Vision

Theories of Color Vision Trichromatic Theory of Vision Opponent Process Theory of Color Vision



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The world would be a pretty boring place if we were unable to discern the beautiful colors that appear before us. How is it that we are able to see in color, or as scientists put it why is it that we have color vision? There are two theories that propose to answer this question: the trichromatic theory of vision and the opponent process theory of vision.

This question has been asked at various times in the past, but the first theory as to why it occurs was put forward by two renowned researchers Thomas Young and Hermann von Helmholtz. In the early 18th century, Young proposed that color vision occurs because of the action of three different color receptors located in the retina of the eye. Helmholtz later found that in order to see colors, humans need to have three different wavelengths of light reflecting off the colored objects.

In order to test this hypothesis, Helmholtz engaged participants in experiments in which they had to match test colors by changing the three different wavelengths of light. The result was that the participants were unable to correctly match the colors if they only used two wavelengths, but by using all three they could successfully match any color in the spectrum. Once the results of these experiments were published, it became known as the Young Helmholtz theory of color vision.

However, it was to take almost another century before researchers and scientists were able to identify the three different color receptors of the retina. It was found that the retina contains cones that are responsible for being able to see different colors. The pigments in these cones have different levels and rates of absorption because of the amount of opsin amino acids in the retina.

While the majority of researchers did accept the Trichromatic Theory of Color Vision, they were not completely satisfied with the complete result because they felt that the explanation of the process of seeing in color was not detailed enough. They claimed that it did not fully explain all possible aspects of color vision. One of these opponents of the theory was Ewald Hering.

Hering noticed that there are aspects of color that are rarely seen by the naked eye, such as a reddish-green, yet these colors do exist. He suggested that two opposing processes work in color vision, which he called a blue-yellow mechanism and a red-green mechanism.

The components of each of these mechanisms work against each other in a process of excitatory and inhibitory responses. The opponent neurons of the retina react in different ways to different colors. According to this theory red is an excitatory color and therefore a positive one, whereas green is an inhibitory color and therefore a negative one.

The experiments for this theory can easily be performed. It was noted that if you stare at a small spot of color on a piece of paper for about a minute and then switch the view to an object of a different color, you will see a patch of the color on the paper on the second object.

In proposing the opponent theory of color vision, Hering expanded on the trichromatic theory. He found that the trichromatic signals from the cones in the retina occurred in subsequent stages. Processes that are spectrally opponent are red and green and blue and yellow. He said that black and white are two non-opponent spectral processes.

 

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