|
Monochromacy (''mono'' meaning one and ''chromo'' color) is, among animals, the condition of possessing only one type of cone cell for color vision, which results in a monochromatic-like (in most cases black and white) vision.〔Cassin, B. and Solomon, S. ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company, 1990.〕 Organisms with monochromacy are called ''monochromats''. The perceptual effect of any arbitrarily chosen light from the visible spectrum can be matched by any pure spectral light. Many species, such as all marine mammals, the owl monkey, and the Australian sea lion (pictured at right) are monochromats under normal conditions. However, in other mammal species, including humans, individuals having such vision are considered carriers of full color blindness disease. In humans that disease is called achromatopsia.〔("Colour Blindness." ) Tiresias.org. Accessed September 29, 2006.〕 ==Causes== There are two kinds of visual receptors in humans: # Rods, which are extremely abundant in numbers (about 120 million) in the periphery of the human retina. Rods only respond to faint levels of light and are very light sensitive, therefore, completely useless in daylight because bright light bleaches them. # Cones, which are mostly near the fovea in the eye and are less active in dim light, more useful in bright light, and essential for color vision.〔 There are three types of cones in normal human eyes (short, medium, and long wavelength, sometimes called blue, green, and red); each detects a different range of wavelengths. Rods outnumber cones by about 20 to 1 in the human retina, but cones provide about 90% of the brain's input.〔 Cones respond faster than rods, and have three types of pigments with different color sensitivities, where rods only have one and so are achromatic (colorless).〔 Because of the distribution of rods and cones in the human eye, people have good color vision near the fovea (where cones are) but not in the periphery (where the rods are).〔 These types of color blindness can be inherited, resulting from alterations in cone pigments:〔 # Anomalous trichromacy, when one of the three cone pigments is altered in its spectral sensitivity, but trichromacy (distinguishing color by both the green-red and blue-yellow distinctions) is not fully impaired. # Dichromacy, when one of the cone pigments is missing and colour is reduced to the green-red distinction only or the blue-yellow distinction only. # Monochromacy when two of the cone pigments are missing and vision is reduced to black and grey-shades and white. # Monochromacy when all three of the cone pigments are missing, and he is seeing only with his rod cells, and vision is reduced to black and grey-shades and white. Out of all ways of inheriting color blindness, this type of monochromacy is the severest of the types because of the lack of cone pigments in the eye. Monochromacy can either be acquired or inherited: Monochromacy is when the organism's retina contains only a single kind of light receptor cell, or that only one kind of light receptor is active at any particular level of illumination. Therefore, monochromacy is caused by either a defect or complete absence of the retinal cones.〔 As for humans who suffer from blue-cone monochromacy, or (BCM), research has shown through examining different families that BCM is inherited as an X-linked recessive trait.〔 Researchers also concluded through multiple studies in the human eye, that there are various pathways to mutation resulting in BCM. People who have red- or green-cone monochromacy, which is a term used to encompass protanomaly, deuteranomaly, protanopia, and deuteranopia, both are X-linked recessive traits. Author J. W. Baird, also argues in his article "The Problems of Color Blindness" that color deficiency in the eyes is in fact transmitted by heredity. He also illustrates that color blindness can "result from certain traumatic conditions, disease, and from the action of chemical agents." 〔 An injury or wound to the eye near the retina for example, where the rods and cones are located, could potentially cause color blindness even if that individuals' vision appears to be normal and images/objects still look "sharp". Finally, he concludes that color blindness can also be caused by "doses of santonin and of other drugs produce typical disturbances of color vision.〔 And it may be mentioned in this connection that a continuous chromatic stimulation, as when one sits in a colored illumination, reduces the sensitivity to that color, and hence produces an abnormal condition of color vision.〔 Another way a form of color blindness can be acquired is through aging. As an individual becomes older, the pigment in the cones and rods weakens over time and it is possible to acquire a form of color blindness. Since all monochromacy types are caused by the x-linked recessive trait including the most common type of color deficiency, the red–green color deficiency, caused by the gene deficiency on the X chromosome,〔 it explains why more men than women inherit one of the forms of monochromacy. Since boys are born with the chromosome makeup XY and girls with XX, it is much less likely for a girl to have a defect the same cone gene in both her X chromosomes than for a boy to have that gene defect in his lone X chromosome. However, it is also shown in research that women with one normal gene and one color-deficient gene (and that includes all women with a red-green color deficient father) are slightly less sensitive to red and green than the average for other people.〔 Another important fact author J. W. Baird addresses about color blindness "is that color-blindness is a defect of color-sensing, not of color-naming." In other words, individuals who have a form of color blindness mentally know what the names of the colors are but can not visually "sense" which colors are which depending on what form of color blindness he/she possesses. Baird explains an example of this in his article when he says, "The circumstance that one individual describes a certain spectral region as green or greenish, while another calls it blue or bluish, indicates nothing more than that, in the vocabularies of these two individuals, the same significance does not attach to these two color-names." 〔 Some individuals have diseases or injuries that lead to nyctalopia, or night blindness, where rod cells stop responding properly to light. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Monochromacy」の詳細全文を読む スポンサード リンク
|