Perfect Explanation of the Trichromatic Theory of Color Vision

Trichromatic theory of color vision
The trichromatic theory of color vision, also known as the Young-Helmholtz theory, states that the retina has three types of receptor cells, which react to light of three different wavelengths - red, green and blue. These cells are responsible for the perception of colors.
Quick Fact
It is estimated that the average human eye can distinguish about ten million colors from one another!
In 1802, Thomas Young proposed that there are three types of photoreceptor cells in the eyes of humans and other primates. These cells allow the eye to perceive colors and differentiate between them. Hermann von Helmholtz further developed this theory in 1850, stating that the cone photoreceptor cells of three types are sensitive to different wavelengths of light. They can be classified as short-preferring, middle-preferring, and long-preferring, based on their response to different wavelengths. The short, middle, and long-preferring cells correspond to the blue, green and red regions of the color spectrum, respectively.

In 1956, Gunnar Svaetichin first showed that there exist three types of receptor cells that are sensitive to three different wavelengths. In 1983, Dartnall, Bowmaker, and Mollon obtained readings from the cone cells of the human eye and Svaetichin's theory was thus validated for the human eye.
What is Trichromacy?
Tri stands for three and chroma is the Greek word for color. The term 'trichromacy' (also called trichromatism) refers to the condition of having three independent channels that convey information about colors, derived from three different types of cones. The three types of cone cells have different absorption spectra. Simply put, 'trichromatic' means 'having perception of three primary colors'. Humans and some other mammals are trichromats. A recent study showed that marsupials possess trichromatic vision. Some insects are trichromats; for example, honeybees that can distinguish between ultraviolet, blue and green light.
How Trichromats Perceive Colors?
Color vision is the ability of an organism to distinguish between colors. Cone cells in the retina of the eye, are responsible for color vision. They are photoreceptor cells sensitive to different wavelengths of light. The activation of cone cells caused by lights of different wavelengths, leads to the perception of many different colors.
Cone cells are responsible for color vision and color sensitivity. They work well in bright light than in dim light. Six to seven million cone cells are found in the retina of a human eye. They are of three types, L-cones, M-cones and S-cones, which have three different types of photosensitive pigments, with a photoreceptive protein called photopsin (or cone opsin). Each pigment is sensitive to a specific wavelength of light and responds to variation in colors in a different way. L-cones respond to light of long wavelengths such as red, M-cones respond to medium wavelengths such as green, and S-cones respond to light of short wavelengths such as blue.
The response of cone cells depends on the wavelength and intensity of light that they react with. The brain cannot differentiate between colors if it receives signals from only one type of cone cells. It is required that a minimum of two types of cones interact with each other so that color vision happens. The brain can examine the signals from at least two cone types and decide the intensity and color of the light.

Light from an object strikes the rod and cone cells in the retina. The rods and cones send neural signals to the bipolar cells. The bipolar cells are located between photoreceptors (rods and cones) and ganglion cells. These cells then transmit visual signals to the ganglionic cells, which carry signals to the brain.

The retinal ganglion cell is a neuron that exists near the inner surface of the retina of the eye. Signals from the eyes are carried to the brain. They are processed in the thalamic region and other parts of the brain, and reach the visual cortex of the cerebrum. From the visual cortex, signals move to other parts of the brain for further processing.

Psychologists say that color perception happens when the neurons in the brain are stimulated. These neurons react to various characteristics of the perceived object like edges, angles, patterns, movement, and luminosity.
Color Blindness
Monochromacy refers to the condition of having only one channel to convey color information. In this case, an individual cannot distinguish between colors and is unable to perceive the difference in brightness of colors. This condition can result from complete absence of cone cells or presence of only one type of cone cells.
Lacking any one type of cone cells leads to a condition called dichromacy. Lack of short-wavelength cones is termed as tritanopia, lack of medium-wavelength cones is referred to as deuteranopia and lack of long-wavelength cones is called protanopia.
Anomalous trichromacy is a condition in which an individual makes color matches that are different from the normal.

Mutation of red or green photoreceptors leads to red-green color blindness pronounced as the inability to distinguish between red and green hues. Those with blue-yellow color blindness cannot differentiate between blue and green hues as also yellow and red hues.
The Opponent Process Theory
Some aspects of color vision cannot be explained by the trichromatic theory. There is a phenomenon of color afterimages. When we stare at a bright green color for some time and then look away at a white wall, we perceive a red color, and the converse happens after looking at red. The same applies to blue and yellow.

Based on the phenomenon of color afterimages, Ewald Hering proposed the opponent process theory of color vision. He said that color vision takes place in three channels where opposite or complementary colors are in competition. The visual system is responsive to three color pairs which are green-red, blue-yellow, and black-white. The color vision is because of the combined response of these components. His theory suggested that the colors in each pair oppose each other. For example, red-green receptors cannot send messages about both colors at the same time. The theory also explained the concept of afterimages. Red and green are complementary colors. When we stare at red for some time, our redness detectors get tired. Their antagonists, the green receptors, take the lead and we see a green afterimage after staring at red for long.
Thus, we saw how the trichromatic theory of color vision explains how photoreceptor cells enable us to perceive and distinguish between colors. It surely helps us understand the amazing mechanism of the eyes and the brain in perceiving colors. It is due their brilliant working that we can appreciate the beauty of colors.