colour vision Flashcards
visible light is part of which spectrum?
electromagnetic
what components make up visible light?
wave and particle
wavelength of visible light
400-750/800nm
what determines the colour of light?
frequency
3 retinal cones
red, green, blue
what is the basis of colour vision?
comparing between cones in two dimensions
ratio of activity across photoreceptors
comparing the activity of neighbouring cones with differing spectral sensitivity
what cones do we make comparisons between?
red vs green
yellow vs blue
yellow is a mixture of red and green
mechanism for red green comparison
colour opponent centre: surround
balance of inhibitory and excitatory input on the central cone
mechanism for blue: yellow comparison
colour opponent ON and OFF bipolars, were red and green synapse with the same bipolar cell
If blue is connected to on bipolar cell and yellow to off…
excited in blue light
If yellow is connected to on bipolar cell and blue to off…
most excited in yellow light
what type of polarisation occurs with colour vision
graded polarisation
opsin protein
- 7 transmembrane domain G protein coupled receptor
- Binds retinaldehyde
- Translates isomerisation of retinal into a ‘biological’ signal
- Determines spectral sensitivity
what is retinaldehyde most sensitive to when not bound to an opsin protein?
UV light
what happens to retinaldehyde when bound to an opsin protein? What causes this change?
its wavelength sensitivity shifts towards the visual light spectrum by adjusting its amino acid structure
long wave cone sensitivity and colour
564nm red
middle wave cone sensitivity and colour
533nm green
short wave cone sensitivity and colour
433nm blue
what determines the wavelength of light that retinaldehyde is most sensitive to?
the amino acid structure of the protein opsin
where are the characteristics of the protein opsin determined?
amino acid sequence encoded in our genome
why can our cone opsin proteins be different from one another?
we dont all have the same amino acid sequence surrounding retinaldehyde as each other, genetic differences
in what ways may cone opsins not work equally well across individuals?
not enough wavelength sensitivity difference between your red and green cone opsins
one of your red or green opsins is malfunctional
In these cases you may loose ability to discriminate between red and green
why do red-green discrimination errors occur more in males than females?
Human Long wavelength sensitive and melanopsin wavelength sensitive opsins are X linked (expressed on our X chromosome)
- If gene on men’s X chromosome is faulty it will be expressed
why is yellow, green colour discrimination less common in
not x linked
what could adding cone types allow?
- Improved spectral resolution within human visible range
- Extend to UV
how many cone opsins do most mammals have? what does this cause
two
red green colour blindness
how many cone opsin receptors do chickens have?
four, they have an additional blue visual receptor
why are mammals the worst at colour vision?
we only have photoreceptors in our eyes
most mammals only have 2 cone opsin genes
inter-individual and species differences in cones:
number of cone classes
their spectral sensitivity
sensory requirements causing differences in species vision capacity
nature of the environment they are living in
land or water
ocean surface species have how many cone opsins
> 4
what wavelength does water transmit more effectively than others?
blue
what range of wavelengths do you get at very deep water
very narrow range, appears blue to us
how many cone opsins do deep sea species have? what so these encode?
two, encoding ‘blue’ sensitive pigments
why do almost all mammals have only 2 cone opsins
many mammals are nocturnal, so would have less use for cone opsins
at the same time in evolution would’ve lost photoreceptors in other parts of the body
what are primates the only group to do?
re-evolve a 3rd cone opsin gene
how many cone opsins did ancestral vertebrate have?
at least 4 cone opsins
What determines species differences in vision capacity
sensory requirements (nature of environment they are living in
evolutionary history
what cone opsin may people lack?
the green cone opsin
what cone opsin is always x linked in mammals?
the green cone opsin/ long wavelength sensitive opsin
how many and what cone opsins does a mouse have?
two cone opsins
green and UV sensitive
how did new world monkeys develop trichromatic vision?
Stable polymorphism on the x chromosome involving two variants of cone opsin gene
same cone opsin gene, but with different amino acids which means it can shift between being sensitive mostly to green or red spectrum
why are new world monkeys colourblind?
the polymorphism allele is X linked so they can only have one version of the allele
what evidence suggests that trichromatic development in primates is under positive selection
it has developed multiple times in new world monkeys
what is the advantage of having females with heterozygous alleles for cone opsins in new world monkeys?
females in the group may lead tasks using colour discrimination. E.g., food selection
how did old world monkey’s develop trichromatic vision?
Duplication of the green cone opsin gene on X chromosome, so X chromosome
accumulation of mutations to shift its spectral sensitivity- so one’s spectral sensitivity is more red and one is more green
do male or female old world monkeys have trichromatic vision?
both
what is the advantage of red/green colour discrimination in primates?
important in discrimination of colour in fruit and vegetation
what is needed for red green discrimination further than cone opsin genes
retina/brain capable of comparing their activation
Engineer female mouse X chromosome green opsin to express human red opsin (result)
- Wild type mice cannot distinguish red from green panel to claim reward
- transgenic mice gradually learn to do this
Engineer female mouse X chromosome green opsin to express human red opsin (conclusion)
Red-green discrimination arises from existing centre:surround organisation
All it requires is that the centre has an over representation of one cone class vs the surround
