Mere trichromats can see distinctly any plant even on the fence background, but from bigger distance they tend to merge. Not for Ms. Y who tells that she easily sees the borderline “from any distance”; if any merging occurs, then, at great distances, the fence merges with blue objects. Also she lacks Fency and her favoured Turquoises in TV’s and of course cannot mix it in computers. She tells that European colour photography rather use Grassy while Japanese ones rather Fency. On a national holyday she detected difference between the national Red-White-Green flags on neighbouring houses: one was Red-White-Grassy, the other Red-White-Fency. She was rather hilarious when discovering that on the signboard of a bio shop the desirable plants and herbs were painted with Fency. She reports that all the Plant Kingdom, with one sure and one possible exception, uses Grassy.
On Tetrachromacy
ReplyDeleteby Ágnes Holba and B. Lukács
Human tetrachromacy is extensively discussed in the literature recently, and it seems that we already understand the genetic background of at least some kinds of it. However its mathematics has not yet been analysed in details. Here we are going to discuss the dimensionality and topology of the colour space. As for its Riemannian geometry (so distances) the data are not yet sufficient.
http://www.rmki.kfki.hu/~lukacs/TETRACH.htm
9. TWO GREENS; THE WORLD OF MS. Y
ReplyDeleteAlthough chance hybridisation of 3 opsin genes might have resulted in 6 types of tetrachromacy, really only the hybridisation discussed in the previous Chapter is possible. Namely for crossover the genes must be on the same chromosome (and very near, in addition). The rod opsin is coded on Chromosome 3, the S gene is on Chromosome 7 and the M and L ones on Chromosome X. So H can result, but no other hybridisation is possible.
And still there is Ms. Y. In her teens she observed various strange vision impressions and gradually discovered that, in spite of the statements of her relatives, “Green is not a boring colour”. Sooner or later she observed that the green leaves of bush can easily be distinguished from the green wire-fencing.
To separate Hungarian cultural exotica from biology & physics, let us write down that it is usual in Hungary to paint the garden fences green. That green is not always the same green, but it is always a chemical product of organic industry. It does not mimic clorophylle green even for trichromats; still the difference is not too great to trichromats between fence green (in this Chapter simply Fency) and some plant green (in this Chapter simply Grassy). We note that in Hungarian this terminology of Ms. Y (mirror-translated) would not be forced at all. Magyar is an Uralic language, but “green” is “zöld” from Osetian “zelde” = “(a kind of) grass”.
Mere trichromats can see distinctly any plant even on the fence background, but from bigger distance they tend to merge. Not for Ms. Y who tells that she easily sees the borderline “from any distance”; if any merging occurs, then, at great distances, the fence merges with blue objects. Also she lacks Fency (and her favoured Turquoises) in TV’s and of course cannot mix it in computers. She tells that European colour photography (or colour prints) rather use Grassy while Japanese ones rather Fency. On a national holyday she detected difference between the national Red-White-Green flags on neighbouring houses: one was Red-White-Grassy, the other Red-White-Fency. (The law defining the national flag needs 2/3 majority so it will be difficult to upgrade it). She was rather hilarious when discovering that on the signboard of a bio shop the desirable plants & herbs were painted with Fency. She reports that all the Plant Kingdom, with one sure and one possible exception, uses Grassy. This is rather natural, being the plant chlorophylle a definite spectrum. The sure exception (according to her) is Blue-Green Algae (or maybe some moss?). And look: blue-green algae aka Cyanophytae, of course contain clorophylle, but also a special chromatine, phycocyan, and for trichromats the result is a blue-green chromatoplasm. Hence the very name; and the Classical Greek “cyanos” when does not mean simply undefined “blue”, means blue-green. (True, you must read and reread Aristotle’s De Anima until understanding this; De Coloribus is not sufficiently detailed.)
The probable exception is lemon fruit, but first we must discuss again mixing. The scheme is rather similar to that of the previous Chapter, but now there seems to be two receptors in the middle region. Let us use now the characteristic colours for the receptor.
We do not really know the peak position of F, but Ms. Y reports a measurement where she had the maximal Fency impression at 513 mm. Because of overlaps we could guess slightly longer value, but hard to tell, what. This value is good enough to tell the story.
This scheme suggests that the trichromat representation of Fency is slightly bluish green, similar to the Scarab Green reported at 510 mm average and 70 mm transmission width in Godlove’s classic article [12]. Indeed, Ms. Y tells that some beetle greens seem more or less Fency for her. If so, Fency seems a simple distribution with one band, but this is not yet sure.
Indeed, in many cases her Fency is more bluish for trichromats than her Grassy; but not always. And of course as with an independent H receptor in the previous Chapter, it is not necessary that Fency have higher colour temperature than Grassy at all. For a while on the two sides of trichromatic M or Green the border of her colour space widens from a line to a ribbon, not very wide but of 2 dimensions. The spectral loop is a line on this ribbon, and the colours of monochromatic lights do have colour temperatures. But not all behind trichromatic Greens. Maybe Fency can be monochromatic and Grassy not (suggested only by the Fency nature of Scarab Green, but that was identified by trichromat Godlove), but we seem to feel the opposite more probable; detailed comparison of European and Japanese colour printing paints might help, but it is not easy to start a new research with a single object of measurements.
ReplyDeleteNow let us see mixing. Obviously in the red, orange, violet and deep blue sectors the G and F receptors are already far, so very handmade stimuli would be needed to feel anything from the doubling of the Middle receptor. There the ribbon of the fully saturated colours has practically narrowed into the usual one-dimensional line. However from Yellow to Turquoise the situation is not hopeless.
For trichromats Yellow = Red + Green. However for Ms. Y two kinds of Yellows should be possible (if not, even the reality of the feeling could be doubted) and Red + Grassy should give a “Yellow” definitely different from Red + Fency (although of course with smaller difference than between Grassy and Fency). Now, the subject does report differences between a “cold” Yellow and a “warm” one. In the best case the terminology is arbitrary. However the subject reports that all yellow fruits are “warm” yellow, with the possible exception of lemon fruit.
Now, if it is true then, this would mean that plant green, Grassy, + Red is the “warm” Yellow, but then the very young “green” lemon fruit should be Fency. The situation is tempting and we are waiting for young lemons on sheltered lemon trees in Budapest, North latitude 47°, but truly lemon yellow is somewhat “colder” yellow even for trichromats (although this means surely that lemon yellow is greener for trichromats). We are translating private terminology of a single individual, not the optimal situation for linguistic evolution.
The shortwave side of Grassy and Fency is rather obscure up to now. The subject should be able to distinguish Grassy + Blue from Fency + Blue; the second would be more frequent and so more familiar than the first, because a single spectral line always would excite F and B stronger than G, and only special 2-band reflection spectra would result in G + B. But so far the subject’s reports are equivocal. Sometimes she reports that she does see “only one kind of Turquoise”, but sometimes she tells that Grassy + Blue “is simply not Turquoise, but a boring Green + Blue”, and sometimes she reports tiredness of eye. However her colour distinction is abnormally good in the green range, not deteriorating where it generally should be, so it would be hard to doubt an extra receptor; but we must admit that we do not have so far viable ideas about the mechanism.
ふたつのみどり
ReplyDeleteby 高橋啓治郎
Radium Software Development – Archive
http://www.radiumsoftware.com/0704.html
「Ｙ女史」は子供の頃から，人とは色の見え方が違うということに，なんとなく気づいていた。
彼女の生まれ育ったハンガリーでは，庭のフェンスを緑色に塗って，草木に溶け込ますのがお決まりだった。でも，彼女はこの風習を理解することができなかった。彼女にとって，「フェンスの緑色」と「草の緑色」は，まったく違う色に見えるのだから。
周りの人は言う――確かにちょっと違う色かもしれないけれど，遠く離れた所から見れば，溶け込んで見えなくなってしまうでしょ，と。でも彼女にしてみれば，「フェンス色」と「草色」はまったく違う色で，いくら離れたところから見ても，そのふたつは簡単に見分けることができた。むしろ，「フェンス色」は青色に溶け込むようなのだという。
そのうち彼女は，「フェンス色」はテレビで再現されないことに気づく。コンピューター上でも「フェンス色」を再現することはできなかった。カメラで写真を撮るときも，ヨーロッパ製のフィルムを使った場合は「草色」が出て，日本製のフィルムを使った場合は「フェンス色」が出てくることに気づいた。休日に掲げられるハンガリーの三色旗も，家によって「赤・白・草色」だったり「赤・白・フェンス色」だったりと，まるで違った国旗を掲げているように彼女の目には見えていた。
四色型色覚
人の目の網膜には，３種類の錐体細胞 (cone cell) が備わっている。一般的な三色型の色覚は，これらの神経細胞がそれぞれ「赤」「緑」「青」の三原色に反応することによって構成される。
これらの錐体細胞のうち，「赤」に反応する錐体細胞と，「緑」に反応する錐体細胞は，元は同じものだったという経緯もあって，遺伝上の変異を生じやすい。どちらかの遺伝子を失ってしまったり，あるいは，両方の中間的な特性を持つ「ハイブリッド型」の錐体細胞を生み出したりすることがある。
赤・緑の錐体細胞の遺伝子はＸ染色体上に位置するため，Ｘ染色体を１本しか持たない男性は，こうした遺伝上の影響を受けやすい。ハイブリッド型の錐体細胞を遺伝した場合，本来は赤と緑に反応するものだった錐体細胞は，赤と緑の中間あたりの色に反応する錐体細胞となって，色覚を偏らせてしまうことになる。
これが女性の場合は，少し状況が異なってくる。女性はＸ染色体を２本持つため，その片方がハイブリッド型を遺伝したとしても，もう片方が正常な赤・緑を遺伝すれば，本来の赤・緑の知覚が失われることはない。しかもこのような場合，「赤」「緑」「青」に加え「ハイブリッド」という４つめの錐体細胞を網膜の中に持つことになる。
４種類の錐体細胞から構成される色覚がどのようなものになるか――三色型色覚を持つ大多数の人間にとって，その世界の有りようは想像することすらできない。恐らくは，哺乳類以外の動物にみられるような四色型色覚 (tetrachromacy) に近いものが見えているんだろう。少なくとも「色を見分ける能力」においては，通常の三色型色覚よりも優れたものを持っていることが確かめられている。
例えば三色型色覚においては，波長 580 nm の黄色の光と，波長 630 nm の赤色の光を混ぜたものは，波長 600 nm のオレンジ色の光と同じ見え方をする。これは，混合光の場合も単色光の場合も，赤・緑錐体細胞の反応の強さに違いが無いためだ。これが，赤・緑の中間色に反応するハイブリッド型の錐体細胞を持つ場合，その細胞は混合光と単色光で大きく異なる反応を返す。そのため，これらの光は明らかに異なった色として知覚されることになる。
四色型色覚は，「色を見分ける能力」においては三色型色覚よりも優れていると言えるのだけれど，むしろ「優れ過ぎている」がゆえに不利益をこうむることもあるのかもしれない。上の例からも分かるように，彼女らには三色型色覚を前提とした加色法が成り立たない。これを前提としたテレビや写真の技術は，彼女らにとってみれば酷く再現性の悪いものとして見えていると考えられる。また，三色型色覚を前提とした色覚検査を通過することができない場合もあり，資格取得などの際に不用な扱いを受ける可能性も考えられる。
４つの色を見る彼女
上に述べたような理由から，四色型色覚はもっぱら女性にのみ発現する。三毛猫のオスが滅多にいないのと同じと言えば，ちょっとは分かりやすいかもしれない（そうでもないか……）。
はたして，このような四色型色覚を持つ女性は，どの程度の割合で存在するんだろう？ なにぶん，これまで本格的な調査が行われたことのない領域であるために，その実態の正確なところはわかっていない。ただ，その発現のメカニズムから推察する限りでは，その割合は決して低くないと考えられる。全女性の 2% から 3% は，このような色覚を持っているのではないかとする指摘もある。いわゆる色弱を持つ男性の母親や娘に発現している可能性が非常に高いのだけれど，自覚していないケースも多いと考えられる。
ただし「Ｙ女史」の例は，その中でもさらに珍しいケースで，青と緑の中間色に反応するハイブリッド型錐体細胞を遺伝していて，それが緑よりも短い波長の「フェンス色」の知覚を可能にしていると考えられる。このような青・緑のハイブリッド型の錐体細胞は，赤・緑のハイブリッド型と同じプロセスでは発現しえないもので，恐らくは突然変異がもたらしたものであると考えられている。