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Color Perception

rkirloskar's picture

                                                                                Color Perception

I am not only fascinated by our ability to see color, but also fascinated  by the fact that different animals perceive color differently. If different animals perceive color differently then different animals perceive the world differently. This does not mean that one animal is superior or inferior to another. It just means that every animal is able to see a different aspect of the world. Animals evolved different color perception to enable them to survive in their differing environments. Color perception plays an important role as it helps animals to distinguish suitable food (ripe food), suitable mates (male plumage) and even to distinguish objects (as in humans). Therefore color perception plays an important role in enabling our survival.

We are able to see color when our eyes have the capability of discriminating light on the basis of its spectral composition. Our eyes are able to do this because they contain three different types of photoreceptors called rods that contain photo-pigments that sense different portions of the visible spectrum. Each cone is able to perceive long wavelengths (like red), middle wavelengths (like green) or short wavelengths (like blue); hence these cones are referred to as L-, M-and S-cones respectively. Each of these photo pigments has a slightly different peak sensitivity, so light at any wavelength in the visible spectrum will excite one or more of these three types of photoreceptors. We are able to see color by comparing the signals that each cone senses. Therefore our ability to see color depends upon comparing the outputs of the three different cones that have different spectral sensitivities

Our retina contains millions of photoreceptor cells called rods and cones. These receptors contain membranes that in turn contain visual pigments that absorb light and undergo a change that triggers an electrical signal. The visual photo pigments contained in both rods and cones are very similar as they both consist of opsins. The three cones in our eyes contain slightly different opsins that have different peak wavelength absorption for each pigment. Rhodopsin is the retinal protein found in rod cells of the eye that that enable night vision. The rods, which are sensitive to light are concentrated in the periphery of the retina, whereas the cones are concentrated in the center of the fovea . Rods are more sensitive to light and a wider range of wavelengths than cones, but cones are more sensitive to color and need high levels of illumination.

 We are able to see color only when the signal from the retina is processed. Our ability to see color depends upon comparing the outputs of the three different cones that have different spectral sensitivities. The signal from the retina consisting of the output of the three cones, is analyzed by nerve cells called the retinal ganglion cells which compare the stimulation of neighboring cones and determine the color of the light reaching a patch of the cones. This signal then travels to the brain where it is divided into numerous pathways branching throughout the cortex. An example of this is when visual signals from the photoreceptors travel to the retinal ganglion cells that code for color information and then to the lateral geniculate nucleus (LGN) in the thalamus and proceed to the primary visual cortex. The primary visual cortex then maintains the relationships of the images on the retina. This is called retinoptic organization.

                The fascinating aspect of color perception is that not all animals perceive color as humans do. Even some humans perceive color differently than others. Birds and fish possess more sophisticated color visual systems than humans. Fish such as piranhas and goldfish are able to see infrared. While humans are trichromats and have photo-pigments with peak sensitivities at three peak wavelengths, birds have photo-pigments that have peak sensitivities at four or five peak wavelengths. Some bird s and can even see ultraviolet light, a property thought to be unique to insects. For example, the peregrine falcon is able to track rodents foraging on the ground by the trail of urine that they leave behind. This urine trail glows a fluorescent yellow color in ultraviolet light, which the peregrine is able to see from many miles above. In fact, predatory birds and songbirds are believed to have the sharpest vision among birds, possessing five times the number of cones per square-millimeter present in the human eye.(The human eye has a maximum of 200,000 cones per square millimeter.) Many nectar feeding birds rely on ultraviolet light to detect flowers suitable for collecting nectar. It is very hard for us to imagine how birds perceive ultraviolet as it lies outside our experience. Many birds that have identical plumage for both sexes, differ when seen in ultraviolet light, a difference only apparent to them.

                Bees on the other hand can only see four colors in “our visible spectrum” which are black, white, yellow and blue. But they can also see ultraviolet. Unlike humans that can see wavelengths ranging from about 400nm to 700nm, bees can see wavelengths ranging from 300nm to 600nm. Therefore bees cannot see the color red, but they can see ultraviolet. So, if you ever thought that red colored flowers were pollinated by bees, you were wrong. The only red colored flower pollinated by bees is the red Poppy, which reflects ultraviolet light. So the bees are attracted to the ultraviolet light that they see and are not attracted to the flower because it is red. Birds on the other hand are very sensitive to the color red, so most red colored flowers are pollinated by birds.

                We know that dogs see in black and white and also in many shades of blue. They also see the world in more shades of grey than humans. Horses have two types of cones in their retinas and see the world similarly to people who suffer from red-green color blindness.

                All these examples show us how differently different animals perceive the world. Every organism only sees an aspect of the world it needs to see to ensure its survival. What intrigues me is that we only study how other organisms see that particular object that we can also see, but there may be so many other aspects of the world that those animals see that we cannot even conceive because it lies outside our experience.

The point that I am trying to make is that we will never see the world from every aspect. In other words, we will never be able to see the complete truth. We are limited by our very biology and we are limited by our unlimited intellect. Our intellect keeps perceiving patterns and defining every object that we see into a form. Therefore we say that everything that we see is actually a construction of the mind.

There is a very apt story in Hindu philosophy about how six blind men tried to describe an elephant. The first blind man touched the leg of the elephant and said that it was like a pillar, the second blind man touched the tail of the elephant and said that it is like a rope, the third blind man touched the  trunk of the elephant and said that it was like the branch of a tree , the fourth blind man touched the ear of the elephant and said that it was like a hand fan, the fifth blind man who touched the elephant’s belly said that it was like a wall and the sixth blind man who touched the elephant’s tusk said that it was like a pipe.

The description given by all the six men was right, but it was only part of the truth. Hence when we talk about color perception and how different organisms see the world, it is not about whether one organism is more right than the other organisms, it’s simply a different perspective. Therefore I feel that all of us are like those blind men in one way or the other, we will never see the complete truth. This is also a problem faced in science, where the very act of observing perturbs what is being observed. As a result, we will never know how what was being observed behaved before we directed a light particle at it. Therefore it is impossible to know the complete truth. But since I am Hindu I will say that there is a way to see the complete truth and that it by meditation and attainment of moksha or nirvana, where you transcend the body and all physical attachments and become part of the universal soul and cosmic energy.

I find the concept of differing perception of color in animals a result of living in parallel worlds a very fascinating idea. Since many animals perceive this world in an entirely different manner than humans, it is possible that we in fact live in a world that has many worlds mapped onto it and that we are just part of one of these worlds. Maybe these worlds are able to be mapped onto each other because the atoms making up these different worlds vibrate at different frequencies and atoms vibrating at different frequencies emit different colors of light. So it is possible that birds that can see ultraviolet light see a whole new world that we cannot ever comprehend because it lies outside our experience. Therefore our biology dictates the world that we will be able to perceive, but Hinduism says that through meditation we can transcend our body and become a part of the cosmic energy. This may be because all these worlds within worlds may have a particular frequency at which all of them converge. If we are able to change the frequency or fine tune the frequency at which the atoms of our body vibrate, maybe we will be able to be in sync with the frequency at which all the worlds converge and transcend our body to become part of the universal soul. According to Hinduism, only meditation will allow you to do so.

I feel that differing color perception has more to it than just ensuring our survival but we will never know as again, we will never be able to experience it (except through meditation). All we can do is study how animals perceive “our world” so that we are able to understand their behavior.