The Color of Vision

Alex Hansen

Prof. Paul Grobstein

Neurobiology and Behavior

April 14, 2007

The Color of Vision

Five essential senses of perception, classified by ancient peoples, exist for the human race. The defining terms include touch, taste, hearing, smell, and sight. Despite the lack of consensus that these five senses are the sole physiological methods that exist, generally, each is eternally exhibited throughout the universe. However, there is no guarantee that every individual will possess all five senses, or that every individual will maintain senses that function with perfect accuracy and without deficiencies. Vision is a primal example of one of the five senses that does not remain homogenous across humans, or across all living organisms. Upon looking at a single image, what one sees may or may not be what another is envisioning. Moreover, the individual may be blind and may be thought to not be able to see at all, yet that person may still appear to retain some internal form of vision that may be unknown to those not diagnosed which such blindness. Such differences in vision arise from a variety of causes including genetics, specific life events, as well as evolution. It is through the development of organisms and evolution that the current classifications of vision have been established. While some types of vision are advantageous for particular animals, such might create detrimental effects for a different species. Thus, although sight is a universal term used for one of the five physiological senses, vision is often specific to varying organisms. With such, according to evolution there often exist biological similarities across different species, and thus there must also exist similarities in the vision of these comparable species as well. Through using these evolutionary differences and similarities as a basis, one can examine this physiological classification of sight across different animals. One particular aspect of vision which appears to demonstrate the heterogeneous quality across species, as well as connect the genetic resemblances within such organisms to possible evolutionary explanations, is color. To the eyes of different animals, color has the ability to vary and change, or could even be lacking due to an abnormality in the animals color vision, or due to a type of development specific to that animal. The developed color vision will often provide advantages to that species, as evolution is a process which occurs to allow for the survival of what is considered favorable. As evolution continues forward to the future, current types of vision will be able to change as certain attributes may develop to become more conducive to success.

The current definition of color vision stands as the capacity of an organism or machine to distinguish objects based on the wavelengths or frequencies of the light they reflect or emit (1). Accordingly, animals with varying types of vision differ in the manner in which they distinguish objects, distinguish the color of objects. While some organisms are able to discriminate between lights of certain different wavelengths, others are not capable of such discrimination at those specific wavelengths. Therefore, the image will not appear the same to the two organisms. Such discrepancies in what is interpreted by the eye, in the discriminations between wavelengths, exist for individuals within the same species. Therefore, as some animals are able to differentiate between many different wavelengths, others do not maintain that ability. The color vision systems vary in strength as each has his/her own spectrum of visible color whose length is determined by his/her visual capabilities. Animals with highly developed color vision systems include coral reef fish and tropical birds (7). Such vision strength among these animals appears to be in concordance with the intensities and magnitudes of color seen by humans on the surfaces of these organisms. On the other hand, this theory cannot be applied universally, as research has concluded that non-primate mammals often have limited color vision, especially in comparison to the color vision of humans (4). A possible different explanation could include that the strength of the color vision is determined by the atmosphere in which the animals live, the environments in which they evolved, the evolutionary process. As apparent differences in color vision arise, it becomes essential to define and distinguish between the types of visions.

Monochromatic, dichromatic, trichromatic and tetrachromatic are four of the developed terms to differentiate vision. As color is derived by the nervous system through the analysis of light responses that develop due to the cone photoreceptors in the eye, it is through these varying cone photoreceptors that the differences are created. Each cone receptor maintains a predetermined sensitivity to a specific section of the light spectrum, a specific range of wavelengths. The number of cones that an animal possesses determines the total range of wavelengths that can be differentiated by that animal, and determines to which type of “chromate” the animal belongs. For example, humans are able to visibly see light of wavelength 380nm to 750nm and have three photoreceptor cones, which distinguish them as trichromates. The three cones are known as the S, M, and L-cones with each letter representing short, medium or long wavelengths of light, which in turn corresponds to red, green or blue light (1). Organisms with four different photoreceptor cones are defined as tetrachromates, while those with two cones retain the classification as dichromatic. These tetrachromates have a larger absorption spectrum than the trichromates and usually are thought to include animals such as arachnids, insects, amphibians, birds, reptiles and fish, while no mammals have been concluded to be known as tetrachromates (10).

A possible explanation for this difference between humans and these reptiles and birds is the similarity of these latter animals to the distinct organisms known as dinosaurs. While evidence has proved the evolution of birds from dinosaurs, the human race does not maintain the same evolutionary similarities, and thus, does not hold the trait of possessing four cones. Comparatively, primates that are defined to be similar to humans such as monkeys and apes all possess three cones (4). Additionally, there exists evidence which illustrates the dichromatic quality of the two similar animals, chipmunks and squirrels. Evolutionary similarities appear to explain the apparent vision similarities across species. For example, specific differences arise in the vision between mammals and vertebrates. One study demonstrates that the photoreceptors of vertebrates have oil droplets at their bases that absorb light which would often stimulate the cells while mammals do not possess this trait. Another study proved that vertebrates have double cones, cones that are joined along their long axes, a feature unknown to mammals (11). Thus, it appears that the number of cones as well as the particular qualities of these cones stays the same for similar species, and thus has can be related to evolution and Darwin. As humans evolved, it appears that a fourth cone could have been lost, for it might not have produced the same advantages for humans as it did and does for vertebrates.

Each animal has evolved to develop a certain type of vision that presents them with advantages. For example, cats, as well as canines, are better apt at seeing in the dark in comparison to humans. This feline quality is due to their tapetum lucidum, which reflects extra light to the retina. However, the day vision of a cat is subsequently restricted and he/she must compensate by developing slit like irises in order to combat light intensity and retinal sensitivity. In turn, such slits often enhance the depth of field for these organisms. In comparing bees and butterflies to humans, the vision spectrum of the insects extends further into the ultraviolet than humans (11). Such proves advantageous for these organisms as they are able to better pollinate flowers, for these flowers often have special ultraviolet patterns that the bees are then able to observe. However, the other end of the light spectrum is essentially lost, the infrared wavelengths, which humans are often able to observe. Overall, each species appears to have developed, evolved, a certain type of vision based on the lives they live.

Nevertheless, vision types are not completely homogeneous across one species, as there exist individuals who do not fit the “normal” standards. The term color-blind is often used to refer to humans who do not posses the standard three photoreceptor cones. Total colorblindness occurs when the individual lacks complete cone function - monochromacy. Typical “colorblind” people possess some type of deficiency or loss of one of the three photoreceptor cones (9). In terms of chromatics, these partial color blind individuals are either dichromatic or anomalous trichromatic. One common type of color-blindness, especially among men, is referred to as red-green color blindness. Such is defined by an inability to differentiate between different wavelengths of light that correspond to the green-yellow-red portion of the spectrum; the color-blind human will often lack the LWS and MWS pigments (5). Interestingly enough, one study determined that chimpanzees can develop, or have developed the same cone dysfunction, the same protanomaly color blindness (4). Such further supports the notion that the evolutionary linkage between humans and apes can be applied to the physiological sense known as vision.

Further examination of color blindness with respect to evolution and the development of advantageous qualities for survival, has been conducted. Although these dichromatic color blind humans have difficulty in differentiating between specific colors, these individuals often develop a stronger ability to differentiate between the shades of white, grey and black. Evidence has shown that in using this such trait, animals with red-green color blindness are largely effective in predator avoidance, illustrating how color blindness could be evolutionarily selected for in certain individuals. In addition, color blind people have a keen ability to detect color-camouflaged objects in comparison to their counterparts who possess color vision, all three photoreceptor cones (5). Military soldiers who are diagnosed with red-green color blindness, excluding pilots for color blindness is a disqualifier, would therefore possess an advantage during time of war in the battle field. Chimpanzees have also been found to discriminate color-camouflaged stimuli in the same manner as humans assisting them as they hunt for insects (2), furthering the former evolutionary-connection notion. What is known as a deficiency may indeed be the cause of survival. Vision has developed and changed within species, within divisions of animals, as certain qualities seem to be evolutionarily chosen.

What society may deem as normal, typical, standard, essential for success, does not always hold true. As organisms have developed through evolution, societal standards have developed through evolution as well. Thus, society’s interpretation of the physiological sense known as vision has developed over time. Currently, there exist accepted definitions to distinguish between types of visions, and dysfunctions of sight, yet there is no guarantee that such will remain valid as time progresses forward. A new species could evolve just as humans evolved, creating a new type of vision, presently unknown to the human race. Moreover, the vision of humans may transform and evolve in order to provide the race with the most beneficial means of survival success, for example, color blindness. Currently, these qualities might appear atypical and shunned upon by society, but with evolution, the atypical can become typical. Although vision appears to be clearly defined, such is never true, for there are differences across species and differences across eras.

References

1) Color Blindness. http://en.wikipedia.org/wiki/Color_blind

2) Barone, Jennifer. The Upside of Color Blindness: Color-blind monkeys make better insect hunters. So what if you can’t be a pilot? http://discovermagazine. com/2007/apr/the-upside-of-color-blindness. April 2, 2007

3) Yokoyama, Shozo. Elephants and Human Color-Blind Deuteranopes Have Identical Sets of Visual Pigments. http://www.genetics.org/cgi/content/full/170/ 1/335. May 2005

4) Terao, Kenichi. Identification of a protanomalous chimpanzee by molecular genetic and electroretinogram analyses. http://sfx.exlibrisgroup.com:9003/brynm ?sid=Entrez:PubMed&id=pmid:15733956. May 2005

5) Yokoyama, Shozo and Takenaka, Naomi. Statistical and Molecular Analyses of Evolutionary Significance of Red-Green Color Vision and Color Blindness in Vertebrate. http://mbe.oxfordjournals.org/cgi/content/full/22/4/968. January 2005

6) Sidjanin, Duska. Canine CNGB3 mutations establish cone degeneration as orthologous to the human achromatopsia locus ACHM3. http://hmg.oxford journals.org/cgi/content/full/11/16/1823. May 2002

7) A. Krauss. Wavelength dependence of the optomotor response in zebrafish (Danio rerio). May 2003. Pubmed

8) SD. Hurn. Day-blindness in three dogs: clinical and electroretinographic findings. Pubmed

9) SS. Deep. Molecular genetics of color-vision deficiencies. Pubmed

10) MS. Loop. Color vision sensitivity in normally dichromatic species and humans. Pubmed

11) Sullivan, Walter. Vision Through Animal Eyes Reveals Surprising Color. http://query.nytimes.com/gst/fullpage.html?sec=health&res=9F06E7D91E38F930A2575BC0A963948260&n=Top%2fReference%2fTimes%20Topics%2fSubjects%2fC%2fColor)http://query.nytimes.com/gst/fullpage.html?sec=health&res=9F06E7D91E38F930A2575BC0A963948260&n=Top%2fReference%2fTimes%20Topics%2fSubjects%2fC%2fColor. August 1985

12) What Color Blind People See. http://colorvisiontesting.com/what%20colorblind %20people%20see.htm)http://colorvisiontesting.com/what%20colorblind%20people%20see.htm. June 2001