Theories of vision

Describe and evaluate the two main colour theories of vision

The two main colour theories of vision are the Trichromatic theory and the Opponent process theory of colour vision. They are both competing theories aiming to explain colour vision. In particular, the relationship between the physical properties of light and the perception of colour in relation to the neural networking system, which is accountable for the conversion of visual imagery into perceptual images. This essay will describe both theories and evaluate them in terms of its advantages and flaws whilst also providing supporting evidence for the key ideas provided. It will conclude whether the theories are complete and unified and which one can be accounted for the most.

Firstly, the trichromatic theory of colour vision was developed by Thomas Young (1802) and H.V. Helmholtz (1852). This theory was based on the observation of colour mixing and proposes that we can create new colours by superimposing on coloured spotlights of the primary colours; blue, red and green. An example of the use of this is TV, computer screens and projectors.

Through research and observation, Young and Helmholtz found that colour vision relies on three key receptor mechanisms in the retina of the eye, which all have different spectral sensitivities to wavelengths. The wavelengths of the three cones are short, medium and long. Each of the cones have specific wavelengths and have different peaks of light absorption, which are "long (560nm), medium (530nm), and short (420nm)". The light of the wavelength stimulates the three receptors to different degrees and different patterns of activity which results in the perception of a colour. In turn, the patterns of activity allow us to forecast what colour would be formed if lights of different wavelengths are combined because the response of each receptor mechanism is known.

The main support for the trichromatic theory stems from the colour matching experiment performed by Young and Helmholtz, a wavelength in one field is matched by altering the proportions of three different wavelengths in another. Although the two fields contain different wavelengths, they are actually perceptually identical. This is termed metamerism. The two indistinguishable fields are called metamers. Metamers are identical as they form the same reaction in all three cone receptors.

Furthermore, in their investigation of colour matching, it was proven that those with normal sight need three wavelengths to produce the standard colour set. The three types of cones that are present in the eyes retina all contain a different type of photosensitive pigment, which is made up of a transmembrane protein called opsin and a light-sensitive molecule. Each of the different pigments are specific to a certain wavelength. 3 However, research has shown that colour vision is still possible with just two receptor mechanisms, the third just allows for more colours to be perceived from the visual spectrum.

Although the trichromatic theory of colour vision succeeds in aiming to explain a number of colour vision phenomena such as colour matching and colour mixing, it fails to account for other colour perceptions such as colour discrimination and opponent colour perceptions, as well as the failure to account for four unique colours - red, green, blue and yellow, and why dichromats with only two types of cone are therefore missing one visual pigment.

This led to the proposal for the opponent process theory of colour vision by E. Herring in the late 19th Century. The theory is based on the visual system, which transfers information about colour by processing signals from rods and cones. The different cones clash in the wavelengths of light in which they react. This allows for a proficient visual system to record the differences between the responses of cones, rather than the cone's response mechanism.

In addition to this, it proposes that colour vision is caused by opposing responses generated by the colour groups 'blue and yellow' and 'red and green'.4 Herring stated that there are three different channels: red-green, blue-yellow and black-white, each responding in an antagonistic way. Meaning there is no greenish-red or blueish-yellow perceived which suggests that 'green and red' and 'blue and yellow' exclude each other in perception. 5

Further support for the blue-yellow and red-green opponent processes comes from electrical recordings from fish retina (Svaetichin 1956). There are also electrical recordings from the LGN (lateral geniculate nucleus) expressing opponent colour processes (DeValois et al, 1996). This supports the theory as the activity of colour perception takes place in the LGN in the brain. 6

However, it has been proven that humans find it easier to visualise reddish-green or bluish yellow, as opposed to greenish-red or yellowish-blue. This was demonstrated in experiments where people were shown sample patches of colour and asked to estimate the percentage of green, blue, yellow and red the see in every patch. The observers hardly reported seeing the colours green and red, or blue and yellow at once. (Abramov & Gordon, 1994)7. This evidence, combined with Herring's proposal that people who are colour blind to seeing red are also colour blind to seeing green, and vice versa for blue and yellow, led to the assumption that red and green are paired, as are blue and yellow. Further evidence for the paired colour groups was found in Herring's observation that a red field forms a green afterimage, and viewing a green field formed a red afterimage. The same results occurred for blue and yellow, thus confirming the paired colours.8 Herring used this as the basis for the opponent process theory of colour vision.

Contrary to this, positive and negative responses are caused by a build up and breakdown of chemicals in retina. White, yellow and red cause the build of chemicals whilst the black, green and blue cause the breakdown. This part of the theory is incorrect as modern physiology research demonstrates that these colours do cause opposite responses.9

Herring accepted and did not challenge the first stages of processing as stated in the trichromatic theory of colour vision, but argued that any theory of colour vision should aim to explain the way we perceive colour.

The opponent-process theory has only recently been taken into account as much as the trichromatic theory. The reason for this is due to the incapability of people in imagining a physiological process that resulted in opposite responses to different wavelengths - S, M, L. However, due to the modern development of physiological techniques, it has been made possible to measure the response of neurons in the retina and LGN, which in turn have made it possible for researchers to confirm Herrings ideas of opponent colours.

While the trichromatic theory coins the way of how the retina permits the visual system to identify colour with three cones, the opponent process theory takes into account the mechanisms that process and receive information from the cones. However, both theories of colour vision were first thought to be competing, it was later discovered that the mechanisms accountable for the opponent process receive signals from the three cones and process them at a more complex level in the neural system.

As well as the cones, which detect the light that enters the eye, the basis of the opponent theory involves two other types of cells which are known as bipolar cells, and ganglion cells. The information from the cones is then passed to the bipolar cells in the retina, which are known to be the cells in the opponent process that convert the information from cones. The information is then passed to ganglion cells which handle the bulk of information about colour. This can be supported by research carried out by Gouras, 1968; Denonasterio and Gouras, 1975; Zrenner and Gouras, 1981 who found that electrical recordings of ganglion cells taken form retinas of primates demonstrate the opponent colour process.

In conclusion, both theories are correct but neither provides a unified and complete theory of colour vision. Both theories fail to explain colour constancy - illusions which look the same colour but are perceptually different. However, both theories provide complimentary explanations that describe different ideas of the visual system by alternative coding strategies at different levels of neural processing. The trichromatic theory explains colour vision at the photoreceptor level, whereas the opponent process theory explains colour vision as when photoreceptors are neurally connected. This suggests neither one can be accounted for more than the other. Given this, a further theory called the stage theory (Juan Pascaual-Leone) was developed to combine both theories and states how both theories explain colour vision but at different stages in the perceptual vision. It consists of two stages, the first being the receptor stage (photo pigments: blue, green and red cones), the second being the neural processing stage where the colour opponent process occurs.10


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