Dichromatic benefits in visual tasks
Short paper for the Creative Research course, about the benefits of some types of color blindness.
Dichromatic benefits in visual tasks
Why colour-vision polymorphism remains
Colour vision deficiency, or colour-blindness comes in many different varieties. In general it can be described as the inability to perceive differences between some of the colours that others can distinguish. Colour-blindness can be caused by eye, nerve or brain damage, but is usually of genetic nature. These conditions of genetic nature are mostly caused by mutations in the X-chromosome of the DNA. Males have only one X chromosome (and one Y-chromosome), while females have two. When one X-chromosome has a mutation which causes some sort of colour vision deficiency, in females the second X-chromosome overrides the mutated one, resulting in regular colour vision. The probability that two X-chromosomes have a certain mutation is of course smaller than the probability of one mutated chromosome. This is why males suffer from colour-blindness more often.
As mentioned before, there are different types of colour-blindness. These types can first be divided into categories by the number of channels from which information about color can be conveyed: one, two, or three. When suffering monochromacy, subjects completely lack the ability to distinguish any colours and perceive only variations in brightness. This is because only one type of cones is actively working. Analogously, dichromats have two out of three functioning types of cones. The third category is called anomalous trichromacy. People suffering from conditions in this category have three types of cones, but one or more types are mutated, which causes anomalous functioning. The most common occurrences of colour-blindness are actually part of this last category. The three dichromatic conditions, deuteranopia, tritanopia and protanopia, are quite common too. In this paper I will discuss dichromats and normally functioning trichromats.
Colour-blindness is usually seen as a mild disability. It can cause problems in tasks such as driving motor vehicles (recognizing colour-coded signals), reading maps, but also in tasks that have been important for millions of years, such as the foraging of food. The most common forms of dichromatic colour-blindness, unable distinguishing between colors in the green-yellow-red spectrum, which makes i.e. finding ripe fruit very hard. So a reasonable question is: considering the disadvantages, why is the incidence of dichromatic colour-blindness so high (as high as 2% in the male population)? Doesn’t the driving principle behind evolution causes these sort of mutations to die out?
The answer to this question may be that dichromatic colour-blindness, although disadvantageous for certain tasks, is beneficial for others. This is the suggestion that is made by Morgan & Mollon1. Their experiment shows that trichromats have more difficulty with discovering patterns in the orientations of objects when these objects have random colours, than when the objects are all the same colour. When this was done with colours which the dichromats couldn’t distinguish, this effect did not occur with the dichromats. Somewhere in the image processing in the brain, the information in the image is segmented into different pieces by some criteria. It seems that colour distinction is of greater importance in this process, causing the pattern in texture to become less noticeable. So, in this task missing information about colour is an advantage.
Related research has been done on advantages of colour vision deficiency in scotopic (low-light) conditions. Verhulst & Maes5 show that colour-blinds have significantly lower light perception treshholds than colour-normals. The research done by Simunovic, Regan & Mollon6 however defies the former research, stating that trichromats and dichromats perform equal in low light. Other research done on this subject show differences too. Caine et. al.4 show that the dichromat advantages in foraging food contribute to the maintenance of the colour vision polymorphism in primates. The research done by Melin et. al.2 however show no signs of these sort advantages in primates.
1 Morgan MJ, Adam A, Mollon JD. Dichromats detect colour-camouflaged objects that are not detected by trichromats. Proc Biol Sci. 1992;248:291–295
2 Melin A, Fedigan L, Hiramatsu C, Kawamura S, Polymorphic color vision in white-faced capuchins (Cebus capucinus): Is there foraging niche divergence among phenotypes? Behavioral Ecology and Sociobiology 2008-03-01, 659-670
3 Osorio D, Vorobyev M, A review of the evolution of animal colour vision and visual communication signals, Vision Research, Volume 48, Issue 20, Vision Research Reviews, September 2008, Pages 2042-2051
4 Caine NG, Osorio D and Mundy NI, A foraging advantage for dichromatic marmosets (Callithris geoffroyi) at low light intensity, Biol. Lett. 23 February 2010 vol. 6 no. 1 36-38
5 Verhulst S, Maes FW, Scotopic vision in colour-blinds, Vision Research, Volume 38, Issue 21, November 1998, P 3387-3390
6 Simunovic MP, Regan BC, and Mollon JD. Is Color Vision Deficiency an Advantage under Scotopic Conditions? Invest. Ophthalmol. Vis. Sci. 42: 3357-3364.