The Perception of Color

The white light is a small fraction of the vast electromagnetic spectrum of the solar radiation, the ultimate source of energy for life forms on the Earth. The energy being quantified is expressed by the equation, E= ηλ, where, E is the energy associated with the electromagnetic radiation (EMR) of specified wavelength (λ) and h is the frequency of the EMR. Fundamentally, a conjugated biomolecule absorbs the light energy while one or more of its electrons reach the excited states. This process conserves the solar energy into chemical energy because the excited electron release energy while coming back to the ground state.The excitation of theelectron(s) and subsequent change in theelectronic configuration associated with photoexcitation of one or more biomolecules may elicit phototropism and visual perception.

The perception of light has evolved as thevisual perception in the kingdom Animalia. Visual perception is the ability of an animal to differentiate the energy of various wavelengths of the visible spectrum of light, to a limited extent, by the developing color patterns. Since the energy of the photon of a specific wavelength is quantized, it can have a certain fixed amount of energy. Therefore, it can only excite a photosensitive biomolecule which excitation energy is lesser or equal to its quantized energy. Further, when associated with their protein counterparts, the photosensitive biomolecules can absorb photon only with limited values of wavelengths.Thus, various combinations of proteins, photosensitive molecules and the quality and quantity of the photon can altogether develop a pattern to sense different colors in the white light. In other words, the photosensitive biomolecules associated with their cognate proteins act as biological prism that distinguish one color from the other in the visible spectrum.It enables the animal to perceive the sense of color and thrive in the ecosystem accordingly. An animal can evade its predator or other threats by perceiving their color or may decide the quality of food (example- theextent of fruit ripening), too. Color perception also helps animals to select a better mating partner, develop avisual memory of objects in the surroundings, express moods and much more depending on the species. The aspect of color perception varies both qualitatively and quantitatively among the species of kingdom Animalia. Several invertebrates including spiders and bees can visualize color in ultraviolet spectrum while most mammals other including humans can't. Nocturnal animals like snakes can also sense the infrared spectrum and use it to capture the preys.

The mechanism of color perception is highly complex and may vary from species to species in one or more aspects. Eye is the organ mediating visual perception of the surroundings. The light reflected from an object is focused on the retina through the lens of the eye. Retina of the eye is responsible for receiving visual stimuli. It consists of two types of photoreceptors namely the rods and cones.Both the cones and rods are essentially neurons that can sense light as a stimulus and convert it into appropriate chemical stimulus. The cones have taped tips, thus being called cones. The cones in thehuman eye are further categorized into S, M or L cones. S cones or small cones are preferably stimulated by relatively smaller wavelengths (l = 440 nanometer, blue color) of the visible spectrum. M cones or medium cones preferably receive visual stimuli from thevisible light of medium wavelengths (l = 535 nanometer, green color). The L cones or large cones are excited by the visible light or relatively larger wavelength ((l = 562 nanometer, red color). Thus, these photoreceptive neurons enable the human eye to visualize three primary colors, blue, green and red (trichromatic vision). The relative intensity and proportion of the three primary colors can generate virtually all possible colors that a human eye can detect in the visible spectrum of light.These cells also provide daylight (photopic) visions and high-resolution color images of the object. The rods, being cylindrical in shape preferentially absorb photons of wavelength 500 nm. These cells remain nonfunctional in even diffused light due to oversaturation by photon irradiance. However, these neurons are essential for providing vision in the dark (scotopic vision). Due to their inability to differentiate light of different wavelengths under dark condition, they only provide amonochromatic vision of gray color.

The photosensitive of cones and rods are photopsin and rhodopsin, respectively. These pigments are exposed to the extracellular matrix through their integration in the plasma membrane. Once excited by the visible light of specific wavelengths, the photosensitive neurons transmit the stimuli to bipolar cells. The stimulated bipolar cells (neurons) transmit the stimuli to the optic nerve fibers through ganglion cells. The optic nerve fibers subsequently transmit the stimuli to the occipital lobe (visual cortex) of the brain through the optical tract. The visual cortex qualitatively and quantitatively processes the stimuli to produce images and provide color perception. The relative abundance of the photopsin pigments and their polymorphism in the population also causes a difference in the pattern of color perception of the same object by different people.

A mutation in the genes encoding photosensitive photopsin molecules in the small, medium or large cones may lead the visual disorder known asanomalous trichromacy, also commonly known ascolor blindness. Dichromacy is a state of color blindness in which the affected individual can perceive only two primary colors because out the mutation in the genes encoding the photopsin perceiving the third color. For example, a mutated photopsin in the medium cones inhibits the green color perception. Thus, a person can perceive blue and red color (dichromacy) but can't see green color. Color blindness may also arise due to insufficient production of the photopsin pigment in one or more cones. Monochromacy is an extremely rare type of color blindness in which the individual can perceive no color at all. In such people, the vision is total due to the activities of the rods (rod monochromatism) that give black and white vision.