The Hubble Space Telescope (HST), the first of its kind, was sent to space to investigate the cosmic matter by analysing the light coming from it. The HST has shown that the distant galaxies are more redder than the nearest galaxy and the phenomena is called redshift. Our trichromatic eye could observe these changes and precisely proved that the universe is expanding and later proposed the Big Bang Theory by reverse calculation of shrinkage of the space and time in the far past 13.7 billion years ago. The Nobel Prize was given to three teams: The Supernova Cosmology Project (Lawrence Berkeley National Laboratory and University of California, Berkeley, CA, USA), The High-z Supernova Search Team (Australian National University, Weston Creek, Australia) and The High-z Supernova Search Team (John Hopkins University and Space Telescope Science Institute, Baltimore, MD, USA) for the discovery of the expanding universe in 2011.
We humans are trichromatic (tri: three, chroma: colour) living beings who can see the sunlight as a seven-colour rainbow in any rainy day. The white sunlight passes through water droplets and undergoes refraction followed by splitting into different spectrums, and we can see only a few colours which were given seven names by Sir Isaac Newton in 1660 — violet, indigo, blue, green, yellow, orange and red. But basically, there are only three colours — red, green, and blue — designated as primary colours. The other colours are derivatives of these three primary colours. But why it is only red, green, and blue, and not any other colour? This is because of three types of receptor genes in our eye expressing at the back surface in the retina. These three receptors are packed in three bags of cells called the cone photoreceptor. These three cone photoreceptor cells are also called the blue cone or short-wave sensitive (SWS) cone, the green cone or medium wave sensitive (MWS) cone and the red cone or long wave sensitive (LWS) cone. The SWS gene is present in autosome (other than the sex chromosome, in 7th chromosome) but the MWS and LWS are present in sex chromosome (in X chromosome, male has XY chromosome and female has XX chromosome). Since the MWS and LWS genes are present in X chromosome, it is shown that the women can see more colour than men. The SWS, MWS and LWS genes encode three types of proteins called blue opsin, green opsin and red opsin which are sensitised by three wavelengths of light that are blue, green, and red spectra, respectively. The other colours in the rainbow are derivative of these three colours created in our head in the visual cortex.
Except humans and primates (monkey), most of the mammals such as dogs, cats, cows are dichromatic creatures. The dichromatic eyes are deficient of LWS cone. These animals cannot see the seven colours of the rainbow rather can see only two colours that are blue and yellow. The monochromatic animals (seals, sea lions, walruses, dolphins, and whales) who have only one type of cone can only visualise the world in grey scale. Professor Jay Neitz, University of Washington, Seattle, USA, says that an ordinary human can see around one million colour hues. It is proven scientifically that every addition of colour sensitive cone increases the colour vision by hundred-fold. For example, the monochromatic eye can only see 100 shades (though it is invariably grey colour), but for dichromatic creatures this is increase to 100-fold that is 10,000 shades (100×100) and for the trichromatic animals the colour shades increase to 1 million (100x100x100).
Linhares et al. in his paper titled “The number of discernible colors in natural scenes” in Journal of Optical Society of America has shown that a normal human can see 2.3 million discernible colours. The photography industry has come up with more shades combination. In the Adobe Photoshop, there are three colour channels that are red, green, and blue and each are having shades ranging from 0-255. The number 0 is black, and number 255 is either blue, green, or red. A normal eye can see 16 million hues, according to Adobe Photoshop photography (255x255x255) in a picture. In this way, theoretically in dichromatic and monochromatic animals the colour visualisation luxury decreases to 65025 (255×255) and 255, respectively. But due to the principle of univariance (comparison of colour in brain), the dichromat can see very less colour as compared to theoretical prediction.
Achromatopsia is a disease where people cannot perceive any colour and for them the world is a dull grey colour.
Patients with achromatopsia are deficient of three types of colour photoreceptors. This is a hereditary disease with very rare occurrence. A neurologist, Oliver Sacks, has described in his book “The Island of Color Blind” about a tribe in Pingelap atoll, a Micronesian island in the South Pacific where everybody is colour blind. But the red-green colour blindness is very common in people which are very similar to dichromatic eyes in mammals. The photosensitive colour receptors originated 550 million years ago in the Cambrian period in the water. Scientists thought that the upgradation of the chromaticity is a gradual evolution for the vision. It is impossible to give a dichromatic eye a trichromatic vision. They argue that the neural wiring is not possible if there is sudden change of dichromatic to trichromatic eye. But this is now possible in a few research studies on mice and owl monkeys (this monkey is nocturnal and does not have red cone).
Dr Jeremy Nathans and Dr Jay Neitz in their research papers published in the peer reviewed journal Science and Nature have engineered the trichromatic vision in the dichromatic eye. In this research, they have shown that supplementation of LWS gene to the mice and owl monkey using different delivery techniques causes upgradation of colour vision. Mice and owl monkeys are deficient in LWS gene and hence cannot recognise the red colour. The behavioural test in mice and owl monkey established that they can see the red colour sweet soya flavour or fruit respectively after supplementation of LWS gene. Theoretically, now we can give mice and monkey the colour vision luxury of 10,000 to two million colours. This discovery will pave the way to cure the inherited colour-blindness disease (those who are colour blind from birth).
One of the assumptions is that every upgradation of chromaticity increases the arena of colourful vision to a very vast order. We know now that mantis shrimp has 16 different types of colour cones. I cannot comprehend how they can see the world in colour. But if I could, then this colorful trichromatic vision of ours is just a black and white image to mantis shrimp.
The writer is Senior Research Scientist, University of Massachusetts Medical School, Massachusetts, USA. Views are personal.