It's more than black and white

Colour blindness is more common than rainbows. Dr Anand Shroff gives us the
full picture on a condition that affects many

What is colour blindness? What causes it?
Colour blindness or colour vision deficiency is the inability to perceive
differences between some or all colours that other people can distinguish.

While it is usually hereditary, it is also possible to acquire colour
blindness through damage to the retina, optic nerve, or higher brain areas.
Transient colour blindness also occurs, though rarely, in some migraine
sufferers.

What are the different kinds of colour blindness?

The human retina contains two kinds of light-sensitive cells: rod cells
(active in low light) and cone cells (active in normal light). When one or
more of the cone systems are either malfunctioning or not functioning at
all, it causes different kinds of colour blindness. When one cone system is
compromised, it is called 'dichromacy', which is the most frequent form of
human colour blindness involving difficulties in distinguishing between
reds, yellows and greens. This is commonly referred to as 'red-green colour
blindness', though the term is somewhat misleading.

Problems in distinguishing between blues and yellows are much rarer, with
the rarest form being complete colour blindness or monochromacy, where one
cannot distinguish any colour from grey, as in a black-and-white movie or
photograph.

Colour blindness is different from colour agnosia (colour confusion), where
the person cannot distinguish colours even though the eyes can. This is a
failure of perception, not of vision.

How is colour blindness detected?
The Ishihara colour test, which consists of a series of coloured spots is
most often used to diagnose red-green colour deficiencies. The full set of
tests helps to diagnose which visual defect is present.
It is important to identify these problems early and explain them to
children to prevent possible problems and psychological trauma.

How common is colour blindness? Who does it affect?
It occurs in about 8 per cent of males and 0.4 per cent of females. Isolated
communities with a restricted gene pool sometimes produce high proportions
of colour blindness. More than 95 per cent of all variations in human colour
vision involve the red and green receptors in male eyes.

Genetic red-green colour blindness affects men more often than women,
because the genes for the red and green colour receptors are located on the
X chromosome. While men have only one X chromosome (and a Y chromosome,
women have two X chromosomes). So women are red-green colour blind only if
both their X chromosomes have a similar deficiency, whereas males are colour
blind if their single X chromosome is defective.
How does it affect everyday activities?

Colour codes present particular problems for colour-blind people. Colour
blind people could also have trouble with a computer interface.
It has been said that in extreme emergencies everyone is colour blind. When
the need to process visual information rapidly arises, as in an aircraft
crash, the visual system may operate only in shades of grey. This is an
important possibility to consider when designing, for example, emergency
brake handles or emergency phones.

How can it be treated?
There is generally no treatment. However, certain types of tinted filters
and contact lenses may help an individual distinguish different colours
better. Additionally, software has been developed to assist those with
visual colour difficulties.

What are the misconceptions about colour blindness?
Colour blindness does not mean swapping of colours in the observer's eyes.
Grass is never red and stop signs are never green. However, dichromats
(red-green blind) often confuse red and green items. For example, they find
it difficult to distinguish a green from a red apple without other cues like
shape.

- Swati PujariIt is a common misconception that colour blindness makes a
person see things only in black and white - Getty Images


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