Many How people may not take the time to ponder the complexities of the color vision that organisms have been graced with over eons of evolution. While some mammals experience monochromacy (total colorblindness or the inability to distinguish the difference between different wavelengths of light), many are dichromatic and can view light in a mixture of two spectrums, because they possess two types of cones in their retinas that can filter both short and long wavelengths of light. The short wavelength cones allow for the perception of blues, while the long wavelength cones allow processing of the red spectrum. Humans, as well as Old world monkeys and apes, are all classified as trichromatic; they have three cone classes that help them to perceive wavelengths of light. These organisms not only possess the short and long receptors for blue and red, but also a third receptor that can process medium wavelengths which are green.

          The way humans view the world is through a mixture of these three wavelengths of light. Interestingly, these are not the only three possibilities when color sight is in question. Some fish, reptile, and avian species have displayed the ability to see the world through tetrachromatic vision. In addition to the three previously mentioned wavelengths, these species have the possibility to see a fourth wavelength that is in the violet range on the light spectrum due to extra genetic information coding for short wavelength sight. Even more interesting is the hypothesis that humans may also have the ability to display tetrachromacy due to the way color sight is inherited on the X chromosome. Studying this possibility of human sight is important because it may or may not be feasible for humans to process the visual information carried by an extra receptor for a fourth wavelength of light. Those individuals who are tetrachromatic may then have an advantage over those with normal trichromatic vision in relation to the colors they can see and their dimension of perceptual experience (Jameson 2007). To understand how humans might display tetrachromatic vision, it helps to know how color vision is inherited, as well as the studies which have been created to discover possible tetrachromats.

            Three separate genes are responsible for color sight and the formation of the photoreceptor protein opsin in its three forms: short, medium, and long wavelength sensitive cones. The gene for the SWS (short wavelength sensitive) cone opsin is located on chromosome 7, but both the MWS (medium wavelength sensitive) and LWS (long wavelength sensitive) genes are found on the q arm situated in a head-to-tail position. The MWS and LWS genes also have a 96% similarity in their amino acid sequence used in the formation of opsin which makes unequal crossing over a likely possibility (Bowmaker 1998). When crossing over occurs, the red pigment (LWS) may line up with the green (MWS) instead of the red with the red, causing an unequal distribution of red and green alleles on what is then a chimeric gene possessing more red or green alleles than normal. When an extra green allele is present the result is protanomoly, or “red-blindness”, and when an extra red allele is present the result is deuteranomoly, or “green-blindness”.

            In males, who will only inherit one X chromosome, these color deficiencies are always manifested if they receive the mutated chromosome from their mother. In women, protonomoly and deuteranomoly are generally not expressed because of X-inactivation; their cells randomly “turn off” the genetic information present in their extra X chromosome. It is because of random X-inactivation of the MWS and LWS opsin genes that the possibility exists for some females to express both the healthy and chimeric forms of these genes, which results in a mosaic of normal and mutant cones in the retina. While the normal cones would give the woman the ability of average trichromatic color vision, the mutant alleles may be able to absorb light somewhere in between the spectrum that is covered by MWS and LWS cones. The genetic possibility to produce a tetrachromat with four cone types is definitely there, but the question that remains is, can the human brain actually process the additional information?

            Nagy, MacLeod, Heyneman & Eisner (1981), as well as Jordan and Mollen (1993) explored the possibility of tetrachromatic humans with the use of Rayleigh matching with an anomaloscope.

 (above) An anomaloscope                                                 (above) An example of what the subjects viewed to match                                                                                                                                       color frequencies

The subjects were given a monocular view of a circle that was split into two fields. They were asked to match the color on one half to the other by adjusting the primaries displayed in the tetrachromatic spectrum on the mismatched side. Their results showed evidence of what would be considered the weak form of tetrachromacy, in which the individuals tested may have the four types of cones necessary for tetrachromatic vision, but are unable to distinguish between four independent color signals. When subjects were asked to produce color matches against a black background, they were happy to accept matches from the trichromatic spectrum. Under different chromatic backgrounds though, the subjects were hesitant to accept a perfect match. This test suggests that an extra cone class creates differences in perception when the match field is paired with varying backgrounds. Given that the subjects could not match the fields while requiring a fourth pigment every time, they could only be described as displaying weak tetrachromacy. In the study done by Jordan & Mollen (1993) 8 subjects displayed characteristics of strong tetrachromats, individuals who both have the four cone classes as well as the capacity to process the visual information they provide. These subjects were able to provide preliminary evidence for the type by refusing the large-field Rayleigh matches and requiring a fourth wavelength to produce an accurate color match. One problem with this study is the fact that the monocular view imposed by the anomaloscope interferes with the normal human binocular view. The intricacy of one’s color experience is dependent on the stimuli involved as well as the complexity of the scene (Jameson 2007).

            In addition to the anomaloscope studies performed, Jameson et al. (2001) developed a test involving spectrum delineation which was recreated by Jakab & Wentzel (2004). In this study, subjects were asked to view the spectrum and trace lines along the borders between the color bands they were able to perceive within that spectrum. In addition to this test, Jameson et al. (2006) also tested for instances of richer color experience in possible tetrachromats and Sayim et al. (2005) explored variations in color naming and color similarity that supported tetrachromatic color variation processing. These methods explored in addition to the anomaloscope may yield more accurate results due to their scene variation and the binocular view allowed in most of the tests performed. One of the newer test methods developed by Dr. Gabriele Jordan (Roth 2006) involves having the subject view three colored discs in rapid succession; although the subjects is not informed, two of these discs are composed of a pure orange wavelength, while the third is a mixture of green and red wavelengths. In this test, a tetrachromat should be able to distinguish which disc is not a color match with the others. Dr. Jordan and her team of researchers have discovered one woman whom they think is a likely candidate for tetrachromacy based on this initial test.

            Even with the information gathered by all of the studies conducted so far, the results on whether or not true human tetrachromats exist are relatively inconclusive. On the one hand, there is evidence of the existence of weak tetrachromats, individuals who may have inherited the four cone types necessary for the condition who can only process the information that the extra cones receive in optimal circumstances. Still, evidence suggests a few (only one or two throughout the history of the research in the field) true tetrachromatic women may exist, though further studies must be done to reach a definite conclusion. Still, the research in this field is relatively minimal and there is a strong possibility that if further studies are carried out with the inclusion of a larger number females who have a genotypic potential to display tetrachromatic vision, more evidence can be compiled to prove or disprove this phenomenon.