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| March 2000 Colour vision deficiency Seeing is not believing By Levina Sung Colour vision deficiency, which is a kind of colour-blindness, poses difficulties for subjects in distinguishing some colours. Human eyes perceive colours with special nerve cells on the retina at the back of the eyeball. There are two kinds of nerve cells — cones and rods — which respond to colours and brightness respectively. Cones are cone-like nerve cells containing three kinds of chemicals responding to the primary colours — red, green and blue. With different chemicals, cones are divided into red, green and blue cones. These cones work together to identify patterns and colours. Colour vision deficiency appears when the number of cones is inadequate or some cones do not function. About one-third of subjects suffer from severe colour vision problems when they lack one of the chemicals in the cones. Some colour vision deficient subjects cannot recognize pale colours under poor lighting or when they are tired. The two most common types of colour vision deficiency are red-green defiance and blue-yellow defiance. According to Dr. Dorothy Fan of the Department of Ophthalmology and Visual Science at The Chinese University of Hong Kong, males are more likely to be colour deficient. The findings were based on two research projects on colour vision of children aged from 2 to 13. One was carried out in two kindergartens in 1996, while the other was done in 19 primary and secondary schools from September 1998 to June 1999. Dr. Fan said about 5.5 percent of males and 1 percent of females in Hong Kong experience colour deficiency. Chromosomes determine the gender of a person. Males have one X chromosome and one Y chromosome, while females have a pair of Y chromosomes. Male have higher risk of inheriting the problem because the X chromosome carries the genes that determine the development of colour sensitive cones. When there are any faults in the genes carried by X chromosome, baby boys will be born with the deficiency. Baby boys sometimes suffer from inherited deficiency when they inherit defected genes from their mothers who possess normal vision. However, boys may not necessary have the problem if their fathers suffered from the deficiency. In addition to inherited colour vision deficiency under these conditions, acquired deficiency also exists. Acquired deficiency is caused by reasons such as macular degeneration, optic nerve diseases, inflammation or detachment of retina. Besides, other diseases like diabetes may also contribute to acquired deficiency. The problem may get worse if the disease becomes more serious. Subjects who suffer from the deficiency may have different degrees of deficiency between the right and left eyes. An inherited deficiency, on the contrary, yields identical degrees of deficiency in both eyes. There are many tests to detect colour vision deficiency. According to Mr. Ricky Lai, a Grade 1 optometrist at the Hong Kong Eye Hospital, Ishihara plates from Japan are the most common test for red-green colour defiance. A full version of Ishihara plates is made up of 38 plates. Out of the 38 plates, 25 plates contain numerals and the remaining plates show the patterns. Each plate consists of a series of spots of varying colours and hues. The figure on the plate can thus be distinguished from the background. Different plates have different functions, such as red-green screening, demonstration of visual tasks and classification of red and green cone deficiencies. People with normal colour vision can recognize the figure from the background without any difficulty. However, for the colour vision deficient subjects, they are either unable to recognize the figure or see a different figure on the plate. For severe colour vision deficiency, subjects are asked to match specified colours from a mixture of different colours or put coloured objects in a correct sequence. Colour vision deficient patients face many difficulties. Colours are constantly used to distinguish colours of surrounding objects — for example, traffic lights. Many jobs require normal colour vision to perform the duties. For instance, pilots are required to have excellent colour vision, and designers ought to be sensitive to colours. Mr. Pete Yeung is a Year 2 university student who has colour vision deficiency. He has problems differentiating red and green objects. Though Mr. Yeung is not much affected by the deficiency in his daily life, he encounters problems when dealing with colours. For instance, when he was in primary school, he had problems in art lessons. He said, “I painted a brown sun. Besides, when the art teacher told us to pick up blue colour pens, I did not know which pen I should pick up.” A similar problem occurred as he was doing assignment for a photojournalism course. “I cannot distinguish the colours easily while checking the photos on a computer, but my classmates can do so,” he said. Although colour vision deficiency poses an inconvenience, it does have some strategic functions. In World War II, people suffering from colour blindness were employed to spot camouflaged camps because they would not be easily confused by the camouflage. No treatment for inherited colour vision deficiency is available. There’s also no test to discover whether people possess colour vision deficient genes or not. |
Japanese Ishihara plates are effective in detecting colour vision deficiencies. (Courtesy of Dr. Dorothy Fan)
These are samples of Ishihara plates
showing figures of numerals and patterns. Each of the figures consists of spots of varying
colours. (Courtesy of Dr. Dorothy Fan)
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