Download Feeble dichromatic

Box 6.3 Evolutionary Psy chology
Fig 6.17 Bowerbird Nests
Animals exhibit different degrees of color vision.
• Four categories of color vision capabilities among mammals:
• Minimal color vision with only a single kind of cone pigment and must rely
on interactions between rods and cones to discriminate wavelength found in
raccoons
• Feeble dichromatic color vision with two kinds of cone pigments but very
few cones found in cats
• Robust dichromatic color vision with two kinds of cones and lots of cones
found in dogs
• Excellent trichromatic color vision
• Certain primates such as humans and old world monkeys have good
trichromatic color vision based on three classes of cone photopigments
• However, unlike mammals most birds and reptiles have
tetrachromatic color vision
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Color Vision Almost Reason Enough for Having Eyes
Jay Neitz, Joseph Carroll and Maureen Neitz
Optics & Photonics News , January 2001 , page 28
Transduction
Evolution of vertebrate visual pigments. James K. Bowmaker , Vision Research 48 (2008) 2022–2041-0
Primate Cone Variation
• New World primates have variable cone phenotypes
• spider monkey are trichromats
• Cebus and squirrel monkeys, the males and some females are
dichromats, while other females are trichromats
• owl monkey are monochromats
• New World monkeys have only one cone pigment gene per Xchromosome
• trichromatic variation in females is based on the presence of
allelic diversity at the X-chromosome opsin gene locus
– only heterozygous females have two genes that encode two different middle-tolong wavelength photopigments
• Humans and Old World monkeys have three different cone classes
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Two photopigment genes, M and L, on the X chromosome
short (S) wavelength sensitive cells with maxima near 415–430 nm
middle (M) with maxima at 530–537 nm (human 530)
long (L) with maxima at 555–565 nm (human 560)
chromosome 7
The Evolution of Primate Color Vision by Gerald H. Jacobs and Jeremy Nathans
Scientific American April 2009 page 60
Organization of the Human Trichromatic Cone Mosaic.
Heidi Hofer, The Journal of Neuroscience, 2005 • 25(42):9669 –9679
Color Vision Deficiency “Color Blindness”
• Most so-called color-blind humans (actually color-deficient
humans) have dichromatic
• Most color-blindness in humans is due to the absence of cones
sensitive to medium-wavelength light (M cones).
– Deuteranopia: missing M-cone, 1.5% males
– Protanopia : missing L-cone, 1% males
– Tritanopia: missing S-cone, rare
• Because women have two X chromosomes, a defective gene
encoding for the medium-wavelength pigment on the X
chromosome is almost always compensated for by a normal
copy of the gene on the other X.
• Introduction of photopigment genes into animals with
dichromatic vision suggests we may be able to correct
dichromatic vision in humans.
Simulating color blindness
Deuteranopia: missing M-cone, 1.5% males
Charts used to test for color-blindness
• There is a number in the center of the circle.
If you can see the number, chances are you are not colorblind.
• Plate 1
Those with normal color vision should read the number 74.
• Plate 2
Those with normal color vision should read the number 6.
The Evolution of Primate Color Vision by Gerald H. Jacobs and Jeremy Nathans
Scientific American April 2009 page 63
Gene therapy in colour Robert Shapley
Replacing a missing gene in adult colour-blind monkeys restores normal colour vision. How the new
photoreceptor cells produced by this therapy lead to colour vision is a fascinating question.
Color Vision Requires Opponent Processing
• Having three cones types does not produce trichromatic vision
• Cones need to be connected in an opponent system
• +L/-M
• -L/+M
• This requires special circuits from Cones to Bipolar to Ganglion
cells
• Primates have an additional class of “midget” retinal ganglion cell
• receives its input from a single cone cell
• midget ganglion cells encode fine spatial detail
• first evolved to connect single cones to the brain
• enabled the detection of separate M and L opsins when they
appeared in primates
• Midget ganglion cells are not present in other mammals.