Analyzers perform a large number of functions or operations on signals. Among them the most important: I. Signal detection. II. Distinguishing signals. III. Signal transmission and conversion. IV. Encoding of incoming information. V. Detection of certain signs of signals. Vi. Recognition of images. As in any classification, this division is somewhat arbitrary.

The detection and discrimination of signals (I, II) is provided primarily by receptors, and the detection and recognition of (V, VI) signals by the higher cortical levels of the analyzers. Meanwhile, the transmission, transformation and coding (III, IV) of signals are characteristic of all layers of the analyzers.

I, Signal detection begins in receptors - specialized cells, evolutionarily adapted to perception from the external or internal environment organism of this or that stimulus and its transformation from physical or chemical form into the form of nervous excitement.

Receptor classification. All receptors are divided into two large groups: external, or exteroreceptors, and internal, or interoreceptors. Exteroreceptors include: auditory, visual, olfactory, taste, tactile receptors; interoreceptors - visceroceptors (signaling a state internal organs), vestibulo- and proprioceptors (receptors of the musculoskeletal system).

By the nature of the contact with the environment, receptors are divided into distant receptors that receive information at a certain distance from the source of irritation (visual, auditory and olfactory), and contact receptors, which are excited by direct contact with it.

Depending on the nature of the stimulus to which they are optimally tuned, human receptors can be divided into 1) mechanoreceptors, k. which include receptors auditory, gravitational, vestibular, tactile receptors of the skin, receptors of the musculoskeletal system, baroreceptors of the cardiovascular system; 2) chemoreceptors, including receptors of taste and smell, vascular and tissue receptors; 3) photoreceptors, 4) thermoreceptors (skin and internal organs, as well as central thermosensitive neurons); five) painful (nociceptive) receptors, in addition to which painful irritations can be perceived by other receptors.

All receptor apparatuses are divided into primary feeling (primary) and secondary-sentient (secondary). The former include olfactory receptors, tactile receptors, and proprioceptors. They differ in that the perception and transformation of the energy of irritation into the energy of nervous excitation occurs in them in the most sensitive neuron. Secondary senses include receptors for taste, vision, hearing, and the vestibular apparatus. They have a highly specialized receptor cell between the stimulus and the first sensitive neuron, that is, the first neuron is not excited directly, but through a receptor (not nerve) cell.

According to their main properties, receptors are also divided into fast and slow adapting, low and high threshold, monomodal and polymodal, etc.

In practical terms, the most important is the psychophysiological classification of receptors by the nature of the sensations that arise when they are irritated. According to this classification, a person distinguishes between visual, auditory, olfactory, taste, tactile receptors, thermoreceptors, receptors for the position of the body and its parts in space (proprio- and vestibuloreceptors) and pain receptors.

Receptor excitation mechanisms. Under the action of a stimulus on a receptor cell, changes occur in the spatial configuration of protein receptor molecules embedded in the protein-lipid complexes of its membrane. This leads to a change in the membrane permeability for certain ions (most often sodium) and the appearance of an ion current that generates the so-called receptor potential. In the primary sensing receptors, this potential acts on the most sensitive areas of the membrane that are capable of generating action potentials - nerve impulses.

In secondary sensing receptors, the receptor potential causes the release of transmitter quanta from the presynaptic end of the receptor cell. A mediator (for example, acetylcholine), acting on the postsynaptic membrane of a sensitive neuron, causes its depolarization (postsynaptic potential - PSP). The postsynaptic potential of the first sensory neuron is called generator potential and it generates an impulse response. In primary sensing receptors, the receptor and generator potentials, which have the properties of a local response, are one and the same.

Most of the receptors have so-called background impulse (spontaneously release a transmitter) in the absence of any irritation. This makes it possible to transmit information about the signal not only in the form of increased frequency, but also in the form of a decrease in the flow of impulses. At the same time, the presence of such discharges leads to the detection of signals against the background of "noise". Under "noises", impulses not associated with external stimulation are captured, arising in receptors and neurons as a result of spontaneous release of transmitter quanta, as well as multiple excitatory interactions between neurons.

These “noises” make it difficult to detect signals, especially when they are low in intensity or when their changes are small. In this regard, the concept of the response threshold becomes statistical: it is usually required to determine the threshold stimulus several times in order to make a reliable decision about its presence or absence. This is true both at the level of the behavior of an individual neuron or receptor, and at the level of the reaction of the whole organism.

In the analyzer system, the procedure of multiple evaluations of a signal to make a decision about its presence or absence is replaced by a comparison of the simultaneous reactions to this signal of a number of elements. The question is resolved, as it were, by voting: if the number of elements simultaneously excited by a given stimulus is greater than a certain critical value, the signal is considered to have occurred. From this it follows that the threshold of the analyzer system's response to a stimulus depends not only on the excitation of an individual element (be it a receptor or a neuron), but also on the distribution of excitation in the population of elements.

The sensitivity of the receptor elements to the so-called adequate stimuli, to the perception of which they are evolutionarily adapted (light for photoreceptors, sound for the receptors of the cochlea of \u200b\u200bthe inner ear, etc.), is extremely high. So, olfactory receptors are able to be excited by the action of single molecules of odorous substances, photoreceptors are able to be excited by a single quantum of light in the visible part of the spectrum, and the hair cells of the spiral (Corti's) organ respond to displacements of the basilar membrane of the order of 1 10 "" M (0.1 A °) , i.e., for the vibration energy equal to 1 ^0~ ^ " r B ^ / cm 2 (^ 10 ~ 9 erg / (s-cm 2). A higher sensitivity in the latter case is also impossible, since the ear would hear the thermal (Brownian) movement of molecules already in the form of constant noise.

It is clear that the sensitivity of the analyzer as a whole cannot be higher than the sensitivity of the most excitable of its receptors. However, in addition to receptors, sensory neurons of each nerve layer, which differ in excitability, are involved in the detection of signals. These differences are very large: for example, visual neurons in different parts of the analyzer differ in light sensitivity by a factor of 10 7. Therefore, the sensitivity of the visual analyzer as a whole depends on the fact that more and more high levels the system increases the proportion of highly sensitive neurons. This contributes to the reliable detection of weak light signals by the system.

I. Distinguishing signals. So far we have been talking about the absolute sensitivity of the analyzers. An important characteristic of how they analyze signals is their ability to detect changes in intensity, timing, or spatial signature of a stimulus. These analyzer system operations are related to to ; ";: parts of the signals, begin already in the receptors, but the following analyzer y, and". \\! .. "are involved in it. It is necessary to provide a different response to the minimum |!" ;! „!! | chi between the stimuli. This minimal difference is the discrimination threshold (times - !; o1:! "!; s;" (threshold, if we are talking about comparing intensities).

In 1834, E. Weber formulated the following law: the perceived increase in irritation (the threshold of discrimination) must exceed the irritation that had been in effect earlier by a certain percentage. So, an increase in the sensation of pressure on the skin of the hand occurred only when an additional load was imposed, constituting a certain part of the load put earlier: if a weight of 100 g lay before, then add (for a person to feel this additive) 3-10 ~ 2 (3d), and if the weight was 200 g, then the barely perceptible addition was 6 g. The resulting dependence is expressed by the formula: D /// \u003d\u003d\u003d const, where / is irritation. A / - its perceptible increase (threshold of discrimination), conv! - constant value (constant).

Similar ratios were obtained for sight, hearing and other human senses. Weber's law can be explained by the fact that with an increase in the level of intensity of the main long-acting stimulus, not only the response to it increases, but also the "system noises", and also the adaptation inhibition deepens. Therefore, in order to again achieve reliable discrimination of additives to this stimulus, they must be increased until they exceed the fluctuations of these increased noises and exceed the level of inhibition.

A formula is derived that in a different way expresses the dependence of sensation on the strength of irritation: E \u003d \u003d a-1o ^ 1 - (- b, Where E - the magnitude of the sensation, / is the strength of stimulation, and and and are constants that are different for different signals. According to this formula, the sensation increases in proportion to the logarithm of the stimulus intensity. This generalized expression, called weber's law- Fechner, confirmed in many different studies.

Spatial discrimination of signals is based on differences in the spatial distribution of excitation in the receptor layer and in the nerve layers. So, if any two stimuli have excited two adjacent receptors, then it is impossible to distinguish between these two stimuli, but they will be perceived as a whole. For spatial differentiation of two stimuli, it is necessary that between the receptors they excite, there is at least one unexcited receptor element. Similar effects occur when hearing stimuli are perceived.

To temporarily distinguish between two stimuli, it is necessary that the nervous processes caused by them do not merge in time and that the signal caused by the subsequent stimulus does not fall into the refractory period from the previous stimulation.

In psychophysiology of the sense organs, such a value of a stimulus is taken as a threshold, the probability of perception of which is 0.75 (the correct answer about the presence of a stimulus in 3/4 of the cases of its action). In this case, it is natural that lower intensity values \u200b\u200bare considered subthreshold, and higher ones - above threshold. However, it turned out that in the "subthreshold" range, a clear, differentiated response to superweak (or ultrashort) stimuli is possible. So, if the intensity of light is reduced so much that the subject himself can no longer say whether he saw the flash or not, then by objectively registered skin-thalvanic reaction it is possible to reveal a clear response of the body to this signal. It turns out that the perception of such superweak stimuli occurs at a subthreshold level.

111. Transfer and transformation. After the transformation of the energy of a physical or chemical stimulus in the receptors into the process of nervous excitement, a chain of processes begins to transform and transmit the received signal. Their purpose is to convey to the higher parts of the brain the most important information about the stimulus and, moreover, in a form most convenient for its reliable and fast analysis.

Signal transformations can be conditionally divided into spatial and temporal. Among the spatial transformations of signals, one can single out a change in their scale as a whole or a distortion of the ratio of different spatial parts. So, in the visual and somatosensory systems at the cortical level, there is a significant distortion of the geometric proportions of the representation of individual parts of the body or parts of the visual field. In the visual cortex, the representation of the central fovea of \u200b\u200bthe retina is sharply expanded with a relative reduction in the periphery of the visual field ("cyclopean eye").

Time transformations of information are reduced mainly to its compression into separate impulse messages, separated by pauses or intervals. In general, for all analyzers, the transition from tonic impulses of neurons to phasic burst discharges of neurons is typical.

Fundamentals of psychophysiology., M. INFRA-M, 1998, pp. 57-72, Chapter 2 Ed. Yu.I. Alexandrov

2.1. The structure and functions of the optical apparatus of the eye

The eyeball has a spherical shape, which makes it easier to rotate for aiming at the object in question and provides good focusing of the image on the entire light-sensitive membrane of the eye - the retina. On the way to the retina, light rays pass through several transparent media of the cornea, lens and vitreous humor. A certain curvature and refractive index of the cornea and, to a lesser extent, the lens determine the refraction of light rays inside the eye. On the retina, an image is obtained that is sharply reduced and turned upside down and from right to left (Fig. 4.1 a). The refractive power of any optical system expressed in diopters (D). One diopter is equal to the refractive power of a lens with a focal length of 100 cm. The refractive power of a healthy eye is 59D when looking at distant objects and 70.5D when looking at close objects.

Figure: 4.1.

2.2. Accommodation

Accommodation is called the adaptation of the eye to clearly see objects located at different distances (like focusing in photography). For a clear vision of an object, it is necessary that its image be focused on the retina (Fig. 4.1 b). The main role in accommodation is played by the change in the curvature of the lens, i.e. its refractive power. When looking at close objects, the lens becomes more convex. The mechanism of accommodation is the contraction of the muscles that change the convexity of the lens.

2.3. Refractive errors of the eye

The two main refractive errors of the eye are myopia (myopia) and farsightedness (hyperopia). These anomalies are not caused by the lack of refractive media of the eye, but by a change in the length of the eyeball (Fig. 4.1 c, d). If the longitudinal axis of the eye is too long (Fig. 4.1 c), then the rays from a distant object will focus not on the retina, but in front of it, in the vitreous body. Such an eye is called myopic. To see clearly in the distance, the nearsighted person must place concave glasses in front of the eyes, which will move the focused image to the retina (Fig. 4.1 e). In contrast, in the far-sighted eye (Fig. 4.1d), the longitudinal axis is shortened, and therefore the rays from a distant object are focused behind the retina.This disadvantage can be compensated for by an increase in the convexity of the lens. However, when examining close objects, the accommodative efforts of farsighted people are insufficient. That is why, for reading, they should wear glasses with biconvex lenses that enhance the refraction of light (Fig. 4.1 e).

2.4. Pupil and pupillary reflex

The pupil is the hole in the center of the iris through which light enters the eye. It sharpens images on the retina, increasing the depth of field of the eye and eliminating spherical aberration. The pupil dilated when darkened in the light quickly narrows ("pupillary reflex"), which regulates the flow of light entering the eye. So, in bright light, the pupil has a diameter of 1.8 mm, with average daylight illumination, it expands to 2.4 mm, and in the dark - up to 7.5 mm. This degrades the image quality on the retina, but increases the absolute sensitivity of vision. The pupil's response to a change in illumination is adaptive, since it stabilizes the retinal illumination in a small range. In healthy people, the pupils of both eyes have the same diameter. When one eye is illuminated, the pupil of the other also narrows; such a reaction is called friendly.

2.5. Retinal structure and function

The retina is the inner light-sensitive membrane of the eye. It has a complex multilayer structure (Figure 4.2). There are two types of photoreceptors (rods and cones) and several types of nerve cells. Excitation of photoreceptors activates the first nerve cell in the retina - the bipolar neuron. Excitation of bipolar neurons activates retinal ganglion cells, transmitting their impulses to the subcortical visual centers. The processes of transmission and processing of information in the retina also involve horizontal and amacrine cells. All of the listed retinal neurons with their processes form the nervous apparatus of the eye, which is involved in the analysis and processing of visual information. That is why the retina is called the peripheral part of the brain.

2.6. Structure and function of retinal layers

Cells pigment epithelium form the outer layer of the retina, which is the most distant from the light. They contain melanosomes, which give them a black color. The pigment absorbs excess light, preventing it from reflecting and scattering, which contributes to a clear image on the retina. The pigment epithelium plays a decisive role in the regeneration of the visual purpura of photoreceptors after its discoloration, in the constant renewal of the outer segments of the optic cells, in the protection of receptors from light damage, as well as in the transfer of oxygen and nutrients to them.

Photoreceptors. A layer of visual receptors: rods and cones adjoins the layer of pigment epithelium from the inside. Each human retina contains 6-7 million cones and 110-125 million rods. They are unevenly distributed in the retina. The central retinal fossa - fovea (fovea centralis) contains only cones. Towards the periphery of the retina, the number of cones decreases and the number of rods increases, so that only rods are present at the far periphery. Cones function in high light conditions, they provide daylight and color vision; more light-sensitive rods are responsible for twilight vision.

Color is perceived best when light acts on the fovea of \u200b\u200bthe retina, which contains almost exclusively the cones. Here is the greatest visual acuity. With increasing distance from the center of the retina, color perception and spatial resolution gradually decrease. The periphery of the retina, on which only rods are located, does not perceive colors. But the light sensitivity of the retinal cone apparatus is many times less than that of the rod apparatus. Therefore, at dusk, due to a sharp decrease in cone vision and the predominance of peripheral rod vision, we do not distinguish color ("at night all cats are gray").

Visual pigments. Human retinal rods contain the pigment rhodopsin, or visual purple, with a maximum absorption spectrum in the region of 500 nanometers (nm). The outer segments of three types of cones (blue-, green- and red-sensitive) contain three types of visual pigments, the maxima of the absorption spectra of which are in the blue (420 nm), green (531 nm) and red (558 nm) regions of the spectrum. The red cone pigment is called iodopsin. The visual pigment molecule consists of a protein part (opsin) and a chromophore part (retinal, or vitamin "A" aldehyde). The source of retinal in the body is carotenoids; with their lack, twilight vision is impaired ("night blindness").

2.7. Retinal neurons

Retinal photoreceptors are synaptically linked to bipolar nerve cells (see Fig. 4.2). When exposed to light, the release of the mediator from the photoreceptor decreases, which hyperpolarizes the membrane of the bipolar cell. From it, a nerve signal is transmitted to ganglion cells, the axons of which are fibers of the optic nerve.

Figure: 4.2. Diagram of the structure of the retina:
1 - sticks; 2 - cones; 3 - horizontal cell; 4 - bipolar cells; 5 - amacrine cells; 6 - ganglion cells; 7 - fibers of the optic nerve

For 130 million photoreceptor cells, there are only 1 million 250 thousand retinal ganglion cells. This means that impulses from many photoreceptors converge (converge) through bipolar neurons to one ganglion cell. Photoreceptors connected to one ganglion cell form its receptive field [Hubel, 1990; Fiziol. view, 1992]. Thus, each ganglion cell summarizes the excitation that occurs in a large number of photoreceptors. This increases the light sensitivity of the retina, but worsens its spatial resolution. Only in the center of the retina (in the region of the central fossa), each cone is connected to one bipolar cell, and that, in turn, is connected to one ganglion cell. This provides a high spatial resolution of the center of the retina, but sharply decreases its light sensitivity.

The interaction of neighboring retinal neurons is provided by horizontal and amacrine cells, through the processes of which signals propagate that change synaptic transmission between photoreceptors and bipolar cells (horizontal cells) and between bipolar cells and ganglion cells (amacrine cells). Amacrine cells carry out lateral inhibition between adjacent ganglion cells. Centrifugal, or efferent, nerve fibers also enter the retina, bringing signals from the brain to it. These impulses regulate the conduction of excitation between the bipolar and ganglion cells of the retina.

2.8. Nerve pathways and connections in the visual system

From the retina, visual information is sent to the brain through the fibers of the optic nerve. Nerves from two eyes meet at the base of the brain, where some of the fibers go to the opposite side (optic chiasm, or chiasm). This provides each hemisphere of the brain with information from both eyes: signals from the right halves of each retina arrive in the occipital lobe of the right hemisphere, and signals from the left half of each retina to the left hemisphere (Fig. 4.3).

Figure: 4.3. Diagram of the visual pathways from the retina to the primary visual cortex:
LPZ - left visual field; PPZ - right visual field; tf - gaze fixation point; lg - left eye; pg - right eye; zn - optic nerve; x - visual chiasm, or chiasm; from - optical path; NKT - external geniculate body; ZK - visual cortex; lp - left hemisphere; pp - right hemisphere

After chiasm, the optic nerves are called optic tracts and most of their fibers come to the subcortical visual center - the external geniculate body (LCC). From here, visual signals enter the primary projection area of \u200b\u200bthe visual cortex (striate cortex, or field 17 according to Brodmann). The visual cortex consists of a number of fields, each of which provides its own, specific functions, receiving both direct and indirect signals from the retina and, in general, preserving its topology, or retinotopy (signals from neighboring areas of the retina enter the adjacent areas of the cortex).

2.9. Electrical activity of the centers of the visual system

Under the action of light in the receptors, and then in the neurons of the retina, electrical potentials are generated, reflecting the parameters of the acting stimulus (Fig. 4.4a, a). The total electrical response of the retina to light is called an electroretinogram (ERG).

Figure: 4.4. Electroretinogram (a) and light-evoked potential (EP) of the visual cortex (b):
a, b, c, d on (a) - ERG waves; the arrows indicate the moments when the light is turned on. Р 1 - Р 5 - positive waves of airspace, N 1 - N 5 - negative waves of airspace on (b)

It can be recorded from the whole eye: one electrode is placed on the surface of the cornea, and the other on the skin of the face near the eye (or on the earlobe). The ERG reflects well the intensity, color, size and duration of the action of the light stimulus. Since the ERG reflects the activity of almost all retinal cells (except for ganglion cells), this indicator is widely used to analyze the work and diagnose diseases of the retina.

Excitation of retinal ganglion cells leads to the fact that electrical impulses rush to the brain along their axons (fibers of the optic nerve). The retinal ganglion cell is the first "classic" type neuron in the retina, generating propagating impulses. Three main types of ganglion cells are described: responding to the light on (on - reaction), turning it off (off - reaction), and both (on-off - reaction). In the center of the retina, the receptive fields of ganglion cells are small, and at the periphery of the retina, they are much larger in diameter. Simultaneous excitation of closely spaced ganglion cells leads to their mutual inhibition: the responses of each cell become smaller than with a single stimulation. This effect is based on lateral or lateral inhibition (see Ch. 3). Due to the round shape, the receptive fields of the retinal ganglion cells produce the so-called pointwise description of the retinal image: it is displayed in a very thin discrete mosaic consisting of excited neurons.

The neurons of the subcortical visual center are excited when impulses from the retina come to them along the fibers of the optic nerve. The receptive fields of these neurons are also round, but smaller than in the retina. The bursts of pulses they generate in response to a flash of light are shorter than those in the retina. At the level of the tubing, there is an interaction of afferent signals from the retina with efferent signals from the visual cortex, as well as from the reticular formation from the auditory and other sensory systems. This interaction helps to highlight the most essential components of the signal and, possibly, participates in the organization of selective visual attention (see Ch. 9).

Pulse discharges of tubing neurons along their axons enter the occipital part of the cerebral hemispheres, in which the primary projection area of \u200b\u200bthe visual cortex (striatal cortex) is located. Here, in primates and humans, information processing is much more specialized and complex than in the retina and tubing. The neurons of the visual cortex have not round, but elongated (horizontally, vertically, or diagonally) receptive fields (Fig. 4.5) of small size [Hubel, 1990].

Figure: 4.5 ... The receptive field of a neuron in the visual cortex of the cat's brain (A) and the responses of this neuron to light strips of different orientations flashing in the receptive field (B). A - pluses mark the excitatory zone of the receptive field, and minuses - two lateral inhibitory zones. B - it can be seen that this neuron most strongly reacts to vertical and close to it orientation

Due to this, they are able to select from the image individual fragments of lines with one or another orientation and location and selectively respond to them. (orientation detectors). In each small area of \u200b\u200bthe visual cortex along its depth, neurons are concentrated with the same orientation and localization of the receptive fields in the visual field. They form an orientation column neurons passing vertically through all layers of the cortex. The column is an example of a functional association of cortical neurons that perform a similar function. A group of adjacent orientation columns, whose neurons have overlapping receptive fields, but different preferred orientations, forms a so-called supercolumn. As studies of recent years show, the functional unification of distant from each other neurons of the visual cortex can also occur due to the synchronization of their discharges. Recently, neurons with a selective sensitivity to cruciform and angular figures, belonging to the second order detectors, have been found in the visual cortex. Thus, the "niche" between simple orientation detectors and higher-order detectors (faces) found in the temporal cortex began to fill in between the simple orientation detectors describing the spatial features of the image.

In recent years, the so-called "spatial-frequency" tuning of neurons in the visual cortex has been well studied [Glezer, 1985; Fiziol. view, 1992]. It lies in the fact that many neurons selectively respond to a lattice of light and dark stripes of a certain width that appears in their receptive field. So, there are cells that are sensitive to a lattice of small strips, i.e. to a high spatial frequency. Cells with sensitivity to different spatial frequencies were found. It is believed that this property provides the visual system with the ability to select areas with different textures from the image [Glezer, 1985].

Many neurons of the visual cortex selectively respond to certain directions of movement (directional detectors) or to some color (color-opposing neurons), and some neurons respond best to the relative distance of the object from the eyes. Information about different signs of visual objects (shape, color, movement) is processed in parallel in different parts of the visual cortex.

To assess signal transmission at different levels of the visual system, registration of total evoked potentials (EP), which in humans can be simultaneously withdrawn from the retina and from the visual cortex (see Fig. 4.4 b). Comparison of the flash-induced retinal response (ERG) and the EP of the cortex makes it possible to assess the work of the projection visual pathway and establish the localization of the pathological process in the visual system.

2.10. Light sensitivity

Absolute vision sensitivity ... For a visual sensation to arise, light must have a certain minimum (threshold) energy. The minimum number of light quanta required for the appearance of the sensation of light in the dark ranges from 8 to 47. One rod can be excited by only 1 light quantum. Thus, the sensitivity of the retinal receptors in the most favorable conditions of light perception is limiting. Single rods and cones of the retina differ insignificantly in light sensitivity. However, the number of photoreceptors that send signals to one ganglion cell in the center and periphery of the retina is different. The number of cones in the receptive field in the center of the retina is about 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.

2.11. Visual adaptation

In the transition 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 the reduced excitability of photoreceptors and visual neurons. Gradually, the contours of objects begin to emerge, and then their details also differ, since the sensitivity of photoreceptors and visual neurons in the dark gradually increases.

The increase in light sensitivity during a stay in the dark is uneven: 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 with peripheral vision. In addition to visual pigments, an essential role in adaptation is played by switching connections between retinal elements. In the dark, the area of \u200b\u200bthe excitatory center of the receptive field of the ganglion cell increases due to the weakening of annular inhibition, which leads to an increase in light sensitivity. The light sensitivity of the eye also depends on the influences coming from the brain. Illumination of one eye lowers the light sensitivity of the unlit eye. In addition, sound, olfactory and gustatory signals also affect sensitivity to light.

2.12. Differential vision sensitivity

If additional illumination dI falls on an illuminated surface with brightness I, then, according to Weber's law, a person will notice the difference in illumination only if dI / I \u003d K, where K is a constant equal to 0.01-0.015. The dI / I value is called the differential light sensitivity threshold. The dI / I ratio is constant at different illumination levels and means that to perceive the difference in illumination of two surfaces, one of them must be brighter than the other by 1 - 1.5%.

2.13. Brightness contrast

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

2.14. Blinding brightness of light

Too bright light causes an unpleasant sensation of dazzle. The upper limit of blinding brightness depends on the adaptation of the eye: the longer the dark adaptation has been, the lower the brightness of the light causes blinding. If very bright (blinding) objects enter 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). For delicate work related to eye strain (prolonged reading, working on a computer, assembling small parts), you should use only diffused light that does not dazzle the eyes.

2.15. Inertia of vision, fusion of flashes, successive images

The visual sensation does not appear instantly. Before sensation arises, multiple transformations and signaling must occur in the visual system. The time of "inertia of vision" required for the appearance of 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 drive a burning match through the air in the dark, then we will see a luminous line, since the light stimuli that quickly follow one after another merge into a continuous sensation. The minimum repetition rate of light stimuli (for example, flashes of light), at which the combination of individual sensations occurs, is called critical frequency of flicker merging. At medium illumination, this frequency is equal to 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 per second in cinema), since the visual sensation from one frame still lasts until the next appears. This provides the illusion of image continuity and movement.

Feelings that continue after the irritation stops are called sequential images. If you look at the turned on lamp and close your eyes, then it is 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 the dark parts are light (negative sequential image). This is due to the fact that excitation from an illuminated object locally inhibits (adapts) certain areas of the retina; if, after that, you turn your gaze to a uniformly illuminated screen, then its light will more strongly excite those areas that were not previously excited.

2.16. Color vision

The entire spectrum of electromagnetic radiation we see is enclosed between shortwave (400 nm wavelength) radiation, which we call violet, and long wavelength radiation (700 nm wavelength), called red. The rest of the colors in 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 colors can be obtained.

The three-component theory of H. 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 still others to blue. Each color affects all three color-sensing elements, but to a different extent. This theory is directly confirmed in experiments in which the absorption of radiation with 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 color vision anomaly was termed "color blindness". Color blindness occurs in 8% of men; it is associated with the absence of certain genes in the male unpaired sex-determining X chromosome. To diagnose color blindness, which is important in professional selection, polychromatic tables are used. People suffering from it cannot be full-fledged transport drivers, 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 a lack of perception of one of the three primary colors. People suffering from protanopia ("red-blind") do not perceive red, the blue-blue rays seem colorless to them. People with deuteranopia ("green-blind") do not distinguish green from dark red and blue. With tritanopia (a rare abnormality of color vision), blue and violet rays are not perceived. All of these 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.

2.17. Perception of space

Visual acuity is the maximum ability to distinguish between individual details of objects. It is determined by the smallest distance between two points that the eye can distinguish, i.e. sees separately, not together. The normal eye distinguishes between two points, the distance between which is 1 arc minute. The center of the retina - the macula - has the maximum visual acuity. 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 terms, with normal acuity taken as a unit. There are people with super-sharpness of vision (visus more than 2).

Line of sight. If you fix a small object with your gaze, then 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, images of which fall on the rest of the retina, are perceived by peripheral vision. The space visible to the eye when fixing the gaze at one point is called field of view. Measurement of the border of the field of view is performed along the perimeter. The boundaries of the field of view for colorless objects are 70 downwards, upwards - 60, inwards - 60 and outwards - 90 degrees. The fields of vision of both eyes in humans partially coincide, which is of great importance for the perception of the depth of space. Fields of view for different colors are not the same and are smaller than for black and white objects.

Binocular vision - This is vision with two eyes. When looking at an 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 a person's perception, the two images merge into one. If you press lightly on one eye from the side, then it will begin to double in the eyes, because the correspondence of the retinas is broken. If you look at a close object, then the image of some more distant point falls on non-identical (disparate) points of two retinas. Disparity plays a large role in assessing 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 by several arc seconds. Binocular fusion or fusion of signals from two retinas into a single neural image occurs in the primary visual cortex of the brain.

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

Distance estimation. The perception of the depth of space and the estimation of the distance to the 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 in monocular vision. For assessing the distance, it is also important that the image of a familiar object on the retina is larger the closer it is.

The role of eye movement for vision. When looking at any objects, the eyes move. Eye movements are carried out by 6 muscles attached to the eyeball. The movement of both eyes occurs simultaneously and in a friendly manner. Considering close objects, it is necessary to reduce (convergence), and when considering distant objects, separate the visual axes of two eyes (divergence). The important role of eye movements for vision is also determined by the fact that image movement on the retina is necessary for the brain to continuously receive visual information. Impulses in the optic nerve occur when the light image is turned on and off. With the continued action of light on the same photoreceptors, impulses in the fibers of the optic nerve quickly cease and the visual sensation with motionless eyes and objects disappears after 1-2 s. If you put a sucker with a tiny light source on your eye, then a person sees it only at the moment of switching 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 examining any object, makes continuous jumps (saccades) that are imperceptible to a person. As a result of each jump, the image on the retina shifts from one photoreceptor to another, again causing the ganglion cells to flicker. 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 under consideration, the more complex the trajectory of eye movement. They seem to "trace" the contours of the image (Fig. 4.6), lingering on the most informative parts of it (for example, in the face these are the eyes). In addition to jumps, the eyes are constantly shaking and drifting (slowly moving from the point of fixation of the gaze). These movements are also very important for visual perception.

Figure: 4.6. Eye movement trajectory (B) when examining the image of Nefertiti (A)

Photochemical processes in the retina associated with the transformation of a number of substances in the light or in the dark. As mentioned above, the outer segments of the receptor cells contain pigments. Pigments are substances that absorb a certain part of the rays of light and reflect the rest of the rays. The absorption of light rays occurs by a group of chromophores that are contained in visual pigments. This role is played by aldehydes of alcohols of vitamin A.

Visual pigment of cones, iodopsin ( jodos - violet) consists of the protein photopsin (photos - light) and 11-cis-retinal, the pigment of the rods - rhodopsin ( rodos - purple) - from the protein scotopsin ( scotos - darkness) and also 11-cis retinal. Thus, the difference between the pigments of receptor cells lies in the peculiarities of the protein part. The processes that occur in sticks have been studied in more detail,

Figure: 12.10. Diagram of the structure of cones and rods

therefore, the subsequent analysis will concern them.

Photochemical processes occurring in rods in the world

Under the influence of a quantum of light absorbed by rhodopsin, photoisomerization of the chromophore part of rhodopsin occurs. This process is reduced to changing the shape of the molecule, the bent 11-cis-retinal molecule turns into a straightened all-trans-retinal molecule. The process of detaching the scopsin begins. The pigment molecule is discolored. At this stage, the discoloration of the rhodopsin pigment ends. Discoloration of one molecule contributes to the closure of 1,000,000 pores (Na + channels) (Hubel).

Photochemical processes in rods in the dark

The first stage is rhodopsin resynthesis - the transition of all-trans-retinal to 11-cis-retinal. This process requires metabolic energy and the enzyme retinal isomerase. As soon as 11-cis-retinal is formed, it combines with the protein of scotopsin, which leads to the formation of rhodopsin. This form of rhodopsin is stable to the next quantum of light (Fig. 12.11). Part of rhodopsin is subject to direct regeneration, part of retinal1 in the presence of NADH is reduced by the enzyme alcohol dehydrogenase to vitamin A1, which, accordingly, interacts with scotopsin to form rhodopsin.

If a person has not received vitamin A for a long time (months), then night blindness, or hemeralopia, develops. It can be treated - it disappears within an hour after the injection of vitamin A. Retinal molecules are aldehydes, therefore they are called retinalums, and vitamins of groups

Figure: 12.11. Photochemical and electrical processes in the retina

group A - alcohols, therefore they are called retinol. For the formation of rhodopsin with the participation of vitamin A, it is necessary for 11-cis-retinal to be converted to 11-trans-retinol.

Electrical processes in the retina

features:

1. MF of photoreceptors is very low (25-50 mV).

2. In the world in the outer segment Na + - the channels close, and in the dark they open. Accordingly, hyperpolarization occurs in the light in photoreceptors, and depolarization occurs in the dark. Closing the Na + -channels of the outer segment causes hyperpolarization by the K + -strum, that is, the emergence of an inhibitory receptor potential (up to 70-80 mV) (Fig. 12.12). As a result of hyperpolarization, the release of an inhibitory mediator, glutamate, decreases or stops, which contributes to the activation of bipolar cells.

3. In the dark: N the a + -channels of the outer segments are opened. Na + enters the outer segment and depolarizes the photoreceptor membrane (up to 25-50 mV). Depolarization of the photoreceptor leads to the emergence of an excitatory potential and enhances the release by the photoreceptor of the mediator glutamate, which is an inhibitory mediator, so the activity of bipolar cells will be inhibited. Thus, the cells of the second functional layer of the retina, when exposed to light, can activate the cells of the next layer of the retina, that is, ganglion cells.

The role of cells of the second functional layer

Bipolar cells as well as receptor (rods and cones) and horizontal, they do not generate action potentials, but only local potentials. There are two types of synapses between receptor and bipolar cells - excitatory and inhibitory, therefore the local potentials produced by them can be both depolarization - excitatory, and hyperpolarization - inhibitory. Bipolar cells receive inhibitory synapses from horizontal cells (Figure 12.13).

Horizontal cells are excited by the action of receptor cells, but they themselves inhibit bipolar cells. This type of inhibition is called lateral inhibition (see Figure 12.13).

Amacrine cells - the third type of cells of the second functional layer of the retina. they are activated

Figure: 12.12. Effect of darkness (A) and light (B) on the transport of Να * ions in photoreceptor cells of the retina:

The channels of the outer segment are open in the dark due to cGMP (A). When exposed to light, due to 5-HMP, they are partially closed (B). This leads to hyperpolarization of the synaptic endings of the photoreceptors (a - depolarization b - hyperpolarization)

bipolar cells, and they inhibit ganglion cells (see Fig. 3.13). It is believed that there are more than 20 types of amacrine cells and, accordingly, they secrete a large number of various mediators (GABA, glycine, dopamine, indolamine, acetylcholine, etc.). The reactions of these cells are also varied. Some react to turning on the light, others to turning it off, others to the movement of the spot along the retina, and the like.

The role of the third functional retinal layer

Ganglion cells - the only classic retinal neurons that always generate action potentials; they are located in the last functional layer of the retina, have a constant background activity with a frequency of 5 to 40 per minute (Guyton). Anything that happens in the retina between different cells affects the ganglion cells.

They receive signals from bipolar cells, in addition, they have an inhibitory effect on amacrine cells. The effect from bipolar cells is twofold depending on whether local potential occurs in bipolar cells. If depolarization, then such a cell will activate the ganglion cell and the frequency of action potentials will increase in it. If the local potential in a bipolar cell is hyperpolarized, then the effect on ganglion cells will be the opposite, that is, a decrease in the frequency of its background activity.

Thus, due to the fact that most retinal cells produce only local potentials and conduction in ganglion cells is electrotonic, this makes it possible to assess the intensity of illumination. All-or-nothing action potentials would not provide this.

In ganglion cells, as in bipolar and horizontal cells, there are receptor sites. Receptor sites are a collection of receptors that send signals to this cell through one or more synapses. The receptor sites of these cells are concentric. They distinguish between the center and the periphery with antagonistic interaction. The sizes of the receptor sites of ganglion cells can be different depending on which part of the retina sends signals to them; they will have fewer fovea receptors compared to signals from the periphery of the retina.

Figure: 12.13. Diagram of functional connections of retinal cells:

1 - photoreceptor layer;

2 - a layer of bipolar, horizontal, amacrine cells;

3 - a layer of ganglion cells;

Black arrows - inhibitory effect, white - exciting

Ganglion cells with an "on" center are activated when the center is illuminated, and when the periphery is illuminated, they are inhibited. On the contrary, ganglion cells with an "off" center are inhibited when the center is illuminated, and when the periphery is illuminated, they are activated.

By changing the frequency of impulses of ganglion cells, the effect on the next level of the visual sensory system will change.

It has been established that ganglionic neurons are not just the last link in signal transmission from retinal receptors to brain structures. The third visual pigment, melanopsin, was found in them! It plays a key role in ensuring the body's circadian rhythms associated with changes in lighting, it affects the synthesis of melatonin, and is also responsible for the reflex reaction of the pupils to light.

In experimental mice, the absence of the gene responsible for the synthesis of melanopsin leads to a pronounced violation of circadian rhythms, a decrease in the intensity of the reaction of the pupils to light, and after the inactivation of rods and cones, it generally disappears. The axons of the ganglionic cells, which contain melanopsin, are directed to the suprachiasmatic nuclei of the hypothalamus.

PRIVATE PHYSIOLOGY OF SENSOR SYSTEMS

Visual system

Vision is evolutionarily adapted to the perception of electromagnetic radiation in a certain, very narrow part of their range (visible light). The visual system provides over 95% of sensory information to the brain. Vision is a multi-stage process that begins with the projection of an image onto the retina of a unique peripheral optical device - the eye. Then, the excitation of photoreceptors, the transmission and transformation of visual information in the neural layers of the visual system, and visual perception ends with the adoption by the higher cortical parts of this system of decisions about the visual image.

The structure and functions of the optical apparatus of the eye. The eyeball has a spherical shape, which makes it easier to rotate it to aim at the object in question. On the way to the light-sensitive membrane of the eye (retina), light rays pass through several transparent media - the cornea, lens and vitreous body. A certain curvature and refractive index of the cornea and, to a lesser extent, the lens determine the refraction of light rays inside the eye (Fig. 14.2).

The refractive power of any optical system is expressed in diopters (D). One diopter is equal to the refractive power of a lens with a focal length of 100 cm. The refractive power of a healthy eye is 59D when looking at distant objects and 70.5D when looking at close objects. To schematically represent the projection of an image of an object onto the retina, you need to draw lines from its ends through the nodal point (7 mm behind the cornea). On the retina, an image is obtained that is sharply reduced and turned upside down and from right to left

Accommodation. Accommodation refers to the adaptation of the eye to clearly seeing objects at different distances. For a clear vision of an object, it is necessary that it be focused on the retina, that is, that the rays from all points of its surface are projected onto the surface of the retina (Fig. 14.4). When we look at distant objects (A), their image (a) is focused on the retina and they are clearly visible. But the image (b) of close objects (B) is vague, since the rays from them are collected behind the retina. The main role in accommodation is played by the lens, which changes its curvature and, consequently, its refractive power. When looking at close objects, the lens becomes more convex (see Fig. 14.2), due to which the rays diverging from any point of the object converge on the retina. The mechanism of accommodation is the contraction of the ciliary muscles, which change the convexity of the lens. The lens is enclosed in a thin transparent capsule, which is always stretched, that is, flattened, by the fibers of the ciliary girdle (Zinn's ligament). The contraction of the smooth muscle cells of the ciliary body reduces the traction of the zinn ligaments, which increases the bulge of the lens due to its elasticity. The ciliary muscles are innervated by the parasympathetic fibers of the oculomotor nerve. The introduction of atropine into the eye causes a disturbance in the transmission of excitation to this muscle, and limits the accommodation of the eye when examining close objects. On the contrary, parasympathomimetic substances - pilocarpine and eserine - cause this muscle to contract.

For the normal eye of a young man, the farthest point of clear vision lies at infinity. He examines distant objects without any tension of accommodation, that is, without contraction of the lumbar muscle. The closest point of clear vision is 10 cm from the eye.

Presbyopia. The lens loses its elasticity with age, and when the tension of the zinc ligaments changes, its curvature changes little. Therefore, the nearest point of clear vision is now not at a distance of 10 cm from the eye, but moves away from it. At the same time, close objects are poorly visible. This condition is called farsightedness, or presbyopia. Elderly people are forced to wear glasses with biconvex lenses.

Refractive errors of the eye. The two main anomalies of refraction of the eye - myopia, or myopia, and farsightedness, or hypermetropia - are caused not by the insufficiency of the refractive media of the eye, but by a change in the length of the eyeball (Fig. 14.5, A).

Myopia. If the longitudinal axis of the eye is too long, then the rays from a distant object will focus not on the retina, but in front of it, in the vitreous body (Fig. 14.5, B). Such an eye is called myopic, or myopic. To clearly see into the distance, it is necessary to place concave glasses in front of myopic eyes, which will move the focused image to the retina (Fig. 14.5, C).

Hyperopia. Farsightedness, or hyperopia, is opposite to myopia. In the far-sighted eye (Fig. 14.5, D), the longitudinal axis of the eye is shortened, and therefore the rays from a distant object are focused not on the retina, but behind it. This lack of refraction can be compensated for by accommodative effort, that is, by an increase in the convexity of the lens. Therefore, a farsighted person strains the accommodative muscle, considering not only close, but also distant objects. When considering close objects, the accommodative efforts of farsighted people are insufficient.

Therefore, for reading, farsighted people should wear glasses with biconvex lenses that enhance the refraction of light (Fig. 14.5, E). Hyperopia should not be confused with farsightedness. All they have in common is that it is necessary to use glasses with biconvex lenses.

Astigmatism. Refractive error also includes astigmatism, i.e. unequal refraction of rays in different directions (for example, along the horizontal and vertical meridians). Astigmatism is caused by a non-strictly spherical surface of the cornea. With severe astigmatism, this surface can approach a cylindrical one, which is corrected by cylindrical spectacle glasses, which compensate for the flaws of the cornea.

Pupil and pupillary reflex. The pupil is the hole in the center of the iris through which rays of light pass into the eye. The pupil sharpens the image on the retina, increasing the depth of field of the eye. Passing only the central rays, it improves the image on the retina also by eliminating spherical aberration. If you cover the eye from the light, and then open it, then the pupil, which has dilated during darkening, quickly narrows ("pupillary reflex"). The muscles in the iris change the size of the pupil by regulating the flow of light into the eye. So, in very bright light, the pupil has a minimum diameter (1.8 mm), with average daylight illumination, it expands (2.4 mm), and in the dark, the expansion is maximum (7.5 mm). This leads to a deterioration in the image quality on the retina, but increases the sensitivity of vision. The limiting change in the diameter of the pupil changes its area by about 17 times. At the same time, the luminous flux changes by the same amount. There is a logarithmic relationship between the light intensity and the pupil diameter. The reaction of the pupil to a change in illumination has an adaptive character, since it stabilizes the illumination of the retina in a small range.

In the iris, there are two types of muscle fibers that surround the pupil: ring (m. Sphincter iridis), innervated by parasympathetic fibers of the oculomotor nerve, and radial (m. Dilatator iridis), innervated sympathetic nerves... Contraction of the former causes constriction, contraction of the latter - dilation of the pupil. Accordingly, acetylcholine and eserine cause constriction, and adrenaline causes pupil dilation. The pupils dilate during pain, during hypoxia, as well as during emotions that increase the excitation of the sympathetic system (fear, rage). Dilation of the pupils is an important symptom of a number of pathological conditions, for example, painful shock, hypoxia.

In healthy people, the size of the pupils of both eyes is the same. When one eye is illuminated, the pupil of the other also narrows; this reaction is called friendly. In some pathological cases, the sizes of the pupils of both eyes are different (anisocoria). The structure and function of the retina. The retina is the inner light-sensitive membrane of the eye. It has a complex multi-layer structure

There are two types of secondary-sensing photoreceptors that differ in their functional significance (rod and cone) and several types of nerve cells. Excitation of photoreceptors activates the first nerve cell in the retina (bipolar neuron). Excitation of bipolar neurons activates retinal ganglion cells, transmitting their impulse signals to the subcortical visual centers. The processes of transmission and processing of information in the retina also involve horizontal and amacrine cells. All of the listed retinal neurons with their processes form the nervous apparatus of the eye, which not only transmits information to the visual centers of the brain, but also participates in its analysis and processing. Therefore, the retina is called the peripheral part of the brain.

The place where the optic nerve leaves the eyeball - the optic nerve head, is called a blind spot. It does not contain photoreceptors and is therefore insensitive to light. We do not feel the presence of a "hole" in the retina.

Let us consider the structure and function of the retinal layers, following from the outer (posterior, most distant from the pupil) layer of the retina to the inner (located closer to the pupil) layer of the retina.

Pigment layer. This layer is formed by one row epithelial cellscontaining a large number of various intracellular organelles, including melanosomes, which give this layer a black color. This pigment, also called screening pigment, absorbs light reaching it, thereby preventing it from being reflected and scattered, which contributes to a clear visual perception. The cells of the pigment epithelium have numerous processes that tightly surround the light-sensitive outer segments of rods and cones.The pigment epithelium plays a decisive role in a number of functions, including in the resynthesis (regeneration) of the visual pigment after its discoloration, in phagocytosis and in the digestion of debris and debris from the outer segments. cones, in other words, in the mechanism of constant renewal of the outer segments of visual cells, in the protection of visual cells from the danger of light damage, as well as in the transfer of oxygen and other substances they need to the photoreceptors. It should be noted that the contact between pigment epithelial cells and photoreceptors is rather weak. It is in this place that retinal detachment occurs - dangerous disease eye. Retinal detachment leads to visual impairment not only due to its displacement from the place of optical focusing of the image, but also due to degeneration of receptors due to a violation of contact with the pigment epithelium, which leads to a serious violation of the metabolism of the receptors themselves. Metabolic disorders are aggravated by the fact that the delivery of nutrients from the capillaries of the choroid of the eye is disrupted, and the layer of photoreceptors does not contain capillaries (avascularized).

Photoreceptors. The pigment layer is adjoined from the inside by a layer of photoreceptors: rods and cones1. The retina of each human eye contains 6-7 million cones and 110-123 million rods. They are unevenly distributed in the retina. The central retinal fossa (fovea centralis) contains only cones (up to 140 thousand per 1 mm2). Towards the periphery of the retina, their number decreases, and the number of rods increases, so that only rods are present on the far periphery. The cones function in high light conditions, they provide daylight. and color vision; much more light-sensitive rods are responsible for twilight vision.

Color is perceived best when light acts on the fovea of \u200b\u200bthe retina, where cones are almost exclusively located. Here is the greatest visual acuity. Color perception and spatial resolution get worse with distance from the center of the retina. The retinal periphery, where only rods are located, does not perceive colors. On the other hand, the light sensitivity of the cone apparatus of the retina is many times less than that of the rod apparatus, therefore, at dusk, due to a sharp decrease in “cone” vision and the predominance of “peripheral” vision, we do not distinguish color (“at night all cats are gray”).

The dysfunction of the sticks, which occurs when there is a lack of vitamin A in food, causes a disorder of twilight vision - the so-called night blindness: a person completely becomes blind at dusk, but during the day his vision remains normal. On the contrary, when the "cones" are damaged, photophobia arises: a person sees in weak "light, but goes blind in bright light. In this case, complete color blindness, achromasia, can also develop.

The structure of the photoreceptor cell. A photoreceptor cell - a rod or cone - consists of a light-sensitive outer segment containing visual pigment, an inner segment, a connecting leg, a nuclear part with a large nucleus, and a presynaptic end. The rod and cone of the retina are directed by their light-sensitive outer segments to the pigment epithelium, that is, to the side opposite to the light. In humans, the outer segment of the photoreceptor (rod or cone) contains about a thousand photoreceptor disks. The outer segment of the rod is much longer than the cones and contains more visual pigment. This partially explains the higher sensitivity of the rod to light: a rod can be excited by just one quantum of light, and more than a hundred quanta are required to activate a cone.

The photoreceptor disc is formed by two membranes connected at the edges. The disc membrane is a typical biological membrane formed by a double layer of phospholipid molecules, between which protein molecules are located. The disc membrane is rich in polyunsaturated fatty acids, which leads to its low viscosity. As a result, the protein molecules in it rotate rapidly and slowly move along the disk. This allows proteins to collide frequently and, when interacting, form functionally important complexes for a short time.

The inner segment of the photoreceptor is connected to the outer segment by a modified cilium, which contains nine pairs of microtubules. The inner segment contains the large nucleus and the entire metabolic apparatus of the cell, including mitochondria, which provide the energy needs of the photoreceptor, and the protein synthesis system, which ensures the renewal of the membranes of the outer segment. This is where the synthesis and incorporation of visual pigment molecules into the photoreceptor membrane of the disc takes place. In an hour, on average, three new discs are formed again at the border of the inner and outer segments. Then they slowly (in humans, for about 2-3 weeks) move from the base of the outer segment of the bacillus to its apex.In the end, the apex of the outer segment, containing up to hundreds of now old discs, breaks off and is phagocytized by the cells of the pigment layer. This is one of the most important mechanisms of protection of photoreceptor cells from molecular defects accumulating during their light life.

The outer segments of the cones are also constantly renewed, but at a slower rate. Interestingly, there is a daily renewal rhythm: the tops of the outer segments of the rods generally break off and phagocytose in the morning and daytime, and the cones - in the evening and night.

The presynaptic end of the receptor contains a synaptic band around which there are many synaptic vesicles containing glutamate.

Visual pigments. The rods of the human retina contain the pigment rhodopsin, or visual purple, the maximum absorption spectrum of which is in the region of 500 nanometers (nm). The outer segments of the three types of cones (blue, green and red sensitive) contain three types of visual pigments, the maxima of the absorption spectra of which are in the blue (420 nm), green (531 nm) and red (558 nm) parts of the spectrum. The red cone pigment is called "iodopsin". The visual pigment molecule is relatively small (with a molecular weight of about 40 kilodaltons), consists of a larger protein part (opsin) and a smaller chromophore (retinal, or vitamin A aldehyde).

Retinal can be in various spatial configurations, i.e. isomeric forms, but only one of them, the 11-cis isomer of retinal, acts as a chromophore group of all known visual pigments. Carotenoids are the source of retinal in the body, therefore their deficiency leads to a deficiency of vitamin A and, as a consequence, to insufficient resynthesis of rhodopsin, which in turn is the cause of impaired twilight vision, or “night blindness”. Molecular physiology of photoreception. Consider the sequence of changes in the molecules in the outer segment of the rod, responsible for its excitation (Fig. 14.7, A). When a quantum of light is absorbed by a molecule of visual pigment (rhodopsin), its chromophore group is instantly isomerized: 11-cis-retinal straightens and turns into completely trans-retinal. This reaction takes about 1 ps (1-12 s). Light acts as a trigger, or trigger, factor that triggers the photoreception mechanism. Following the photoisomerization of retinal, spatial changes occur in the protein part of the molecule: it becomes discolored and turns into the state of metarodopsin II.

As a result, the visual pigment molecule acquires the ability to interact with another protein - the membrane guanosine triphosphate-binding protein transducin (T). In a complex with metarodopsin II, transducin becomes active and exchanges guanosine diphosphate (GDP) bound to it in the dark for guanosine triphosphate (GTP). Metarodopsin II is able to activate about 500-1000 transducin molecules, which leads to an increase in the light signal.

Each activated transducin molecule bound to a GTP molecule activates one molecule of another membrane-bound protein, the enzyme phosphodiesterase (PDE). The activated PDE breaks down the molecules of cyclic guanosine monophosphate (cGMP) at a high rate. Each activated PDE molecule destroys several thousand cGMP molecules - this is another step in signal amplification in the photoreception mechanism. The result of all the described events caused by the absorption of a light quantum is a drop in the concentration of free cGMP in the cytoplasm of the outer segment of the receptor. This, in turn, leads to the closure of ion channels in the plasma membrane of the outer segment, which were open in the dark and through which Na + and Ca2 + entered the cell. The ion channel is closed due to the fact that, due to a decrease in the concentration of free cGMP in the cell, cGMP molecules, which were bound to it in the dark and kept it open, leave the channel.

A decrease or cessation of entry into the outer segment of Na + leads to hyperpolarization of the cell membrane, i.e., the emergence of a receptor potential on it. In fig. 14.7, B shows the directions of ion currents flowing through the plasma membrane of the photoreceptor in the dark. The concentration gradients of Na + and K + are maintained on the bacillus plasma membrane by the active work of the sodium-potassium pump localized in the membrane of the inner segment.

The hyperpolarizing receptor potential, which has arisen 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 release of the transmitter (glutamate). Thus, the photoreceptor process ends with a decrease in the rate of neurotransmitter release from the presynaptic end of the photoreceptor.

The mechanism for restoring the initial dark state of the photoreceptor, that is, its ability to respond to the next light stimulus, is no less complex and perfect. This requires reopening the ion channels in the plasma membrane. The open state of the channel is provided by its bond with cGMP molecules, which in turn is directly caused by an increase in the concentration of free cGMP in the cytoplasm. This increase in concentration is provided by the loss of metarodopsin II of the ability to interact with transducin and the activation of the enzyme guanylate cyclase (GC), which is able to synthesize cGMP from GTP. The activation of this enzyme is caused by a drop in the concentration of free calcium in the cytoplasm due to the closure of the membrane ion channel and the constant operation of the exchange protein, which releases calcium from the cell. As a result of all this, the concentration of cGMP inside the cell increases and cGMP binds again to the ion channel of the plasma membrane, opening it. Through the open channel, Na + and Ca2 + again begin to enter the cell, depolarizing the receptor membrane and transferring it to the "dark" state. The release of the mediator is again accelerated from the presynaptic end of the depolarized receptor.

Retinal neurons. Retinal photoreceptors are synaptically associated with bipolar neurons (see Fig. 14.6, B). When exposed to light, the release of a neurotransmitter (glutamate) from the photoreceptor decreases, which leads to hyperpolarization of the membrane of the bipolar neuron. From it, a nerve signal is transmitted to the ganglion cells, the axons of which are fibers of the optic nerve. Signal transmission from both the photoreceptor to the bipolar neuron and from it to the ganglion cell occurs in a pulseless way. The bipolar neuron does not generate impulses due to the extremely small distance over which it transmits the signal.

For 130 million photoreceptor cells, there are only 1 million 250 thousand ganglion cells, the axons of which form the optic nerve. This means that impulses from many photoreceptors converge (converge) through bipolar neurons to one ganglion cell. Photoreceptors connected to one ganglion cell form the receptive field of the ganglion cell. The receptive fields of various ganglion cells partially overlap each other. Thus, each ganglion cell summarizes the excitation that occurs in a large number of photoreceptors. This increases light sensitivity, but degrades spatial resolution. Only in the center of the retina, in the region of the central fossa, each cone is connected to one so-called dwarf bipolar cell, to which only one ganglion cell is also connected. This provides a high spatial resolution here, but sharply decreases light sensitivity.

The interaction of neighboring retinal neurons is provided by horizontal and amacrine cells, through the processes of which signals propagate that change synaptic transmission between photoreceptors and bipolar cells (horizontal cells) and between bipolar and ganglion cells (amacrine cells). Amacrine cells carry out lateral inhibition between adjacent ganglion cells.

In addition to afferent fibers, the optic nerve also contains centrifugal, or efferent, nerve fibers that bring signals from the brain to the retina. It is believed that these impulses act on the synapses between the bipolar and hanliose cells of the retina, regulating the conduction of excitation between them.

Nerve pathways and connections in the visual system. From the retina, visual information is sent to the brain through the fibers of the optic nerve (II pair of cranial nerves). The optic nerves from each eye meet at the base of the brain, where their partial intersection (chiasm) forms. Here, part of the fibers of each optic nerve passes to the side opposite from its eye. Partial intersection of fibers provides each cerebral hemisphere with information from both eyes. These projections are organized so that signals from the right halves of each retina are sent to the occipital lobe of the right hemisphere, and signals from the left halves of the retinas to the left hemisphere.

After the optic chiasm, the optic nerves are called the optic tracts. They are projected into a number of cerebral structures, but the main number of fibers comes to the thalamic subcortical visual center - the lateral, or external, geniculate body (NCT). From here, the signals go to the primary projection area of \u200b\u200bthe visual cortex (styar cortex, or field 17 according to Brodman). The entire visual area of \u200b\u200bthe cortex includes several fields, each of which provides its own, specific functions, but receives signals from the entire retina and generally retains its topology, or retinotopy (signals from neighboring areas of the retina enter neighboring areas of the cortex).

Electrical activity of the centers of the visual system. Electrical phenomena in the retina and optic nerve. Under the action of light, electrical potentials are generated in the receptors, and then in the neurons of the retina, reflecting the parameters of the acting stimulus.

The total electrical response of the retina to light is called an electroretinogram (ERG). It can be recorded from the whole eye or directly from the retina. To do this, one electrode is placed on the surface of the cornea, and the other on the skin of the face near the eye or on the earlobe. On the electroretinogram, several characteristic waves are distinguished (Fig. 14.8). Wave a reflects the excitation of the inner segments of photoreceptors (late receptor potential) and horizontal cells. The b wave occurs as a result of the activation of glial (Müllerian) cells of the retina by potassium ions released during the excitation of bipolar and amacrine neurons. Wave c reflects the activation of pigment epithelial cells, and wave d - horizontal cells.

The ERG reflects well the intensity, color, size and duration of the action of the light stimulus. The amplitude of all ERG waves increases in proportion to the logarithm of the intensity of the light and the time during which the eye was in the dark. The d wave (reaction to switching off) is the larger, the longer the light has been acting. Since the ERG reflects the activity of almost all retinal cells (except ganglion cells), this indicator is widely used in the clinic of eye diseases to diagnose and control treatment for various retinal diseases.

Excitation of retinal ganglion cells leads to the fact that impulses rush along their axons (fibers of the optic nerve) into the brain. The retinal ganglion cell is the first "classic" type neuron in the photoreceptor-brain chain. Three main types of ganglion cells are described: responding to switching on (on-reaction), to switching off (off-reaction) of light, and to both (on-off-reaction) (Fig. 14.9).

The diameter of the receptive fields of ganglion cells in the center of the retina is much smaller than on the periphery. These receptive fields are circular and concentrically constructed: a circular excitatory center and an annular inhibitory peripheral zone, or vice versa. With an increase in the size of the light spot flashing in the center of the receptive field, the response of the ganglion cell increases (spatial summation). Simultaneous excitation of closely spaced ganglion cells leads to their mutual inhibition: the responses of each cell are made less than with a single stimulation. This effect is based on lateral, or lateral, inhibition. The receptive fields of neighboring ganglion cells partially overlap, so that the same receptors can participate in the generation of responses from several neurons. Due to the round shape, the receptive fields of the retinal ganglion cells produce the so-called pointwise description of the retinal image: it is displayed in a very thin mosaic of excited neurons.

Electrical phenomena in the subcortical visual center and visual cortex. The picture of excitation in the neural layers of the subcortical visual center - the external or lateral geniculate body (NCT), where the optic nerve fibers come, is in many ways similar to that observed in the retina. The receptive fields of these neurons are also round, but smaller than in the retina. The neuronal responses generated in response to a flash of light are shorter here than in the retina. At the level of the lateral geniculate bodies, the afferent signals from the retina interact with the efferent signals from the visual cortex, as well as through the reticular formation from the auditory and other sensory systems. These interactions provide the allocation of the most essential components of the sensory signal and the processes of selective visual attention.

Pulse discharges of neurons of the lateral geniculate body along their axons enter the occipital part of the cerebral hemispheres, where the primary projection area of \u200b\u200bthe visual cortex (striatal cortex, or field 17) is located. Here, much more specialized and complex processing of information takes place than in the retina and in the lateral geniculate bodies. The neurons of the visual cortex have not round, but elongated (horizontally, vertically, or in one of the oblique directions) receptive fields of small size. Due to this, they are able to select from the whole image separate fragments of lines with one or another orientation and location (orientation detectors) and selectively react to them.

In each small area of \u200b\u200bthe visual cortex along its depth, neurons are concentrated with the same orientation and localization of the receptive fields in the visual field. They form a column of neurons that runs vertically through all layers of the cortex. The column is an example of a functional association of cortical neurons that perform a similar function. As the results of recent studies show, the functional unification of distant from each other neurons of the visual cortex can also occur due to the synchronization of their discharges. Many neurons in the visual cortex selectively respond to certain directions of movement (directional detectors) or to some color, and some neurons respond best to the relative distance of the object from the eyes. Information about different signs of visual objects (shape, color, movement) is processed in parallel in different parts of the visual cortex.

To assess the transmission of signals at different levels of the visual system, the registration of total evoked potentials (EP) is often used, which in animals can be simultaneously withdrawn from all departments, and in humans - from the visual cortex using electrodes applied to the scalp (Fig.14.10).

Comparison of the retinal response (ERG) evoked by a light flash and the EP of the cerebral cortex allows us to establish the localization of the pathological process in the human visual system.

Visual functions. Light sensitivity. Absolute vision sensitivity. For the appearance of a visual sensation, it is necessary that the light stimulus has a certain minimum (threshold) energy. The minimum number of light quanta necessary for the appearance of the sensation of light, under conditions of darkness adaptation, ranges from 8 to 47. It is calculated that one rod can be excited by only 1 light quantum. Thus, the sensitivity of the retinal receptors under the most favorable conditions of light perception is physically extreme. Single rods and cones of the retina differ insignificantly in light sensitivity, however, the number of photoreceptors sending signals to one ganglion cell in the center and at the periphery of the retina is different. The number of cones in the receptive field in the center of the retina is about 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. In the transition from darkness to light, temporary blindness occurs, and then the sensitivity of the eye gradually decreases. This adaptation of the visual sensory system to bright light conditions is called light adaptation. The opposite phenomenon (dark adaptation) is observed when moving from a bright room to an almost unlit one. At first, a person sees almost nothing due to the reduced excitability of photoreceptors and visual neurons. Gradually, the contours of objects begin to emerge, and then their details also differ, since the sensitivity of photoreceptors and visual neurons in the dark gradually increases.

The increase in light sensitivity during stay in the dark is uneven: 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. Pigments of cones in the dark are restored faster than rhodopsin of rods, therefore, in the first minutes of stay in the dark, adaptation is due to processes in the cones. This first period of adaptation does not lead to large changes in the sensitivity of the eye, since the absolute sensitivity the apparatus is small.

The next period of adaptation is due to the restoration of rod rhodopsin. This period ends only at the end of the first hour in the dark. The restoration of rhodopsin is accompanied by a sharp (100,000-200,000 times) increase in the sensitivity of the rods to light. Due to the maximum sensitivity in the dark of only the rods, a dimly lit object is visible only with peripheral vision.

In addition to visual pigments, a significant role in adaptation is played by the change (switching) of connections between the elements of the retina. In the dark, the area of \u200b\u200bthe excitatory center of the receptive field of the ganglion cell increases due to the weakening or removal of horizontal inhibition. This increases the convergence of photoreceptors on bipolar neurons and bipolar neurons on the ganglion cell. As a result, due to spatial summation at the periphery of the retina, light sensitivity in the dark increases. The light sensitivity of the eye also depends on the influences of the central nervous system. Irritation of some areas of the reticular formation of the brain stem increases the frequency of impulses in the fibers of the optic nerve. The influence of the central nervous system on the adaptation of the retina to light is also manifested in the fact that the illumination of one eye lowers the light sensitivity of the unlit eye. Sensitivity to light is also influenced by sound, olfactory and taste signals.

Differential visual sensitivity. If additional illumination (dI) is applied to the illuminated surface, the brightness of which is I, then, according to Weber's law, a person will notice the difference in illumination only if dI / I \u003d K, where K is a constant equal to 0.01-0.015. The dI / I value is called the differential light sensitivity threshold. The dI / I ratio is constant at different illumination levels and means that for perception of the difference in illumination of two surfaces, one of them must be brighter than the other by 1-1.5%.

Brightness contrast. Mutual lateral inhibition of visual neurons underlies the general, or global, brightness contrast. So, a gray strip of paper lying on a light background seems darker than the same strip lying on a dark background. The reason is that the light background excites many retinal neurons, and their excitation inhibits the cells activated by the stripe. Therefore, against a brightly lit background, the gray stripe appears darker than against a black background. Lateral inhibition acts most strongly between closely spaced neurons, producing local contrast. There is an apparent increase in the brightness difference at the border of surfaces of different illumination. This effect is also called outlining: at the border of the bright field and the dark surface, you can see 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 an unpleasant sensation of dazzle. The upper limit of blinding brightness depends on the adaptation of the eye: the longer the dark adaptation has been, the lower the brightness of the light causes blinding. If very bright (blinding) objects fall into the field of view, they impair the discrimination of signals in a large part of the retina (on a night road, drivers are blinded by the headlights of oncoming cars). For fine visual work (prolonged reading, assembling small parts, the work of a surgeon), you should use only diffused light that does not dazzle the eyes.

Inertia of vision, fusion of flashes and sequential images. The visual sensation does not appear instantly. Before sensation arises, multiple transformations and signaling must occur in the visual system. The time of "inertia of vision" required for the appearance of visual sensation is on average 0.03-0.1 s. This sensation also does not disappear immediately after the irritation has stopped - it lasts for some time. If we drive through the air with any bright point (for example, a burning match) in the dark, then we will see not a moving point, but a luminous line. Light stimuli that quickly follow one after another merge into one continuous sensation.

The minimum repetition rate of light stimuli (for example, flashes of light), at which the fusion of individual sensations occurs, is called the critical flicker fusion frequency. Cinema and television are based on this property of vision: we do not see gaps between individual frames ("/ 24 s in cinema), since the visual sensation from one frame still lasts until another appears. This provides the illusion of the continuity of the image and its movement.

The sensations that continue after the irritation has stopped are called sequential images. If you look at the switched on lamp and close your eyes, then it is 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, that is, its light parts are dark, and the dark parts are light (negative sequential image). The reason for this is that excitation from an illuminated object locally inhibits (adapts) certain parts of the retina; if, after that, the gaze is directed to a uniformly illuminated screen, then its light will more strongly excite those areas that were not previously excited.

Color vision. The entire spectrum of electromagnetic radiation we see is enclosed between short-wave (wavelength from 400 nm) radiation, which we call violet, and long-wave radiation (wavelength up to 700 nm), called red. The rest of the colors of the visible spectrum (blue, green, yellow, orange) have intermediate wavelength values. Mixing rays of all colors gives white. 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 colors can be obtained.

Theories of color perception. The three-component theory (G. Helmholtz) enjoys the greatest recognition, according to which color perception is provided by three types of cones with different color sensitivity. Some of them are sensitive to red, others to green, and still others to blue. Each color has an effect on all three color sensing elements, but to a different extent. This theory has been directly confirmed in experiments where the absorption of radiation with different wavelengths in single cones of the human retina was measured with a microspectrophotometer.

According to another theory, proposed by E. Goering, there are substances in the cones that are sensitive to white-black, red-green and yellow-blue radiation. In experiments where the pulses of the ganglion cells of the retina of animals were withdrawn with a microelectrode when illuminated with monochromatic light, it was found that the discharges of most neurons (dominators) arise under the action of any color. In other ganglion cells (modulators), impulses occur when illuminated with only one color. Seven types of modulators have been identified that respond optimally to light with different wavelengths (from 400 to 600 nm).

Many so-called color-opposing neurons are found in the retina and visual centers. The action on the eye of radiation in some part of the spectrum excites them, and in other parts of the spectrum it slows down. It is believed that such neurons most efficiently encode color information.

Consistent color imagery. If you look at a painted object for a long time, and then turn your gaze to white paper, then the same object is seen painted in a complementary color. The reason for this phenomenon is color adaptation, i.e., a decrease in sensitivity to this color. Therefore, from the white light, as it were, is subtracted the one that acted on the eye before, and a feeling of additional color arises.

Color blindness. 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 called color blindness). Color blindness occurs in 8% of men and much less often in women: its occurrence is associated with the absence of certain genes in the sexually unpaired X chromosome in men. To diagnose color blindness, which is important in professional selection, polychromatic tables are used. People suffering from this disease cannot be full-fledged transport drivers, since they cannot 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 a lack of perception of one of the three primary colors.

People suffering from protanopia ("red-blind") do not perceive red, the blue-blue rays seem colorless to them. People with deuteranopia ("green-blind") do not distinguish green from dark red and blue. With tritanopia, a rare anomaly of color vision, blue and violet rays are not perceived.

All of these types of partial color blindness are well explained by the three-component theory of color perception. Each type of this blindness is the result of the absence of one of the three cone color-sensing substances. There is also complete color blindness - achromasia, in which, as a result of damage to the cone apparatus of the retina, a person sees all objects only in different shades gray.

Perception of space. Visual acuity. Visual acuity is the maximum ability of the eye to distinguish between individual details of objects.

Visual acuity is determined by the smallest distance between two points that the eye distinguishes, that is, it sees separately, and not together. The normal eye distinguishes between two points, visible at an angle of 1 ". The maximum visual acuity has a macula. To the periphery of it, visual acuity is much lower (Fig. 14.11). Visual acuity is measured using special tables, which consist of several rows of letters or open The visual acuity, determined according to the table, is usually expressed in relative values, and the normal acuity is taken as 1. There are people with hyperacuity of vision (visus more than 2).

Line of sight. If you fix a small object with your gaze, then 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 1.5-2 °. Objects, images of which fall on the rest of the retina, are perceived by peripheral vision. The space seen by the eye when fixing the gaze at one point is called the field of view. The boundary of the field of view is measured with the perimeter. The boundaries of the field of view for colorless objects are 70 ° downwards, 60 ° upwards, 60 ° inwards and 90 ° outwards. The fields of vision of both eyes in humans partially coincide, which is of great importance for the perception of the depth of space. Fields of view for different colors are not the same and are smaller than for black and white objects.

Distance estimation. The perception of the depth of space and the estimation of the distance to the 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 in monocular vision. For the assessment of distance, it is also important that the image of an object on the retina is the larger, the closer it is. The role of eye movement for vision. Eyes move when looking at any objects. Eye movements are carried out by 6 muscles attached to the eyeball somewhat anterior to its equator. These are 2 oblique and 4 rectus muscles - external, internal, upper and lower. The movement of both eyes occurs simultaneously and in a friendly manner. Considering close objects, it is necessary to reduce (convergence), and when considering distant objects, separate the visual axes of two eyes (divergence). The important role of eye movements for vision is also determined by the fact that image movement on the retina is necessary for the brain to continuously receive visual information. As already mentioned, impulses in the optic nerve occur when the light image is turned on and off. With the continued action of light on the same photoreceptors, impulses in the fibers of the optic nerve quickly cease and the visual sensation with motionless eyes and objects disappears after 1 - 2 s. To prevent this from happening, the eye, when examining any object, makes continuous jumps (saccades) that are not felt by a person. As a result of each jump, the image on the retina shifts from some photoreceptors to new ones, again causing the ganglion cells to flicker. The duration of each jump is equal to hundredths of a second, and its amplitude does not exceed 20 °. The more complex the object under consideration, the more complex the trajectory of eye movement. They seem to trace the contours of the image, lingering on its most informative areas (for example, in the face - these are the eyes). In addition, the eye constantly shakes and drifts (slowly moves from the point of fixation of the gaze), which is also important for visual perception.

Binocular vision. 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. Images of all objects fall on the so-called corresponding, or corresponding, areas of two retinas, and in human perception, these two images merge into one. Press lightly on one eye from the side: immediately begins to double in the eyes, because the retinas are out of alignment. If you look at a close object, converging the eyes, then the image of some more distant point falls on non-identical (disparate) points of two retinas. Disparity plays a large role in assessing distance and, therefore, in seeing the depth of the relief. A person is able to notice a change in depth, creating a shift in the image on the retinas by several arc seconds. Binocular fusion or fusion of signals from two retinas into a single neural image occurs in the primary visual cortex.

Estimation of the size of the object. The size of the object is estimated as a function of the size of the image on the retina and the distance of the object from the eye. In the case when the distance to an unfamiliar object is difficult to estimate, gross errors in determining its value are possible.

Under the action of light in the receptors, and then in the retinal neurons [?], Electrical potentials are generated, reflecting the parameters of the acting stimulus. The total electrical response of the retina to light is called an electroretinogram (ERG). It can be recorded from the whole eye or directly from the retina. To do this, one electrode is placed on the surface of the cornea, and the other on the skin of the face near the eye or on the earlobe. On the electroretinogram, several characteristic waves are distinguished (Fig. 13.4).

Figure: 13.4. Electroretinogram (according to Gravit).

a, b, c, d - ERG waves; the arrows indicate the moments of switching on and off the flash of light.

Wave a reflects the excitation of the inner segments of photoreceptors (late receptor potential) and horizontal cells. Wave b arises as a result of the activation of glial (Müllerian) cells of the retina with potassium ions released when bipolar and amacrine neurons are excited. Wave from reflects the activation of cells of the pigment epithelium, and the wave d - horizontal cells.

The ERG reflects well the intensity, color, size and duration of the action of the light stimulus. The amplitude of all ERG waves increases in proportion to the logarithm of the intensity of the light and the time during which the eye was in the dark. Wave d (reaction to switching off) the more, the longer the light was on. Since the ERG reflects the activity of almost all retinal cells (except ganglion cells), this indicator is widely used in the clinic of eye diseases to diagnose and control treatment for various retinal diseases.

Excitation of retinal ganglion cells leads to the fact that impulses rush to the brain along their axons (fibers of the optic nerve). The retinal ganglion cell is the first "classic" type neuron in the photoreceptor-brain chain. Three main types of ganglion cells are described: responding to the on (on-reaction) and off (off-reaction), as well as to both (on-off-reaction) (Fig. 13.5). [!]

Rns. 13.5. [!] Impulse of two retinal ganglion cells and their concentric receptive fields. The inhibitory zones of the receptive fields are shaded. The reactions to the switching on and off of the light are shown when the center of the receptive field and its periphery are stimulated by the light spot.

The diameter of the receptive fields of ganglion cells in the center of the retina is much smaller than on the periphery. These receptive fields are circular and concentrically constructed: a circular excitatory center and an annular inhibitory peripheral zone, or vice versa. With an increase in the size of a light spot flashing in the center of the receptive field, the response of the ganglion cell increases (spatial summation).

Simultaneous excitation of closely spaced ganglion cells leads to their mutual inhibition: the responses of each cell are made less than with a single stimulation. This effect is based on lateral, or lateral, inhibition. Due to the circular shape, the receptive fields of the retinal ganglion cells produce the so-called pointwise description of the retinal image: it is displayed in a very thin mosaic consisting of excited neurons.