1. The visual system Lecture 1: Structure of the eye
Photoreceptors: transduction and adaptation
Lecture 2:
Retinal processing
Primary visual cortex: simple and complex cells, edge and feature detection
Colour vision, depth perception 1

2. Visual processing

3. Processing visual information - pathways
Retina (vertical pathway)
Lateral geniculate nucleus of thalamus
(Subcortical areas)
Primary visual cortex
Secondary visual cortex 2

4. 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) 3

5. Retinal wiring

6. Retinal connections
(Guyton & Hall) 4

7. Vertical pathway in the fovea
ON and OFF bipolar cells in the dark
(Kolb) 5

8. Vertical pathway in the fovea
ON and OFF bipolar cells in light
(G protein coupled glutamate receptors can be either excitatory or inhibitory) 6

9. 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) 7

10. Convergence in the vertical pathway
Fovea:
no convergence
One cone  two ganglion cells
Periphery: convergence
Many photoreceptors  one ganglion cell 8

11. Vertical and horizontal pathways
Vertical pathway:
Transmission of the image
Photoreceptorbipolar cell ganglion cell
Action potentials generated in ganglion cells
Axons of ganglion cells form the optic nerve
Horizontal pathway:
Horizontal cells responsible for lateral inhibition
This allows for contrast and edge detection 9

12. Central visual pathways

13. Outputs from the retina
(Kandel et al) 10

14. 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) 11

15. Primary visual cortex

16. The visual cortex
(Guyton & Hall) 12

17. Medial surface of occipital cortex: large foveal representation
Primary visual cortex
Medial surface of occipital cortex: large foveal representation
(Kandel et al) 13

18. Edge detection

19. Edge detection
Edge detection: perhaps the most important aspect of image processing
Begins with ganglion cells, continues in the cortex 14

20. 15

21. 16

22. 17

23. Edge detection in primary visual cortex
Various types of cells continue the processing:
we’ll look at one type, the simple cells 18

24. How the recordings were made19

25. Simple cell
(Kandel et al) 20

26. 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) 21

27. Colour vision

28. Spectral sensitivity of rods and cones22

29. ...depends on sequence differences in opsins23

30. 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) 24

31. Colour “blindness”
Subject can’t distinguish colours in this frequency range 25

32. Testing colour vision
Ishihara test
(Kandel et al) 26

33. Testing colour vision Constructing the Ishihara test 3. Sum of 1 + 2
1. blue-yellow
2. red-green
3. Sum of 1 + 2 27

34. Depth perception

35. Binocular disparity
(Kandel et al) 28

36. Binocular disparity neurones
(Kandel et al) 29

37. A pure binocular disparity stimulus
(Kandel et al) 30