Evolution and Genetics of Colour Vision

Question 1

Colour appearance and wavelength composition are dissociable

Answer 1

◦ colour appearance cannot simply be accounted for by the spectral profile (wavelength composition) of the image
‣ colour constancy shows that wavelength can change whiele colour appearance remains constant
‣ simultaneous colour contrast shows that colour appearance can change even though wavelength remains constant
◦ spectral changes in the illuminant would be expected to lead to changes in relative cone signals and changes in signalling from RGCs to LGN
‣ opponent processing along retinogeniculate pathway seems suited to signal wavelength composition of light
‣ perhaps further cortical processing is required to account for perceived colour - colour constancy

Question 2

How are different attributes of the visual image processed?

Answer 2

In parallel by distinct cortical areas

◦ motion, depth, form, shape, colour * modular view that these attributes processed in parallel by specialised cortical areas
◦ areas can be divided into D and V streams w dorsal stream concerned w location, depth, motion, and ventral stream concerned w object recognition * 2 streams served by projections from early cortical visual  areas to various parietal and temporal areas * from ventral stream via V4, to temporal areas crucial for object recognition * from dorsal stream MT to parietal areas crusial for knowing where things are and visuallguided action

Question 3

How is colour processed?

Answer 3

• initial reports suggested colour-sensitive cells confined to blobs in V1
  ◦ circular RFs, no orientation sensitivity
  ◦ orientation sensitivity in interblobs
  ◦ later work cast doubt
• onward connections maintained segregation to specialised colour processing areas

V4- fourth visual area
* intermediate between V1-3 and temporal areas crucial for object recognition
* early recordings hinted at specialisation for colour
* classified cells as wavelength coding or colour coding in monkey visual cortex
◦ found only wavelength in V1 but some in V4 were colour coding
◦ V4 lesions had little effect on colour vision and greater effect on shape discrimination
◦ V4 may be involved in colour processing but not the cortical colour centre

Question 4

What is cerebral achromatopsia?

Answer 4

• rare condition assoc w damage to ventral visual areas
  ◦ does not depend on damage to V1
  ◦ includes homologues of monkey extrastriate ventral stream areas including V4
• usually complicated w assoc deficits in object perception
• 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 5

What are single opponent cells?

Answer 5

opposed inputs based on wavelength (parvocellualr in LGN)

Question 6

What are double opponent cells?

Answer 6

opposed inputs based on both wavelength and spatial location in RF

• double opponent cells originally reported in V1 blobs - circular RFs w’o orientation sensitivity
• later reports questioned their existence
• recent evidence suggest DO cells found in V1 but not confined ot blobs and have oriented RFs

Question 7

What are visual pigments?

Answer 7

• photon absorbing molecules that enable photoreceptors to produce electrical signals in response to light
• consist of an opsin bound w 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

Question 8

Describe the spectral tuning of opsin gene families

Answer 8

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

Question 9

What are mammalian opsin families like?

Answer 9

• the base mammalian condition is dichromatic
• modern mammals have retained the rod and two of the cone opsins
• recent duplication of the LWS gene in African primates diverged to give the M and L cone opsins (both members of the ancestral LWS family)
• ancient subsystem. - s cone w single calss of M/L cone supports dichromatic colour vision
• divergence of M/L into distinct M and L cones means centre and surround will have spectrally opponent inputs

Question 10

Where are mammalian LWS opsin gene found?

Answer 10

X chromosome

• old world primates have 2 similar genes head to tail (M and L opsins)
• thought that dupliation of original LWS opsin gene followed by divergence gave rise to distinct M and L opsins
  LCR randomly activates promoter of L and M opsins - only one expressed in a single cone