1. Processing visual information - pathways
Retina (vertical and horizontal pathways)
Lateral geniculate nucleus of thalamus
(Subcortical areas)
Primary visual cortex
Secondary visual cortex 1

2. Processing visual information
- processes
Contrast and edge detection (retina)
Straight edges, curves and corners (primary cortex)
Colour (retina, secondary cortex)
Movement and complex form (secondary cortex)
Depth (primary and secondary cortex)
Important concept: parallel processing (different aspects of the same image are processed simultaneously by different cortical regions) 2

3. Retinal wiring 3

4. Retinal connections
(Guyton & Hall) 4

5. LIGHT  Photoreceptor hyperpolarisation
Visual transduction
REMEMBER
LIGHT  Photoreceptor hyperpolarisation 5

6. Vertical pathway in the fovea
ON and OFF bipolar cells in the dark
(Kolb) 6

7. Vertical pathway in the fovea
ON and OFF bipolar cells in light
(G protein coupled glutamate receptors can be either excitatory or inhibitory) 7

8. Vertical pathway in the fovea
Fovea: each cone connects to both on and off bipolar cells –
Connections from bipolar to ganglion cells are all excitatory + +
Ganglion cells produce the first action potentials in the visual pathway
(Kandel et al) 8

9. Convergence in the vertical pathway
Fovea:
no convergence
One cone  two ganglion cells
Periphery: convergence
Many photoreceptors  one ganglion cell 9

10. Function of horizontal cells
Effects of light on cone connecting directly to an “on” bipolar cell:
Cone is hyperpolarised
Bipolar cell is depolarised
Ganglion cell is depolarised – 10

11. Function of horizontal cells
Effects of light on area surrounding an “on” bipolar cell:
Edge cone is hyperpolarised
Horizontal cell is hyperpolarised
Centre cone is depolarised
Bipolar cell is hyperpolarised
Ganglion cell is hyperpolarised  – –  +
Light on edge of receptive field antagonises light on centre
Horizontal cells produce surround inhibition 11

12. Result: ganglion cell receptive fields
(Kandel et al) 12

13. Result: ganglion cell receptive fields
(Kandel et al) 13

14. Result: ganglion cell receptive fields
Responds poorly to diffuse light
(Kandel et al) 14

15. Central visual pathways15

16. Outputs from the retina
(Kandel et al) 16

17. Outputs from the retina
Pretectum
Pupil constriction/ dilation
Superior colliculus
Eye movement in pursuit of objects
Lateral geniculate nucleus (LGN)
Perceived visual image (relay station in cortical pathway)
(Kandel et al) 17

18. Outputs from the retina
LGN:
“Relay station”
Receptive fields similar to those of ganglion cells
Input from each eye into separate layers
Right worldleft LGN Left worldright LGN
(Guyton & Hall) 18

19. Vertical specificity of pathways from retina to cortex
(Kandel et al) 19

20. Effects of visual pathway damage
(Kandel et al) 20

21. Primary visual cortex 21

22. The visual cortex
(Guyton & Hall) 22

23. Medial surface of occipital cortex: large foveal representation
Primary visual cortex
Medial surface of occipital cortex: large foveal representation
(Kandel et al) 23

24. Primary visual cortex Area of cortex devoted to fovea is huge...
because
High convergence in periphery - no convergence in fovea
(Mountcastle) 24

25. Edge detection 25

26. Edge detection
Edge detection: perhaps the most important aspect of image processing
Begins with ganglion cells, continues in the cortex 26

27. 27

28. 28

29. 29

30. Edge detection begins in the retina
Ganglion cell responds best when the edge just touches the central region of the receptive field
(Kandel et al) 30

31. Edge detection in primary visual cortex
Three types of cells continue the processing:
Simple cells
Complex cells
End-stopped complex cells 31

32. How the recordings were made32

33. Simple cell
(Kandel et al) 33

34. Simple cell A simple cell: responds to a straight edge
at a specific angle
in a specific position in the visual field
(Kandel et al) 34

35. Varieties of simple cell receptive field
(Kandel et al) 35

36. Constructing the simple cell receptive field
LGN neurones
Simple cell
(Kandel et al) 36

37. Complex cell
(Kandel et al) 37

38. Complex cell 38

39. Complex cell A complex cell: responds to a straight edge
at a specific angle
in any position within a large receptive field
The edge stimulus can continue beyond the edge of the receptive field
(Kandel et al) 39

40. End-stopped complex cell
Like a complex cell...
...but inhibited if a straight edge stimulus continues beyond the receptive field
Not inhibited by corners
(Hubel) 40

41. Edge detection in primary visual cortex
Simple cells
Straight edges in a specific place
Complex cells
Straight edges within a defined region
End-stopped complex cells
Corners and curves 41

42. Colour vision 42

43. Colour opponent ganglion cells
see 43

44. Colour “blindness”
Genes on X chromosome for L (red) and M (green): recombination may make mutant forms or else cause gain/loss
Males, having only one X, may lack red or green cones
(Kandel et al) 44

45. Colour “blindness”
Subject can’t distinguish colours in this frequency range 45

46. Testing colour vision
Ishihara test
(Kandel et al) 46

47. Testing colour vision Constructing the Ishihara test 3. Sum of 1 + 2
1. blue-yellow
2. red-green
3. Sum of 1 + 2 47

48. Simulated colour “blindness”
Normal
Protanope
(no red cones) 48

49. Simulated colour “blindness”
Normal
Deuteranope
(no green cones) 49

50. Simulated colour “blindness”
Normal
Tritanope
(no blue cones) 50

51. Depth perception 51

52. Monocular depth cues
(Kandel et al) 52

53. Monocular depth cues 53

54. Binocular disparity
(Kandel et al) 54

55. Binocular disparity neurones
(Kandel et al) 55

56. A pure binocular disparity stimulus
(Kandel et al) 56