Re: FWD (SK) Color Vision (was: The night side of science)

From: avatar (avatar@renegadeclothing.com.au)
Date: Wed Jan 15 2003 - 21:00:28 MST


 Absolutely fascinating stuff...

I found this somewhat spurious observation. "I found it interesting that a tetrachromat, looking at a special computer screen based on four primary colors rather than the standard three, could theoretically dump data into her head faster than the rest of us. So I guess there could be advantages of being a tetrachromat!"

The Achromatopsia Network services rod monochromasts of various sorts (no colour vision).

Ironically I have become somewhat vision impaired myself (though this process has been arrested) due to complications from tissue rejection and require sunglasses and (if I'm not being stubborn) a sunvisor to walk around in sunlight. I've often wondered whether the fact that the brain can detect a single photon through the eye has any implications in terms of quantum physics and object perception. There's an interesting article on how objects are rendered in animation in the June issue of Wired. This makes me think that all sorts of quantum effects may be happening in particular areas of object observation (with fluid edges etc.) via the single photon aspect. But I'm probably completely wrong cause I'm a non-scientist and this is just a feeling.

Towards Ascension
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34 After Armstrong
----- Original Message -----
From: "Terry W. Colvin" <fortean1@mindspring.com>
To: <extropians@extropy.org>; "Forteana /Alternate Orphan/" <forteana@yahoogroups.com>
Sent: Monday, January 13, 2003 5:28 PM
Subject: FWD (SK) Color Vision (was: The night side of science)

> The first thing to say is that human vision, from cornea through optic
> nerve is extraordinarily complex. Second, while I read about everything
> published on vision during graduate school, over the last 3 decades my
> reading in the area has been hit-and-miss.
>
> Young (1801) and Helmholtz (1852) postulated a tri-chromatic theory of
> color vision, that humans have 3 types of rods. Helmholtz drew theoretical
> curves of sensitivity to the various wavelengths for each . . . the middle
> one a "bell curve" maximally sensitive to green and the two others rather
> skewed toward the central wavelengths and maximally sensitive to red/orange
> and to blue/violet. The theory was strongly supported at the retinal level
> in 1964 when Marks, Dobelle & MacNichol reported in _Science_ that the
> absorption curves of single monkey and human cones fell into 3 groups.
> [They extracted single cones and passed various wavelengths of light
> through it "the long way" . . . important both because the photochemical
> molecules are aligned for maximum reaction to light from that direction and
> because of the limited sensitivity of the instrument that measured how much
> came out the other end, i.e., was not absorbed. To appreciate the
> difficulty of dealing with a single cone, consider that each human eye
> contains 6 to 7 million cones and 75 to 150 million rods.] While their
> data showed that the cones maximally sensitive to blue and to green were
> more similar than Helmholtz had proposed a century earlier, most accepted
> the idea of 3 types of cones, differing in their relative sensitivity to
> the various wavelengths.
>
> Thaddeus Cowan <tc.pw@Relia.Net> pointed to Cornsweet's statement
> that there is a fourth sensitivity spectrum in the normal human eye, that
> of the rods.
> Quite correct, but generally considered irrelevant to color vision.
> The clearest data demonstrating that the rods play no role in the
> perception of color comes from comparing what humans see when a small
> colored stimulus is focused on the dead center of the eye (fovea centralis,
> which contains no rods) and when the same stimulus is focused sufficiently
> off center so that it hits many rods (including about 20 degrees off
> center, where the greatest rod concentration is found and rods are more
> numerous than cones) . . . there is no difference in color naming / color
> perception by many different measures, hence no impact of the rods.
> Unfortunately for direct demonstration of this point, and vision research
> generally, there is no place on the retina where only rods occur.
> While it is possible that rods play some role in the perception of
> large colored stimuli, which have been rarely studied, most vision
> researchers consider that unlikely . . . even though the neural paths
> leading from rods and cones are not completely separate even within the
> retina much less in the optic nerve. Incidentally, rods and cones are
> distinguished either by differential histologic staining reactions or
> absorption distributions . . . their name was assigned before we found that
> the cones in the fovea are rod shaped.
> With 3 types of cones (the only receptors generally considered to be
> involved in color vision) the typical human is trichromatic . . . and also
> has rods. The term tetrachromatic is reserved for those rare individuals
> thought to have four types of cones (plus rods).
> While it has long been recognized that rods react differently to the
> various wavelengths [for example, the amount of 510 nanometer or
> "blue-green" light that can be detected by the rods under the most ideal
> conditions is minuscule in contrast with the amount of light of around 680
> nm or "orange" required before it is seen; both appear gray]. But this
> fact is primarily detected under special laboratory conditions whereas
> every day we name colors . . . hence we focus on the color-naming measure.
>
>
> One set of figures at hand shows 6% of males and 0.4% of females as
> Deuteranopes (i.e., weak or nonfunctioning "green" cones). Since we can't
> yet directly measure the retina without destroying vision, the most precise
> measures of color weakness come from asking people to match colors.
> Deuteranopes with milder problems simply require more green than typical to
> match yellow with a mixture of red and green. In its stronger forms what
> most of us see as bluish-red, green, and gray are said to be the same color
> (i.e., perfect matches).
> Protanopes (2% of males and 0.04% of females, with "red" cone
> problems) range from those requiring more red than typical to match yellow
> with a combination of red and green to those who report red, bluish-green,
> and gray to be the same color.
> The extremely rare Tritanope (0.0001% of males, too few females to
> calculate) range from requiring more blue than typical to match blue-green
> with a mixture of blue and green to a handful who report purplish blue,
> greenish yellow, and gray as identical.
>
> Historically the names trichromat, dichromat, and monochromat come
> from a slightly different color matching task. Consider the "color
> circle," formed by twisting the linear spectrum of wavelengths into a
> circle so that hues directly opposite each other, when mixed in about equal
> proportions, are seen as gray [i.e., green and red yield gray; blue and
> yellow yield gray . . . reference here to the "additive mixing" of shining
> two separate lights on the same surface, not to the "subtractive mixing"
> produced by two color filters in front of a single bulb or when mixing
> paint].
> How many different wavelengths, spaced reasonably apart on the circle,
> are required for a human observer to match all possible single wavelengths
> of light?
> ** an individual who has no cones will only require one . . . the
> entire world appears gray, the only variation in vision is brightness;
> hence the monochromat. One can argue whether "1" color is seen or "0", but
> historically "mono" has been used.
> ** a few individuals are satisfied to match all possible hues when
> only 2 wavelengths fairly far apart on the color circle are mixed.
> ** most humans require only three lights to make a good match for all
> possible wavelengths.
> ** I have been told (but never seen in a refereed journal or standard
> text) of a few individuals, all women, who are not satisfied with the
> matches unless four colors from the circle are used; presumably these are
> the tetrachromats referred to by Emlyn [sorry, I've lost the fuller
> attribution]. While color weakness affects roughly 16 times more men than
> women, some of the most interesting variations are found in females.
> Another one, also with only a handful of cases, consist of humans with
> normal color vision in one eye and marked color weakness in the other eye.
>
> One set of figures shows 92.0% of males and 99.6% of females to be
> trichromats. Anomalous trichromats, those presumed to have "weakness" in
> one of their cone types, constitute 5.9% of males and 0.4% of females.
> Dichromats, those with only 2 functioning cone types, make up 2.1% of males
> and only 0.03% of females. Rod monochromats, those with cones either
> absent or not functioning . . . and the only ones mentioned here that are
> truly "color blind", are 0.003% of males and 0.002% of females.
>
>
> If everything seems clear so far, watch out. The major rival to the
> Young-Helmholtz tri-chromatic theory of color vision was Hering's 1874
> opponent-process theory. Following in the tradition of astute observations
> by Leonardo da Vinci and Goethe, he postulated four basic colors (in
> mutually antagonistic pairs) and three receptors:
> * a red--green receptor, red from breakdown or dissimilation and green
> from assimilation
> * a yellow--blue receptor, the yellow being dissimilation
> * a white--black receptor, the white being dissimilation
> Until the single cone sensitivity data of Marks et al in the mid-1960s
> there was no clear winner between the two theories, as some color vision
> effects supported one theory, other effects its rival. For a short period
> after Marks et al it appeared that Hering was wrong. But then the vision
> theorists started asking why several phenomena so strongly supported the
> Hering notion. For example, the negative after-image:- stare at a vivid
> red patch for a minute then immediately look at a white or light gray . . .
> the gray appears greenish (and vice versa); clearly an opponent-process
> sort of event.
> After some additional work the currently accepted understanding
> arose:- the Young-Helmholtz tri-chromatic theory describes how the retina
> works (i.e., there really are 3 types of cones in the typical human eye)
> but the neural pulses are converted into something very much like Hering's
> opponent-process for the passage through the optic nerve to the brain.
> Hence color vision is a combination of a tri-chromatic physiology (retinal)
> and a two-part opponent-process physiology (neural). Depending on the
> task, one of these may play the major role and the other be almost
> undetectable . . . hence the age-old inability to decide between the two
> theories.
>
> And then we have, at about the same time as Marks et al, the Nobel
> prize-winning work of Hubel and Wiesel, who showed that some visual
> receptors more readily detect vertical bars of light whereas others are
> more sensitive to horizontal or 90-degree angle bars. How does this tie in
> to color vision and the Young-Helmholtz / Hering notions? And we haven't
> even mentioned moving stimuli . . . .
>
> Charles
>
> ---------------------
>
> [...]
>
> > With 3 types of cones (the only receptors generally considered to be
> > involved in color vision) the typical human is trichromatic . . . and also
> > has rods. The term tetrachromatic is reserved for those rare individuals
> > thought to have four types of cones (plus rods).
>
> The only reference I could find for "tetrachromatic" was in Cornsweet's
> book. Even though I've taught this stuff for 30+ years, it would not
> surprise me to find a few lacunae in my knowledge. Here is your
> opportunity to teach an old dog a new trick. I would like to know the
> following:
>
> 1) What retinal area is sensitive to the "second" interpretation of
> green? The retinal areas sensitive to red and green are small and
> overlap (pretty much). The areas sensitive to blue and yellow are
> larger an overlap (pretty much). Does the retinal area for the new
> color enjoyed by the tetrachromat overlap the red-green area or fall
> somewhere between the blue-yellow area?
>
> 2) What does the spectral absorption curve for the fourth cone look
> like?
>
> 3) What is the new color's complement?
>
> These are very basic questions. Surely these data are available
> somewhere.
>
> Finally, how does the existence of a fourth cone change our
> understanding of the opposing process mechanisms in the LGN? Normally,
> for the trichromat there are three centers--a blue-yellow, red-green,
> and black-white (light present-light absent). Is there a fourth center
> for "new green" and its complement?
>
> Thad Cowan
>
>
> --
> Terry W. Colvin, Sierra Vista, Arizona (USA) < fortean1@mindspring.com >
> Alternate: < fortean1@msn.com >
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