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Deficiente cromatice

Atentie: Acest articol nu poate substitui o consultatie medicala. Cu exceptia cazurilor de tricromatie anomala, va recomandam sa consulati medicul. Evitati traume psihologice, mai ales in cazul copiilor.

La oameni, deficienta in perceptia culorilor, este incapacitatea de a percepe culorile in acelasi mod ca majoritatea populatiei. In general ea este determinata genetic, dar e posibil de asemenea sa fie dobandita, daca este afectata retina, nervul optic, sau anumite regiuni cerebrale. Prima lucrare stiintifica asupra subiectului a fost publicata de chimistul englez John Dalton in 1798, "Extraordinary facts relating to the vision of colours". [1] Din aceasta cauza fenomenul in intregime este uneori incorect numit Daltonism, desi medical daltonismul se refera la un tip specific de deficienta in perceptia culorilor: deuteranopie.

Cuprins

[modifică] Tipuri de deficiente si perceptii diferite ale culorilor

Exista trei forme majore de perceptie diferita a culorilor: tricromatie anomala, dicromatie si monocromatie. In general doar ultimile doua sunt considerate deficiente, in timp ce conditiile tricromatice sunt considerate variatii genetice in cadrul speciei umane.

Procentajul de oameni dupa tipul de perceptie a culorilor[1] [2]
Tipul de perceptie a culorilor Barbati Femei Total
Tricromatie standard 91.3% 99.6% -
Tricromatie anomala 6.3% 0.37% -
Protanomalie (variatie in conul L) 1.3% 0.02% -
Deuteranomalie (variatie in conul M) 5.0% 0.35% -
Tritanomolie (variatie in conul S) 0.0001% 0.0001% -
Dicromatie 2.4% 0.03% -
Protanopie (absenta conului L) 1% - 1.3% 0.02% -
Deuteranopie (absenta conului M) 1% - 1.2% 0.01% -
Tritanopie (absenta conului S) 0.001% 0.03% -
Monocromatie (absenta conurilor ) - - -
Monocromatie conala (absenta conurilor L si M) - - -
Acromatopsie (absenta conurilor) 0.00001% 0.00001% -
Agnosie cromatica - - -

Nota: - indica faptul ca autorul inca nu a gasit datele

[modifică] Cauzele si manifestarile

Absorbtia normalizata a spectrelor la oameni, in celule de tip conuri (S,M,L) si bastonete (R)
Extinde
Absorbtia normalizata a spectrelor la oameni, in celule de tip conuri (S,M,L) si bastonete (R)

Retina umana normala contine doua tipuri de celule sensibile la lumina: celule-bastonete (active la limina de intensitate mica) si celule-conuri (active la lumina zilnica normala). Exista trei tipuri de conuri, fiecare continand un pigment diferit. Cand pigmentii absorb lumina, conurile sunt activate. Spectrul lungimilor de unda in care conurile sunt activate difera: tipul S (de la short in engleza) este mai sensibil la lumina de lungime scurta de unda, avand maxima in zona albastra; tipul M (de la medium in engleza) este mai sensibil la lumina de lungime medie de unda, avand maxima in zona verde; tipul L (de la long in engleza) este mai sensibil la lumina de lungime lunga de unda, avand maxima in zona galbena, alaturi de zona rosie. E important de realizat ca spectrul de absorbie al fiecarui din cele trei tipuri de conuri cuprinde aproape intreg spectrul vizibil, si doar maxima se afla in zona indicata. Simplist, lumea vorbeste de receptori "albastru", "verde" si "rosu", desi receptorul rosu de fapt isi mare maxima in zona galbena. Sensibilitatea in perceptia culorilor depinde de suprapunerea celor trei sisteme: diferite culori sunt recunoscute cand tipuri diferite de conuri sunt stimulate in mod diferit. De exemplu, lumina rosie stimuleaza receptorul de lungime lunga de unda mult mai mult decat ceilalti receptori, dar schimarea treptata a colorii, percepute atunci cand lungimea de unda este treptat redusa, este rezultatul stimularii din ce in ce mai mari si a celorlalti doi receptori.

Cele mai frecvente forme de deficienta in perceptia culorilor la oameni rezulta din faptul ca receptorii de lungime medie sau scurta au maxime in lungimi de unda putin deplasate, sau au careva din acesti receptori lipsa.

Genele care codeaza receptorii verde si rosu se afla in cromozomul X, ceea ce expica faptul ca deficientele rosii-verzi sunt mai frecvente la barbati decat la femei. Jeremy H. Nathans de la Howard Hughes Medical Institute a demonstrat (2006) ca gena care codeaza receptorul albastru se afla in cromozomul 7, distribuit la fel la barbati si la femei.

[modifică] Deficienta cromatica dobandita

Regiunile cerebrale implicate in procesarea culorilor includ calea parvocelulara a nucleului geniculat lateral a talamusului (the parvocellular pathway of the lateral geniculate nucleus of the thalamus in engleza) si zona vizuala V4 a cortexului vizual.

O deficienta cromatica dobandita este in generl foarte atipica deficientelor genetice. De exemplu, ea se poate dobandi si partial, adica doar intr-o parte a campului vizual, in timp ce restul ramane cu o viziune cromatica normala. Unele forme de deficienta cromatica dobandita sunt reversibile. Deficiente trecatore pot aparea (foarte rar) la unii pacienti care sufera de migrena.

[modifică] Tricromatie

Unul din receptori are maxima deplasata fata de frecventele considerate "standard": 420nm, 534nm, 564nm.

[modifică] Deuteranomalie

Cea mai frecventa deficienta in perceptia culorilor, aproximativ 6% din barbati, variind de la natie la natie (8-10% la Europeni, 5% la Asiatici, 4% la Africani; cifrele variaza foarte mult si de la o sursa la alta, astfel incat nu exista o cifra general acceptata). Maxima pigmentului de lungime de unda medie este mai deplasata spre rosu, prin urmare sensibilitatea la lumina verde este mai scazuta. Se manifesta prin discriminarea mai dificila a diferentelor mici in lungimile de unda rosu, portocaliu, galben si verde. Uneori individul numeste gresit culorile din acest spectru, doarece lui ele ii apar mai rosii. Percep un pic mai multe variatii de albastru decat indivizii "standard". Spre deosebire de celelalte tipuri de deficiente, individul percepe intensitatea culorilor la fel ca si indivizii normali. Este cea mai putin severa forma de deficienta.

[modifică] Protanomalie

Aproximativ 1% din barbati. Maxima pigmentului de lungime de unda lunga (galben-rosu) este mai deplasata spre verde. Individul este mai putin sensibil la lumina rosie decat indivizii "standard". Ei deosebesc culorile mai rau dacat majoritatea oamenilor. Lumina combinata le apare de o culoare ceva diferita decat "standard". Sufera de o innegrire a partii rosii a spectrului. Rosul se reduce mai rapid in intensitate, potential devenind negru. Se intampla ca semaforul rosu sa le apara stins. Ca si la deuteranomalici, discrimineaza mai dificil diferentele mici in lungimile de unda rosu, portocaliu, galben si verde, si numestc uneori gresit culorile din acest spectru, deoarece lor ele le par mai verzi decat celorlalti indivizi.

[modifică] Genetica deficientelor cromatice rosu-galbene

Protanomalia si deuteranomalia sunt uneori numite deficiente "rosii-verzi", deoarece indivizii numesc "gresit" (diferit fata de normal) culorile din acest spectru.

Deficientele rosii-verzi afecteaza barbati (46,XY) de aproximativ 60 de ori mai des dact femeile (46,XX). O femeie trebuie sa mosteneasca gena, situata in cromozomul X, de la ambii parinti, pentru ca sa aiba o astfel de deficienta.

Un barbat cu deficienta rosu-verde si o femeie cu o viziune normala (si nepurtatoare) vor avea fiici cu viziune normala, dar purtatoare de trasatura (si fii cu viziune normala). Fiicile aceastea (femei purtatoare cu viziune normala), la randul lor, pot avea atat fii cu viziune normala, fii cu viziune deficienta, fiice nepurtatoare cu viziune normala, fiici purtatoare cu viziune normala, cat si (mai rar; doar daca si tatal are deficienta cromatica) fiice cu viziune deficienta. In fine, fii femeilor cu deficienta vor mosteni trasatura, iar fiicele vor fi purtatoare, dar o vor avea doar daca si tatal este deficient.


The gene for red-green color blindness is transmitted from a color blind male to all his daughters who are heterozygote carriers and are perceptually unaffected. In turn, a carrier woman passes on a mutated X chromosome region to only half her male offspring. The sons of an affected male will not inherit the trait, since they receive his Y chromosome and not his (defective) X chromosome.

Because one X chromosome is inactivated at random in each cell during a woman's development, it is possible for her to have four different cone types, as when a carrier of protanomaly has a child with a deuteranomalic man. Denoting the normal vision alleles by P and D and the anomalous by p and d, the carrier is PD pD and the man is Pd. The daughter is either PD Pd or pD Pd. Suppose she is pD Pd. Each cell in her body expresses either her mother's chromosome pD or her father's Pd. Thus her red-green sensing will involve both the normal and the anomalous pigments for both colors. Such women are tetrachromats, since they require a mixture of four spectral lights to match an arbitrary light.

[modifică] Tritanomalie

Se mai numeste si deficienta comatica in albastru-galben. Este foarte rara, deoarece ambele parti ale cromozomului 7 trebuie sa contina anomalia. Receptorul de lungime de unda albastru-violet are maxima putin deplasata fata de normal. Se manifesta la detectarea culorilor in spectrul albastru-galben. Punctul neutru la tritanopi este la 570 nm; la lungimi mai scurte ei percep verde, la lungimi mai lungi - rosu. Gena care codeaza receptorul albastru, aflata in cromozomul 7, nu are nici o vecina cu secventa ADN similara, si deficienta cromatica esta cauzata de o simpla mutatie a acesti gene (2006, Jeremy H. Nathans, Howard Hughes Medical Institute).

[modifică] Detectarea tricromatiei anomale

Indivizii tricromatici percep toate culorile, dar compararea lor difera de cea standard. Pentru a detecta aceeasi notiune de galben intr-o amestecatura rosu/verde, observatorii protanomalici au nevoie de o intensitate mai mare in rosu, iar cei deuteranomalici - de o intensitate mai mare in verde decat observatorii normali. Din punct de vedere practic, tricromatii trec prin viata fara nici o problema si efectueaza sarcini care cer o viziune normala a culorilor. Multi sintre ei nici macar nu stiu ca ei poseda o perceptie a culorilor diferita de cea normala. Unica lor problema este cand sunt pusi in fata unui test de viziune cromatica.

Protanomalia si deuteranomalia pot fi usor detectate cu ajutorul unui insrument numit anomaloscop, care amesteca limini din spectrele rosu si verde in proportii diferite, pentru a compara rezultatul cu o lumina galbena fixa. Daca cele doua culori (amesticatura si galbenul pur) sun prezentate in fata unei audiente largi de barbati, cand proportia de rosu treptat creste, la inceput o proportie mica (observatorii deuteranomalici) vor declara ca cele doua culori sunt la fel, in timp de marea masa va spune ca culoarea amestecata este inca verzuie. Apoi, cu cresterea proportiei de rosu, majoritatea va spune ca culorile se potrivesc. In sfarsit, dupa ce si mai mult rosu este adaugat, si indivizii protanomalici vor declara ca culorile se potrivesc, in timp ce restul lumii va spune ca amestecatura este sigur mai rosietica. Anomaloscopul este folosit la diagnosticarea tricromatiei. O alta metoda de detectie este testul lui Ishihara (vezi mai jos).

[modifică] Dicromatie

This is a sample image. Cifrele din toate figurele de mai jos apar cu aproximativ aceeasi intensitate indivizilor cu o viziune cromatica normala, dar unele din ele nu vor putea fi detectate de unii indivizi cu deficiente cromatice. Aceasta figura contine numarul 83.
This is a sample image. Cifrele din toate figurele de mai jos apar cu aproximativ aceeasi intensitate indivizilor cu o viziune cromatica normala, dar unele din ele nu vor putea fi detectate de unii indivizi cu deficiente cromatice. Aceasta figura contine numarul 83.
Aceasta imagine contine numarul 37. Protanopicii s-ar putea sa nu-l vada.
Aceasta imagine contine numarul 37. Protanopicii s-ar putea sa nu-l vada.
Aceasta imagine contine numarul 49. Deuteranopicii s-ar putea sa nu-l vada. Cifra 9 poate fi dificila chiar si pentru unii indivizi cu viziune normala.
Aceasta imagine contine numarul 49. Deuteranopicii s-ar putea sa nu-l vada. Cifra 9 poate fi dificila chiar si pentru unii indivizi cu viziune normala.
Aceasta imgine contine numarul 56. Tritanopicii s-ar putea sa nu-l vada.
Aceasta imgine contine numarul 56. Tritanopicii s-ar putea sa nu-l vada.

Dichromacy is a moderately severe color vision defect in which one of the three basic color mechanisms is absent or not functioning. It is hereditary and sex-linked, affecting predominantly males.[7] Dichromacy occurs when one of the cone pigments is missing and color is reduced to two dimensions.[6]

Dichromats are lacking one of the three cone types entirely. Dichromacy is much less common than anomalous trichromacy, but for protanopia and deutanopia at least, the effect is similar. (This is because in anomalous trichromacy the L-cone and M-cone have abnormally similar sensitivity curves, almost as though the two cone types have merged into one.)

Color blindness is usually classed as a disability; however, in select situations color blind people may have advantages over people with normal color vision. There is anecdotal evidence that color blind individuals are better at penetrating color camouflage and at least one scientific study confirms this under controlled conditions.[2] ^ Morgan MJ, Adam A, Mollon JD. "Dichromats detect colour-camouflaged objects that are not detected by trichromats." Proc Biol Sci. 1992 Jun 22;248(1323):291-5. PMID 1354367.

Protanopes and deuteranopes are dichromats; that is, they can match any color they see with some mixture of just two spectral lights (whereas normally humans are trichromats and require three lights).These individuals normally know they have a color vision problem and it can effect their lives on a daily basis. They see no perceptible difference between red, orange, yellow, and green. All these colors that seem so different to the normal viewer appear to be the same color for this two percent of the population.

[modifică] Protanopie

Protanopia is a severe type of color vision deficiency caused by the complete absence of red retinal photoreceptors. It is a form of dichromatism in which red appears dark. It is congential, sex-linked, and present in 1% of all males.[7]

Protanopia (1% of the males): Lacking the long-wavelength sensitive retinal cones, those with this condition are unable to distinguish between colors in the green-yellow-red section of the spectrum. They have a neutral point at a wavelength of 492 nm—that is, they cannot discriminate light of this wavelength from white. For the protanope, the brightness of red, orange, and yellow is much reduced compared to normal. This dimming can be so pronounced that reds may be confused with black or dark gray, and red traffic lights may appear to be extinguished. They may learn to distinguish reds from yellows and from greens primarily on the basis of their apparent brightness or lightness, not on any perceptible hue difference. Violet, lavender, and purple are indistinguishable from various shades of blue because their reddish components are so dimmed as to be invisible. E.g. Pink flowers, reflecting both red light and blue light, may appear just blue to the protanope. Very few people have been found who have one normal eye and one protanopic eye. These unilateral dichromats report that with only their protanopic eye open, they see wavelengths below the neutral point as blue and those above it as yellow. This is a rare form of color blindness.

[modifică] Deuteranopie

Deuteranopia is a color vision deficiency, moderately affecting red-green hue discrimination in 1% of all males. It is hereditary and sex-linked form of dichromatism in which there are only two cone pigments present.[7]

Deuteranopia(1% of the males): Lacking the medium-wavelength cones, those affected are again unable to distinguish between colors in the green-yellow-red section of the spectrum. Their neutral point is at a slightly longer wavelength, 498 nm. The deuteranope suffers the same hue discrimination problems as the protanope, but without the abnormal dimming. The names red, orange, yellow, and green really mean very little to him aside from being different names that every one else around him seems to be able to agree on. Similarly, violet, lavender, purple, and blue, seem to be too many names to use logically for hues that all look alike to him. This is one of the rarer forms of colorblindness making up about 1% of the male population, also known as Daltonism after John Dalton. (Dalton's diagnosis was confirmed as deuteranopia in 1995, some 150 years after his death, by DNA analysis of his preserved eyeball.) Deuteranopic unilateral dichromats report that with only their deuteranopic eye open, they see wavelengths below the neutral point as blue and those above it as yellow.

[modifică] Tritanopie

Tritanopia is an exceedingly rare color vision disturbance in which there are only two cone pigments present and a total absence of blue retinal receptors.[7]

[modifică] Monocromatie

Monochromacy, also known as "total color blindness"[6], is the lack of ability to distinguish colors; caused by cone defect or absence.[7] Monochromacy occurs when two or all three of the cone pigments are missing and color and lightness vision is reduced to one dimension.[6]

Color blindness is usually classed as a disability; however, in select situations color blind people may have advantages over people with normal color vision. Monochromats may have a minor advantage in dark vision, but only in the first five minutes of dark adaptation.

Complete inability to distinguish any colors is called monochromacy. It occurs in three forms:

[modifică] Monocromatia conului albastru

Cone monochromacy is a rare, total color blindness that is accompanied by relatively normal vision, electoretinogram, and electrooculogram.[7]

Blue cone monochromacy is a condition in which the L-cones and M-cones are missing, leaving just the S-cones and the rods. Because the S-cones do not contribute to our perception of brightness, blue cone monochromats have the same problems with bright lights as do achromatopes, although they are able to distinguish a small range of colors.

Cone monochromacy cone monochromacy is the condition of having both rods and cones, but only a single kind of cone. A cone monochromat can have good pattern vision at normal daylight levels, but will not be able to distinguish hues. Blue cone monochromacy (X chromosome) is caused by a complete absence of L- and M-cones. It is encoded at the same place as red-green color blindness on the X chromosome. Peak spectral sensitivities are in the blue region of the visible spectrum (near 440 nm). They generally show nystagmus ("jiggling eyes"), photophobia (light sensitivity), reduced visual acuity, and myopia (nearsightedness).[10] Visual acuity usually falls to the 20/50 to 20/400 range

[modifică] Acromatopsie

Rod monochromacy (achromatopsia) is a rare, nonprogressive inability to distinguish any colors as a result of absent or nonfunctioning retinal cones. It is associated with light sensitivity (photophobia), involuntary eye oscillations (nystagmus), and poor vision.[7]

Achromatopsia is a form of colorblindness characterized by the total loss of all color vision. It is quite rare, but the effects are quite severe. Because an achromatope's vision comes entirely from the rods, it works best at low light levels, and at the periphery of view. Achromatopes must wear dark sunglasses in daylight or bright indoor conditions.

achromatopsia or rod monochromacy, where the retina contains no cone cells, so that in addition to the absence of color discrimination, vision in lights of normal intensity is difficult. While normally rare, achromatopsia is very common on the island of Pingelap, a part of the Pohnpei state, Federated States of Micronesia, where it is called maskun: about 1/12 of the population there has it. The island was devastated by a storm in the 18th century, and one of the few male survivors carried a gene for achromatopsia; the population is now several thousand, of whom about 30% carry this gene.

[modifică] Acromaptosie centrala sau agnosie cromatica

Color agnosia or "central achromatopsia", where the person cannot perceive colors, even though the eyes are capable of distinguishing them. Some sources do not consider this to be true color blindness, because the failure is of perception, not of vision. It is a form of visual agnosia.

[modifică] Detectare si diagnostic

Metoda standard de detectie a deficientilor cromatice este testul lui Ishihara, care consta dintr-o serie de desene formate din pete (cerculete) colorate. Este testul cel mai des folosit pentru a detecta deficiente rosii-verzi. Un numar de pete (cerculete) sunt colorate un pic diferit, si pot fi percepute cu o viziune normala, dar nu cu o anumita deficienta colora. O figura, de obicei un numar scris cu cifre arabe, apare. Setul complet de teste consta din mai multe tipuri de combinatii figura/fundal, si permite diagnosticarea tipului exact de deficienta prezenta.

Testul lui Ishihara este uneori criticat deoarece contine doar numere, fiind astfel inutil pentru copii mici. Se considera ca identificarea problemelor posibile este imporanta sa fie facuta cat mai timpuriu, pentru a le fie explicate copiilor si a preveni probleme posibile sau traume psihologice. In acest scop, teste alternative s-au dezvoltat, care folosesc doar simbolurile patrat, cerc, masina. Testele clinice sunt gandite sa fie rapide, simple si efective in identificarea unor categorii cat mai largi de deficiente cromatice. In studii stiintificem dimpotriva, interesul este de a dezvolta teste mai flexibile [3] pentru a colecta date, a identifica puntele copunctale, a masura diferente abia perceptibile.

[modifică] Tratament si acomodare

There is generally no treatment to cure color deficiencies, however, certain types of tinted filters and contact lenses may help an individual to distinguish different colors better. Additionally, software has been developed to assist those with visual color difficulties.

[modifică] Implicari pentru design

Design implications of color blindness

Color codes present particular problems for color blind people as they are often difficult or impossible for color blind people to understand.

Good graphic design avoids using color coding or color contrasts alone to express information, as this not only helps color blind people, but also aids understanding by normally sighted people. The use of Cascading Style Sheets on the world wide web allows pages to be given an alternative color scheme for color-blind readers. This color scheme generator helps a graphic designer see color schemes as seen by eight types of color blindness. It is sometimes claimed that in extreme emergencies everyone is color blind. When the need to process visual information as rapidly as possible arises, for example in a train or aircraft crash, the visual system may operate only in shades of grey, with the extra information load in adding color being dropped. This is an important possibility to consider when designing, for example, emergency brake handles or emergency phones.

[modifică] Conceptii gresite

Misconceptions and compensations

Color blindness is not the swapping of colors in the observer's eyes. Grass is never red, stop signs are never green. The color impaired do not learn to call red "green" and vice versa. However, dichromats often confuse red and green items. For example, they find it difficult to distinguish a Granny Smith from a Braeburn or the red and green of a traffic light without other cues (for example, shape or location). This is demonstrated nicely in this simulation of the two types of apple as viewed by a trichromat or by a dichromat.

Color blindness almost never means complete monochromatism. In almost all cases, color blind people retain blue-yellow discrimination, and most color blind individuals are anomalous trichromats rather than complete dichromats. In practice this means that they often retain a limited discrimination along the red-green axis of color space although their ability to separate colors in this dimension is severely reduced.