Dyschromatopsia is the medical term for the inability to distinguish colours. This sight defect is usually known as daltonism, in homage to the important English chemist and physicist John Dalton (1766 – 1844), who suffered from colour blindness, studied it and accurately described it for the first time (1794).
The most common form of daltonism is the difficulty to distinguish between red and green. Generally this defectiveness is genetic, but some lesions in the visual organs and the nervous system can affect the vision of colours.
Concise anatomy of the eye
The human eye is an optical system that receives luminous stimuli, transforms them into electrical impulses and transmits these to the visual cortex, where they are decoded and turned into sensory impressions according to the experience of each person.
Light is received as an electromagnetic wave. The optical system of the human eye, formed by the ocular outer surface (cornea and lacrimal film) and by the crystalline lens, is responsible for the focusing of the “light wave” on the retina, a very thin neurosensorial membrane. This membrane acts like a sort of photographic film: it is impressed by light and transforms it into a nerve impulse codified according to the shapes and colours of objects.
The capacity of the retina to codify colours is due to the presence of specialized nerve cells that are able to receive a variety of light wavelengths. These cells, named cones (sensitive to bright light) and rods (responsive to faint light), have pigments which are impressed by several wavelengths of the electromagnetic radiation (light). Thus they recognize and transmit to the brain the colours of objects.
In the central area of the retina, cones sensitive to red and green are predominant, the reason why a third red light (STOP) was introduced on the back of cars, at the middle, to increase safety: the time of reaction for braking is 0.2 seconds shorter.
Towards the periphery, the retina sensitiveness to the blue – yellow spectrum increases. So, in motor vehicles, the light indicating the level of fuel is yellow, because its perception is easier when the driver is not looking directly at it. The contrast between blue and yellow is also strong during movement. Therefore, in Portugal, road signs present yellow and black arrows for lateral guidance in tollgates, detours, etc. and the ambulances of the public medical emergency services are now painted blue and yellow. These colours are also applied, for instance, to volleyball balls …
Daltonism is a recessive disorder associated with the X chromosome.
The human genome presents 23 pairs of chromosomes (46 chromosomes), and 1 pair defines the gender. This pair is composed of one X and one Y chromosome in men (XY) and two X chromosomes in women (XX).
During fertilization, each individual receives one chromosome from its father and another one from its mother, to create its own pairs of chromosomes.
The term recessive means that it is necessary to inherit two chromosomes with an identical characteristic (or defect), for a child to present a trait (or disease) transmitted by its parents.
Since female individuals inherit one X chromosome from their father and another
X chromosome from their mother, they will only have the genetic disease if both chromosomes are equally defective: a very rare occurrence. But, women may inherit a defective chromosome and a normal one: then, despite not being affected by the disease, they carry it and may transmit it to their children.
If male individuals inherit the defective X chromosome from their mother, they will always have the genetic disease, because they only possess one X chromosome.
According to this transmission pattern, the probability of a woman carrier (not affected) transmitting the genetic disease to her sons is 50%; her daughters may only inherit the disease if their father has it. Also, a woman affected by a genetic disease always transmits it (100% of cases) to her male children; her female children always receive a defective chromosome, being thus carriers, but they will only have the disease if their father also bears it.
Men are never carriers of this genetic disease; they may only be affected by it. If they have it, they will transmit their defective X chromosome to their daughters, who will be affected by the disease or just carriers whether they inherit from their mother a defective chromosome or a normal one. Men never transmit the disease to their sons, because these only receive the Y chromosome.
This genetic transmission pattern of daltonism explains why this disease is extremely rare in women and relatively frequent in men. In fact, nearly 8% of men suffer from altered chromatic vision versus less than 0.5% of women. Also, 97% of the individuals affected by this disease are men.
There are three primary colours: blue, green and red. Not only the mixture of these three colours produces the white colour, but also all the other colours or hues are a result of their combination.
Thus, an error or defect in a cell type sensitive to one of these three colours alters specifically the vision of that colour, and affects also the vision of all compound colours deriving from it.
The seriousness of daltonism is very variable. It may be due to an extremely severe defect, with total absence of chromatic vision – black and white vision – in case of nearly inexistence of photoreceptor cells. Visual acuity is then very low, since this defectiveness comes with a very weak capacity for discrimination: cases of legal blindness with the development of nystagmus (tremulous movement of the eyes in an attempt to focus the objects).
Fortunately these severe types of daltonism are very rare, and the mild forms prevail, characterized by the difficulty to recognize compound colours.
Generally, according to the chromatic defect, there are three types of daltonism: protanopia (defect in the red cones), deuteranopia (defect in the green cones) and tritanopia (defect in the blue cones).
These designations are used in case of a total defect; for a partial defect the terms are: protanomaly, deuteranomaly and tritanomaly.
In any case, these defects have consequences in all the other colours resulting from a combination of primary colours.
Most of the times the defectiveness is not absolute and, despite the existence of a chromatic defect, the individual is able to distinguish these colours though less saturated. Luminosity itself can change the chromatic perception of a colour-blind individual.
The picture below at the left presents an original painting by Mondrian. The two pictures at the right are reproductions made by a colour-blind person at two different hours of the day with different luminosity: the changes in colour saturation resulting from a chromatic defect and light brightness are obvious.
It is curious the way colour-blind individuals paint: in general, they avoid using transition colours, creating thus nearly monochromatic paintings, and sometimes colours are surrounded by dark outlines to evade those transitions.
The great majority of colour-blind patients present insensitivity to one colour – dichromatism, because only two colours are perceived, or to two colours – monochromatism, as only one colour is recognized.
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