by Joe Makintosh

Created on: April 24, 2009

Both the Trichromatic Theory Of Vision and the Opponent Process Theory Of Vision deal with how colour vision is achieved, however, they are not the same.

The Trichromatic Theory began when ThomasYoung - a renowned researcher - theorised that colour vision was only possible because of three different types of receptors. Hermann von Helmholtz, another renowned researcher, later contributed to Young's theory by discovering that people with normal colour vision (i.e not colour blind) need three different wavelengths to see colour.

On the other hand, the Opponent Process Theory attempts to answer the questions that the Trichromatic Theory does not. Ewald Hering, the creator of the theory, questioned why we do not see certain mixes of colours such as Blueish-Yellow or Redish-Green. This theory suggests that colour vision is achieved through two opposing mechanisms. The red-green mechanism and the yellow-blue mechanism.

The ways the theories have been tested are different as well. The Trichromatic Theory was tested by Helmholtz in a colour-matching experiment. Subjects would attempt to match a colour by altering the amounts of the three wavelengths of light. It was discovered that the subjects found it impossible to match the colours if they only used two wavelengths but could match any colour if they used three.

The Opponent Process Theory was tested through the use of Afterimages. If a subject was to stare at a red square for roughly a minute and then looks at a white surface a green after image would be seen. Another test used was devised by Leo Hurvitch and Dorothea Jameson called Hue Cancellation. If one was to mix green and red light together yellow would be produced as opposed to a reddish-green. If one was also to mix yellow and blue light together white light would be produced instead of a blueish-yellow.

Another difference is the cause of each of the theories. The Trichromatic Theory Of Vision's cause are the receptors that receive the light. The three different types of Cone Receptors depend on the wavelength of the light. There are Short-wavelength Cone Receptors, Mid-wavelength Cone Receptors and Long-Wavelength Cone Receptors. These cones are found behind the retina and have a different levels of absorption because of the amounts of opsin amino acids in the cones. This has been discovered by researchers.

However, the Opponent Process Theory is due to different responses of the neurones called Opponent Neurones. These nuerones have an excitatory (or posotive) response to some wavelengths and an inhibitory response (negative response) to other wavelengths. The colour red, for example, causes an excitatory response whereas green makes an inhibitory response.

To conclude, both theories are very different because they handle different things. The Trichromatic Theory is about the cone receptors behind the retina in your eye, whereas the Opponent Process Theory deals with the neurones in your brain. They also have the similarity that they both try and find out why we see reds, blues, yellows, greens and all of the other colours you can imagine.

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