Colour vision - The science behind perception of colours
Can you imagine a world without colours?
The pure joy of mixing two random colours to make another, is a sentiment I’m sure every artist can resonate with. Having painted a few canvases myself, I wanted to see what my paintings would look like if I were colour blind. A quick online simulation program produced this result!
Normal Vision
If I were colour blind
Interesting, isn’t it?
So let's rewind back to the 1700s, when an English chemist threw some light on colour blindness (Pun intended!). John Dalton, also known for the famous atomic theory, realised that he and his brother perceived colours differently than the rest of the world. His quest to understand this phenomenon led to the release of his first paper “Extraordinary facts relating to the vision of colours” at the age of 28! He was quick to realise that the reason he shared this condition with his brother was because colour blindness is hereditary and not merely a coincidence. He also postulated that it was due to a blue liquid in the aqueous humor of the eye, which acted as a filter to other colours. His theory was soon disproved when they dissected his eyes after he died (one of his last wishes!) and found no blue liquid. However, colour blindness has been commonly referred to as Daltonism in his honour since then.
Now before we begin to deal with the pathology, let us understand how humans perceive colours. Light, as we all know, is an electromagnetic wave ranging from the high-frequency gamma rays to the low-frequency radio waves. Only a very small portion of this spectrum is visible to the human eyes. The retina has 2 types of specialised cells to perceive this visible spectrum – Rods and Cones. The rods sense how much darker or brighter the surrounding looks and the cones are responsible for colour vision. When visible light falls on an object only those waves of a specific wavelength which are reflected by the object are picked up by the retina. So an apple appears red because its surface reflects waves from the red side of the spectrum and absorbs the rest of the spectrum. However the human eye has only three types of cones, which are sensitive to the 3 primary colours. They are:
- Red pigment cones (sensitive to 560nm),
- Green pigment cones (sensitive to 530nm) and
- Blue pigment cones (sensitive to 430nm).
So when light of approximately 430nm hits the retina, only the blue cones are stimulated. Now here’s the tricky part. If there are only 3 types of cones, how are we able to perceive so many different colours? The answer is simple. Imagine the retina to be your colour palette where you can mix different proportions of the 3 primary colours to produce other colours. For example, when the red and green cones are stimulated together equally, we perceive it as yellow. This can be represented in the form of R:G:B ratios. So when red and green cones are stimulated to 83% of their maximum potential while blue cones remain dormant, we see yellow (83:83:0). An equal stimulation of all the cones gives us the sense of white.
Now that we have got the basics figured, let us see what could possibly go wrong here. You see, the red and green pigment cones are encoded by genes on the X chromosome, while the blue pigment cones are coded by genes on chromosome 7. So a mutation in the X chromosome can mean decreased number of red and green pigment cones or the complete absence of them. This explains why almost all of the colour blind people are men. Males inherit the X chromosome from their mothers. Being an X linked recessive disorder, all males who inherit the defective X are colour blind while the females are only carriers of this defective gene. However in an extremely rare circumstance, females born to a colour blind father and a carrier mother might also be colour blind.
Does that mean that people with colour blindness do not perceive colours at all? Well, maybe. Maybe not. It depends on the severity of the condition. But to understand this, we need to familiarise ourselves with some terminologies. In medical terms, a total absence of the pigment is referred to as -anopia and decreased sensitivity is called -anomaly.
|
|
Total Absence |
Decreased Sensitivity |
|
Red Cone Pigments |
Protanopia |
Protanomaly |
|
Green Cone Pigments |
Deuteranopia |
Deuteranomaly |
|
Blue Cone Pigments |
Tritanopia |
Tritanomaly |
The most severe form is the complete absence of red and green cone pigments. This state is known as Monochromacy where the individual perceives everything in shades of white and black. A less severe form is Dichromacy in which one of the three pigments is either deficient or absent. Red-green colour blindness occurs when an individual completely lacks either the red pigment (protanopia) or the green pigment (deuteranopia).
People who have defective green pigments with decreased sensitivity to green colour, called as Deuteranomaly, find it difficult to differentiate between red and green hue as these wavelengths are closer in the spectrum. This is in fact the most common form of colour blindness. Tritanomaly and tritanopia are extremely rare.
Being colour blind may sound like a trivial issue, but it can pose some dangers. In fact, in 1875, Swedish physiologist Alarik Firthiof Holmgren investigated a train crash and attributed the incident to the driver probably being colour blind. He later went on to devise the Holmgren wool test for colour vision. However the more commonly used test of colour vision is the Ishihara chart designed by Dr. Shinobu Ishihara. These charts have different coloured dots arranged in a confusing pattern. While an obvious pattern may strike a normal eye, colour blind individuals will be able to see a hidden pattern.
Normally one should see 74.
Dichromacy perceives it as 21.
Can we cure colour blindness? Unfortunately, no. However in recent times, there are specific glasses available that help people with deuteranomaly to differentiate between red and green better.
So the next time you see a rainbow or you work on that palette, take a moment to appreciate it, because a world without colours is truly unimaginable!
Author: Soundarya V (Facebook)
Sources and citations
Hall, John E., and Arthur C. Guyton. "Unit X : Chapter 50 - The Eye : Receptor and neural function of the retina” . Guyton and Hall Textbook of Medical Physiology. 12th ed. 614-616. Print.
Jonathan C.Horton. "Section 4: Chapter 39: Disorders of eyes, ears, nose and throat”. Harrison's Principles of Internal Medicine. 19th ed. Vol. 1. 197. Print.