1. Evolution of colour vision After J Neitz, J Arroll, M Neitz Optics & Photonics News, pp. 26-33, Jan 2001.

2. Neural mechanisms of seeing colour zLight sensitive receptors zneural components for processing yextracting relative responses from neighbouring receptors ywavelength sensitive encoding youtput to labelled lines

3. Black and white perception zSmall cluster of receptors illuminated by a small spot of light zinformation gathered yfrom illuminated receptors yfrom their immediate neighbours zBrain nerve fibres receive output from ycluster of receptors from the “white” labelled lines ycluster of receptors from the “black” labelled lines zone of the two outputs is inverted compared to the other

4. Hue perception zEncoding in two components, each of them responsible for a pair of sensations, sensations in each pair are opposed to one another, yblue-yellow hue system yred-green hue system zeach draws yfrom a common set of photoreceptors: L, M, S; youtputs via different neural components: different labelled lines.

5. Cone photoreceptors

6. Hue systems zblue-yellow(B-Y): output from the S cones, comparing it to L + M cone responses zred-green(R-G): output from the L cones, comparing it to M cone responses zonly blue-yellow system draws from S cones, S cones differ from M and L in physiology and retinal distribution zB-Y more vulnerable: toxic exposure, eye diseases, trauma zDifferent evolutionary history

7. Blue-Yellow colour vision system zTrichromatic colour vision in mammals: only in man and some subset of primates zSome mammals are monochromats zMost mammals are dichromats, e.g. dog, system is homologous to the “blue-yellow” system

8. Cone photopigment sensitivity of dogs Dogs have two types of cone- pigments most similar to human S and L pigments. The bar at the bottom approximates how a dog can distinguish among colours

9. Tomatoes: which one is ripe, seen by a dog

10. Tomatoes: which one is ripe, seen by a trichromat

11. Photopigments and their genes zComposition of the photopigments ychromophore: 11-cis-retinal yprotein component, covalently bound: opsin zIn terrestrial animals the chromophore is the same, the opsin varies ythe opsin tunes the absorption maximum ythe opsins belong to a comon family

12. Photopigments and their genes zMolecular genetic methods can deduce the amino acid sequencees of photopigment opsins zThe two classes of dichromatic pigments have strikingly different amino acid sequences (50 %): Indication for early differentiation of the S and L photopigments in evolutionary terms

13. Photopigments and their genes - evolution of colour vision zS and L pigments amino acid sequences different zSeven amino acid changes produce the 30 nm difference between the M and L pigments zExtrapolation and speculation: 6 % difference in amino acid sequence required for the 100 nm shift between S and L cones

14. Speculation on evolution zComparison: differences in rod pigments of species as clock, constant rate genetic drift S and L/M cone differenti- ation about 1000 million years ago (MYA) Oldest fossils: 6000MYA

15. Speculation on evolution zDichromacy almost as old as vision zDistinction among colours, humans see y200 grey levels yDichromacy: 50 discernible chromatic steps, provides 10.000 steps zWavelength sensing is as fundamental to vision as is light detection

16. Red - Green colour vision system zL and M photopigments individually polymorchic, on average difference: 15 amino acids zGenetic clock estimate: L and M difference 50 MYA (Old and New World primates split about 60 MYA) zThree neuronal line pairs: (Black-White, Y-B, R-G) y100 steps in R-G direction: 10 6 distinguishable colours

17. Beyond trichromacy zNon-mammal diurnal vertebrates (birds, fish, etc.) have four photopigments: also UV zMammals were nocturnal when appeared at the time of the dominance of dinosaurs zNocturnal ancestors of modern primates were reduced to dichromacy zPrimates invented trichromacy separately

18. Neural circuits for red- green colour vision zDiurnal primates: acute spatial vision: small receptive fields (midgets), contacting single cones zOpponent signals from surrounding neighbours: new receptor (L or M) compares also colour, no new wiring needed zMammalian visual cortex molded by experience

19. Directions of colour vision research zL and M photopigment genes might misalign during meiosis and recombine: mixed sequences might occur zVariants common in L gene, females have two X chromosomes, the two might have different L pigments zX-chromosome inactivation can produce two L cones in females: four spectrally different receptors.

20. Directions of colour vision research zThe two L cones are very similar: few steps of colour discrimination zFemales found who showed increased colour discrimination ability zL/M cone ration can change from 1:1 to 4:1, with no measurable colour vision difference: plasticity of nervous system? zChromatically altered visual environment has long term influence on colour vision

21. Further speculation zIf neural circuits for colour vision are sufficiently plastic gene therapy ycould replace missing photopigments ycould add a fourth cone type