Evolution and genetics of colour vision

Question 1

Colour appearance and wavelength composition are dissociable. Explain.

Answer 1

• Colour constancy shows that wavelength can change while colour appearance remains constant
• Simultaneous colour contrast shows that colour appearance can change even though wavelength remains constant
• Opponent pressing along retinogeniculate pathway seems suited to signal wavelength composition of light
• Perhaps further cortical processing is required to account for perceived colour e.g. colour constancy

Question 2

DORSAL stream: ?

Answer 2

-where?
Onward connections via MT to parietal areas crucial for visually guided action

Question 3

VENTRAL stream: ?

Answer 3

-what?
Onward connections via V4 to temporal areas crucial for visual recognition

Question 4

Cortical processing of colour

Answer 4

• Initial reports suggested colour-sensitive cells confined to blobs in V1
  
  • Circular RFs, no orientation sensitivity
  • Orientation sensitivity, no colour sensitivity in interblobs
  • Later work cast doubt
• Onward connections maintained segregation to specialised colour processing areas

Question 5

Is V4 specialised for colour?

Answer 5

• Fourth visual area (Zeki, 1971) discovered from orderly projections from V2, V3
• Early recordings hinted at specialisation for colour
• was sometimes discussed as ‘the colour centre’
• Zeki classified cells as wavelength coding to colour coding in monkey visual cortex; found only wavelength in V1 but some in V4 were colour coding
• However, V4 lesions had little effect on colour vision and greater effect on shape discrimination
• V4 may be involved in colour processing but it’s not ‘the cortical colour centre’ (no evidence)

Question 6

Colour coding vs wavelength coding cells

Answer 6

• Wavelength and colour are dissociated - responses of colour cells would correlate with perceived colour, while wavelength cells would correlate with wavelength composition
• Zeki (1980, 1983) found only wavelength cells in V1 but significant proportion of colour sensitive in V4

Question 7

Explain the difference between single opponent and double opponent cells

Answer 7

SINGLE:
-opposed inputs based on wavelength e.g. parvocellular in LGN
-could contribute to colour constancy
DOUBLE:
-opposed inputs based on both wavelength and spatial location in RF
-could contribute to colour constancy
-originally reported in V1 blobs - circular RFs without orientation sensitivity
-later reports questioned they existence
-recent evi suggests DO cells are found in V1 but not confined to blobs and have oriented RFs

Question 8

describe Visual pigments

Answer 8

• Photon-absorbing molecules that enable photoreceptors to produce electrical signals in response to light
• consists of an opsin bound with a chromophore
• opsins are members of the GPCR superfamily
• light sensitivity is conferred by the absorption of a photon by the chromophore and subsequent conformational change (11-cis retinal to trans retinal)
• Spectral sensitivity is conferred by the AA sequence of the opsin (determined by opsin)
• Likelihood of absorbing photons of different energy/wavelength is determined by oxygen

Question 9

Sequence analysis suggests 5 vertebrate opsin families. Distinguish between cone and rod opsins

Answer 9

• cone opsins very ancient ~ 540 MYA
• rod opsin family emerges after divergence of cone opsins

Question 10

how many opsin family cone vs rod

Answer 10

4 cone opsin families and 1 rod opsin family

Question 11

describe Spectral tuning of opsin gene families

Answer 11

• each opsin within a family can be tuned by AA substitutions
• peak spectral sensitivity can be shifted (by a few nm) by single AA substitutions at certain key positions

Question 12

Mammalian opsin families

Answer 12

• The base mammalian condition is dichromatic
• modern mammals (eutharians) have retained the rod opsin and two of the cone opsins
• a recent (~30 MYA) duplication of the LWS gene in African primates diverged to give the M and L cone opsins (both members of the ancestral LWS family)

Question 13

Cerebral achromatopsia

Answer 13

• Rare condition associated with damage to ventral visual areas
  
  • does not depend o damage to V1
  • includes homologues of monkey extrastriate ventral stream areas including V4
• usually complicated with associated deficits in object perceptions
• complete loss of colour sensations but normal acuity
  
  • patients can detect borders defined only by chromaticity (not luminance) indicating intact wavelength discrimination despite loss of colour

Question 14

Mammalian LWS opsin gene is on the ?

Answer 14

x-chromosome
- old world primates (catarrhine) have two similar genes head to tail (M and L opsins)
- thought that duplication of original LWS opsin gene followed by divergence (<5 AA substitution) gave rise to distinct M and L opsins
- LCR (randomly) activates promoter of L or M opsins - only one expressed in a single cone

Question 15

Genetics of human opsins

Answer 15

• two AA substitutions in exon 5 account for most of the spectral difference between human M and L opsins
• substitutions in other positions confer allelic variation such that the peak sensitivity of M and L can vary by a few nm across individuals

Question 16

Human colour deficiencies

Answer 16

• Dichromat
  
  • single X chromosome (in males) has only M or L genes
• cone sensitivities:
  
  • S, M (protanope)
  • S, L (deuteranope)
• Anomalous trichromat
  
  • allelic copies of M or L genes
  • S, M, M’ (protanomalous)
  • S, L, L’ (deuteranomalous)

Question 17

Could centre-surround organisation in the retina exploit the emergence of a new spectral class of cone?

Answer 17

• Ancient subsystem: S-cone opposed with single class of M/L cone supports dichromatic colour vision
• recent subsystem: antagonistic inputs to centre and surround from single class of M/L cone
• divergence of M/L into distinct M and L cones means centre and surround will have spectrally opponent inputs

Question 18

Polymorphic colour vision in new-world primates

Answer 18

• old-world primates (including humans) are trichromats (A)
• new-world primates are dichromats (B,C) but in several species LWS opsin gene comes in 3 alleles with sensitivity similar to human M, L or intermediate
• males are homozygous females are dichromats
• heterozygous females are trichromats (depends on X-inactivation; allelic gene from only one of the X-chromosomes is expressed)

Question 19

Implications with polymorphic colour vision?

Answer 19

• possible scenario for evolution of trichromacy in human lineage (old-world primates)
• if a gene for a new class of photoreceptor evolves, do we also need genes to modify the circuitry it feeds into? or is the nervous system flexible enough to exploit it straight away?
• is early developmental experience needed to ensure a new photoreceptor can work with existing circuitry?

Question 20

Genetic engineering of trichomacy in dichromatic male monkeys

Answer 20

• squirrel monkeys (new world, heterozygous female trichromats (S/M/L), males and homozygous females dichromate (S/M)
• transfected adult males with recombinant human L opsin
• behavioural demonstration of red-green colour discrimination after 20 weeks (coinciding with significan levels of transgene expression)
• adult CNS is sufficiently plastic to exploit new spectral cone class with existing circuitry