Have you ever wondered if everyone around you saw the world in the same colours as you do? What if what is red to you, is blue to someone else? Well, how do we see colours in the first place?
We use our sensory organ for vision, the “eye” to see. At the back of the eyeball, there is a layer called the retina, which contains photoreceptor cells; ‘rods’ and ‘cones’. These photoreceptor cells contain light-sensitive pigments that get activated when light falls on them. They then send signals to the brain, wherein the information is processed that allows us to finally see what is in front of us.
Rods and cones
Rods contain the photopigment ‘rhodopsin’ and are generally located in the peripheral region of the retina. There are three types of cones that allow us to see the three different colours of light: blue, green and red, corresponding to short, medium and long wavelengths of light.
Cones contain the photopigments called ‘iodopsin’. They are located around the region of the eye called the macula and are abundant in the ‘fovea’ which is centrally placed in it. Rods and cones work in tandem to allow us to see in all light conditions. Rods help us to see in low light conditions and provide us with ‘scotopic vision’, whereas cones help us see colour in bright light conditions, providing us with ‘photopic vision’. Visual acuity or ‘clarity of vision’ is provided more by cones. Hence, it is much easier to see sharper boundaries and well-defined shapes and sizes in bright light.
What is the ‘blind spot’ of the eye?
Neurons that carry information received by the photoreceptors to the brain, leave the eye through the optic nerve. The point of entry of the optic nerve is called the optic disk. There are no photoreceptors present in this region, and hence, when light falls on this region, it cannot be detected, forming the ‘blind spot’ of the eye.
Colour blindness
Colour blindness is an often wrongly used term to describe someone with a colour vision deficiency, which is the inability of a person to differentiate between different shades of colour. Very rarely are people absolutely colour blind.
Do you see what I see
Which numbers can you see?
The numbers are 42, 74 and 12 respectively.
Types of colour vision deficiencies
Colour vision deficiencies may occur due to deficiency in one, two or all three types of cones. The most common type of colour vision deficiency is called the ‘red-green’ colour vision deficiency wherein an individual has difficulty in differentiating between red, yellow and green. A rarer type of deficiency is the ‘blue-yellow’ colour vision deficiency wherein the individual has difficulty in differentiating between shades of blue-green and black. A very rare and severe form of colour vision deficiency is called ‘blue cone monochromacy’ which causes very reduced colour vision. Achromatopsia is the condition where a person can only see in black, white and shades of grey.
How do you get colour vision deficiencies?
Colour vision deficiencies are inherited or sometimes caused by random mutations that occur in the regions of the X chromosome that are responsible for producing cones. There are two copies of the X chromosome in females whereas only one copy in males, hence all males that inherit a defective copy of the gene experience colour blindness. On the other hand, since females have two copies of the gene, only if both the copies are defective, will she be affected. Most often, the normally functioning gene out of the two copies compensates for the defect and leads to the production of normal cones. Such a female, is however, a called a ‘carrier’ and is capable of passing on the defective gene to her sons. It has been found that 1 in 12 men and 1 in 200 women are affected by some sort of colour vision deficiency.
There are some very rare cases, especially in women, that possess a fourth type of cone. This condition is called tetrachromacy and this allows them to see many different shades distinctly!
A deficiency with rods would cause night blindness.
How do you diagnose colour vision deficiency?
It is very difficult to diagnose colour vision deficiency, as you would never know you have it. A pseudoisochromatic plate is used, which contains hundreds of different coloured dots. A person with normal vision would be able to see a number that stands out from the colours whereas a person with a colour vision deficiency would possibly see another number.
Can colour vision deficiency be treated?
Traditionally, nothing much could be done if you had colour blindness. Nowadays, tinted glasses are available, that are made up of special optical materials that can block certain wavelengths of lights that fall in the overlap regions of different colour wavelengths. This helps the person better distinguish between different colours.
Read about 8 Surprising Facts About Colour and Colour Vision Deficiency
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About Author
Saunri Dhodi Lobo is pursuing M.Sc in Life Sciences with specialization in Neurobiology. Her interests include writing poetry, going for nature walks and swimming. Currently she is involved in research on Alzheimer’s Disease in fruit flies.
Read all Articles by Saunri Dhodi Lobo