More about colour vision deficiency
Firstly you need to know a little about how normal colour vision works. There are four kinds of receptor in the retina of the human eye. The rod receptors are for night vision and do not send any information about colours to the brain. We are all colour blind at night. Try recognising printed colours under moonlight.
The other three receptors, the cones, operate at brighter levels of light and send information about colour to the brain. Each contains a different light absorbing photo-pigment. One cone is preferentially excited by blue light and is known as the ‘blue’ or S-cone; the second is most sensitive to green light and is known as the ‘green’ or M-cone, while the third, the ‘red’ or L cone is most sensitive to yellow, orange and red light (see picture). They send electrical signals to the receptors and the brain constructs the perception of colour using the relative strength of the three neural signals.
On the way to the brain the neural signals from the cones are transformed into three different signals: (1) a brightness signal, (2) a red or green signal and (3) a blue or yellow signal, as shown in the diagram.
In abnormal colour vision the photopigment in one of the cones is missing or has an abnormal photo-pigment.
1% men. Red cone photo-pigment is missing. Red-green channel does not work. Can only perceive blues and yellows. Reds appear very dark because of lack of sensitivity to red light.
1% men. Green cone photo-pigment is missing. Red-green channel does not work. Can only perceive blues and yellows.
1 in 13,000 men and women. Blue cone pigment missing. Blue-yellow channel does not work. Can only perceive reds and greens.
The terms protanopia, deuteranopia and tritanopia are derived from Greek and simply mean the first, second and third kinds of vision. (opia = vision deficiency)
People with these three kinds of colour vision deficiency are known as dichromats, (di=2) meaning they have only 2 channels for colour and brightness, instead of the normal three. However, they perceive more than 2 colours. They perceive a variety of blues and yellows that vary in brightness / darkness and paleness / vividness (see pictures). Green is perceived as white or bluish-grey or yellowish-brown depending on whether the green is pure green, blue-green or yellowish-green. Reds are perceived as dark yellow (that is a brown colour).
It gets a bit more complicated because some people (about 6% of men) have three cone photopigments, but one of them is abnormal. They are called anomalous trichromats. Their colour vision is usually a lot better than that of the dichromats. Sometimes their deficiency is so mild that they have only a few problems with colour in everyday life. However, about half do have a colour vision deficiency that is severe enough to cause practical problems in everyday life. (see refs 10 and 11). Anomalous trichromats have a reduced ability to differentiate colours and they see colours a bit differently from those with normal colour vision.
Protanomaly 1% men. The red light absorbing photopigment is anomalous: it has reduced sensitivity to red light.
Deuteranomaly 5% men. The green light absorbing photopigment is anomalous: it has enhanced sensitivity to green light.
Tritanomaly Rare. Caused by a reduced number of blue cones in the retina.
Some people are totally colour blind: they see no colours, but this is very rare. This is known as monochromasy because there is only one neural channel in the visual system, the one that signals brightness. Monochromats also have poor visual acuity.
The red-green colour vision deficiencies are inherited. The mode of inheritance is sex-linked recessive, which means that the abnormal gene is carried by the mother, who has normal colour vision, but there is a 50% chance that her sons will have the colour vision deficiency and that her daughters will be carriers.
Abnormal colour vision can also occur as a result of eye disease. This is known as acquired colour vision deficiency. Diseases that disrupt the retina of the eye cause a blue-green colour vision deficiency and those that affect the optic nerve usually cause a red-green deficiency.