There are articles by Tersteege (1972), Hunt (1976), Bartleson (1978), Wright (1981), Lenny and D'Zmur (1988).

Good luck to the inquisitive reader in studying this glorious literature!

8.1 LIGHT, DARK AND CHROMATIC ADAPTATION

Adaptation is the body's ability to change its sensitivity to a stimulus in response to changes in stimulation conditions.

Note that the general concept of adaptation covers all areas of perception.

Adaptation mechanisms can be ultra-short in duration (on the order of milliseconds) or vice versa - ultra-long, lasting weeks, months and even years. In general, adaptation mechanisms serve to reduce the observer’s sensitivity to a stimulus as the physical intensity of the latter increases (for example, one can clearly hear the ticking of a clock in the middle of a quiet night

And You can’t hear it at all in a noisy reception).

IN In relation to vision, three types of adaptation are important: light, dark and chromatic.

Light adaptation

Light adaptation- this is the process of decreasing the sensitivity of vision as the general level of illumination increases.

TO For example: on a clear night it is easy to see millions of stars, but at noon there are just as many of them in the sky - but during the day no stars are visible. This happens because during the day the total brightness of the sky is several orders of magnitude higher than at night, and therefore during the day the sensitivity of vision is reduced in comparison with night sensitivity. Thus, the difference in the brightness of the night sky and stars is able to provide visual perception of the latter, whereas during the day it is not large enough.

Another example: imagine waking up in the middle of the night and turning on a bright light. At first you are blinded, unable to make out anything

And you may even feel a slight pain, but after just a few tens of seconds you begin to gradually distinguish objects. This happens because in the dark the vision mechanisms were in the most sensitive state and immediately after turning on the light (due to their increased sensitivity) they are overloaded, but after a short time they adapt, reducing sensitivity and thereby ensuring normal vision.

Dark adaptation

Dark adaptation similar to light, except that the process goes in the opposite direction, that is:

Dark adaptation is the process of increasing the sensitivity of vision as the level of photometric brightness decreases.

Despite the fact that the phenomena of light and dark adaptation are similar to each other, they are still two independent phenomena, caused by different mechanisms and performing different visual work (for example, light adaptation occurs much faster than dark adaptation).

Anyone can experience a dark adaptation by entering a twilight cinema from a sunlit street: at first the room seems completely dark, and many simply stop at the threshold because they cannot see anything. However, after a short period of time, objects in the room (chairs, spectators) begin to emerge from the darkness. After a few more minutes, they will become clearly visible, and it will not be difficult to recognize familiar figures, find the right chair, etc., since dark adaptation mechanisms gradually increase the overall sensitivity of the visual system.

We can talk about light and dark adaptation as an analogy to automatic exposure control in cameras.

Chromatic adaptation

The processes of light and dark adaptation radically influence the color perception of stimuli and are therefore taken into account by many models of color perception. However, the third type of vision adaptation - chromatic adaptation - is the most important, and all models must take it into account.

Chromatic adaptation is a process of largely independent adjustment of the sensitivity of color vision mechanisms.

Moreover, it is often believed that chromatic adaptation is based only on an independent change in the sensitivity of the three types of cone photoreceptors (while light and dark adaptation are the result of a general change in the sensitivity of the entire receptor apparatus). However, it is important to remember that there are other mechanisms of color vision (acting, for example, at the opponent level and even at the level of object recognition) that are capable of changing sensitivity, which can also be classified as chromatic adaptation mechanisms.

As an example of chromatic adaptation, let's take a sheet of white paper illuminated by daylight. If this sheet is moved into a room lit by incandescent lamps, it will still be perceived as white, despite the fact that the energy reflected from the sheet has changed from predominantly “blue” to predominantly “yellow” (this is the same change that which color reversible photographic film cannot accommodate, as we discussed in the introduction to this chapter).

Rice. 8.1 illustrates this situation: in Fig. 8.1(a) shows a typical daylight scene; in Fig. 8.1 (b) - same scene, illuminated by lamp

Rice. 8.2 Example of post-images caused by local retinal adaptation.

Fix your gaze on the black dot for 30 seconds, and then move it to a uniform white surface. Pay attention to the colors of the post-images and compare them with the colors of the original stimuli.

mi incandescent and perceived by a certain visual system incapable of adaptation; in Fig. 8.1 (c) - again the same scene under the light of incandescent lamps, perceived by a certain visual system capable of adaptation like the human visual system.

The second illustrative example of chromatic adaptation is the so-called. post-image times shown in Fig. 8.2: Focus on the black dot in the center of the figure and remember the positions of its colors; After about 30 seconds, move your gaze to a lighted white area, such as a white wall or a blank sheet of paper. Pay attention to the colors that appear and their relative positions. The resulting post-images are the result of an independent change in the sensitivity of color mechanisms. For example, areas of the retina exposed to the red stimulus of Figure 8.2 decrease their sensitivity to “red” energy as the exposure adapts, causing the retinal area to fail in the “red” response (normally expected when exposed to white stimuli), resulting in when looking at A blue post-image appears on the white surface. The appearance of other colors in post-images is explained in a similar way.

So, if we can talk about light adaptation as an analogy to automatic exposure control, then we talk about chromatic adaptation as an analogy to automatic white balance in video or digital photo cameras.

Wright (1981) provides a historical overview of why and how chromatic adaptation was studied.

Looking at objects with both eyes. When a person looks at an object with both eyes, he cannot perceive two identical objects. This is due to the fact that images from all objects during binocular vision fall on the corresponding, or identical, areas of the retina, as a result of which in a person’s imagination these two images merge into one

Binocular vision is of great importance in determining the distance to an object and its shape. Estimation of the size of an object is related to the size of its image on the retina and the distance of the object from the eye

Lack of binocular vision often leads to strabismus

Pupillary reflex

The eye's response to light (constriction of the pupil) is a reflex mechanism for limiting the amount of light reaching the retina. Normal pupil width is 1.5 – 8 mm

The degree of room illumination can change the width of the pupil by 30 times. When the pupil narrows, the flow of light decreases, spherical aberration disappears, which gives self-scattering circles on the retina. In low light, the pupil dilates, which improves vision. The pupillary reflex takes part in the adaptation of the eye

Adaptation

Adaptation of the eye to seeing objects in conditions of different intensities of room lighting

Light adaptation. When moving from a dark room to a light one, blinding occurs at first. Gradually, the eye adapts to light by reducing the sensitivity of the photoreceptors of the retina. Lasts 5 – 10 minutes.

Mechanisms of light adaptation:

Reduced sensitivity of photoreceptors to light

Narrowing of the receptor field due to the severing of connections between horizontal cells and bipolar cells

Rhodopsin decay (0.001 sec.)

Constriction of the pupil

Dark adaptation. When moving from a light room to a dark room, a person initially sees nothing. After some time, the sensitivity of the retinal photoreceptors increases, the outlines of objects appear, and then their details begin to be distinguished. lasts 40 – 80 minutes.

Dark adaptation processes:

Increases the sensitivity of photoreceptors to light by 80 times

Rhodopsin resynthesis (0.08 sec.)

Pupil dilation

Increase in the number of connections between rods and retinal neurons

Increase in receptive field area

Rice. 6.11. Dark and light adaptation of the eye

Color vision

The human eye perceives 7 primary colors and 2000 different shades. The mechanism of color perception is explained by different theories

Three-component theory of color perception(Lomonosov-Jung-Helmholtz theory of color perception) - suggests the existence in the retina of three types of photosensitive cones that respond to different lengths of light rays. This creates different color perception options.

the first type of cones reacts to long waves (610 - 950 µm) - sensation Red

the second type of cones - for medium waves (460 - 609 µm) - sensation Green colour

the third type of cones perceives short waves (300 - 459 microns) - sensation of blue color

The perception of other colors is determined by the interaction of these elements. Simultaneous excitation of the first and second types creates the sensation of yellow and orange colors, and the second and third give violet and bluish colors. Equal and simultaneous stimulation of three types of color-perceiving elements of the retina gives the sensation white, and inhibition forms them black color

The decomposition of light-sensitive substances found in cones causes irritation of nerve endings; the excitation that reaches the cerebral cortex is summed up, and a sensation of one uniform color arises

The complete loss of the ability to perceive colors is called anopia, while people see everything only in black and white

Color perception disorder - color blindness (color blindness) - Mostly men suffer - about 10% - lack of a certain gene on the X chromosome

There are 3 types of color vision disorders:

protanopia – lack of sensitivity to red color (have loss of perception of waves with a length of 490 microns)

deuteranopia – to green color (have a loss of perception of waves with a length of 500 microns)

tritanopia – to blue color (loss of perception of waves with a length of 470 and 580 microns)

Complete color blindness – monochromacy rare

Color vision testing is carried out using Rabkin tables

3-11-2012, 22:44

Description

Range of brightness perceived by the eye

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

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

Depending on the sign of the change in brightness, a distinction is made between light adaptation - adjustment to a higher brightness and dark adaptation - adjustment to a lower brightness.

Light adaptation

Light adaptation proceeds much faster than the dark one. Coming out of a dark room into bright daylight, a person is blinded and sees almost nothing in the first seconds. Figuratively speaking, the visual device is off scale. But if a millivoltmeter burns out when trying to measure a voltage of tens of volts, then the eye refuses to work only for a short time. Its sensitivity automatically and quite quickly drops. First of all, the pupil narrows. In addition, under the direct influence of light, the visual purple of the rods fades, as a result of which their sensitivity drops sharply. The 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, within a few seconds a person begins to see the surrounding picture in general terms, and after five minutes the light sensitivity of his vision comes into full compliance with the surrounding brightness, which ensures normal functioning of the eye in new conditions.

Dark adaptation. Adaptometer

Dark adaptation has been studied much better than light, which is largely explained by the practical importance of this process. In many cases, when a person finds himself in low light conditions, it is important to know in advance how long it will take and what he will be able to see. In addition, the normal course of dark adaptation is disrupted in some diseases, and therefore its study has diagnostic value. Therefore, special devices have been created to study dark adaptation - adaptometers. The ADM adaptometer is commercially produced in the Soviet Union. Let us describe its structure and method of working with it. The optical design of the device is shown in Fig. 22.

Rice. 22. ADM adaptometer diagram

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

When measuring light sensitivity, lamp 3 is turned off and shutters 6 and 33 are opened. Lamp 22 is turned on and the centering of its filament is checked using the image on plate 20. Lamp 22 illuminates milk glass 25 through a condenser 23 and a daylight filter 24, which serves as a secondary light source for a milk glass plate 16. The 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 aperture 26, the area of ​​which changes when drum 17 rotates. Filter 31 has an optical density of 2, i.e., a transmittance of 1%, and the remaining filters have a density of 1. 3, i.e. transmission 5%. Illuminator 7-11 serves for side illumination of the eyes through hole 5 when studying visual acuity under blinding conditions. When removing the adaptation curve, lamp 7 is turned off.

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

The basic procedure for measuring the progress of dark adaptation is as follows. In a darkened room, the patient sits in front of the adaptometer and looks inside the ball, pressing his face tightly to the half mask. The doctor turns on lamp 3, using filters 4 to set the brightness Lc to 38 cd/m2. The patient adapts to this brightness within 10 minutes. By turning disk 15 to set the circular diaphragm, visible to the patient at an angle of 10°, the doctor, after 10 minutes, extinguishes lamp 3, turns on lamp 22, filter 31 and opens hole 32. With the diaphragm and filter 31 fully open, the brightness L1 of glass 16 is 0.07 cd /m2. The patient is instructed to look at fixation point 14 and say “I see” as soon as he sees a bright spot at the site of plate 16. The doctor notes this time t1 reduces the brightness of plate 16 to the L2 value, waits until the patient says “I see” again, notes the time t2 and decreases the brightness again. The measurement lasts 1 hour after the adaptive brightness is turned off. A series of ti values ​​is obtained, each of which has its own L1, which makes it possible to construct 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 when aperture 26 is fully opened and with filters turned off. Let us denote the total transmission of the filters and diaphragm? The optical density Df of a system that attenuates brightness is equal to the logarithm of its reciprocal value.

This means that the brightness with the introduced attenuators is L = Lm ?ph, a logL, = logLm - Dph.

Since light sensitivity is inversely proportional to 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 dark adaptation time 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 also of the whole body. The average values ​​of Df and permissible limit values ​​that do not yet go beyond the norm are given. Based on the values ​​of Df, we calculated using formula (50) and in Fig. 24

Rice. 24. Normal course 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 the greater complexity of this process. The course of the curve depends on many factors: from the brightness of the preliminary illumination of the eyes Lc, from the place on the retina onto which the test object is projected, from its area, etc. Without going into details, we will point out the difference in the adaptive properties of cones and rods. In Fig. 25

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

shows a graph of decreasing threshold brightness taken from Pinegin's work. The curve was taken after strong exposure of the eyes to white light with Lс = 27,000 cd/m2. The test field was illuminated with green light with? = 546 nm, a 20" test object was projected onto the periphery of the retina. The abscissa axis represents the dark adaptation time t, the ordinate axis is lg (Lп/L0), where L0 is the threshold brightness at t = 0, and Ln is at any other time. moment. We see that in about 2 minutes the sensitivity increases 10 times, and over the next 8 minutes - another 6 times. At the 10th minute, the increase in sensitivity accelerates again (threshold brightness decreases), and then becomes slow again. Explanation of the progression The curve is like this. At first, the cones quickly adapt, but they can only increase sensitivity by about 60 times. After 10 minutes of adaptation, the capabilities of the cones are exhausted. But by this time, the rods have already been disinhibited, providing a further increase in sensitivity.

Factors that increase light sensitivity during adaptation

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

In § 30 we indicated the brightness limits 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 Lп = 10-6 cd/m2. And at the top? At a high level of adaptation L, the threshold brightness Lп can be called the minimum brightness that can still be distinguished from complete darkness. Using the experimental material of the work, we can conclude that Lp at high brightnesses is approximately 0.006L. So, it is necessary to evaluate the role of various factors when reducing the threshold brightness from 60 to 10_6 cd/m2, i.e. "... 60 million times. Let's list these factors:

1. Transition from cone to rod vision. From the fact that for a point source, when we can assume that light acts on one receptor, En = 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. Increasing the visual inertia time from 0.05 to 0.2 s, i.e. 4 times.
4. An increase in the area over which the effect of light on the retina is summed. At high brightness, what is the angular resolution limit? = 0.6", and at low? = 50". An increase in this number means that many receptors unite to jointly perceive light, forming, as physiologists usually say, one receptive field (Gleser). The receptive field area increases 6900 times.
5. Increased sensitivity of the brain's vision centers.
6. Increasing the concentration a of the photosensitive substance. It is this factor that we want to evaluate.

Let us assume that the increase in brain sensitivity is small and can be neglected. We will then be able to estimate the effect of increasing a, or at least the upper limit of 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 of 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 room to a light one, blinding occurs at first. Eye sensitivity gradually decreases; they adapt to light. This adaptation of the eye to bright light conditions is called light adaptation.

The opposite phenomenon is observed when a person moves from a bright room, in which the sensitivity of the eye to light is greatly dulled, into a dark room. At first, due to reduced excitability of the eye, he sees nothing. 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 an adaptation of the eye to low light conditions, is called dark adaptation.

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

When in the dark light adaptation, i.e., the decrease in retinal sensitivity, 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; therefore, 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 process of increasing sensitivity when staying in the dark is presented in rice. 221. In the first 10 minutes, the sensitivity of the eye increases by 50-80 times, and then within an hour by many tens of thousands of times. The increased sensitivity of the eye in the dark has a complex mechanism. Of great importance 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 is completed by the end of the first hour in the dark. The restoration of rhodopsin is accompanied by a sharp increase in the sensitivity of the retinal rods to light. After a long stay in the dark, it becomes 100,000 - 200,000 times greater than it was in harsh lighting conditions. Since rods have maximum sensitivity after a long stay in the dark, very dimly lit objects are visible only when they are not in the center of the visual field, that is, when they irritate 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 from 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 meets with some objections. They are due to the fact that when the eye is exposed to high-brightness light, the amount of rhodopsin decreases only slightly and, according to calculations, this cannot cause such a large decrease in the sensitivity of the retina as occurs during light adaptation. Therefore, it is now believed that the phenomena of adaptation depend not on the splitting and resynthesis of photosensitive pigments, but on other reasons, in particular, on the processes occurring in the neural elements of the retina. This can be supported by the fact that adaptation to long-term stimulation is a property of many receptors.

It is possible that when adapting to illumination, the ways in which photoreceptors connect to ganglion cells are important. 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 believed that in the dark, the so-called horizontal neurons of the retina begin to function - Dogel stellate cells, the processes of which end on many photoreceptors.

Thanks to this, the same photoreceptor can be connected to different bipolar and haiglio cells, and each such cell becomes associated with a large number of photoreceptors ( ). Therefore, in very low light conditions, 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 stops and then a smaller number of photoreceptors are associated with the ganglion cell and, therefore, 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 irritation 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 sensitivity to light in the other, unilluminated eye. Irritations of other senses act similarly, for example, weak and medium-strength sound signals, olfactory and gustatory irritations.

If the effect of light on a dark-adapted eye is combined with some indifferent stimulus, for example a bell, then after a series of combinations, just turning on 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 by a conditioned reflex pathway, that is, that they are subject to the regulatory influence of the cerebral cortex (A. V. Bogoslovsky).

The retinal adaptation processes are also influenced by the sympathetic nervous system. Unilateral removal of the cervical sympathetic ganglia in humans causes a decrease in the rate of dark adaptation of the desympathetic eye. Injecting adrenaline has the opposite effect.

Adaptation is the adaptation of the eye to changing lighting conditions. Provided by: changes in the diameter of the pupil opening, movement of black pigment in the layers of the retina, different reactions of rods and cones. The pupil can vary in diameter from 2 to 8 mm, while its area and, accordingly, the luminous flux change by 16 times. The pupil contracts in 5 seconds, and its full dilation occurs in 5 minutes.

Color adaptation

The perception of color can change depending on external lighting conditions, but human vision adapts to the light source. This allows the lights to be identified as the same. Different people have different eye sensitivity to each of the three colors.

Dark adaptation

Occurs during the transition from high to low brightnesses. If bright light initially entered the eye, the rods were blinded, the rhodopsin faded, and the black pigment penetrated the retina, blocking the cones from the light. If suddenly the brightness of the light decreases significantly, the pupil will first dilate. Then the black pigment will begin to leave the retina, rhodopsin will be restored, and when there is enough of it, the rods will begin to function. Since cones are not sensitive to low brightness, at first the eye will not distinguish anything until a new vision mechanism takes effect. The sensitivity of the eye reaches its maximum value after 50-60 minutes of being in the dark.

Light adaptation

The process of adaptation of the eye during the transition from low to high brightness. In this case, the rods are extremely irritated due to the rapid decomposition of rhodopsin, they are “blind”; and even the cones, not yet protected by grains of black pigment, are too irritated. Only after sufficient time has passed does the adaptation of the eye to new conditions end, the unpleasant feeling of blindness ceases and the eye acquires the full development of all visual functions. Light adaptation lasts 8-10 minutes.