As a sub-area of vision, color perception is the ability to perceive differences in the spectral composition of light . It is based on the fact that the eye has different types of receptors , each of which is sensitive to certain areas of the light spectrum. The excitation patterns of these receptor cells form the basis for the complex processing in the retina and brain, which ultimately leads to color perception .
Different spectral compositions of the color stimulus can lead to the same color perception. The composition of the color stimulus cannot therefore be inferred from the perceived color alone . Only stimuli from monochromatic light of a certain wavelength can also be characterized by the perceived color, as its spectral color .
History of exploration
- 1672: Isaac Newton discovers that light is composed of different color components and describes the phenomenon of metamerism (light composed differently can produce the same color impression).
- 1794: John Dalton reports on his color blindness. He saw something red for others as a shadow, and he perceived orange, yellow and green only as different shades of yellow. The red-green blindness is therefore also referred to as " Daltonism ". This is not to be confused with red-green poor eyesight, where red, green, orange, and yellow are distinguishable, but not all of their shades.
- 1801: Thomas Young suspects that the possibility of composing all colors from three primary colors is based on physiological processes in the retina , and suggests three types of receptors that match the primary colors. This model was developed into a three-color theory by Hermann von Helmholtz around 1850 .
- 1855: James Clerk Maxwell identifies two types of "Daltonism" and explains them using his three-receptor theory .
- 1874: The physiologist Ewald Hering publishes his four-color theory as a counter thesis. At the end of the 19th century , von Kries succeeded in resolving the contradiction and advancing the view with the Kries zone theory .
Color stimulus, color valence and color sensation
|Site of action
|Type of action
|Area of Expertise
|Eye (entrance pupil)
|Development of colors / optics
Pin and Stäb-
surfaces of the retina
of the retinal receptors
|Eye and visual cortex
|Physiology / Psychology
- The color stimulus is the spectral distribution of the radiation power to which the cones of the retina of the eye are sensitive. This adequate stimulus is the physical cause of color valence and color sensation. The color stimulus itself results from the spectral (and direction-dependent) reflection characteristics of the observed object, modified by the spectral composition of the incident light.
- The color valence is the spectrally specific physiological effect of radiation. It is characterized by the respective states of excitation of the three types of cones in the human eye, which depend on the (physical) color stimulus.
- The color sensation arises from the interaction of the incoming “average” overall brightness and the matching color constancy of the brain . The receptor- related trichromatic reaction - as a direct stimulus response of the three types of cones - does not reach consciousness. The parameter pairs black / white (light value) as well as red / green, blue / yellow (two contrasting colored pairs) are formed along the signal processing stations from the sensory cells to the cerebrum.
- A spectral color is the perception of color that is triggered by monochromatic radiation. The colors produced by the refraction of light on a prism are spectral colors.
Humans can only perceive electromagnetic radiation visually in the area in which the solar spectrum contains the predominant part of its energy. The perceptible range begins at approx. 380 nm and ends at approx. 780 nm. Since the term “light” is occasionally used in physics for other sections of the electromagnetic spectrum, the term “ visible light” has become established for the visible spectral range . It includes the colors from blue-purple to green and yellow to dark red. You can e.g. B. be made visible through a prism . Some animals can see beyond what is visible to humans, such as bees, who can also see the nearby UV light.
As a rule, light is composed of light waves of different wavelengths. When all wavelengths are represented in the same way as it is approximately true for sunlight, one speaks of "white light". On the other hand, light that only consists of radiation of a certain wavelength is called monochromatic . Light sources that emit monochromatic light practically do not occur in nature. The rainbow is not considered a source of light because the water droplets reflect sunlight, whereby the refraction of light also creates monochromatic colors. Technically, however, monochromatic light sources can be realized, e.g. B. with sodium vapor lamps or lasers . However, humans cannot distinguish with their eyes whether light of a certain color is monochromatic light or a (equivalent) “mixture” of light from different spectral ranges. It is therefore important to conceptually separate the physical properties of light from the perception of color.
The perception system must have at least two different types of “light receptors” in order to be able to determine different compositions of light. With only one type of receptor, distinctions according to wavelength are not possible, which W. Rushton in 1970 pointed out as the principle of univariance .
Humans have two different systems of visual receptors. The chopsticks are more sensitive, but there is only one type. Consequently, no colors can be distinguished with these receptors alone. The second system consists of the cones , the receptors of color vision. In humans there are three types with different spectral sensitivity. You are responsible for daytime vision ( photopic vision ). Your stimulus response needs a luminance of at least approx. 3 cd / m². Below this threshold, only light-dark differences are perceptible through the rod receptors (scotopic or night vision ). Cones alone play a role in the perception of the central fovea . The extrafoveal color perception can also be influenced by the rods under certain twilight conditions, but in full daylight these are saturated due to the high light intensity and do not contribute to color perception.
The sensitivity of the sensory cells to light of different wavelength ranges is made possible by the molecules of specific visual pigments contained in their membrane . These consist of a protein component, called opsin , and the retinal molecule bound to it , which forms the light-sensitive component. The latter is covalently bound as a ligand to opsin . If a photon hits the retinal molecule with the appropriate energy, it changes its spatial structure, from the angled 11-cis to the elongated all-trans isomer of the molecule. This structural change is known as the primary photochemical reaction . It lasts about 2 · 10 −14 seconds and triggers several subordinate processes in the sensory cell that significantly amplify the signal and ultimately result in a change in its membrane potential ( receptor potential , here through hyperpolarization ), which then triggers a nervous signal chain.
The visual pigments of the rods are called rhodopsin and consist of scotopsin and retinal. The visual pigments of the three types of cones are called iodopsins and each consist of one of the three types of photopsin and retinal. The latter is kept correspondingly different by the three, slightly differently built, types of photopsins, whereby spectrally different reaction maxima are given in each case. This is the reason for the different spectral sensitivity of the three types of cones (S, M and L types).
In addition, photosensitive ganglion cells are also found in the retina in humans , which are sensitive to light due to the photopigment melanopsin and transmit their signals to neurons of the epiphysis and those of the suprachiasmatic nucleus in the hypothalamus , where they become decisive as timers for the circadian rhythm .
Sensory cells of the retina
Photons can therefore cause deformation of the visual pigment in the sensory cells of the outer layer of the retina , the photoreceptors , and trigger a receptor potential through the subsequent phototransduction processes . This in turn triggers a signal for a complex network of neurons that lies between the sensory cells and the retinal ganglion cells , which form the innermost layer of the retina. After massive cross-processing in this network, signals then go to the ganglion cells and from there are passed on to the brain via the neurites of the optic nerve . Here finally, after further processes, they contribute to the perception of the color impressions experienced.
For visual perception in humans, as already mentioned, two systems of photoreceptors must be distinguished:
- The rods are still active when the light intensity is less than 0.1 cd / cm² and are responsible for night vision .
- The three different types of cones register different color valences . Each type of cone has a specific spectral sensitivity range.
- S cones (S for short) are sensitive to shorter wavelengths (around 400–500 nm). The absorption maximum is around 420 nm, which corresponds to a violet-tinted blue. S-cones are also called blue cones and are only represented in humans with a share of twelve percent of all cones.
- M cones (M for medium) are sensitive to medium wavelengths (around 450–630 nm). The absorption maximum is around 530 nm, which corresponds to a turquoise-tinted green. M cones are also called green cones.
- L-cones (L for Long) are sensitive to longer wavelengths (around 500–700 nm). The absorption maximum is around 560 nm, which corresponds to a greenish yellow. L-cones are also called red cones.
The different absorption spectra come about because the three types of cones each contain a different sub-form of the visual pigment. Their protein component - the so-called opsin - is encoded by genes that are located on the 7th chromosome (in the case of S-opsin) and on the X chromosome (in the case of L- and M-opsin). Since a mutated L or M-opsin gene is responsible for the red-green weakness , their gonosomal inheritance means that the red-green weakness is much more common in men than in women.
The absorption curves of all three types of cones are relatively broad and overlap strongly. One type of cone alone does not provide sufficient information for color vision. This is only possible through the neuronal processing of the excitation patterns of at least two different types of cones.
The density of cones is greatest approximately in the center of the retina, at the point of sharpest vision ( fovea centralis ). The density decreases towards the outside, and at the edge of the field of vision there are hardly any cones left, but many rods. Again, there are no rods in the fovea centralis. For example, you can only see faintly shining stars at night if you “look past” them, and then without color.
Neural processing of color stimuli
The first steps of information processing already take place in the retina , which is embryologically a part of the brain created from the eye cup . One group of sensory cells (cones or rods) converges via interconnected nerve cells ( bipolar cells , horizontal cells , and amacrine cells ) onto one retinal ganglion cell , the third neuron. Such a group of sensory cells forms a receptive field , and one distinguishes between a center and its periphery. The photoreceptors in the center act in opposite directions to those in the periphery on the ganglion cell connected downstream. If the center is exciting and the periphery is inhibiting, one speaks of an on-center ganglion cell, in the opposite case of an off-center ganglion cell. This type of connection is used to enhance the contrast.
There are essentially three sub-systems.
- Diffuse bipolar cells transmit signals from both L and M cones to so-called parasol ganglion cells (also called M cells), whose axons move into the magnocellular layers of the lateral geniculate body (CGL). They show a broad spectral response. The information that they pass on is achromatic and presumably serves primarily to distinguish between light and dark.
- The so-called midget ganglion cells (also called P cells), on the other hand, receive signals (via midget bipolar cells) from just one L-cone or one M-cone in the center. The receptive fields of these cells are very small and react differently to long-wave and medium-wave light. The axons of the midget ganglion cells move into the parvocellular layers of the CGL. They mainly process the red / green contrast. In terms of evolutionary history, this is the youngest subsystem; it was not until the primates that the opsins of the L and M cones emerged from gene duplication.
- Blue bipolar cells converge on the bistratified ganglion cells and form an on center of S cones; diffuse bipolar cells conduct signals from L and M cones, which have an inhibitory effect (off). This can be used to highlight blue / yellow contrasts in particular. The axons of these retinal ganglion cells project onto the coniocellular (sub-) layers of the CGL.
In addition to the differentiation of the color qualities, further processing processes are known.
- The fast-working red-green system of the M and L cones, which were developed jointly in ancestral history, also serves to highlight edges in the image pattern. The difference between the L (red) and M (green) signals is compared with the sum of both. If, under laboratory conditions, both types of cones are stimulated with red and green light of the same strength (isoluminance), the ability to perceive sharp edges is greatly reduced ( minimally distinct border phenomenon).
- The blue-yellow system, which works less quickly, is also responsible for color constancy .
- The red cone signal alone is presumably used for motion detection, especially in slow processes.
Special features of color perception
Metameric color equality
Color stimuli are generated by combinations of different wavelengths of the electromagnetic spectrum. The same color stimulus can be generated by different combinations. This effect is called metamerism . Two color samples can therefore look completely identical (under the same lighting), although they absorb different spectral components of the light. If the color samples are illuminated with colored light - that is, with light in which spectral components are missing - the difference can become visible, provided that the missing spectral component in one color sample contributes more to its appearance than in the other. This is a problem in the production of things and objects from different materials, which should also look the same color under different lighting conditions.
Color constancy belongs to the group of so-called constancy phenomena of human perception, in addition to constancy of form and constancy of size . Color constancy is the property of the sense of sight to perceive the body color of objects as almost independent of changes in the color spectrum of natural lighting. Such changes happen at intervals of seasons, times of day, changes in cloud cover and shadows when changing location or view.
Color constancy has also been proven in fish and bees. The advantage of the ability becomes particularly clear here, as the color of the lighting can change quickly and intensively under water and in the area of flowers. The search for food is considerably simplified here, or only made possible at all if what you are looking for is always seen in almost the same colors.
In photography , effects of changes in natural or artificial lighting can be reproduced by taking pictures with artificial light films during the day or daylight films with artificial light. Such effects can be observed with a digital camera when the white balance changes .
Neurophysiology of color constancy
The elucidation of the mechanism of color constancy is a particular challenge within the neuroscience of the sense of sight. Since the ability is based on the fact that neural representations of widely spaced areas of the visual field influence one another, higher cortical processes must be involved. The visual cortex area V4 plays a proven key role for the mechanism of color constancy . But adaptation processes are also involved at deeper levels, for example within the retina. It is also known that there are inter-individual differences in the implementation of color constancy in humans. The perspectives of painters, for example more impressionistic like Claude Monet in his famous series of pictures with views of the cathedral of Rouen , make it clear that both attentional and learning processes play a role here.
UV perception in humans
The rhodopsin of human rods has two absorption maxima, one in the visible range at 500 nm ( turquoise ) and a secondary maximum in the UV range at 350 nm. Due to the absorption of UV light in the lens of the eye, a stimulus in the UV Area largely prevented. This represents a protective function for the retina, which can be damaged by the high-energy UV radiation. People who have had their lenses removed (e.g. because of cataracts ) can perceive UV light stimuli as bright, but without this as color to see.
Defects in color perception
Color ametropia occurs in different forms.
- Red-blind people without red receptors are called protanopes (gr. Protos , first; gr. An- , not; gr. Open look)
- Green-blind people as deuteranopes (Greek deuteros , second), they both show the phenomenon of dichromacy, so they only have two instead of three types of cones.
- Red weakness ( protanomaly ) and green weakness ( deuteranomaly ) are based on changed sensitivities of the corresponding receptors.
These ametropia occur when the opsin genes are changed. But lens discolouration (yellowing) can also affect color perception. There are different ways of detecting defects, such as the Ishihara color chart or the Farnsworth test . Whether someone is suitable for a certain profession (pilot) can also be determined with other special tests ( Beyne lantern test ).
Color perception in the animal kingdom
Color vision differs considerably between animal species. In the history of evolution, seeing has developed several times - and independently of one another. There are differences in the number of receptor types and in their spectral sensitivity. Most mammals have two different types of color receptors, some primates three, reptiles and the birds evolved from these often four.
Distribution in the animal kingdom
- Color vision was investigated in insects, especially in honeybees . Karl von Frisch showed that you can “ask” bees about their color sensations by training them on colored plates with food rewards. For proof of true color vision, it is not enough for an animal to keep returning to the color it once experienced as being able to eat, because it could have learned the gray level. The sensory stimulus color is only recognized if it is chosen again and again regardless of the brightness . Frisch checked this by offering the bees color tokens of different brightness of the rewarded color in competition with other colors to choose from, and found that the “color” had priority in the decision.
- The mantis shrimp Neogondodactylus oerstedii eight different types of receptors in the visible and four in the ultraviolet range.
Lower vertebrates, and among the mammals the marsupials , usually have four types of cones. They are therefore called tetrachromats . In addition to the L, M and S cones, they have an ultraviolet cone that reacts in the range of less than 380 nm. Since this tetrachromatic color system is found in marsupials, birds and fish, which is more complex than humans, it is assumed that it represents an evolutionarily early type within the vertebrate color systems.
The different types of bony fish have developed different systems to adapt to the different lighting conditions in their habitats . Most of the fish examined so far are tetrachromats. The number of cones and their absorption maxima usually match their way of life: with increasing depth in water bodies, the lighting is increasingly monochromatic due to the stronger absorption of long and short-wave light. In clear seas or lakes, the blue portion of the light reaches depths of over 60 meters. In freshwater lakes with a high density of plankton , yellow-green light prevails at depths of 25 meters. In black water rivers and moor lakes , the red component of the light reaches a depth of no more than three meters. At the same time, in all waters, the intensity of the light decreases with depth. For example, twilight-active fish or fish living in dark regions have sensitive cones mainly in the red area, while diurnal fish living in the upper, light-flooded regions have more blue and green cones.
- Rod monochromats have no cones. You can see in very low light intensities, but only grayscale. The lightest gray is provided by objects in shades of green.
- Dichromates also have two different types of cones. Example: Common golden mackerel (Coryphaena hippurus).
- Similar to humans, trichromats have three types of cones. Example: cichlid ( Cichlasoma longinasus ).
Whether di- and trichromats can perceive and differentiate between different colors depends on the further neuronal processing in the retina and brain.
In addition to the rhodopsin in the rods, chickens have four cone pigments: red (maximum sensitivity at approx. 570 nm), green (approx. 510 nm), blue (approx. 450 nm) and violet (approx. 420 nm). In addition, there is another pigment in the pineal organ ( pineal gland / epiphysis ), pinopsin (approx. 460 nm).
Birds and reptiles alike have carotenoid- colored and colorless oil droplets in their cones , which act as color filters. These filters narrow the absorption spectra of the cones and thus improve the differentiation of colors. Mammals, and thus also humans, do not have these color filters.
- In addition to the rod pigment rhodopsin, mice only have two cone pigments for green (absorption maximum approx. 510 nm) and ultraviolet (approx. 350 nm). The cone pigments are also unevenly distributed. In the upper half of the retina, which looks at the ground, there are only green cones next to rods; in the lower half of the retina, which looks at the sky, only UV cones next to rods. This is an adaptation for foraging for food or for easier recognition of the UV patterns in the plumage of predators.
- Even popular science articles still maintain that dogs have no color sense, i.e. they see in black and white. But the house dog also has two types of cones with sensitivities in the green and blue spectral range.
- Primates can see "in color". As studies on monkeys at the Japanese National Research Institute in Tsukuba have shown, the ability to perceive color tones regardless of brightness is not innate. This was found in monkeys raised in monochromatic light. They were always unable to recognize a colored object if it reflected light of different wavelengths under different lighting conditions.
Evolution of cone types in terrestrial vertebrates
- Absorption is determined here as the number of photons picked up by a cone per second.
- The mentioned absorption maxima are only guidelines; There are differences not only between species, but also from individual to individual.
Birds have four types of cones, the absorption maxima of which are 370 nm (UV type), 445 nm (S type), 508 nm (M type) and 565 nm (L type). On the basis of comparisons of the DNA sequences of different opsin types in different recent animals, it is assumed that the common ancestors of birds and mammals also had four types of cones. In an early phase of mammalian evolution , the middle S and M types were lost. It is assumed that these animals were nocturnal and could therefore tolerate this change in the visual system. As an estimate of the evolutionary time with the help of the molecular clock showed, with the transition to diurnal activity 30 to 40 million years ago in the ancestors of the primates of the Old World a third type of cone was created through gene duplication , so that again an M-type (530 nm) was available, but its absorption maximum differs only slightly from the L-type (560 nm). A selective advantage may have been that with three types of cones, fruit as a food source can be better distinguished than with two.
Many insects, birds, lizards, turtles and fish have receptors in their retina that are also stimulated by light with wavelengths shorter than 400 nm - i.e. by ultraviolet. But some invertebrates also have receptors for UV: Sir John Lubbock , a friend and neighbor of Charles Darwin , discovered as early as 1882 that ants under ultraviolet (UV) pick up their pupae and carry them out of the radiation range. Karl von Frisch recognized in the 1950s that bees and ants perceive UV light as color.
Due to the fourth type of cone, which has its absorption maximum in ultraviolet (UV) or violet (V), certain animals such as some insects, almost all fish ( goldfish ), reptiles, the ancient mammals of Australia and birds can distinguish more colors than humans. One speaks of tetrachromasia. Studies on Budgie ( Melapsittacus undulatus ) showed that the bird can not only the colors that also distinguishes humans perceive, but also mixtures of different UV component. For example, a bird distinguishes between different colors for a certain blue, depending on the UV component, where humans can only perceive a single color.
From the number of cone types, however, it is by no means possible to deduce what abilities an animal has of differentiating colors. This depends very much on the processing of the cone reactions in the retina and brain and can only be investigated through behavioral experiments.
The ability to perceive ultraviolet plays a role in courtship for some birds .
- Measurements of the UV reflection showed that of 139 species in which males and females cannot be distinguished by the human eye, in more than 90% of the species the sexes differ in the UV pattern.
- In males of 108 Australian bird species, those parts of the plumage that play a role in courtship reflect more UV than other plumage areas.
- In the case of the blue tit ( Parus caeruleus ), the females prefer those males that glow most strongly in the ultraviolet range. Since the UV reflection depends on the microstructure of the feathers, it can provide information about the health of the males.
- In the Azure Bishop ( Guiraca caerulea ), the males with the highest UV reflection occupy the largest and most productive territories and feed their young most frequently.
But the perception of ultraviolet or its respective expression also plays a role in the acquisition of food.
- The surface of many fruits reflects ultraviolet. This makes it easier for animals with UV perception to find such fruits or to assess their respective quality.
- Kestrels ( Falco tinnunculus ) discover the trail of their prey (e.g. earth mice ) using their markings, since urine and feces reflect ultraviolet.
Ripe yellow bananas fluoresce blue in ultraviolet light, unlike the leaves of this or other plants. This is an indication that some banana-eating animals could use this property with the help of their UV perception to assess the ripeness of the fruit.
Although the neural pathways and mechanisms of processing color information in humans are known in principle, the emergence of consciousness about color is - similar to the emergence of consciousness about other things - dependent on many other, and often unknown, influences. “Apparently it is not the primary developmental task of the sense of sight to produce aesthetic sensations. Rather, his most important task seems to be to ensure the survival of the individual through safe orientation and optimal recognition. That is why the sense of sight has developed in such a way that it can adapt to the lighting conditions as far as possible. ”( Harald Küppers ) The naming of colors and the division of the color spectrum give indications that there are cultural differences and thus differences influenced by learning processes at this level of perception in color groups.
Color names and color sorts
- Empedocles understood white and black as colors.
- In his work De sensu (“About the Senses”) Aristotle equates the brightness of the air with the color white of bodies, while darkness corresponds with the color black. So he comes to the assumption that colors are composed of different mixtures of white and black.
According to these ideas, the colors were arranged according to a brightness scale until the 17th century: white - yellow - red - blue - black. While nowadays a color is defined by hue, saturation and lightness, hue was only seen as a result of lightness until then. Goethe was still influenced by this view when he rejected Newton's results and valued his own color theory even higher than his literary work.
In so-called natural languages , the huge number of distinguishable color nuances are assigned to a few color categories, for example in European languages often: violet, blue, green, yellow, orange, red, pink, brown. Research found that the berimos in Papua New Guinea used only five categories. They assigned a wide range of color nuances, which Europeans divided into the two categories green and blue , to just one term.
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