Why does my Optometrist check my colour vision?
Colour and culture
Colours are deeply ingrained in human society and culture. Throughout history colours have been used to indicate prosperity (purple), purity (white), danger (red), health (green) among many others. National identity is always closely associated with colours (Orange for Netherlands, Red for Wales, Green for Ireland). Among its other uses colour can be used as a non-verbal command (traffic lights the obvious example) or to indicate gender (pink and blue).
Pick up any fashion magazine in a supermarket and there are pages devoted to the colours which are ‘in’ this season, and colours not to wear together. Crossing the road at the traffic lights there is a green man and a red man. In HMV there are piles of colourful album covers to catch your eye. That is before you even see the name of the artist (Green Day, Black Sabbath, Pink Floyd, Blue) or songs (Blue Suede Shoes, Yellow Submarine [left], Purple Rain).
Colour is ingrained so deep into our psyche that is incredibly difficult to complete dissociate it from anything in your daily life.
Why do we have colour vision?
As with many other creatures in the animal kingdom, through millions of years of evolution humans have developed the ability to discriminate colour. But why is this? Why should this trait have b een passed on throughout history? Following the Darwinian theory of evolution it is only logical to assume that the ability to discriminate colour proved advantageous, hence the ‘fitter’ individuals prospered and passed on their traits.
The best way to indicate how colour vision would be an advantage would be to put a few questions to the reader:
1) Could you tell if a fruit is ripe or past its best?
2) Could you differentiate between cooked and uncooked meat?
3) Would you know if the frog you are holding is a poison dart frog or local amphibian?
Hopefully the answer to all 3 is yes! However I challenge you to try again without using colour as a guide. It is a lot harder. Therefore it is clear to see colour vision is an advantage
Based on this mechanism, through evolution we have seen animals develop some fantastic coloured displays for attracting a mate (as seen to the right), warding off predators and camouflaging into the background to hunt prey.
How does colour vision work?
Animals possess two types of light sensitive cells – rods & cones. Rods are mainly responsible for sensing brightness. They are most sensitive to movement and seeing incredibly dark images, but give poor detail. Cones are responsible for sensing colour and fine detail. Humans possess 1 type of rod and 3 types of cone, other mammals possess more or less.
Cones are sensitive to one particular colour, hence are highly stimulated by a light of that exact colour. Hence an animal with a cone sensitive to blue light would only be sensitive to blue objects. However this does not mean this animal would not see a red object. It just means that objects are either a shade of blue, or not blue at all. For sake of argument imagine an artist only using blue paint. He may water down the blue paint to change its shade, but cannot make the paint red.
In the case of humans, we have cones sensitive to blue, green and red. Therefore we can not just determine if an object is red or not, blue or not and green or not, but determine if it is a combination. This means that instead of being restricted to blue paint only, the artist could not mix a whole palette of colours, hence giving us colour vision!
As humans have 3 cones, we can perceive a larger range of colours than an animal with only 2 cones (for example a dog) and a smaller range than the mantis shrimp (pictured right) that has an incredible 12 cones! That being said even 2 animals with the same number of cones do not possess the same colours vision. Using the analogy of paint from above, you may use 2 different pots of paint that another artist, hence still use plenty of colour, just different ones.
As a matter of pure interest a recent study showed that 2% of women possess a 4th cone. Although in most cases this cone does not work, a few women in the study demonstrated superior colour differentiation than observed in a normal human. This study may explain the age old argument why women use a ridiculous number of words for purple (lavendar, thistle, plum, fuchsia, magenta, violet, in digo, amethyst etc).
So what is colour blindness?
As a child, a friend of mine tried convincing me that being colour blind was like watching black and white TV all day. You just saw 50 shades of gray and that was it. Although his logic and reasoning seemed sound, with a firmer understanding of the visual system as explained above, I would hope that you would know this is not the case!
In colour ‘blindness’ a perfectly normal person is either missing a photoreceptive cone (dichromat), or has a cone which does not work properly (anomalous trichromat). Due to in essence possessing only 2 cones, a person will not be able to perceive colours as well as someone with 3 cones. Brushing with 2 paints not 3 as it were.
This is caused due to a damaged gene carried on the X chromosome. As women carry two X chromosomes, a duplicate gene is present. In men however, as there is only one X chromosome, if the gene is damaged, there is no replacement. This is reflected in the prevalence of colour blindness – less than 0.1% of girls are colour blind, yet approximately 8% of men are colour blind. This means that an incredible 578 million men world wide are colour blind (assuming a world population of 7.2 billion). It should also be noted that as the X chromosome is passed on down the generations, it is common to find colour blindness runs in families.
Why does my Optometrist test my colour vision?
As illustrated in the previous paragraph, a whopping 8% of the male population is colour blind. Due to the pure numbers at work it is highly likely that in every school classroom that there is a boy who is colour blind. In general optometrists will test a childs colour vision at their first eye examination to check for colour blindness.
If a child is colour blind, it is very important for the parents to know this. As a child is in school they can be embarrassed and ridiculed for colouring in a picture the wrong colour. Handouts to the class and wall displays may also be incredibly difficult for a colour blind child to read (despite being perfectly legible to the ‘normal’ teacher). For the fashion conscious, colour blindness can cause a wardrobe catastrophe. A nightmare for many teenagers. For the very youngest children, even learning colours can be confusing and difficult as can be seen in the two images below:
The image to the left is the original, the image to the right is that seen by a red/green colour deficiency
In my own personal experience, I found a geography text book filled with pie charts and diagrams that used green and red as the primary colours. These charts were impossible to read and highly confusing to those who are colour blind. It is easy to see how an inability to actually see a clear diagram could be seen by the teacher as not being able to interpret the information.
People with colour vision defects are ineligible for certain professions, for example the fire services, train drivers, aircraft pilots and electrical engineers within the armed forces. The reasons are normally founded on safety concerns. A fireman would not be able to see glowing embers in the aftermath of a fire, and this may result in another fire. Train drivers need to read signals at a moments notice (pictured to the right is a standard signal). Failure to do so may be fatal. Aircraft pilots must be able to clearly read beacons, aircraft position lights, charts and approach slope indicators. In the case of engineers, confusing two wires could cause a serious malfunction to equipment with potential fatal consequences.
With this in mind, it is important that a child know that they are colour blind at a young age, and not to find out when applying for a job 20 years later.
On the author
My name is James Brawn, I am a pre-registation optometrist with an independent practice in Wales. I will be starting my PhD in Vision Science in October at my alma mater Cardiff University.
Feel free to follow me on twitter @brawnybalboa