The process of color vision
Color perception is a complex physiological process, carried out by the retina.
The retina is covered with millions of photoreceptors: rods, which are more numerous, and cones.
The rods enable grayscale vision because they are only sensitive to light intensity, so they play a role in low-light vision.
Cones are used for color vision, and there are 3 types: S sensitive to blues, M to greens and L to reds.
They convert light energy into nerve impulses and transmit them to the brain via the optic nerve, where they are decoded.
In some cases, color perception may be incomplete or even completely absent.
These are called dyschromatopsias.
Hereditary dyschromatopsia: colour blindness
Hereditary dyschromatopsias are better known as colour blindness. They are characterised by an absence of colour perception or an inability to differentiate between certain colours.
On average, 8% of men and 0.5% of women are colour blind.
Colour blind people do not have the 3 normal colour-forming channels.
- or one of the channels is missing, in which case the colour blind person is a "dichromat", forming colours using only 2 channels.
- if red is missing, the subject is called a protanope, if green is missing, a deuteranope (most common), if blue is missing (extremely rare), a tritanope.
or one of the channels is present but deficient, in which case the colour blind person is an "abnormal trichromat".
- if red is deficient, they are called protanomalous
- if green, it is deuteranomal
- if it is blue, it is tritanomal.
As visual cells do not regenerate, these vision problems are permanent and incurable.
Acquired dyschromatopsias are disorders of colour vision caused by damage to the cones, optic pathways or visual cortex as a result of disease.
They develop over time and may be associated with a deterioration in the visual field.
The visual diseases responsible for these disorders include: certain retinal detachments, macular degeneration, trauma to the optic nerve, Leber's disease and excessive sun exposure.
Unlike hereditary dyschromatopsias, these anomalies can develop positively thanks to the treatment offered for the disease that caused them.
The different tests
Pseudo-isochromatic plates: the Ishihara test
This test was created in 1917 by Ishihara Shinobu.
It consists of 38 "pseudo isochromatic" plates, i.e. plates made up of small colored dots forming a background on which a symbol (letter or number) appears.
Patients without dyschromatopsia can clearly distinguish the symbol, while those with the disease are unable to read it.
Its purpose is to highlight the existence of dyschromatopsia, if any, and its axis. It cannot be used to assess the severity of the deficiency.
These tests use color samples to be classified in a certain order.
There is the Holmgren test, in which the patient must select a number of similar colors from colored threads.
There are also Farnsworth tests, in which colored tokens have to be sorted.