雅思阅读第087套P2-Colour Blindness

Colour Blindness

Colour blindness results from an absence or malfunction of certain colour-sensitive cells in the retina. The retina is a neuro-membrane lining the inside back of the eye, behind the lens. The retina contains both rod cells (active in low light or night vision but which cannot distinguish colour) and cone cells (active in normal daylight, sensitive to colour). Cone cells, also called photoreceptors, are concentrated mostly in the central part of the retina, in an area called the macula. Cone cells provide clear, sharp colour vision. The cones contain light-sensitive pigments that are sensitive to the range of wavelengths. There are three different types of cones with one sensitive to short wavelengths, or the colour blue, one sensitive to medium wavelengths, or the colour green, and the other sensitive to higher wavelengths, or the colour red. All of these cells send information about colour to the brain via the optic nerve which connects to the¹ retina at a point very close to the macula. Normal persons, referred to as trichromats, are able to match all colours of the spectrum by using a combination of these three fundamental colour sensitivities. Hence, the huge variety of colours we perceive stems from the cone cells’ response to different compositions of wavelengths of light.

There are many types of colour blindness. When there are deficiencies in the cones, either at birth or acquired in other ways, the cones are not able to distinguish the particular wavelengths and thus, that colour range is seen differently. Those with defective colour vision have a deficiency or absence in one or more of the pigments. People with a deficiency in one of the pigments (the most common type of colour vision problem) are called anomalous trichromats. When one of the cone pigments is absent and colour is reduced to two dimensions, dichromacy occurs. These individuals normally know they have a colour vision problem and it can affect their lives on a daily basis. They see no perceptible difference between red, orange, yellow, and green. All these colours that seem so different to the normal viewer appear to them to be the same colour. Missing the cones responsible for green and red hues can also affect the sensitivity to brightness.

Most cases of colour blindness, about 99%, are inherited, resulting from partial or complete loss of function in one or more of the different cone systems and affect both eyes without worsening over time. The most common are red-green hereditary (genetic) photoreceptor disorders collectively referred to as “red-green colour blindness”. It affects 8% of all males of European origin and 0.4% of all females. The gene for this is carried in the X chromosome. Since males have an X-Y pairing and females have X-X, colour blindness can occur much more easily in males and is typically passed to them by their mothers. In other words, females may be carriers of colour blindness, but males are more commonly affected. People with this disorder cannot identify red or green by itself but can if among a coloured group. Other forms of colour blindness are much more rare. They include problems in discriminating blues from yellows. Both colours are seen as white or grey. This disorder occurs with equal frequency in men and women and usually accompanies certain other physical disorders, such as liver disease or diabetes.

The rarest form of all is total colour blindness, monochromacy, where one can only see grey or shades of black, grey and white as in a black-and-white film or photograph. Monochromacy occurs when two or all three of the cone pigments are missing and colour and lightness vision is reduced to one dimension. Another term for total colour blindness is achromatopsia, the inability to see colour.

Inherited colour vision problems cannot be treated or corrected. Some acquired colour vision problems can be treated with surgery, such as the removal of a cataract, depending on the cause. Certain types of tinted filters and contact lenses may also help an individual to distinguish different colours better. Additionally, computer software has been developed to assist those with visual colour difficulties and those with mild colour deficiencies to learn to associate colours with certain objects and are usually able to identify colour in the same way as everyone else. One frequent problem encountered is with traffic lights, and worst of all, warning lights: colour-blind people always know the position of the colours on the traffic light – in most situations; red on top, yellow in the centre, green on the bottom. But warning lights present an entirely different problem. In this situation there is only one light; no top or bottom, no right or left, just one light that is either red or yellow.

Colour vision problems can have a significant impact on a person’s life, learning abilities and career choices. On an everyday basis, there are some annoyances and frustrations: not being able to differentiate between green or ripe tomatoes when preparing food, for example, or buying clothes that to the ‘normal’ eye seem positively garish. However, people with colour vision problems usually learn to compensate for their inability to see colours. Although there is little or no treatment for colour blindness, most colour deficient persons compensate well for their defect and may even discover instances in which they can discern details and images that would escape normal-sighted persons. At one time the US Army found that colour-blind persons can spot camouflage colours in cases where those with normal colour vision are typically fooled.

SECTION 2: QUESTIONS 14-26

Questions 14-20

Complete each sentence with the correct ending A – K from the box below.

Write the correct letter A – K in spaces 14-20 below.

14 __________ Colour blindness can be caused by a birth defect, or

15 __________ Surprisingly, some people who are colour blind

16 __________ People with hereditary colour blindness

17 __________ Because of our genetic make-up, colour blindness

18 __________ Red-green genetic photoreceptor disorders mean that people

19 __________ People with monochromacy

20 __________ The inability to see certain lights

A	can see better at night than during the day
B	cannot be treated by surgery
C	can affect men much more easily than women
D	can affect their sensitivity to bright lights
E	can see no colour at all, other than shades of black, grey and white
F	can see things that people with normal vision cannot
G	can have very dangerous consequences for colour-blind people.
H	can be acquired or inherited
I	can mean having to wear contact lenses.
J	cannot distinguish certain colours if they stand alone
K	can match all colours of the spectrum