3 types of cone photoreceptors
- red / green / blue
- maximally responsive to different parts of the spectrum
what is the basis of colour vision
comparing between the 3 cones
- comparing activity of neighbouring cones with differing spectral sensitivity
how can you create the percept of yellow
- scaling the red and green cones
comparisons
- either : red vs green
- or: blue vs yellow (red+green)
2 filters between physical stimulus and our percept
- 3 cone comparisons
- information channels used to compare activity
centre:surround - red:green
if centre cone is red, it will be more depolarized/excited by red light (less excited by green), and vice verse if it was green
blue:yellow discrimination - ON vs OFF
- ON - blue cone
- OFF - green and red
- yellow light will cause stronger hyperpolarisation (OFF) in ganglion cell
- minimal depolarization from ON cell
- blue light causes strong depolarization of ganglion cell, minimal hyperpolarisation
- more depolarisation from ON
opsin role in spectral sensitivity
- determines what wavelengths of light retinaldehyde is most sensitive to
- prefers shorter wavelengths (blue/ UV violet) ~ 380 nm which we can’t see due to UV lens
- but when bound to opsin, amino acids change charge across chromaphore (retinaldehyde) shifting it to longer wavelengths that we can see
where do we change the amino acids
- cone opsins
- red: long wave ~ 564 nm
- green: middle wave ~ 533 nm
- blue: short wave ~ 433 nm
what are the colour vision characteristics determined by
cone opsin amino acid sequence in genome
Colour blindness
- shows ‘colour’ is inherited e.g. part of genome
- some men have bad red / green dscrimination as LWS and MWS opsins are X-linked
- more prevelent as only one copy of each gene, therefore higher likelyhood it is defected
species differences
- indication of it being inherited
- chicken have one extra blue cone, therefore better spectral resolution within human visual range (tetra-chomatic vision) and beyond to UV
most mammals
- have only 2 cone opsins, so are red/green colour blind
what determines species differences in colour vision capacity
- sensory requirements
- evolutionary history
sensory requirements
- e.g. deep sea species 2 cone opsin genes encoding ‘blue’ sensitive pigments, as more ‘blue’ wavelengths
- surface species >4 cone opsin genes
- ecological niches
evolutionary history
- ancestral vertebrate had at least 4 cone opsins
- almost all mammals have only 2
- primates are the only group to have re-evolved a 3rd cone opsin gene
primate colour vision - new world monkeys
- red/green discrimination has re-evolved
- in female monkeys, polymorphism in green cone opsin, so can detect also red light (3 different opsins)
- males only have one copy, so no additional discrimination
primate colour vision - old word monkeys & apes
- duplication of the green cone opsin gene on X chromosome
- mutation can shift its spectral sensitivity
- all individuals have 3 cone opsin types
advantage of red:green discrimination for primates
- distinguishing food e.g. ripe or unripe
Genetic experiment in mice
- attempted to introduce red cone opsin into mice (only 2 opsins)
- transgenic mice gradually learn colour discrimination
… due to retinal centre:surround organisation
