Re: [Leica] Re: Trichromatic vision

[Aaron Sandler <aaron.sandler@duke.edu>]

Thanks for the very impressive summary.  I didn't know about the subtypes 
of M and L cones.  I think you took care of the horse pretty well!

I also believe that subcortical and cortical structures (programmed by a 
combination of genetic inheritance and individual experience/environment) 
normally have a greater influence on human perception of color than does 
the retina.  (Leaving out such situations as a complete lack of a class of 
cone, of course.)  The challenge for neurobiologists is to figure out where 
in the brain and (more interestingly) how that processing takes place.

- -Aaron

At 07:15 PM 2/5/2004, you wrote:
>In a senior moment, I don't recall if I sent this post. If it a duplicate, I
>apologize for the wasted bandwidth.
>
>Larry Z
>---------------------------
>
>To continue to beat a horse of a different color, this is a long post,
>extracted from current research literature, explaining the assertion that 
>there are
>more than three types of color receptors in the human eye. The trichromatic
>theory of color vision is not invalidated, as such, but it has been
>significantly modified. It helps explain why we can see colors that we 
>cannot normally
>photograph and why people disagree on the exact color match of pieces of 
>fabric.
>(This is my problem when my wife takes me shopping with her.)
>
>A long accepted fundamental property of human vision is trichromacy. The
>trichromatic theory helps to explain our color perceptions and color
>discriminations. The anatomical basis of trichromacy begins with the 
>complement of cone
>photoreceptors in the retina. For over one hundred years researchers 
>thought that
>the color-normal eye contained three cone types, designated as S, M, and L,
>whose photopigments were later psychophysically estimated to have peak 
>spectral
>sensitivities near 440, 540, and 560 nanometers. There is considerable
>overlap in sensitivity of the middle wavelength sensitive and long wavelength
>sensitive cone types.
>
>Over the years, however, sensory psychologists questioned whether subtle
>variations may exist in normal color vision based on small individual 
>differences
>in the spectral sensitivities of the photopigments (Alpern & Wake, 1977; 
>Neitz
>& Jacobs, 1986). The findings of the early studies were viewed with some
>skepticism, however, because of the difficulty in ruling out measurement 
>error and
>confounding factors. As the psychophysical evidence grew, researchers began
>to investigate this possibility from many angles.
>
>Today, psychophysical (Neitz & Jacobs, 1990; Mollon, 1992),
>microspectrophotometric (Dartnall, Bowmaker, & Mollon, 1983), and 
>molecular genetic studies
>(Nathans, Piantanida, Eddy, Shows, & Hogness, 1986; Winderickx et al., 1992)
>provide evidence of substantial variation in the number and spectral 
>sensitivity
>of the cone types in the color-normal eye (also see Mollon, Cavonius, and
>Zrenner, 1998). The evidence now suggests the presence of three broad 
>families of
>normally occurring cone photopigments. There is thought to be only one
>photopigment with a peak spectral sensitivity in the short wavelengths 
>(blue), but
>there is now evidence that there are multiple middle wavelength (green)
>photopigments and multiple long wavelength (red) photopigments. The 
>difference in
>spectral sensitivity among the middle wavelength pigments or among the long
>wavelength pigments has been estimated to be approximately 5-7nm(Neitz, 
>Neitz, &
>Jacobs, 1995). In fact there may be as many as 9 different cone types with 
>various
>peaks in photosensitivity among the middle and long wavelength families.
>
>Molecular genetic analyses show that individuals may inherit a surprisingly
>large number of different X-linked, recessive genes that encode the 
>production
>of these photopigments (Neitz, Neitz, & Grishok, 1995). An obvious 
>question is
>why do we have so many color vision genes? The genes that encode the middle
>and long wavelength sensitive pigments reside near the end of one of the arms
>of the X chromosome and they have very similar DNA sequences. In fact, the
>substitution of one amino acid in the DNA of a photopigment gene is 
>sufficient to
>cause a change in the spectral sensitivity of that photopigment and in our
>color perceptions. The location and similarity of these genes makes them
>susceptible to the kinds of genetic errors that produce multiple gene 
>copies, as well
>as hybrid genes that are genetic composites of the original ones (Nathans, et
>al., 1986).
>
>At present, it appears that normal color vision results from inheriting at
>least one cone type from each cone class (short, middle, and long). It is
>unclear, however, which complement of genes and cone types result in 
>specific types
>of color vision deficiency. There is a great deal of genetic variation among
>individuals with the same type of color defect, making this work difficult.
>However, it appears that both the type and severity of a color vision 
>defect can
>be linked to the complement of different cone types in the retina. Hybrid
>genes, which have been associated with small differences in the spectral
>sensitivity of the photopigments, are thought to be involved.
>
>These findings lead to an interesting question: if humans possess more than
>three cone types in their retina, do they still have trichromatic vision? The
>answer appears to be yes, presumably because the outputs of the different
>middle or longwavelength cone photoreceptors are summed together before 
>leaving the
>retina. The resulting signals differ to a small but significant degree across
>individuals, though, because they affect color perception in some situations.
>Individuals with different complements of cone pigments will not accept each
>other's color matches in the long wavelength end of the spectrum and they 
>will
>disagree on color names for certain wavelengths of light (Neitz, Neitz, &
>Jacobs, 1993). For example, a particular mixture of red and green light might
>appear a perfect yellow to your eye, but appear a greenish-yellow or slightly
>orange to someone else. This type of color vision assessment, called the 
>Rayleigh
>Match, is the most accurate method for measuring color discrimination and
>diagnosing the congenital color vision defects.
>
>The distribution of photoreceptors in the retina appears to be nearly random.
>The ratio of R / G / B cone types varies, but the long wavelength cones are
>the most prevalent; short wavelength cones the least prevalent in the retina.
>Thus some people with a normal complement of color vision genes, may have a
>mosaic retina: a patchwork of color-normal and color-deficient regions (Cohn,
>Emmerich, & Carlson, 1989). The nature of this mosaic depends on the 
>inherited
>complement of color vision genes and on the point in development that
>X-chromosome inactivation occurred. That is, some may develop a color vision
>deficiency while others may develop normal color vision (Miyahara, 
>Pokorny, Smith,
>Baron, & Baron, 1998). And, in fact, there are reports in the literature of
>identical (monozygotic) twins where one twin has normal color vision and 
>the second
>is color-deficient (Jorgenson, et al., 1992).
>
>In addition to teasing out the exact mechanism of color vision, work for
>sensory psychologists and physiologists also involves investigating the 
>extent to
>which these individual differences in color vision affect interactions with
>the world. Society uses color to code information in a variety of settings,
>including art. photography, education and transportation. In many occupations
>color discrimination is critical, for example, in discriminating 
>electrical wiring
>and colored signal lights or in medical research. While these individual
>differences are small, they may prove to be problematic in some settings.
>
>In contrast to the research directed at the earliest stages of sensory
>processing, today there is also substantial exciting research interest at 
>the other
>end of the sensation to perception continuum: This research is directed at
>higher level perceptual processes and phenomena in the gray area where 
>perception
>and cognition meld. Culture, desire, expectation, and learning are as
>important in determining what we see as the sensation itself. I suspect 
>that this
>area is of greater interest to photographers than understanding the specific
>mechanism of color vision. In short, each of us sees color slightly 
>differently
>and there are some colors that humans can see which cannot be duplicated 
>by any
>trichromatic process using fixed primary colors.
>
>Larry Z
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