Sample Medical Science Research Paper on Color Sensing Pigments

Color Sensing Pigments

Human beings must have three different color-sensing pigments to have color vision, which allows for the perception of about 2-7 million colors. This feature, also known as trichromacy refers to possessing of three independent channels for conveying color information from three different forms of cone cells in the eye. This study seeks to find out why these organisms must have the three-color sensing pigments for color vision. The study discusses these types of cones, trichromacy, mechanism of trichromatic vision, color perception, theories related to vision, color of the brain,

Trichromacy

 Human beings are organisms with trichromacy and thus, referred to as the trichromats because their retina have three types of color receptors referred to as the cone cells which have different absorption spectra (Hyper Physics, 2010). In reality, these receptors may be greater than three may, since varying forms of these receptors are active in varying light intensities. The three types of cone cells in trichromats at low light intensities are the rod cells that contribute to color vision (Wyszecki & Stiles, 2002).

Mechanism of Trichromatic Vision

            Color vision refers to the ability of an organism to distinguish objects based on the wavelengths of light reflected. Colors are measured and quantified in several ways, thus, making the perception of colors of a person a subjective process through which the brain responds to. Stimuli are produced in the event light reacts with numerous forms of cone cells in the eye. Naturally, diverse people perceive the same illuminated object in diverse ways.

            According to Jacobs & Nathans (2009), Isaac Newton made a discovery that white light splits into various colors when passed in dispersive prism. In addition, he discovered that these colors could be recombined by passing them through different prism to make white light. These colors, red, orange, yellow, green, blue, indigo, and violet are derived from long and short wavelengths from low to high frequency respectively. The differences in wavelengths result to perceived hue, from one nm in blue-green and yellow wavelengths; to longer, red and shorter blue wavelengths. When the pure spectral colors are mixed together, chromaticites that are quite high can be distinguished, although the human eye only distinguishes hundred hues (Widermann, Barton, & Russel, 2011). In very low light levels, vision is scotopic since light is detected by rod cells of the retina, which are maximally sensitive to wavelengths of about 500 nm. The rods play minimal role in color vision. In brighter light, vision is phototopic, to imply that light is detected by cone cells that are highly essential in color vision. Cones are sensitive to various wavelengths and more sensitive to wavelengths of about 555 nm. In between, there is the mesopic vision where both the rods and the cones offer signals to the retinal ganglion cells. The perceived white is derived by the comprehensive spectrum of visible light. White color is therefore perceived by coalescing red, green, and blue wavelengths or complementary colors such as blue and yellow.

Perception of Color

            Perception of the color commences with the specialized retinal cells that contain pigments of varied spectral sensitivities referred to as the cone cells. These three forms of cones that is sensitive to three varied spectra results to trichromatic color vision (Leong, 2013). From the cone cells there are pigments that are made of opsin apoprotein, which is covalently related to 11-cis-hydroretinal or the seldom 11-cis-dehysroretinal (Web Exhibits, 2016). These three types of cones are labeled according to the ordering of wavelengths of the spectral sensitivities, starting from short (S), medium (M), and long (L) types. The three forms do not comprehensively respond to particular colors, rather the perception of the colors are derived from a complicated process that commences with differential output of the cells in the retina and completed in the visual cortex and associated brain regions (Strettoi, et al, 2010). For instance, the L cones are referred simply to as the red receptors, while their sensitivity according to microspectrophotometry the greenish-yellow region within the spectrum. The S and the M do not openly respond to the blue and green, even though they are often described in such spectra. A convenient approach of representing color is the RGB (Red Green Blue) color model, which on the contrary is based indirectly on the cone type of the human eye. The peak response of the cone cells varies even among the people with normal color vision (Jameson, Bimler & Wasserman, 2006).

Cone Cells

Cones form part of the photoreceptor cells in the retina and are responsible for color vision (Diana, Robert & Russel, 2011). Cone cells are found in the fovea centralis at the retina and operate maximally in bright light unlike rods that operate best in dim light. At least six million cones are present in the eye and are more concentrated in the macula (Kandel, Schwartz, & Jessell, 2000). Rod cells are more sensitive to light but cones allow perception of color and perception of finer details and more speedy modifications in images due to the response times to stimuli than the rods (Schacter, 2009). The S, L, M cones are sensitive to visible wavelengths that respond to the short-wavelengths, medium wavelengths, and long wavelengths. The table attached in the Appendix 1 summarizes the cones types and their related features.

The three pigments responsible for detecting light vary in their precise chemical composition due to genetic mutation, variation of the cones with assorted color sensitivity (Osorio, & Vorobyev, 2008). When the cone cells are destroyed by disease, blindness ensues. It is assumed that cones possess heightened visual acuity because every cone cell carries a lone connection to the optic nerve, and hence they can easily differentiate two diverse stimuli.

Unlike rods, cones cells are shorter but wider and tapered (Eysenck & Keane, 2005). They are countable than rods in other parts of the retina apart from the outnumbered rods at the fovea. Structurally, these cells are cone like in shape at one end where the pigment filters incoming light, thus providing different response curves. Cones are typically 40–50 µm long while their diameter are around 0.5 – 4.0 µm as the smallest. They are tightly packed at the centre of the eye, specially the fovea. The S-cone is however larger than the rest. To determine the arrangement of the cones, a process referred to as photo bleaching is used (Purves, Augustine, & Fitzpatrick, 2001). Photo bleaching is achieved by depicting dark-adapted retina to a given wavelength of light that paralyzes the particular form of sensitivity to the wavelength up to 30 minutes from its ability of adapting to darkness that makes it appear white. This contrasts the grey dark-adapted cones that appear when an image of the retina is taken. Findings from the images usually illustrate that the S-cones are randomly positioned and are less in number than the M and L cones. The ratio of the M and L cones fluctuates among various people with regular vision.

Similar to the rods, the out portions of the cones have paginations in their membranes that further affect the pigments. The disks attached to the outer membranes, are pinched off, and survive separately in rods. Neither the rods nor the cones divide however; their membranous disks wear out at the end of the outer segment and are consumed and recycled by the phagocyte cells. The response of the cells to light is besides nonuniform as the center of the pupil peaks as a result of stiles- Crawford effect (Sayim, Jameson, Alvarado & Szeszel, 2005).

Theories relate to vision

There are two complementary models of color vision; the trichromatic (Young-Helmholtz) theory and the opponent process model (Hunt, 2004). The trichromatic model affirms the three types of cones in the retina as preferentially blue, green, and red (Sharpe et al, 2006). The opponent theory affirms that the visual system interprets color antagonistically; red vs. green, blue vs. yellow, and black vs. white.

An assortment of wavelengths of light stimulates these receptors in varying levels. The yellowish-green light stimulates L and M cones similarly but only S cone weakly. The Red light stimulates L cones more than the M cones whereas the blue-green light stimulates M cones more than the L cones (Hunt, 2004). The brain combines information derived from every receptor to produce varied perceptions of different wavelengths of light. The photo pigments are usually present in the L and M cones, which are encoded on the X chromosome. Conversely, defective encoding of these chromosomes results to the common forms of color blindness.

Perception of color in the Brain

Processing of the color in the brain commences at a very early stage within the visual system in the retina through color opponent mechanisms. Trichromacy arises from the receptors while the opponent model arises at the retinal ganglion cells and beyond (Vita, 2016). In the opponent theory, the mechanisms refer to the opposing color effect whereas in the visual system, the activities of the varied receptors are opposed. From the retinal ganglion cells, visual information is sent usually to the brain through the optic nerve to the optic chiasma. From the chiasma, visual tracts also known as optic tracts enter the thalamus to synapse at the lateral geniculate nucleus (LGN) (Roorda  & Williams, 2009).

Lateral geniculate nucleus is divided into M-laminae, koniocellular laminae, and the P-laminae (Wyszecki & Stiles, 2002). The M-laminae consists of M-cells, , while the P-laminae consists of the P-cells, that both receive input from the L and M cones from the retina. The Konioellualr laniane receives axons from the small bistratified ganglion cells. Synapsing occurs at the LGN, as the visual tract moves on to the primary visual cortex (V1) at the back of the brain within the occipital lobe. At the V1, the three-color separation breaks down as the associated cells respond to some parts of the spectrum better than others do. Since color tuning is unstable, varied, relatively small populations of the neurons within V1 assume the role of color vision.

Color formation from the V1 blobs is sent to the cells in V2, which are most strongly tuned for the clustered in thin stripes. Within V2, neurons synapse onto cells in V4 and posterior inferioi temporal cortex. Color processing within V4 takes place in globs, which is the first portion where color is processed in full ranges of hues in color space. Other studies prove that neurons in extended V4 offer input to the inferior temporal lobe (Oyster, 2009).

Conclusion

            From the study, human beings have three different color-sensing pigments for color vision. This feature, also referred to as trichromacy is useful for transmitting color information from three different forms of cone cells in the eye. Trichromacy is a feature of the human vision since their retina has three types of color receptors referred to as the cone cells with different absorption spectra. Humans possess other useful cells for light vision known as the rods cells, which perceive light at low light intensities. Colors are measured and quantified in numerous ways. The perception of colors therefore, of a person is a subjective process through which the brain responds to. Perception of the color commences with the specialized retinal cells that contain pigments of varied spectral sensitivities referred to as the cone cells. The peak response of the cone cells varies among every person. Cone cells are a component of the photoreceptor cells in the retina and are responsible for color vision. Cone cells are found in the fovea centralis at the retina and operate maximally in bright light unlike rods that operate best in dim light.

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Appendix

Appendix 1: The table below summarizes the cone cells of the eye

Table 1: Cones Cells

Cone TypeNameRangePeak Wavelength
Sβ400-500 nm420-440 nm
Mγ450 -630 nm534-555 nm
Nρ500-700 nm564 -580 nm