“Colors, like features, follow the changes of the emotions” – Pablo Picasso
The world is colorful. Thanks to the visual stimulus receptor – the eye filled with light and color receptors – people are able to see the palette of hues. Did you know that some of us see colors differently than others?
Not everyone is able to see the full richness of colors, while some people can see a broader spectrum than others. Within this paper, we explain what color vision is and what kind of mechanisms occur in the human eye during this process. We also describe the physical phenomena that allow for color perception, and we characterize the diseases which impair it.
Image credit: Pixabay (Free Pixabay license)
Adaptive vision mechanism
The eye is a very complicated vision detector [1]. It influences the perception of taste, objects recognition, enables reading and writing, and that is just the beginning of a very long list [2]. Basically, it is like a miniaturized camera that gives us information about the surroundings.
The human eye consists of three very delicate layers: an outer, middle, and inner one, see Figure 1. The outer layer, called a fibrous tunic, consists of the cornea and sclera, which are the outer, protective coat of the eyeball. The middle layer, called the uvea, consists of elements like the choroid, ciliary body, and iris. That layer supplies the nutrients and blood to all other structures of the eye.
We produce tears in response to chemical and emotional stimuli. Image credit: PublicDomainPictures via Pixabay (Free Pixabay licence)
The innermost layer is the retina – this layer consists of multiple specialized cells responsible for converting light into biological signals and transmitting them through the optic nerve for further processing in the brain. The retina is able to do so through cells called rods and cones [3].
The human eye has over 120 million rods which are the primary type of cell responsive to light. When the light passes through an eye, it is bent and forms an image directly on the retina – this mirror image stimulates retinal cells differently depending on the intensity and color of the perceived light. While the rods are responsive to light intensity, to recognize colors, our eye needs cells even more specialized than rods.
Figure 1. Schematic image of the eye structure. Image credit: Magdalena Osial
A little bit of theory behind the color vision
Color vision is possible thanks to cells called cones, see Figure 2. There are about 7 million of them inside the retina, and they are specialized in detecting the richness of colors. The light that passes through the cones stimulates them, depending on their color, and cones that are responsive to a particular wavelength of visual light emits an electric signal for the next layer of the retina.
This layer, composed of bipolar cells, is responsible for connecting cones and rods with ganglion cells that are attached to the optic nerve. The first theory of color vision, the so-called Young-Helmholtz trichromatic theory, assumes that color vision depends on these receptors [4,5,6].
These tiny cells are able to process the light and separate them into three colors: blue, green, and red. However, another theory called the opponent-process theory proposed by Ewald Hering and later extended by Richard Solomon assumes that the color vision is controlled by three opposing systems [7], where four unique colors, blue, yellow, red, and green, are vital to characterize color perception.
Moreover, there are also three opposite color channels: blue versus yellow, red versus green, black versus white. Our eyes detect color hues based on maximal two colors at the same time. On the other hand, it is possible to detect one of the opposing colors at the same time.
Figure 2. Three light receptors inside the retina. Blue edge represent epithelium that is ideally black internal layer inside eye thanks to which the light is not reflected. Image credit: Magdalena Osial
The nature of color
The color is a combination of light beams that, from the physiological point of view, can be described by three features, i.e., hue, saturation, and brightness. Knowing what receptors in the eye translate information, which reaches the human eye into the image that we see, but have you ever wondered what exactly the color is?
A long time ago, Sir Isaac Newton first figured out that objects do not inherently have a color. When the light hits an object, certain wavelengths are absorbed, while these are reflected from the surface our eye perceives. For example, an apple would absorb all the wavelengths corresponding to all the colors besides red. Therefore, the apple looks red. If an object absorbs all wavelengths, it will look black, while when the surface reflects all colors, then it is recognized as white.
In fact, color is light of a particular range of wavelengths that is reflected more strongly than the light of other wavelengths. Basically, color vision is connected with physical phenomena like reflection, absorption, transmission, refraction scattering [8].
Reflection of light occurs when the light bounces off from boundaries. The absorption takes place when the light does not bounce off, and the energy of the incident light is absorbed. Transmission means that the light can pass through an object like the water or our eye, while refraction is bending of the light when it crosses the boundary like air-water. The scattering bases on the interaction of light with small particles like water dispersed in the atmosphere, resulting in optical phenomena such as rainbows, the blue color of the sky, and even halos. It happens because the light interacts with matter.
What if our natural camera malfunctions?
Did you know that our eyes are unique, and people may see colors differently? That may be caused by the smaller than average amount of color receptors that lead to the deficiency of color vision. But what does it mean that someone can see fewer or even more colors?
Well, let’s start with what causes people to see fewer colors. As stated before, normal vision uses all three types of light receptors in your eye; however, in some individuals, this ability is disrupted. This phenomenon of color blindness can be separated into three categories: monochromatism, dichromatism, and anomalous trichromacy [9].
The first one, i.e. monochromatism, is the inability to see any color. People who suffer from this form of colorblindness perceive the world as black and white, with shades of grey in between. This can happen either when all of their cones do not work, or just one type of them do.
Dichromatism happens when two of your light receptors function normally, but the third one is completely missing [10]. You could think that the person with this sort of distortion in vision would not be able to see a specific color. However, a more appropriate description is that certain parts of the light spectrum are not seen and typically appear as gray.
Dichromatism can be separated into three different categories: tritanopia, where the blue light receptor is missing; deuteranopiain which the green receptor is missing; and protanopia, where the red light receptor is missing. Individuals with protanopia and deuteranopia see the world in a very similar way, with yellow and blue being the most prominent colors. Did you know that these disorders are more often observed in men?
Anomalous trichromacy is fairly similar to dichromatism, as, in individuals with this form of color blindness, two of three light receptors work properly [11]. The major difference is that instead of color missing, the third receptor has a reduced sensitivity to the color it should be received. Just like dichromacy, anomalous trichromacy is divided into three subcategories: protanomaly, deuteranomaly, and tritanomaly.
There is an additional form of disordered color vision, tetrachromacy [12,13], such people are able to see more colors than the average person. Such individuals may distinguish all possible shades of colors that can be recognized. For example, the grey color for an average population can be recognized by the person with tetrachromacy as the mixture of many colors.
It is only found in women, and their ability to see more colors is caused by the additional fourth type of cone found in their eyes. That is because the mutation that causes tetrachromacy is passed down through the X chromosome. Since men have only one such chromosome, it would result in anomalous trichromacy.
Summary
Did you know that people are able to distinguish even up to 10 million colors and all that thanks to three types of receptors? Surprisingly, 12% of the women population have four types of receptors and are able to see 100 times more colors than others. Our eyes see only visible light, while many birds, insects, and fishes also have the four types of receptors, but this time they can see even ultraviolet light.
Color helps us remember the objects, learn and evoke various emotions. There are also a number of disorders that result in some people not seeing colors or only seeing certain colors. So far, most of them are not curable, but one thing is for sure, a world without colors would lose its irresistible charm.
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