1. Vision in vertebrates How do we see ?

2. Aims nTo describe how sensory systems fulfil their need to u transduce energy u compare inputs u lead to appropriate behaviour nusing the vertebrate visual system

3. Main parts of visual system neye u lens, retina nbrain - primates u lateral geniculate nucleus u visual cortex nbrain - other vertebrates u superior colliculus / optic tectum

4. Warning nNot all vertebrates work the same way u differences in anatomy u differences in retinal & CNS function F even between closely related species, e.g. frog and toad u always look and see what species the work was done on!

5. Visual systems nfrog - optic tectum nman

6. Lens nlight is a wave 3 * 10 8 m/s nfocus light onto the retina nmaps places in outside world to places on retina in 1:1 fashion

7. Retina nstructure nphotoreceptors - at back of retina nlayers of cells noutput from ganglion cells - at front of retina nBlindspot

8. Structure of retina nretina at back of eye photoreceptors ganglion cells

9. Structure of retina nElectron - micrograph

10. Structure of retina nDiagram light

11. Photoreceptors nRods and Cones nTransduction nCan we perceive photons? nColour vision

12. Rods and Cones nrods 100 * more sensitive nmost cones at fovea nrod density highest around fovea nTherefore, turn eye to see in different places

13. Photoreceptor nrhodopsin in membrane discs inside outer segment nmembrane voltage determined by cell membrane light

14. Transduction nin the dark, channels are open

15. Transduction nTransduction movie

16. Transduction nSensitivity increased by u gain in enzymes u gain in channel u gain at synapse F vesicle ribbon increases number of vesicles released F this reduces quantal noise at synapse

17. Physiological recording nsuction pipette records inward current in outer segment bright dim

18. Can we perceive photons? npeople can see light flashes when 1 in 100 rods will get a photon with 0.2s n Macaque rods able to detect individual photons

19. Colour vision nmost common form is red-green deficiency

20. Colour vision nDiurnal animals & birds have colour vision nHumans and Old- World monkeys are tri-chromatic u most monkeys dichromatic u May have evolved to detect when fruit is ripe

21. Colour vision nDiurnal animals/birds have colour vision nHumans and Old-World monkeys are trichromatic u most monkeys dichromatic u May have evolved to detect when fruit is ripe nother mechanisms of color vision exist u oil droplets in amphibians, turtles ngene homology

22. Colour vision ngene duplication on X chromosome

23. Summary so far nAt retina, u world is spatially mapped u light level is encoded by current u color is used (but not in all animals) u very sensitive

24. Retina is Layered nDiagram light

25. Physiology nDowling - Necturus (mudpuppy) light

26. Physiology nreceptor is inhibited by light nSign conserving /reversing synapses nhorizontal cells mediate lateral inhibition light

27. Physiology nganglion cells signal to brain ndifference in light between adjacent receptors namacrine cells signal on or off nNot light level light

28. On-Off responses nganglion cells usually respond to changes in light nresults from lateral inhibition

29. Lateral inhibition nHermann Grid ncommon to vision, touch, hearing...

30. Summary so far nAt retina, u world is spatially mapped u light level is encoded by current u color is used (but not in all animals) u very sensitive nAt ganglion cells u on/off & surround /center u not a 1:1 relation between light level and signal u this enhances dynamic range

31. Blindspot naxons of ganglion cells run over surface and turn to give optic nerve

32. Ganglion cells project to LGN nLateral geniculate nucleus

33. Mapping of cells nvisual field mapped spatially ndifferent ganglion cells project to different layers

34. LGN sensitive to lines nganglion cells respond to spots nLGN to lines ndifferent line orientations for each LGN cell

35. LGN projects to Visual cortex nvisual = striate cortex = V1

36. Orientation selectivity in cortex nLGN orientation is maintained, with a pinwheel pattern

37. Ocular dominance nLGN kept data from the eyes separate nin visual cortex, data converges. nSome cells have dominant input from R, some from L ncells in same column have same dominance

38. V1 Cortex norientation and ocular dominance work together u grey is contralateral eye

39. Depth perception nUse both eyes to calculate how far away objects are nHypothesis 1: rangefinder nHypothesis 2: measure overlap of images

40. Disparity... n1) rotate eyes n2) compare signals from different parts of the retina n2 wins out

41. Depth perception nMuller - Lyern Ponzo which lines are longer ? Hering

42. Blindsight nloss of visual cortex may show evidence for blindsight npatient cannot “see” but can follow targets with their eyes npatient can discriminate words nprojection to superior colliculus may be responsible

43. Deconstruction of signal nVisual system does not froward a direct “photographic copy”

44. Reconstruction ? nKanizsa illusion ntemporal lobe

45. Summary ntransduction well understood; high gain system nretina well understood; lateral inhibitory mechanism nLGN and V1 cortex fairly well understood; lateral & temporal inhibition; binocular vision nfurther processing still to be elucidated