Colour blindness is a condition where a person is unable to identify a certain colour or differentiate between one or more colours. The word ‘blindness’ is actually a misnomer. Although the person may be blind to see a particular colour, in actuality the person experiences no actual blindness but a deficiency in colour vision. In this article, we discuss the causes, symptoms and treatment of colour blindness.
HOW YOUR EYES SEE COLOUR
Light enters your eye through the pupil. The iris changes the size of the pupil, depending on how bright the light is. The lens focuses the light onto the back of the eye, which is called the retina. The retina contains rods and cones. The rods are involved in distinguishing between light and dark, the cones are involved in the perception of colour. You have three different types of cones: 65% of cones can be sensitive to red; 33% of cones can be sensitive to green; 2% of your cones are sensitive to blue.
These cones are scattered throughout the retina. Deficiencies in these cones causes colour blindness. When light from an object hits the cones, it stimulates them to varying degrees. The resulting signal moves along the optic nerve to the visual cortex of the brain, which processes the information and identifies the colour of the object.
THE TYPES OF COLOUR BLINDNESS
The most common type of colour blindness is red-green colour blindness. This is where someone sees little or no difference between red and green hues. Yellow-blue colour blindness is less common. This is where someone can’t distinguish between blue and yellow hues. Complete colour blindness is called achromatopsia and affects about 1 in 33,000 people. With this condition someone sees only black, white and shades of grey. The condition is commonly known as ‘day blindness’, as people with this condition see better and sharper images in subdued light. In bright light, their picture of the world is often blurry.
THE CAUSES OF COLOUR BLINDNESS
The cause of colour blindness is an inherited genetic disorder. The information (genes) for the green and red photosensitive pigments are located on the X-chromosome. Men have an X and a Y chromosome, so if the X chromosome carries the gene mutation he will be colour blind. Because women have two X chromosomes, both X’s would have to contain the gene mutation for her to become colour blind. Because colour blindness on the X chromosome is inherited, males are more often colour blind than women (1 in 12 for men and 1 in 100 for women). People with colour blindness often have no other vision problems.
DIAGNOSIS & TREATMENT
Colour blindness can be difficult to detect, particularly in children with inherited colour vision deficiency as they may be unaware that they have any problems with their colour vision. A child may have been taught the colour of objects from an early age and will know for example that grass is green and strawberries are red even if they have no concept of their true colours. Everyone should have their eyes tested as a matter of routine. Ask your optometrist about colour vision testing. Your optometrist will examine you and ask about any symptoms you may be experiencing. A common test to detect colour vision defects is the Ishihara Plate test. This can test for red/green colour blindness but not blue colour blindness.
Currently there is no treatment or cure for colour blindness. People are often taught to recognise colours in other ways. For example, traffic lights usually have the red light on top and green on the bottom. There are also some corrective lenses on the market that claim to help with colour perception, but they don’t “fix” colour blindness. For most, however, colour blindness is more a disruption than a danger to their everyday lives.