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Thread: Vision in humans and other living creatures, some random info from wikipedia.

  1. #1
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    Default Vision in humans and other living creatures, some random info from wikipedia.

    Heya all

    A 'red' apple does not emit red light.Rather, it simply absorbs all the frequencies of visible light shining on it except for a group of frequencies that is perceived as red, which are reflected. An apple is perceived to be red only because the human eye can distinguish between different wavelengths. The advantage of color, which is a quality constructed by the visual brain and not a property of objects as such.

    The perception of "white" is formed by the entire spectrum of visible light, or by mixing colors of just a few wavelengths, such as red, green, and blue, or even by mixing just a pair of complementary colors such as blue and yellow.

    The retina contains two major types of light-sensitive photoreceptor cells used for vision: the rods and the cones.
    Rods cannot distinguish colors, but are responsible for low-light (scotopic) monochrome (black-and-white) vision; they work well in dim light as they contain a pigment, rhodopsin (visual purple), which is sensitive at low light intensity, but saturates at higher (photopic) intensities. Rods are distributed throughout the retina but there are none at the fovea and none at the blind spot. Rod density is greater in the peripheral retina than in the central retina.
    (this may explain why a rainbow i saw at night appeared monochromatic).

    The nervous system derives color by comparing the responses to light from the several types of cone photoreceptors in the eye. These cone photoreceptors are sensitive to different portions of the visible spectrum. For humans, the visible spectrum ranges approximately from 380 to 740 nm, and there are normally three types of cones.

    The cones are conventionally labeled according to the ordering of the wavelengths of the peaks of their spectral sensitivities: short (S), medium (M), and long (L) cone types, also sometimes referred to as blue, green, and red cones. While the L cones are often referred to as the red receptors, microspectrophotometry has shown that their peak sensitivity is in the greenish-yellow region of the spectrum. Similarly, the S- and M-cones do not directly correspond to blue and green, although they are often depicted as such. It is important to note that the RGB color model is merely a convenient means for representing color, and is not directly based on the types of cones in the human eye.

    In most primates closely related to humans there are three types of color receptors (known as cone cells). This confers trichromatic color vision, so these primates, like humans, are known as trichromats.

    Many invertebrates have color vision. Honey- and bumblebees have trichromatic color vision, which is insensitive to red but sensitive in ultraviolet. Papilio butterflies possess six types of photoreceptors and may have pentachromatic vision. The most complex color vision system in animal kingdom has been found in stomatopods (such as the mantis shrimp) with up to 12 different spectral receptor types thought to work as multiple dichromatic units.

    Vertebrate animals such as tropical fish and birds sometimes have more complex color vision systems than humans. In the latter example, tetrachromacy is achieved through up to four cone types, depending on species. It has been suggested that it is likely that pigeons are pentachromats.

    Reptiles and amphibians also have four cone types (occasionally five), and probably see at least the same number of colors that humans do, or perhaps more. In addition, some nocturnal geckos have the capability of seeing color in dim light. The sensitivity of the helmet gecko eye was calculated to be 350 times higher than human cone vision at the color vision threshold. The optics in the gecko's eyes, having distinct concentric zones of different refractive powers that constitute a multifocal optical system.

    In the evolution of mammals, segments of color vision were lost, then for a few species of primates, regained by gene-duplication. Eutherian mammals other than primates (for example, dogs, cats, mammalian farm animals) generally have less-effective two-receptor (dichromatic) color perception systems, which distinguish blue, green, and yellow—but cannot distinguish reds. The adaptation to see reds is particularly important for primate mammals, since it leads to identification of fruits, and also newly sprouting leaves, which are particularly nutritious.

    However, even among primates, full color vision differs between new-world and old-world monkeys. Old-world primates, including monkeys and all apes, have vision similar to humans. New World Monkeys may or may not have color sensitivity at this level: in most species, males are dichromats, and about 60% of females are trichromats, but the owl monkeys are cone monochromats, and both sexes of howler monkeys are trichromats.Visual sensitivity differences between males and females in a single species is due to the gene for yellow-green sensitive opsin protein (which confers ability to differentiate red from green) residing on the X sex chromosome.

    Several marsupials such as the fat-tailed dunnart have been shown to have trichromatic color vision.
    Marine mammals, adapted for low-light vision, have only a single cone type and are thus monochromats.

    Some animals can distinguish colors in the ultraviolet spectrum. The UV spectrum falls outside the human visible range. Birds, turtles, lizards, and fish have UV receptors in their retinas. These animals can see the UV patterns found on flowers and other wildlife that are otherwise invisible to the human eye. So far, there has not been enough evidence to show that any mammals are capable of UV vision.

    Aphakia is the absence of the lens of the eye, due to surgical removal, a perforating wound or ulcer, or congenital anomaly.
    Aphakic people are reported to be able to see ultraviolet wavelengths that are normally excluded by the lens.

    The pigments present in the L and M cones are encoded on the X chromosome; defective encoding of these leads to the two most common forms of color blindness. The OPN1LW gene, which codes for the pigment that responds to yellowish light, is highly polymorphic (a recent study by Verrelli and Tishkoff found 85 variants in a sample of 236 men), so up to twenty percent of womenhave an extra type of color receptor, and thus a degree of tetrachromatic color vision.Variations in OPN1MW, which codes for the bluish-green pigment, appear to be rare, and the observed variants have no effect on spectral sensitivity.

    In humans, two cone cell pigment genes are located on the sex X chromosome, the classical type 2 opsin genes OPN1MW and OPN1MW2. It has been suggested that as women have two different X chromosomes in their cells, some of them could be carrying some variant cone cell pigments, thereby possibly being born as full tetrachromats and having four different simultaneously functioning kinds of cone cells, each type with a specific pattern of responsiveness to different wave lengths of light in the range of the visible spectrum.One study suggested that 2–3% of the world's women might have the kind of fourth cone that lies between the standard red and green cones, giving, theoretically, a significant increase in color differentiation. Another study suggests that as many as 50% of women and 8% of men may have four photopigments.

    People with four photopigments have been shown to have increased chromatic discrimination in comparison to trichromats.[6] Each of the three cone types in a trichromatic human retina can pick up about 100 different gradations of color, and the brain can combine those variations so that the average human can distinguish about 1 million different colors; a true human tetrachromat would have another type of cone, and its 100 shades theoretically would allow them to see 100 million different colors.


    cheers

    robert

  2. #2
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    Very interesting

    So that is why she's never happy with the decor?
    http://img62.imageshack.us/img62/3985/laser.gif

    Doc's website

    The Health and Safety Act 1971

    Recklessly interfering with Darwinís natural selection process, thereby extending the life cycle of dim-witted ignorami; thus perpetuating and magnifying the danger to us all, by enabling them to breed and walk amongst us, our children and loved ones.





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