Colour is an attribute of an item which comes from the light it reflects or emits; this light causes a visual perception based on its wavelength. There are three perceptual dimensions to colour; hue, saturation and brightness. These three dimensions work together, not independently to perceive the correct colour, for example the hue yellow is seen as brighter than red or blue. But the four colour hues are unique; they cannot be described as mediators of other colours. During this essay the different theories that describe how humans perceive colour will be critically discussed, such as; Young & Helmholtz’s (1802, 1852, as cited in Goldstein, 2010) trichromatic theory, Hering’s (1872, as cited in Goldstein, 2010) opponent-process theory, the dual process theory of colour vision and how the brain processes the information provided.
Around 1802, Young proposed that colour vision was from the outcome of the action of three different colour receptors. Helmholtz then later discovered that three different wavelengths of light are needed to create different colours (1852, as cited Goldstein, 2010). Helmholtz came to this conclusion by doing colour-matching experiments; he made participants alter the amounts of three different wavelengths until it matched the colour of one wavelength. It was found that participants could not match the colour if they only used two wavelengths, but could match any colour using three wavelengths. So in accordance with this theory a wavelength of light stimulates three receptors in the eye to different amounts, the activity caused by the receptors results in the perception of colour.
100 years later, researchers had identified the three receptors which recognize wavelengths; they had found that cone pigments have different levels of absorption; small cone receptors have a maximum absorption level of 419nm, middle cone receptors have a maximum absorption level of 531nm and large cone receptors with a maximum absorption level of 558nm. Cone pigments are receptors found in the retina of the eye, which are responsible for the vision of colour and detail. The reason as to why the cone receptors have different levels of absorption is because of the amount of opish amino acids present in each receptor (Nathans et al., 1986). Verrelli and Tishkoff (2004) had found evidence for Nathan et al.’s suggestion that cone receptors are made of opish amino acids. Verelli and Tishkoff (2004) had found that the amino acids are encoded on the X chromosone
Dartnell, Bowmaker and Mollon (1983) did an experiment where they measured the response of cone receptors when different wavelengths were absorbed by them, from this it was found that blue has a large response in the small cone receptor, a smaller response in the middle cone receptor and an even smaller response in the large receptor. Also that yellow has a very small response in the small cone receptor, and a large, almost the same response in the middle and large cone receptors. White has an equal response in all the cone receptors.
The findings of Dartnell, Bowmaker and Mollon (1983) explain the results found by Helmholtz’s (1852) colour-matching experiment. In Helmholtz’s colour-matching experiment (1852) even though the wavelengths are different in the two separate fields of light, they are still perceived as the same colour by us humans, this is called metamerism and the two identical fields of light are called metamers. Metamers look alike because they result in the same pattern of activity in the three cone receptors, for example a 530nm green light causes a large response in the middle cone receptor and 620nm red light causes a large response in the large cone receptor. And when they are mixed together they match the colour of a 580nm light which is perceived as yellow because it has the same pattern response in the cone receptors as the 530 and 620 nm light mixture.
However the trichromacy theory does not explain all the aspects of colour vision, for example it does not explain colour afterimages. A colour afterimage is when you see an afterimage of a different colour, for example if you stare at a blue dot for a little while, then look at a white wall you will see and yellow dot and vice versa. Another problem with the trichromacy theory is that people with red-green colour blindness are still able to perceive yellow even though, according to this theory, yellow is produced by the green and red receptors.
Hering (1872, as cited in Goldstein, 2010) proposed the opponent-process theory after the trichromacy theory did not explain colour afterimages. The opponent-process theory says that colour vision occurs by opposing responses produced by blue and yellow, and by green and red. A study done by Abramov & Gordon (1994) supports this theory, they asked participants to estimate the percentage of blue, green, yellow and red in each patch, the results showed that participants rarely reported seeing blue and yellow, or red and green at the same time. These results supports an observation by Hering (1872, as cited in Goldstein, 2010) that people who are colour-blind to red are also colour-blind to green, and same for blue-yellow. These observations led to the conclusion that red and green and paired and so are blue and yellow, and so the opponent-process theory was proposed.
There has been modern physiological research which provides evidence that neurones respond in opposite ways to blue and yellow and to red and green. Hering proposed that black and white mechanisms responded negatively to black light and positively to white light, the red and green mechanism respond positively to red and negatively to green, and then yellow and blue mechanism responds positively to yellow and negatively to blue. Only recently has there been research which supports Hering’s (1872, as cited in Goldstein, 2010) phenomenological observation, it was found that there were opponent neurones in the retina which are called retinal ganglion cells, and the lateral geniculate nucleus that responds to certain wavelengths of lights with an excitatory response, and certain wavelengths with an inhibitory response (DeValois,1960; Svaetichin, 1956). This physiological evidence describes provides evidence for both the trichromacy theory, and opponent-process theory showing they are both correct.
Hurvich and Jamesson (1957) did an investigation which supports the opponent-process theory; they found that participants seeing a blue light could cancel it out using a yellow light, and the same for a red light with a green light. This supports the theory as it shows a hue can be cancelled out using the opposite hue, showing they are paired. Also there is neurological evidence which provides empirical evidence for the opponent-process theory, it was confirmed that colour opponent ganglion cells are present within the retina and lateral geniculate nucleus (Neitz and Neitz, 2008).
However, Gegenfurter (2003) argued that the opponent channels in the lateral geniculate nucleus and retina do not provide a very good account of perception of hue by itself. He argued that the lateral geniculate nucleus circuitry does not explain for unique hues, and there must be additional processing involved in the perception of colour, and suggests that colour processing occurs further than the lateral geniculate nucleus.
The most modern theory for colour vision is the dual-process theory, which combines the trichromacy theory and opponent-process theory to explain the perception of colour. This theory takes the trichromatic theory to describe what is happening at the start of the visual system, with the cone receptors in the retina. Where a wavelengths causes a different response in each of the three cone receptors. It then uses the opponent-process theory to describe what happens after in the visual system. The cone receptors in the retina from the trichromatic theory trigger the retinal ganglion cells which send input to the brain which perceives the correct colour in people with normal colour vision. Knoblauch (2002, as cited in Passer et al., 2009) supports the dual-process theory as he used microelectrodes to record singles cells, and found that there are neurones not only in the retina, but in the visual relay stations and visual cortex that respond in the way an opponent-process mechanism would by altering their rate of firing.
So once the information from the receptors is sent to the brain, what happens with it? Well the retinal ganglion cells send the visual information via the optic nerve to the optic chiasma, from then it reaches the lateral geniculate nucleus. After the information synapses at the lateral geniculate nucleus it goes to the primary visual cortex (V1) which is in the occipital lobe. The V1 contains double-opponent cells, which are clustered within a region of the V1 called blobs. The blobs come in two different ways, red-green and blue-yellow. The red-green blobs compare the amounts of red-green in one scene to an adjacent part of the scene, and the blue-yellow does the same but for blue and yellow hues, this was proposed by Land (1977), in his retinex theory of colour vision. From the blobs in V1 the colour information is sent to the second visual area in V2. Here in V2 there are stripes of wavelength sensitive neurones with other stripes that are sensitive to other visual information such as motion. The neurones in V2 then synapse onto cells in V4. In V4 there are very small millimetre sized areas called globs, which is the first part of the brain that colour is processed in the terms of hues (Conway, Moeller & Tsao, 2007).
There is research which supports the theory of the way colour information travels through the brain, Vaina (1994) had found that people with damage to the V4 can distinguish wavelengths but can’t label them as colours, which supports Conway, Moeller & Tsao’s (2007) observation that the V4 processes hues. There is more research supporting this, by Walsh et al. (1983) who had found that lesions to the V4 impairs colour constancy supporting the Land’s (1977) retinex theory that colour constancy is processed in the brain.
So in conclusion, there are 2 main theories as to how colour is perceived, Young-Helmholtz trichromatic theory (1802, 1852) and Hering’s (1872) opponent-process theory. Both were supported with plenty of research to support each theory, but there were evident flaws with the trichromatic theory; as it could not explain colour afterimages. And thus the opponent-process theory was proposed. However I believe that the best theory for colour perception is the most modern; the dual-process theory, which undertakes the trichromatic theory and opponent-process theory. This theory is the best to explain colour processing as it not only explains what happens in the retina, but goes on to explain how the brain processes the information provided by the receptors in the retina. Also it shows how the two different cone receptors and retinal ganglion cells work together to perceive the correct colour.