HEALTH / Common complaints: Colour blindness

ABOUT one in 12 boys has difficulty in distinguishing red and green lights and so cannot become a sailor, a pilot, or an engine driver. This red-green colour blindness is, in fact, due either to a defect in red vision or in green vision, rarely in both. These defects are much less common in girls, affecting fewer than one in 100. Complete colour blindness, in which the world is seen as if in a black-and-white film, does occur but is extremely rare.

The light-sensitive structures in the retina at the back of the eye are of two kinds, rods and cones, and it is the cones which perceive colour. The rods do not distinguish between the colours, being mostly concerned with vision in poor light and at the edges of the visual fields - things seen out of the corner of the eye.

Each eye has between four and seven million cones, most of which are packed into the central region of the retina called the macula. This is the area on to which the eye focuses vision from straight ahead, and colour vision is most accurate for objects viewed directly. The cones work best in strong light - colours are hard to distinguish at twilight and all cats are grey by night.

When light falls on to a cone it causes a structural change in the pigment within it, and this causes the cone to emit an electrical impulse. Each of the millions of cones responds most strongly to light of a particular colour (or wavelength), and there are three main types, responding to the light wavelengths 445 nanometres (blue), 535nm (green) or 570nm (red). Impulses from all the cones are integrated by specialist nerve cells deep in the retina before the information is transmitted to the brain.

The common type of colour blindness is due to the retina lacking one of the three main types of cone. A lack of red cones makes reds look dull, almost grey. Lack of green cones makes green look grey and makes oranges, browns, and other colours hard to distinguish. Lack of blue cones is much less common, though it may develop after childhood as a result of poisoning from chemicals or drugs.

The common red and green deficiencies are inherited as gender-linked disorders - the defective gene is on the X sex chromosome and the disorder affects males. Women may be symptomless carriers who may pass the defect on to their male children. Colour deficiencies are much more common in whites than among black or Asian peoples.

People with defective colour vision do not realise there is anything wrong with their sight unless they are tested or their work requires them to be accurate in colour discrimination. Most of those affected have only a mild defect which shows itself, if at all, by their making mistakes in matching close shades of a colour. Children nowadays are tested by asking them to look at colour plates made up of multiple dots and asking them which numerals they see. More elaborate tests using coloured lights may be used

to confirm the results of this simple

screening test.

There is no treatment for deficiencies in colour vision, but recognition of the defect is important not only in choice of careers but also in mundane tasks such as changing an electric plug.