|
The color-blindness is not strictly speaking a visual pathology, but rather a particularity that modifies the perception of the colors. Transmitted genetically, this anomaly is going to last the whole life without aggravation, nor improvement. While observing a geranium in light of a candle, the English chemist John Dalton had the feeling that he didn't discern the colors correctly. This is how was identified, since the end of the 18th century, the anomaly of perception of the colors named color-blindness. A masculine anomaly transmitted by the motherThis "blindness of the colors" is bound to a relatively frequent hereditary genetic anomaly, but of which the consequences are essentially masculine since 8% of the men are only reached for 0,5% of the women. This disparity is due to the fact that the gene of the color-blindness is carried by the chromosome sexual X and that it is recessive, that means that the presence of a normal gene is sufficient so that the anomaly doesn't express itself. The woman having two X chromosomes, it is only color-blind if these two chromosomes, inherited of his two parents, carry the deficient gene, rare possibility. On the other hand, the man possessing only one X chromosome, inherited of his mother, it is sufficient that this last transmitted him the abnormal gene so that he is color-blind. This transmission bound to the sex permits to understand why the men transmit their color-blindness never to their son (since they only give him the chromosome OF IT), while the mothers can transmit the anomaly without being themselves color-blind. Three colors for the view Larger than the simple confusion between the red and the green, to which it is often reduced, the color-blindness can don varied shapes. To understand this diversity, let's come back to the mechanism of the colorful vision. This one results from the perception of three colors, the bruise, the green and the red, whose combination is sufficient to form all a nuance of hues. The colorful impression elaborated by our brain results from the information on the respective intensities of these three colors transmitted by cells of the retina: the cones. One distinguishes "blue", "red" and "green" cones thus. If one of these types of cones doesn't function, the brain won't have any information on the corresponding color. The missing coneThus, people having a normal vision are "normal trichromates", that means that they possess the three types of cones and see the set of the colors. While the color-blind are dichromates: they only possess two types of cones, generally blue and green. Of this fact, they only discern three hues: the bruise, the yellow and a white or gray intermediate hue. The absence of blue or green cones is a lot rarer. In an exceptional manner, two or the three types of cones can miss, the person seeing then in black and white. One speaks of "monochromatisme" or "achromatopsie." To the inverse, sometimes the cones are well present but don't transmit the corresponding signal perfectly to their color, what entails a light change of the colorful vision. The reached people have a "abnormal trichromatisme." The tracking of the color-blindness is extremely simple and often gets used since the first medical visit in the school. The anomaly is marked easily with the help of boards representing the numbers or drawings formed of colorful points on a bottom of another color. The color-blind cannot distinguish the drawing. An exam ophtalmologique more deepened will be able to specify the type of color-blindness. A normal visual acutenessNo treatment is able, currently, to re-establish a normal vision of the colors. However, the color-blindness doesn't entail real handicap, the person having his own frame of reference, where the missing nuances are replaced by different hues of gray. The existence of this anomaly doesn't increase the risk of visual failing and don't impose any particular surveillance. In fact, the main inconvenience of the color-blindness is to forbid some professions, as the public transportation. In counterpart, the absence of vision of some colors is probably an advantage in graphic domains. Finally, some illnesses, as the diabetes or a glaucoma, can drag a decrease of the vision of the colors, often non discerned by the patients. To the difference of the color-blindness, these "dyschromatopsies" can worsen with the evolution of the illness. 2009-07-06
|
