Why do we have the colour vision system that we have? Essay

“The rays are not colour’d” – according to Isaac Newton, colour is something we perceive ourselves (Thompson, et al, 2006). In order to understand why we perceive colour the way we do, we must ask why we have the colour vision system that we have, what colour is, what it enables us to do, and the repercussions of the alternative options to colour vision. Advantages of having a colour vision system, and more specifically a trichromatic colour vision system, appear to be firmly grounded in the evolutionary approach.

White light consists of many different wavelengths, most clearly seen in a rainbow, where the raindrops, as prisms, split the different wavelengths so we can see their individual paths, or individual colours. Newton described these wavelengths in terms of their colour, ranging from short to long wavelengths: red, orange, yellow, green, blue, indigo and violet.

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Light is merely a combination of all these wavelengths, and so in everyday life, we see colour based on which wavelengths are reflected when lights hits an object, and which are absorbed, not what colour the object is. We cannot see many different wavelengths, and which we can see is based primarily on the photoreceptors that we have. Although some animals have the ability to see in the infra-red, and on first glance, this would seem to be a highly valuable skill, because of our warm blood, this would merely result in us only seeing the glow from our own blood (Bowmaker, 1983).

Colour vision relies on us possessing these photoreceptors in our retinas, otherwise known as ‘cones’, that have sensitivity to certain wavelengths along the visual spectrum. A photoreceptor that responds most to a wavelength of 550-570nm, for example would be able to detect a yellowy-green colour. The sensitivity of this receptor peaks at 560nm, yet the further away the wavelength from this peak, the less efficient this receptor will be at distinguishing colour (Thompson et al, 2006). Possessing only a single receptor such as described above, is known as having monochromatic vision, and leads to many problems.

Because photoreceptors can’t distinguish between colour and intensity, and light of a certain wavelength at one receptor’s peak sensitivity could appear identical to a light with a shorter wavelength, yet twice the intensity. This is known as univariance. With only one receptor, unable to tell the difference between high intensity and high sensitivity of wavelength, we can receive only very limited information about the visual world. Humans would benefit far more from a richer type of visual information, and this is therefore why, in general, we do not possess monochromatic vision (Wolf, 2002).

A more sophisticated, yet still quite lacking alternative to monochromatic vision is dichromatic vision. Having a second cone type, one that responds to either a significantly shorter or longer wavelength, for example, allows differences in colour to be recognised. Because of this, the majority of mammals have this dichromatic vision. One aspect of vision to be sacrificed with this added cone, blue in this case, however, is that in order to have two cone types, there must be fewer of the original cones to accommodate any more. Having fewer of a certain type of cone means compromising visual acuity, as high acuity relies on many identical adjacent cones (Snowden, 2002).

For dichromatic vision to be successful, therefore, there must be only a small number of this second cone, and none in the fovea: our central focus point and area of the retina where visual acuity is at its highest. Having a higher number of blue cones actually would appear to be useless – a typical flaw in our vision system is that blue light is never in focus, otherwise known as chromatic abberation. This means that it would be futile to have blue cones in the fovea, as they would be unable to process fine detail in any case (Thompson, et al, 2006).

The second alternative to monochromatic vision allows more advanced colour detection and is the type of vision possessed by most humans. Research (Mollen, 1989; Wolf, 2002) suggests that one example of needing trichromatic vision for survival is identification of fruit in comparison with its surroundings; more generally, finding objects against a speckled background. Animals have even been found to exhibit colour changes as signs of fertility of sexual readiness, so this is certainly useful from an evolutionary perspective (Domb & Pagel, 2001).

This trichromatic vision occurs as a result of taking the dichromatic two-cone system and dividing one cone to generate a total of three cones, two of which respond to similar wavelengths. These cones have been termed the ‘blue’ cone, which responds best to long wavelengths, the ‘green’ cone, which responds best to medium wavelengths, and the ‘red’ cone, which responds best to short wavelengths. Trichromacy, though appearing particularly useful for distinguishing colour, is only possessed by humans and some other primates; other species possess an even larger number of cones, however, with shrimps having more than ten different types (Jacobs, 1993).

In terms of why we have a trichromatic visual system, there are many advantages over other types that are specifically useful for humans. In order to retain a good level of visual acuity, as well as not including blue cones in the fovea as mentioned previously, we do not possess any more than three cones – those with many cones will be incredibly good at identifying a magnitude of colours, yet have horrendous acuity. As we have little need for the former ability, and life would be very difficult without fine detail, trichromatic vision seems most suitable.

Advantages of having a cone system, irrespective of the number of cones involved lie in the visual system’s processes beyond the retina. Using a ratio system, or ‘opponent coding’, to compare the activity of each cone, means that colour can be distinguished even when the light intensity changes (Hering, 1964). Another useful aspect of our colour vision system is colour constancy: the ability to continue to view colours as relatively constant despite changes in intensity of colour of light. Instead of viewing each object in our visual field separately, our brain compares every object, and so when the light changes, and all the objects behave similarly because of this change, all reflecting short wavelengths, for example. The brain can understand that it is the light, and not the colour of the objects that is changing, and the same is true for changes in illumination (Jameson & Hurvich, 1989).

Perhaps the most useful way of illustrating the advantages of our own colour vision system is to compare it with that of those with colour-blindness, and the detriments of a lack of colour. As briefly mentioned earlier, Mollen’s (1989) paper on the advantages of colour vision illustrates that without colour vision, detecting objects or edges on a background of a different chromaticity is almost impossible. This also makes sense in terms of why we have this colour vision system, from an evolutionary perspective; if humans were not able to identify brightly coloured fruit, or danger signals, it greatly reduces their chance of survival (Gouras & Eggers, 1983).

Perceptual segregation and identification are two further advantages of colour vision. Grouping items of the same colour can help to identify objects made up of different sections, as well as being able to recognise separate objects that are all alike in colour. Faced with a large selection of objects with similar texture, shape and luminance, colour can be the only way to group different objects together quickly: a clearly useful ability in a wide variety of tasks (Mollen, 1989).

The trichromatic system, as well as many other specialised features of colour vision appear to have their foundations in evolutionary psychology; so many aspects can be justified through aiding survival, and Mollen (1989) implies that this is no accident. His theory suggests that the trichromatic system of many primates has evolved over millions of years from a far more basic monochromatic system. The need to attract attention, identify, distinguish, group, and see colour in fine detail has encouraged this system to become more advanced to adapt to primates’ more sophisticated needs.

References

Bowmaker, J.K. (1983). Trichromatic colour vision: why only three receptor channels? Trends in Cognitive Sciences, 6, 41-43.

Domb, L.G. and Pagel, M. (2001). Sexual swellings advertise female quality in wild baboons. Nature, 410, 204-206.

Gouras, P. & Eggers, H. (1983). Responses of primate retinal ganglion cells to moving spectral contrast. Vision Res. 23, 1175-1182.

Hering, E. (1964) Outlines of a Theory of the Light Sense. Harvard University Press, Cambridge, Mass.

Jacobs, G.H. (1993). The distribution and nature of colour vision among the mammals. Biological Review, 68, 413-471.

Jameson, D. and Hurvich, L.M. (1989). Essay concerning color constancy. Annual Review of Psychology, 40, 1-22.

Mollen, J.D. (1989). ‘Tho’ she knee’d in that place where they grew…’: the uses and origins of primate colour vision. Journal of Experimental Psychology, 146, 21-38.

Snowden, R.J. (2002). Visual attention to color: parvocellular guidance of attentional resources? Psychological Science, 30(13), 180-184.

Snowden, R., Thompson, P., & Troscianko, T. (2006) Basic Vision. Oxford: Oxford University Press

Wolf, K. (2002). Visual ecology: coloured fruit is what the eye sees best. Current Biology, 12, 253-255.

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