after switching off the adaptive brightness. A series of values ​​ti is obtained, each of which corresponds to its own, L1, which makes it possible to plot the dependence of the threshold brightness Ln or light sensitivity Sc on the dark adaptation time t.

Let us denote by Lm the maximum brightness of plate 16, i.e., its brightness at full aperture 26 and with the filters turned off. The total transmission of filters and apertures will be denoted by ?f. The optical density Df of a system that attenuates brightness is equal to the logarithm of its reciprocal.

This means that the brightness with the introduced attenuators L = Lm ?f, a lgL, = lgLm - Df.

Since the light sensitivity is inversely proportional to the threshold brightness, i.e.

In the ADM adaptometer, Lm is 7 cd/m2.

The description of the adaptometer shows the dependence of D on the time of dark adaptation t, which is accepted by doctors as the norm. Deviation of the course of dark adaptation from the norm indicates a number of diseases not only of the eye, but of the whole organism. The average values ​​of Df and the permissible limit values ​​are given, which do not yet go beyond the limits of the norm. Based on the values ​​of Df, we calculated by formula (50) and in Fig. 24

Rice. 24. Normal behavior of the dependence of Sc on the dark adaptation time t

we present the dependence of Sc on t on a semilogarithmic scale.

A more detailed study of dark adaptation indicates a greater complexity of this process. The course of the curve depends on many factors: on the brightness of the preliminary illumination of the eyes Lc, on the place on the retina on which the test object is projected, on its area, etc. Without going into details, we point out the difference in the adaptive properties of cones and rods. On fig. 25

Rice. 25. Dark adaptation curve according to N.I. Pinegin

shows a graph of the decrease in threshold brightness, taken from the work of Pinegin. The curve was taken after strong illumination of the eyes with white light with Lc = 27000 cd/m2. The test field was illuminated with green light = 546 nm, a 20" test object was projected onto the periphery of the retina The abscissa shows the time of dark adaptation t, the ordinate shows lg (Lp/L0), where L0 is the threshold brightness at the moment t = 0, and Ln is at any other We see that in about 2 minutes the sensitivity increases by a factor of 10, and over the next 8 minutes another factor of 6. At the 10th minute, the increase in sensitivity accelerates again (threshold brightness decreases), and then becomes slow again. curve is like this. At first, the cones quickly adapt, but they can increase the sensitivity only by a factor of 60. After 10 minutes of adaptation, the possibilities of the cones are exhausted. But by this time, the rods are already disinhibited, providing a further increase in sensitivity.

Factors that increase light sensitivity during adaptation

Previously, studying dark adaptation, the main importance was attached to an increase in the concentration of a photosensitive substance in the receptors of the retina, mainly rhodopsin. Academician P. P. Lazarev, in constructing the theory of the process of dark adaptation, proceeded from the assumption that the light sensitivity Sc is proportional to the concentration a of the light-sensitive substance. Hecht held the same views. Meanwhile, it is easy to show that the contribution of an increase in concentration to the overall increase in sensitivity is not so great.

In § 30, we indicated the limits of brightness at which the eye has to work - from 104 to 10-6 cd/m2. At the lower limit, the threshold brightness can be considered equal to the limit itself Lp = 10-6 cd/m2. And at the top? With a high level of adaptation L, the threshold brightness Lp can be called the minimum brightness, which can still be distinguished from complete darkness. Using the experimental material of the work, we can conclude that Lp at high brightness is approximately 0.006L. Thus, it is necessary to evaluate the role of various factors when the threshold brightness decreases from 60 to 10_6 cd/m2, i.e., by a factor of 60 million. Let's list these factors.:

1. Transition from cone vision to rod vision. From the fact that for a point source, when it can be considered that light acts on one receptor, Ep = 2-10-9 lux, and Ec = 2-10-8 lux, we can conclude that the rod is 10 times more sensitive than the cone.
2. Pupil dilation from 2 to 8 mm, i.e. 16 times in area.
3. An increase in the time of inertia of vision from 0.05 to 0.2 s, i.e. 4 times.
4. An increase in the area over which the summation of the effect of light on the retina is performed. At high brightness, the angular resolution limit? \u003d 0.6 "and with a small? \u003d 50". An increase in this number means that many receptors are combined to perceive light together, forming, as physiologists usually say, one receptive field (Gleser). The area of ​​the receptive field is increased by 6900 times.
5. Increased sensitivity of the brain centers of vision.
6. Increasing the concentration of a photosensitive substance. It is this factor that we want to evaluate.

Let us assume that the increase in the sensitivity of the brain is small and can be neglected. Then we can estimate the effect of increasing a, or at least an upper limit on the possible increase in concentration.

Thus, the increase in sensitivity, due only to the first factors, will be 10X16X4X6900 = 4.4-106. Now we can estimate how many times the sensitivity increases due to an increase in the concentration of the photosensitive substance: (60-106)/(4.4-10)6= 13.6, i.e., approximately 14 times. This number is small compared to 60 million.

As we have already mentioned, adaptation is a very complex process. Now, without delving into its mechanism, we have quantitatively assessed the significance of its individual links.

It should be noted that deterioration in visual acuity with a decrease in brightness, there is not just a lack of vision, but an active process that allows, with a lack of light, to see at least large objects or details in the field of view.

The sensitivity of the eye depends on the initial illumination, i.e. on whether a person or animal is in a brightly lit or dark room.

When moving from a dark to a light room, blindness occurs at first. Gradually, the sensitivity of the eyes decreases; they adapt to the light. This adaptation of the eye to bright light conditions is called light adaptation.

The reverse phenomenon is observed when a person passes from a bright room, in which the sensitivity of the eye to light is greatly dulled, into a dark room. At first, due to the reduced excitability of the eye, he does not see anything. Gradually, the contours of objects begin to appear, then their details begin to differ; retinal excitability gradually increases. This increase in the sensitivity of the eye in the dark, which is the adaptation of the eye to low light conditions, is called dark adaptation.

In experiments on animals with registration or impulses in the optic nerve light adaptation manifests itself in an increase in the threshold of light irritation (decrease in the excitability of the photoreceptor apparatus) and a decrease in the frequency of action potentials in the optic nerve.

When staying in the dark light adaptation, i.e., the decrease in the sensitivity of the retina, which is constantly present in conditions of natural daylight or artificial night lighting, gradually disappears, and as a result, the maximum sensitivity of the retina is restored; consequently, dark adaptation, i.e., an increase in the excitability of the visual apparatus in the absence of light stimulation, can be considered as a gradual elimination of light adaptation.

The course of increasing sensitivity when staying in the dark is shown in rice. 221. In the first 10 minutes, the sensitivity of the eye increases 50-80 times, and then within an hour many tens of thousands of times. Increasing the sensitivity of the eye in the dark has a complex mechanism. Important in this phenomenon, according to the theory of P. P. Lazarev, is the restoration of visual pigments.

The next period of adaptation is associated with the restoration of rhodopsin. This process proceeds slowly and comes to the end by the end of the first hour of stay in darkness. The restoration of rhodopsin is accompanied by a sharp increase in the sensitivity of retinal rods to light. It becomes after a long stay in the dark 100,000 - 200,000 times more than it was in conditions of harsh lighting. Since rods have maximum sensitivity after a long stay in the dark, very dimly lit objects are visible only when they do not lie in the center of the field of view, that is, when they stimulate the peripheral parts of the retina. If you look directly at a source of weak light, it becomes invisible, since the increase due to dark adaptation in the sensitivity of the cones located in the center of the retina is too small for them to perceive irritation with light of low intensity.

The idea of ​​the significance of the decomposition and restoration of visual purple in the phenomena of light and tempo adaptation encounters some objections. They are related to the fact that when the eye is exposed to light of high brightness, the amount of rhodopsin decreases only slightly, and this, according to calculations, cannot cause such a large decrease in the sensitivity of the retina, which occurs during light adaptation. Therefore, it is now believed that the phenomena of adaptation do not depend on the splitting and resynthesis of photosensitive pigments, but on other causes, in particular, on the processes occurring in the nerve elements of the retina. This can be supported by the fact that adaptation to a long-acting stimulus is a property of many receptors.

It is possible that the methods of connecting photoreceptors to ganglion cells are important in adaptation to illumination. It has been established that in the dark the area of ​​the receptive field of a ganglion cell increases, i.e., a larger number of photoreceptors can be connected to one ganglion cell. It is assumed that the so-called horizontal neurons of the retina begin to function in the dark - Dogel's stellate cells, the processes of which terminate in many photoreceptors.

Due to this, the same photoreceptor can be connected to different bipolar and haiglioid cells, and each such cell becomes associated with a large number of photoreceptors ( ). Therefore, in very low light, the receptor potential increases due to summation processes, causing discharges of impulses in ganglion cells and optic nerve fibers. In the light, the functioning of horizontal cells ceases, and then a smaller number of photoreceptors are associated with the ganglion cell and, consequently, a smaller number of photoreceptors will excite it when exposed to light. Apparently, the inclusion of horizontal cells is regulated by the central nervous system.

Curves of two experiments. The time of stimulation of the reticular formation is marked with a dotted line.

The influence of the central nervous system on the adaptation of the retina to light is illustrated by the observations of S. V. Kravkov, who found that illumination of one eye leads to a sharp increase in the sensitivity to light of the other, unlit eye. Similarly, stimuli of other sense organs act, for example, weak and medium-strength sound signals, olfactory and gustatory stimuli.

If the action of light on a dark-adapted eye is combined with some indifferent stimulus, such as a bell, then after a series of combinations, one switching on of the bell causes the same decrease in the sensitivity of the retina, which was previously observed only when the light was turned on. This experience shows that adaptation processes can be regulated in a conditioned reflex way, that is, that they are subject to the regulatory influence of the cerebral cortex (AV Bogoslovsky).

The sympathetic nervous system also influences the adaptation processes of the retina. Unilateral removal of the cervical sympathetic ganglia in humans causes a decrease in the rate of dark adaptation of the sympathetic eye. The introduction of adrenaline has the opposite effect.

Mechanisms of light perception. visual adaptation. (dark and light).

Light causes irritation of the light-sensitive elements of the retina. The retina contains light-sensitive visual cells that look like rods and cones. There are about 130 million rods and 7 million cones in the human eye.

Rods are 500 times more sensitive to light than cones. However, rods do not respond to changes in the wavelength of light; do not show color sensitivity. Such a functional difference is explained by the chemical features of the process of visual reception, which is based on photochemical reactions.

These reactions proceed with the help of visual pigments. The rods contain the visual pigment rhodopsin or "visual purple". It got its name because, when extracted in the dark, it has a red color, as it absorbs green and blue light rays especially strongly. The cones contain other visual pigments. Molecules of visual pigments are included in ordered structures as part of the double lipid layer of the membrane discs of the outer segments.

Photochemical reactions in rods and cones are similar. They begin with the absorption of a quantum of light - a photon - which transfers the pigment molecule to a higher energy level. Next, the process of reversible change in pigment molecules is started. In rods - rhodopsin (visual purple), in cones - iodopsin. As a result, the energy of light is converted into electrical signals - impulses. So, under the influence of light, rhodopsin undergoes a number of chemical changes - it turns into retinol (vitamin A aldehyde) and a protein residue - opsin. Then, under the influence of the reductase enzyme, it turns into vitamin A, which enters the pigment layer. In the dark, the reverse reaction occurs - vitamin A is restored, passing through a series of stages.

Directly opposite the pupil in the retina is a rounded yellow spot - a retinal spot with a hole in the center, in which a large number of cones are concentrated. This area of ​​the retina is the area of ​​the best visual perception and determines the visual acuity of the eyes, all other areas of the retina determine the field of view. Nerve fibers depart from the light-sensitive elements of the eye (rods and cones), which, when combined, form the optic nerve.

The point where the optic nerve exits the retina is called the optic disc. There are no photosensitive elements in the region of the optic nerve head. Therefore, this place does not give a visual sensation and is called a blind spot.

Visual adaptation is the process of optimizing visual perception, which consists in changing the absolute and selective sensitivity depending on the level of illumination.

Light visual adaptation is a change in the sensitivity thresholds of photoreceptors to an active light stimulus of constant intensity. In the course of light visual adaptation, there is an increase in absolute thresholds and discrimination thresholds. Light visual adaptation is completely completed in 5-7 minutes.

Dark visual adaptation - a gradual increase in visual sensitivity during the transition of light to twilight. Dark visual adaptation takes place in two stages:

1- within 40-90 sec. increases the sensitivity of cones;

2- as the visual pigments in the cones are restored, the sensitivity to the light of the rods increases.

Dark visual adaptation is completed in 50-60 minutes.

Mechanisms of light perception. visual adaptation.

Absolute light sensitivity is a value that is inversely proportional to the smallest brightness of light or illumination of an object, sufficient for a person to experience light. The light sensitivity will depend on the illumination. In low light, dark adaptation develops, and in strong light, light adaptation develops. With the development of dark adaptation, the AFC will increase, the maximum value will be reached in 30-35 minutes. Light adaptation is expressed in a decrease in light sensitivity with increasing illumination. Develops in a minute. When the illumination changes, BURMezanisms are activated, which provide adaptation processes. The size of the pupil is regulated by the mechanism of the unconditioned reflex during dark adaptation, the radial muscle of the iris will contract and the pupil will expand (this reaction is called mydriasis). In addition to absolute light sensitivity, there is also a contrast one. It is evaluated by the smallest difference in illumination that the subject is able to distinguish.

3. Dynamics of blood pressure, linear and volumetric blood flow velocity along the systemic circulation.

37.) Theories of color perception. Color vision ,

color perception, the ability of the human eye and many species of animals with daytime activity to distinguish colors, i.e., to feel differences in the spectral composition of visible radiation and in the color of objects. The human eye contains two types of light-sensitive cells (receptors): highly sensitive rods responsible for twilight ( night) vision, and less sensitive cones responsible for color vision.

There are three types of cones in the human retina, the maximum sensitivity of which falls on the red, green and blue parts of the spectrum, that is, it corresponds to the three “primary” colors. They provide recognition of thousands of colors and shades. The spectral sensitivity curves of the three types of cones partially overlap. Very strong light excites all 3 types of receptors, and therefore is perceived as blindingly white radiation (the effect of metamerism).

Uniform stimulation of all three elements, corresponding to the weighted average daylight, also causes a sensation of white.

Color perception is based on the property of light to cause a certain visual sensation in accordance with the spectral composition of the reflected or emitted radiation.

Colors are divided into chromatic and achromatic. Chromatic colors have three main qualities: color tone, which depends on the wavelength of light radiation; saturation, depending on the proportion of the main color tone and impurities of other color tones; color brightness, i.e. degree of proximity to white. A different combination of these qualities gives a wide variety of shades of chromatic color. Achromatic colors (white, gray, black) differ only in brightness. When two spectral colors with different wavelengths are mixed, the resulting color is formed. Each of the spectral colors has an additional color, when mixed with which an achromatic color, white or gray, is formed. A variety of color tones and shades can be obtained by optically mixing just the three primary colors red, green and blue. The number of colors and their shades perceived by the human eye is unusually large and amounts to several thousand.

Mechanisms of color perception.

The visual pigments of cones are similar to rod rhodopsin and consist of a light-absorbing retinal molecule and opsin, which differs in amino acid composition from the protein part of rhodopsin. In addition, cones contain a smaller amount of visual pigment than rods, and their excitation requires the energy of several hundred photons. Therefore, cones are activated only in daylight or sufficiently bright artificial light, they form a photopic system, or daytime vision system.

In the human retina, there are three types of cones (blue-, green- and red-sensitive) that differ from each other in the composition of amino acids in the opsin of the visual pigment. Differences in the protein part of the molecule determine the features of the interaction of each of the three forms of opsin with retinal and specific sensitivity to light waves of different lengths (Fig. 17.7). One of the three types of cones absorbs as much as possible short light waves with a wavelength of 419 nm, which is necessary for the perception of blue. Another type of visual pigment is most sensitive to medium wavelengths and has an absorption maximum at 531 nm, it serves to perceive green. The third type of visual pigment maximally absorbs long wavelengths with a maximum at 559 nm, which allows red to be perceived. The presence of three types of cones provides a person with the perception of the entire color palette, in which there are over seven million color gradations, while the scotopic system of rods makes it possible to distinguish only about five hundred black and white gradations.

Receptor potential of rods and cones

A specific feature of photoreceptors is the dark current of cations through the open membrane channels of the outer segments (Fig. 17.8). These channels open at a high concentration of cyclic guanosine monophosphate, which is a second messenger of the receptor protein (visual pigment). The dark current of cations depolarizes the photoreceptor membrane to approximately -40 mV, which leads to the release of the mediator at its synaptic ending. The visual pigment molecules activated by the absorption of light stimulate the activity of phosphodiesterase, an enzyme that breaks down cGMP, therefore, when light acts on photoreceptors, the concentration of cGMP in them decreases. As a result, the cation channels controlled by this mediator close, and the flow of cations into the cell stops. Due to the continuous release of potassium ions from the cells, the photoreceptor membrane hyperpolarizes to approximately -70 mV, this hyperpolarization of the membrane is the receptor potential. When a receptor potential occurs, the release of glutamate in the synaptic endings of the photoreceptor stops.

Photoreceptors form synapses with bipolar cells of two types, which differ in the way they control chemodependent sodium channels in synapses. The action of glutamate leads to the opening of channels for sodium ions and depolarization of the membrane of some bipolar cells and to the closure of sodium channels and hyperpolarization of bipolar cells of another type. The presence of two types of bipolar cells is necessary for the formation of antagonism between the center and periphery of the receptive fields of ganglion cells.

Adaptation of photoreceptors to changes in illumination

Temporary glare during a quick transition from dark to bright light disappears after a few seconds due to the light adaptation process. One of the mechanisms of light adaptation is the reflex constriction of the pupils, the other depends on the concentration of calcium ions in the cones. When light is absorbed in the membranes of photoreceptors, cation channels close, which stops the entry of sodium and calcium ions and reduces their intracellular concentration. A high concentration of calcium ions in the dark inhibits the activity of guanylate cyclase, an enzyme that determines the formation of cGMP from guanosine triphosphate. Due to the decrease in calcium concentration due to the absorption of light, the activity of guanylate cyclase increases, which leads to additional synthesis of cGMP. An increase in the concentration of this substance leads to the opening of cation channels, the restoration of the flow of cations into the cell and, accordingly, the ability of cones to respond to light stimuli as usual. A low concentration of calcium ions contributes to the desensitization of cones, i.e., a decrease in their sensitivity to light. Desensitization is due to a change in the properties of phosphodiesterase and cation channel proteins, which become less sensitive to the concentration of cGMP.

The ability to distinguish between surrounding objects disappears for a while with a rapid transition from bright light to darkness. It is gradually restored in the course of dark adaptation due to the dilation of the pupils and the switching of visual perception from the photopic system to the scotopic one. Dark adaptation of rods is determined by slow changes in the functional activity of proteins, leading to an increase in their sensitivity. The mechanism of dark adaptation also involves horizontal cells, which contribute to an increase in the central part of receptive fields in low light conditions.

Receptive fields of color perception

The perception of color is based on the existence of six primary colors that form three antagonistic or color-opponent pairs: red - green, blue - yellow, white - black. Ganglion cells that transmit color information to the central nervous system differ in the organization of their receptive fields, which consist of combinations of the three existing types of cones. Each cone is designed to absorb electromagnetic waves of a certain wavelength, but they themselves do not encode information about the wavelength and are able to respond to very bright white light. And only the presence of antagonistic photoreceptors in the receptive field of a ganglion cell creates a neural channel for transmitting information about a certain color. In the presence of only one type of cones (monochromasia), a person is not able to distinguish any color and perceives the world around him in black and white, as in scotopic vision. In the presence of only two types of cones (dichromasia), color perception is limited, and only the existence of three types of cones (trichromasia) ensures the completeness of color perception. The occurrence of monochromasia and dichromasia in humans is due to genetic defects of the X chromosome.

Concentric broadband ganglion cells have rounded on- or off-type receptive fields that are formed by cones but are designed for photopic black and white vision. White light entering the center or periphery of such a receptive field excites or inhibits the activity of the corresponding ganglion cell, which ultimately transmits information about the illumination. Concentric broadband cells sum up signals from cones that absorb red and green light and are located in the center and on the periphery of the receptive field. The input of signals from cones of both types occurs independently of each other, and therefore does not create color antagonism and does not allow broadband cells to differentiate color (Fig. 17.10).

The strongest stimulus for concentric anticolor ganglion cells of the retina is the action of antagonistic colors on the center and periphery of the receptive field. One variety of anticolor ganglion cells is excited by the action of red on the center of its receptive field, in which cones sensitive to the red part of the spectrum are concentrated, and green on the periphery, where there are cones sensitive to it. In another variety of concentric anticolor cells, cones are located in the center of the receptive field, sensitive to the green part of the spectrum, and on the periphery - to the red. These two varieties of concentric anticolor cells differ in their responses to the action of red or green color on the center or periphery of the receptive field, just as on- and off-neurons differ depending on the impact of light on the center or periphery of the receptive field. Each of the two varieties of anticolor cells is a neural channel that transmits information about the action of red or green, and the transmission of information is inhibited by the action of the antagonistic or opponent color.

Opponent relations in the perception of blue and yellow colors are provided as a result of the combination in the receptive field of cones that absorb short waves (blue) with a combination of cones that respond to green and red, which, when mixed, gives the perception of yellow. Blue and yellow colors are opposite to each other, and the combination of cones that absorb these colors in the receptive field allows the anticolor ganglion cell to transmit information about the action of one of them. How exactly this neural channel turns out to be, i.e., transmitting information about blue or yellow, determines the location of the cones within the receptive field of the concentric anticolor cell. Depending on this, the neural channel is excited by blue or yellow color and inhibited by the opponent's color.

M- and P-types of retinal ganglion cells

Visual perception occurs as a result of coordination with each other of various information about the observed objects. But at the lower hierarchical levels of the visual system, starting from the retina, an independent processing of information about the shape and depth of the object, about its color and its movement is carried out. Parallel processing of information about these qualities of visual objects is provided by the specialization of retinal ganglion cells, which are divided into magnocellular (M-cells) and parvocellular (P-cells). In a large receptive field of relatively large M-cells, consisting mainly of rods, a whole image of large objects can be projected: M-cells register rough signs of such objects and their movement in the visual field, responding to stimulation of the entire receptive field with short impulse activity. P-type cells have small receptive fields, consisting mainly of cones, and designed to perceive small details of the shape of an object or to perceive color. Among the ganglion cells of each type, there are both on-neurons and off-neurons, which give the strongest response to stimulation of the center or periphery of the receptive field. The existence of M- and P-types of ganglion cells makes it possible to separate information about the different qualities of the observed object, which is processed independently in parallel paths of the visual system: about the fine details of the object and its color (paths start from the corresponding receptive fields of P-type cells) and about movement objects in the visual field (path from M-type cells).

Light perception (light perception) is the most important function of the visual analyzer, which consists in the ability to perceive light, as well as to distinguish its lightness (brightness).

Impairments associated with light perception are the first symptoms of many diseases, both of the eye and other organs and systems (for example, liver disease, hypo- and beriberi).

Light perception is mostly answered by rod photoreceptors, which are most located in the peripheral parts of the retina. That is why the sensitivity to light is higher in the periphery of the retina than in its central region.

As you know, cones are responsible for daytime vision, rods - for twilight (night).

Only 1 photon of light can excite the photoreceptors of the retina, but the ability to distinguish light appears only with the action of at least 6 photons.

Light perception is responsible for the following characteristics:

• irritation threshold - the minimum luminous flux that causes irritation of the retinal receptors;
• discrimination threshold - the ability of the visual analyzer to distinguish the minimum difference in light intensity.

Light adaptation

A very important ability of the eye is light adaptation - an adaptation to increase the brightness of light (illuminance). The process of adaptation itself lasts about a minute (the brighter the light, the longer it takes). Initially (in the first seconds after the increase in illumination), the sensitivity decreases sharply, and returns to normal only after 50-70 seconds.

This is the ability of the visual organ to adapt to a decrease in brightness. With a decrease in illumination, the photosensitivity first sharply increases, but after 15-20 minutes it begins to weaken, and after about an hour complete dark adaptation occurs.

Study of light perception

The most commonly used technique for determining impaired light perception is the Kravkov test. In a darkened room, the patient is shown a square (dimensions - 20 × 20 cm), on the corners of which small squares (3 × 3 cm) of green, yellow, blue and blue colors are glued. If light perception is not disturbed, a person in 40-60 seconds will be able to distinguish between yellow and blue, otherwise he will not determine the blue color, but instead of a yellow square he will see a light area.

Also, to determine the pathology of light sensitivity, special devices are used - adaptometers. The essence of the technique.

The patient should adjust to the light by looking at a bright screen for at least 15 minutes. Then the lights are turned off in the room. The patient is shown a slightly illuminated object, gradually increasing its brightness. When the patient can distinguish the object, he presses a special button (in this case, a dot is placed on the form of the adaptometer). The brightness of the object is changed first after three minutes, and then every five minutes. The study lasts an hour, after which all the points on the form are connected, as a result, a curve of the patient's photosensitivity is obtained.

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The sensitivity of the receptor cells of the eye is not constant, but depends on the illumination and the previous stimulus. So, after the action of intense light, the sensitivity decreases sharply, and in the dark it increases. The process of adaptation of vision is associated with the gradual "appearance" of objects when moving from a well-lit room to a dark one and, on the contrary, too bright light when returning to a lighted room. Vision adapts to light faster - within a few minutes. And dark adaptation occurs only after a few tens of minutes.. This difference is partly explained by the fact that the sensitivity of "daytime" cones changes faster (from 40 s to several minutes) than "evening" rods (completely ends only after 40-50 minutes). In this case, the rod system becomes much more sensitive than the cone system: in absolute darkness, the threshold of visual sensitivity reaches the level of 1-4 photons per second per photoreceptor. Under scotopic conditions, light stimuli are better distinguished not by the central fovea, but by its surrounding part, where the density of rods is highest. By the way, the difference in the rate of adaptation is quite understandable, since in nature, the illumination after sunset decreases rather slowly.

The mechanisms of adaptation to changing illumination begin with the receptor and optical apparatus of the eye. The latter is associated with the reaction of the pupil: constriction in the light and expansion in the dark. This mechanism is activated by the ANS. As a result, the number of receptors on which the light rays fall changes: the connection of rods at dusk worsens visual acuity and slows down the time of dark adaptation.

In the receptor cells themselves, the processes of decreasing and increasing sensitivity are due, on the one hand, to a change in the balance between the decaying and synthesized pigment (a certain role in this process belongs to the pigment cells that supply the rods with vitamin A). On the other hand, with the participation of neural mechanisms, the sizes of receptor fields are also regulated, switching from the cone system to the rod system.

The involvement of receptor cells in the process of adaptation can be easily verified by examining Fig. 6.30. If at the beginning the eye is fixed on the right half of the drawing, and then transferred to the left, then within a few seconds it will be possible to see the negative of the right drawing. Those areas of the retina, on which rays fell from dark places, become more sensitive than neighboring ones. This phenomenon is called in a consistent way.

Rice. 6.30. A drawing that allows you to determine the gradual decomposition of the visual pigment: after looking at the black cross for 20-30 seconds, look at the adjacent white field, where you can see a lighter cross.

The sequential image can also be colored. So, if you look at a colored object for a few seconds, and then look at a white wall, you can see the same object, but painted in complementary colors. Apparently, this is due to the fact that the white color contains a complex of light rays of different wavelengths. And when rays of the same wavelength act on the eye, even earlier, the sensitivity of the corresponding cones is reduced, and this color seems to be separated from white.