Characteristics of color vision
The ability to distinguish colors is a feature of the human eye. The visual apparatus is capable of perceiving electromagnetic waves of different lengths. The main components of the color spectrum are:
- red,
- violet,
- orange,
- blue,
- yellow,
- blue,
- green.
There are only three main colors: red, green and blue; when mixed, different tones are obtained. Color perception exists due to the fact that there are three significant receptors in the retina that perceive the main tones, while being irritated by the other two, and this is how the colors are mixed.
Tones are divided into chromatic and achromatic.
Distinctive features of the first category are:
- Color tone;
- brightness;
- saturation.
The second group is distinguished exclusively by brightness (white and black).
Test with answers: “Visual analyzer”
I option.
1. What is the perceiving element of any analyzer? a) Pathways b) Cerebral cortex c) Receptors+
2. Where does the analysis of external stimuli take place? a) in pathways b) in the cerebral cortex + c) in receptors
3. What protects the eyes from dust: a) Eyebrows and eyelashes + b) Eyelids + c) Lacrimal glands
4. What is the name of the outer layer of the eyeball? a) Vascular b) Fibrous (protein)+ c) Reticular
5.What does the pupil do in low light? a) narrows b) expands+ c) does not change d) sometimes expands, sometimes narrows
6. What is the name of the pigmented part of the choroid? a) Cornea b) Retina c) Iris+
7.Where is the image of visible objects formed? a) Cornea b) Iris c) Retina+
8. What provides the ability to expand and contract, allowing the required amount of light to pass through? a) Lens b) Pupil+ c) Vitreous body
9. What makes color vision work? a) Rods b) Cones+ c) Iris cells
10.Where is the maximum number of receptor cells located on the retina? a) Sclera b) Blind spot c) Macula+
11. What is the place where the optic nerve exits that does not perceive light rays called: a) white spot b) yellow spot c) dark area d) blind spot+
12. What is the name of the transparent part of the outer shell of the eye? a) retina b) cornea + c) iris
13. What does the lens do? a) participates in the nutrition of the eye b) perceives light c) refracts light rays + d) protects the eye
14. Photosensitive receptors - rods and cones are located in: a) the white membrane of the eye b) the choroid of the eye c) the vitreous body and the lens of the eye d) the retina of the eye +
15. A blurry image of nearby objects is a sign of: a) Myopia b) Farsightedness+ c) Cataracts
Option II.
1. Select where the analysis of external stimuli takes place? a) in pathways b) in the cerebral cortex + c) in receptors
2. Mark which organ protects the eyes from dust? a) Eyebrows and eyelashes + b) Eyelids + c) Lacrimal glands
3. Choose what is the name of the outer shell of the eyeball? a) Vascular b) Fibrous (protein)+ c) Reticular
4. Note what provides the ability to expand and contract, allowing the required amount of light to pass through? a) Lens b) Pupil+ c) Vitreous body
5. Color vision works because there are: a) Rods b) Cones+ c) Iris cells
6. Remember where the light-sensitive receptors are located - rods and cones? a) In the white membrane of the eye b) In the choroid of the eye c) In the vitreous body and lens of the eye d) In the retina of the eye +
7. Select what does the pupil do in low light? a) narrows b) expands+ c) does not change d) sometimes expands, sometimes narrows
8. What is the name of the pigmented part of the choroid? a) Cornea b) Retina c) Iris+
9. Choose the name of the place where the optic nerve exits that does not perceive light rays? a) white spot b) yellow spot c) dark area d) blind spot+
10. The transparent part of the outer shell of the eye is: a) retina b) cornea + c) iris
11. Select what the lens contributes to? a) participates in the nutrition of the eye b) perceives light c) refracts light rays + d) protects the eye
12. Select where the image of visible objects is formed? a) In the Cornea b) In the Iris c) In the Retina+
13. Note what is the perceiving element of any analyzer? a) Pathways b) Cerebral cortex c) Receptors+
14. A blurry image of nearby objects is a sign of what? a) Myopia b) Farsightedness+ c) Cataracts
15. Indicate where the maximum number of receptor cells is located on the retina? a) Sclera b) Blind spot c) Macula+
Color perception
The human eye is a complex and at the same time the most advanced visual system among all mammals. Distinguishes more than 150 thousand colors and shades. Perception is carried out through photoreceptors. Photoreceptors contain iodopsin, which is responsible for sensitivity to tones of the visual apparatus. A person with full vision has 6-7 million cones in the eyeball. If their number is smaller or pathologies are observed in their composition, then color vision disturbances occur.
It has been proven that vision differs greatly between women and men. Women distinguish more tones and shades, while men are better at recognizing moving objects and are able to focus their gaze on a specific object for a longer period of time.
The sense of smell and color vision developed in antiphase in the evolution of mammals
Color vision test. A person with normal trichromatic vision should see the number 6 here. A person who has difficulty distinguishing shades of red and green sees the number 5. For example, I see five well, but have difficulty seeing six. This depends on how different the two opsin genes located on the X chromosome are from each other. There are many more colorblind people among men because they have only one X chromosome (women have two). Among monkeys there are almost no colorblind people, because natural selection eliminates those who are unable to distinguish a ripe fruit from an unripe one and a nutritious reddish leaf from an inedible green one. Rice. from www.uni-mannheim.de
Mammals lost color vision at the very beginning of their evolution, losing two of the four genes for light-sensitive proteins - opsins. Color vision subsequently reappeared in monkeys due to a duplication of one of the two remaining opsins. As it turned out, in parallel with the loss of color vision genes in ancient mammals, as a result of numerous duplications, the number of olfactory receptor genes sharply increased. Apparently, both processes - the weakening of color vision and the development of smell - were associated with the fact that the expansion of dinosaurs at the end of the Triassic - beginning of the Jurassic period forced ancient mammals to switch to a nocturnal lifestyle.
The evolution of color vision in vertebrates has been studied in sufficient detail. The ability to distinguish colors is determined by light-sensitive cone proteins called opsins, which can be “tuned” to different wavelengths. Depending on which amino acids are in certain “key” positions in the opsin molecule, the protein selectively reacts to light waves of one or another length. Even before reaching land, vertebrates had developed a very advanced color vision system based on four opsins (tetrachromatic vision). This system is preserved in many terrestrial vertebrates, including birds, which are excellent at distinguishing colors. Perhaps if humans had such vision, we would find the trichromatic color display system used in our televisions and computer monitors to be poor. Humans, like all Old World monkeys, have trichromatic vision. In most other mammals, of the four opsins present in ancient vertebrates, only two were retained (dichromatic vision). The ancestors of monkeys also had dichromatic vision (which means they could not distinguish red from green).
It is believed that the loss of two opsins by mammals was due to the fact that their ancestors once switched to a nocturnal lifestyle. Most likely, this happened at the dawn of their history - at the end of the Triassic or at the beginning of the Jurassic - and was associated with the vicissitudes of a long competitive struggle between the two main evolutionary trunks of terrestrial vertebrates - synapsids and diapsids. During the Permian period, synapsid reptiles—the ancestors of mammals—were the dominant group. In the next, Triassic period, their dominance was shaken, as young active competitors appeared on the scene - archosaurs belonging to the group of diapsid reptiles. Archosaurs relied on their large size, fast running and sharp teeth, while synapsid reptiles, meanwhile, were rapidly undergoing “mammalization”—the development of mammalian features. They gradually became smaller and went “into the shadows.”
At the end of the Triassic - beginning of the Jurassic, power on land finally passed to one of the groups of archosaurs, namely dinosaurs. Synapsids became almost extinct, with the exception of one small group that gave rise to mammals. Throughout the Jurassic and Cretaceous periods, until the extinction of the dinosaurs, mammals had to lead a predominantly nocturnal lifestyle, and also remain small in order to be less likely to be seen by the dominant daytime predators. Under these circumstances, color vision became useless and two opsin genes were lost. Natural selection cannot look into the future - it preserves only those traits and genes that are needed here and now. When, after the extinction of the dinosaurs, many mammals became diurnal again, they had to make do with dichromatic vision, since there was nowhere to get new opsin genes to replace the lost ones.
Until recently, scientists assumed that both opsin genes were lost almost immediately and a very long time ago, even before the division of mammals into monotremes and therians (= marsupials + placentals). However, one of the lost genes was found in the platypus genome. This means that the genes were not lost all at once, but one by one, and not so quickly. The common ancestor of all modern mammals still had three opsins, but the common ancestor of therians had only two. Some Australian marsupials appear to have full color vision, but neither of the two missing genes could be found in their genomes, despite targeted searches. This means that if they really have color vision, they acquired it secondarily and on a different genetic basis.
How this can happen in principle is perfectly illustrated by the example of monkeys. In the common ancestor of Old World monkeys, who lived 30–40 million years ago, one of the two surviving opsin genes underwent duplication, and natural selection quickly “tuned” the resulting copies to different wavelengths. To do this, it was necessary to fix only three mutations (amino acid substitutions). As a result, the monkeys' vision became trichromatic, which gave them the ability to distinguish ripe fruits from green ones and fresh foliage (the most nutritious) from old ones (in many tropical plants, young leaves have a reddish tint). In parallel and completely independently, color vision developed in some New World monkeys. Their missing third opsin arose as an allelic variant of one of two old opsin genes. This opsin gene is located on the X chromosome, so only females have a chance to receive three different opsins (and trichromatic vision) from their parents, and not all of them. But monkeys lead a social lifestyle, and the presence in the herd of at least several females who can distinguish red from green turns out to be very useful for the entire team.
Takushi Kishida from Kyoto University in his article published in the journal PLoS ONE
, showed that the genes for the olfactory receptors of terrestrial vertebrates have no less fascinating evolutionary history than the genes for visual proteins. As it turned out, a decrease in the number of opsins was accompanied by an increase in the number of olfactory receptors, and vice versa.
Mammals differ from other terrestrial vertebrates not only by problems with color vision, but also by a much more developed sense of smell. For example, a rat has up to 1,600 functioning olfactory receptor genes, while a chicken has only about 80. Unlike the colors of the spectrum, three or four genes are not enough to distinguish between numerous odors: each volatile molecule needs its own receptor. Researchers have long established that numerous mammalian olfactory receptor genes arose through multiple duplications from an original small set. It was natural to assume that the development of smell in mammals, like the loss of color vision, was associated with the transition to a nocturnal lifestyle. In this case, most of the olfactory gene duplications would have occurred around the same time as the loss of the two opsin genes.
To test this hypothesis, Kishida conducted a thorough comparative analysis of olfactory receptor genes in the genomes of six land vertebrates: frog, chicken, platypus, opossum, dog and mouse. Analysis of these six species allows us to reconstruct the situation at the most important branch points in the evolutionary tree of terrestrial vertebrates. Comparison of the frog with other species sheds light on the common ancestor of modern amphibians and amniotes (= reptiles + birds + mammals), who lived about 340 million years ago (the beginning of the Carboniferous period). The chicken, as a direct descendant of archosaurs, helps to form an idea of the common ancestor of synapsid and diapsid tetrapods, who lived about 310 million years ago (the second half of the Carboniferous period). The platypus will tell about the common ancestor of monotremes and therians (180 million years ago, Early Jurassic), the opossum - about the common ancestor of marsupials and placentals (140 million years ago, Early Cretaceous). The evolutionary paths of the ancestors of dogs and mice diverged about 85 million years ago (Late Cretaceous). As for you and me, we are closest to the mouse on this simplified evolutionary tree.
Kishida's results suggest that the common ancestor of amphibians and amniotes had approximately 100–110 olfactory receptor genes. Initially, most of these genes were on one chromosome, but from time to time they jumped to other chromosomes. This process of “dispersal” of olfactory genes across chromosomes apparently took place already in the first terrestrial vertebrates and practically ceased by the time the monotreme and therian lineages diverged (180 million years ago). As a result, in all mammals, olfactory genes are present on almost all chromosomes. In humans, for example, they are not present on only two chromosomes: the 20th and Y chromosomes. Kishida suggests that the dispersal of olfactory genes across chromosomes facilitated their subsequent multiple duplications.
In the common ancestor of synapsids and diapsids, the number of olfactory genes remained the same (about a hundred). In the common ancestor of monotremes and therians there were already about 330 of them, in the common ancestor of placentals and marsupials their number increased to about 670. The common ancestor of mice and dogs had about 740 olfactory receptor genes.
The most important result obtained by Kishida is that almost all duplications of olfactory genes in the evolution of tetrapods were confined to a segment of the evolutionary tree concluded between the common ancestors of diapsids and synapsids (310 million years ago) and placentals and marsupials (140 million years ago). . In general, duplications, compared to dispersal, began later and ended later.
Taking into account the resolution of the applied methods, we can conclude that, within the limits of this resolution, the period of mass duplications of olfactory genes is exactly
coincides with the period of loss of opsin genes. The first opsin gene was lost during the period between the common ancestor of diapsids and synapsids and the common ancestor of monotremes and therians, that is, in the early stages of the formation of mammals. The second opsin gene was lost between the common ancestor of monotremes and therians and the common ancestor of marsupials and placentals, that is, early in the evolution of therians. Duplications of olfactory genes are also confined to these two segments. If there were any modern animal whose ancestors would have separated from “our” evolutionary trunk later than the chicken, but earlier than the platypus, the dating could be significantly clarified. But, unfortunately, there is no such animal.
Kishida also notes that the restoration of color vision in Old World apes was accompanied by the loss of a significant part of the olfactory genes (or their transformation into non-functioning pseudogenes). Obviously, the development of vision and smell occurred in antiphase. When ancient mammals switched to a nocturnal lifestyle, the role of vision decreased and the role of smell increased. When the monkeys returned to daily life and began to rely heavily on their senses again, their sense of smell weakened.
Another interesting detail is that one of the families of olfactory genes (known as family No. 7; in primates this is the largest family of olfactory genes) underwent increased duplication after
separation of placentals and marsupials. Some receptors in this family are known to respond to sex pheromones.
PS
Readers may have a reasonable question: is it really enough to add a new receptor to improve vision or smell? Where will the new brain structures that are supposed to process the signals from this new receptor come from? It seems that this does not require new brain structures - the existing structures use some generalized, universal algorithms for interpreting incoming signals. Apparently, in the process of individual development, the brain automatically learns to distinguish between signals coming from different receptors and interpret them precisely as different signals. Without changing anything in the structure of the brain, you can add a new opsin to the retina, and the brain itself will figure out what to do with the new type of signals. See about this: The mouse brain is ready to see the world in a human way, “Elements”, 03.29.2007, The brain is in charge of color perception, “Elements”, 10.28.2005.
Source:
Takushi Kishida.
Pattern of the Divergence of Olfactory Receptor Genes during Tetrapod Evolution // PLoS ONE
. 2008. V. 3. P. e2385.
For the evolutionary role of gene duplication, see also:
The evolutionary history of one of the human genes is traced, “Elements”, 06/17/2008.
Alexander Markov
Diagnosis of disorders
Color vision disorders are both acquired and congenital. Congenital abnormalities are more common in men. In women, such deviations are much less common.
Acquired pathologies are observed when problems arise with:
- retina;
- central nervous system;
- optic nerve.
A person who fully perceives three main tones is a trichromat. A dichromat distinguishes two out of three tones, and people who distinguish only one color are called monochromats.
Color discrimination ability is determined by:
- polychromatic tables
- anomaloscopes - special devices used in ophthalmology;
Other diagnostic methods are also used.
Does visual acuity depend on the color of the iris?
Sharpness is determined by the quality of the optical system; it depends on the size of the cones in the center of the fundus. The smaller the size, the higher the resolution of visual perception.
Experts have confirmed that people with different eye colors do not have the same predisposition to certain diseases. They found that the shade of the iris does not affect the eye's ability to distinguish objects. This is just a feature of a person’s appearance. People have different colors due to different amounts of melanin.
Eye color and night vision
Numerous studies have shown that blue-eyed people see better at dusk and at night than brown-eyed/black-eyed people. The theory is confirmed by a boy from China. Nong Yusui has a very light color, which is not typical for the population of the country.
Experts have found that the child sees perfectly even in complete darkness. He reads books, writes and draws indoors with the lights off. The cause is leucoderma. This is a congenital disease characterized by a small amount of melanin in the eyes. Therefore, Nong Yusui can see well even at night.
Based on this, we came to the conclusion that if a person with light eyes, then in the dark he sees better than brown-eyed/black-eyed people, since he lets in more light.
Primary data processing
The information received by the retina undergoes primary processing by photoreceptors. Specialized cells analyze the image and encode it into electromagnetic pulses. The processed data is sent to the brain along nerve fibers.
Rods and cones of the human retina
Cones are nerve cells located on the retina of the eye that are responsible for color vision. There are three types of photoreceptors. Some react to red, others to green, and others to blue. Only their combination allows a person to notice other colors and their shades.
Color discrimination by cones is carried out thanks to the pigment iodopsin. Each type of photoreceptor has a certain modification: chlorolab and erythrolab (they are isolated from the retina and have been well studied), as well as the theoretically existing cyanolab, which has not yet been found.
Although cones are less sensitive to light, they respond better to movement and moving objects.
Color blindness is a visual impairment that results in the inability to distinguish one or more colors. The reason for the deviation is the lack of corresponding pigments in the cones. In most cases, color blindness is a hereditary disorder (passed from a carrier mother to her son), and only in some cases is it an acquired disease. It affects about 10% of men and less than 0.5% of women.
Rods are nerve cells, thanks to which a person is able to see in the dark and adapt to it (the adaptation process takes up to 20 minutes). They do not respond to color and display a black and white image. In the daytime, due to the rhodopsin pigment present in them, they help the cones to record the blue color and its shades.
Nyctalopia or night blindness is an ophthalmological disease characterized by deterioration of twilight vision. This pathology can be either congenital or acquired. The second form is caused by a deficiency of vitamin A, thiamine and niacin. With nyctalopia, the process of rhodopsin restoration is disrupted. As a result: disruption of the formation of corresponding impulses by the rods, which are transmitted to the brain.
