All The Colors We Cannot See: Tetrachromacy in Humans
One million colors, that is the approximate number the typical human eye can see . This is due primarily to the fact that our eyes contain three different types of photoreceptors, known as cones, that are responsible for detecting wavelengths in the visible spectrum, with different yet overlapping ranges. They are known as short (S), middle (M) and long-wave (L) cones, because each has its peak sensitivity in the respective range. Because humans possess these three types of cones, they are described as trichromats (tri = 3, chroma = color). The perception of color is not simply a matter of wavelength detection, as it involves various stages of neuronal processing. But the ability to detect and discriminate colors is affected by the variety of cones in the eye. Dogs, and the majority of mammals for example, have only two types of cones, so they are known as dichromats. As a result they can detect and discriminate fewer colors than humans. On the other hand, some animals such as certain species of birds, fish, butterflies and bees, are known as tetrachromats, because they have four types of cones and can see and discriminate many more colors than humans. By some estimates they can see about 100 million different colors, or about 99 million more than we can! This is due largely to the fact that many tetrachromats have a fourth cone sensitive to the ultraviolet part of the spectrum, which is invisible to the human eye. We don’t exactly know what colors such animals can see in this range, but we do know that they can make distinctions that humans can’t, in other words what we see as one color, they see as several. This can be demonstrated with the use of ultraviolet photography as in the following example (fig. 1).
Fig.1 What a human sees (top) what a butterfly ‘sees’ (bottom). Image taken with an ultraviolet camera. Image source: http://www.webexhibits.org/causesofcolor/17C.html
Recent research has opened up the possibility that some humans might in fact be tetrachromats, and therefore seeing a greater range of colors than the rest of us. The idea of tetrachromacy in humans was first broached by the Dutch scientist H. L. de Vries in 1948  when he speculated that men who had a particular form of color-blindness, known as anomalous trichromacy, would have inherited their conditions from mothers who might be carrying an extra cone. This was later developed in the work of John Mollon and Gabriel Jordon , at the University of Cambridge in the 1990’s as they set out to find women who might be tetrachromats. The idea works something like this. The genes responsible for our M and L-cones are carried on the X-chromosome. Women have two X-chromosomes, whereas men have only one X and a Y chromosome. The vast majority of color-blindness is caused by a subtle change in the DNA sequence (mutation) of the L or M-cone and is why color-blindness occurs mostly in men – who have only the one X-chromosome. Since women carry two X- chromosomes, any defect or mutation on one is typically compensated for by the other. However, in some cases, that mutation can create a cone whose spectral sensitivity is different enough from the other three (anomalous trichromacy) to qualify as a fourth cone. It becomes possible therefore for a woman to carry both the standard set of S, M and L-cone sensitivities and a fourth with an altered spectral sensitivity. And that is exactly what Mollon et al. set out to find. They began by identifying the mothers of color-blind men and subjected them to genetic and color matching tests. What they found was that the fourth cone sat somewhere between the M and L cones, in terms of peak sensitivity (between 530nm and 560nm respectively). Unfortunately, the fourth cone is not in the ultraviolet range and so does not bestow super human bee-vision, but it should confer the ability to discriminate more colors in the red, yellow, green part of the spectrum – they should see discriminations in color that most of us don’t recognize. But to their surprise, this did not seem to be the case. They concluded that the altered spectral sensitivity of the fourth cone in the women they looked at, was not different enough from the others to bestow what has been termed functional or strong tetrachromacy. The women studied had the fourth cone, but it made no difference.
After nearly twenty five years of searching, Jordon and Mollon finally discovered what they were looking for in 2007. Having administered a set of color matching tests to a group of women identified as having sons with anomalous trichromacy they discovered one person who passed the color matching tests successfully. The successful candidate, hereafter known as cDa29, is the first functional tetrachromat to have been identified. She had an extra cone that sat midway between the tradition M and L-cones (fig.2)
Fig. 2 Simulated cone fundamentals for cDa29. Source: Jordan et al. (2010)
Jordan estimates that 12% of women are candidates for tetrachromacy but that functional tetrachromats are rarer - 2-3% by some estimates . One reason for the rarity is that besides having a spectral sensitivity that is distinct from the typical M and L-cone sensitivities, things like the optical density of the photopigments and the proportion of such cones also play a part. So even though tetrachromacy is not necessarily rare, most carriers do not exhibit four-dimensional color vision. In addition, being able to identify such people has become a real challenge, as many of them probably assume we all see the same world as they do. Today there are only a small handful of identified functional tetrachromats – but seeing what they see is still largely a mystery, especially since our language, our world and our science is designed for trichromats. All we have are anecdotal descriptions of a world of colors the rest of us just simply cannot see.
 J.D. Mollon, (1999) Color vision: Opsins and options, Proceedings of the National Academy of Sciences. USA 96, 4743-5 .
 De Vries, H. (1948). The fundamental response curves of normal and abnormal dichromatic and trichromatic eyes. Physica, 14, 367–380.
 Jordan G., Deeb S., Bosten J., Mollon J.D. (2010) The dimensionality of color vision in carriers of anomalous trichromacy, Journal of Vision, 10 (2010), p. 12
 Neitz, J. C., and Neitz, M. (2001) Color Vision: Almost Reason Enough for Having Eyes, Optics & Photonics News 12(1), 26-33