2. Aqueous Humour
Vitreous Humour

3. Rods and cones in more detail
Pigment epithelium

4. OK, what about transforming light into nerve impulses? (transduction)
Distinctions:
Cones Rods
~ 6 million/eye
~120 million / eye
-mostly in fovea
-mostly in periphery
-wavelength-sensitive
-insensitive to wavelength
-insensitive to light intensity
-very sensitive to light intensity
-high acuity
-low acuity
-few-to-one relationship to ganglions (6:1)
-many-to-one relationship to ganglions (120:1)
OK, what about transforming light into nerve impulses?
(transduction)

8. Rods & cones are similar to typical neurons, but also different…
discs

9. My Visual Pigment diagram
Retinal
opsin

10. My Visual Pigment diagram
Retinal
opsin

11. My Visual Pigment diagram
opsin

12. At rest (no light):
Na+
Na+
Na+
Na+
Na+

13. A substance called cGMP is holding the Na+ channels open.
The cascade of activity when pigment bleaching occurs results in the breakdown of cGMP, thus closing the Na+ channels.
The cell hyperpolarizes, turning off the “dark current”.

14. -receptors are releasing inhibitory neurotransmitter
In the dark:
-receptors are releasing inhibitory neurotransmitter

15. In the dark: In the light:
-receptors are releasing inhibitory neurotransmitter
In the light:
-hyperpolarized receptors stop sending inhibitory signals
-results in increases in AP activity bipolars, ganglions

16. Functional architecture or How to build a feature detector
(e.g.: a length detector)

17. A Firing rate of neuron A 2 3 4 5 6 7 Receptors stimulated1
Receptors stimulated A
This way to the brain

18. -can see how differing convergence leads to differences in aspects of rods and cones:
e.g. acuity differences

19. This way to the brain

20. This way to the brain

21. This way to the brain

22. This way to the brain

23. This way to the brain

24. This way to the brain

25. e.g.: acuity differences
-can see how differing convergence leads to differences in aspects of rods and cones:
e.g.: acuity differences
e.g.2: light sensitivity

26. Assume it takes 10 units of activity for a ganglion to fire
This way to the brain

27. Result: No response in brain
2 units 2 2 2 2 2 2
Result: No response in brain
This way to the brain

28. Result: Response in brain!!
10 units
2 units 10 10 10 10 10 10
Result: Response in brain!!
This way to the brain

29. This way to the brain

30. 2 units
This way to the brain

31. Net activity in ganglion: 12 units
Net activity in ganglion: 12 units
This way to the brain

32. Net activity in ganglion: 12 units
Net activity in ganglion: 12 units
This way to the brain

33. A Firing rate of neuron A 2 3 4 5 6 7 Receptors stimulated1
Receptors stimulated A
This way to the brain

34. A Firing rate of neuron A 2 3 4 5 6 7 Receptors stimulated1
Receptors stimulated A
This way to the brain

35. Time

36. Firing rate
Time

37. Firing rate
Time

38. Firing rate
Time

39. Firing rate of converged-on neuron

40. Firing rate of converged-on neuron

41. Problem: the firing rate is influenced by more than just orientation
Problem: the firing rate is influenced by more than just orientation. Intensity of the stimulus also influences channel activity

42. But what if we used a less intense line?
Notice, it fires maximally at 20 cps. So, if it is firing at 10cps, this neuron “knows” it isn’t a vertical line.
Firing rate of converged-on neuron
But what if we used a less intense line? 30 25 20 15 10 5
Is 10 cps because it isn’t vertical, or because it isn’t very intense?

43. Firing rate of converged-on neuron
The solution: look at the pattern of activity across several differently-tuned fibres.
Firing rate of converged-on neuron 30 25 20 15 10 5

44. OK, that’s fine in theory, but what actually exists in the brain?
Well, centre-surround receptive fields are everywhere
Moving higher up (into cortex), can find ‘simple’ cells,
complex cells, end-stopped hypercomplex cells, etc.

45. The processing of the signal begins at the level of the retina in the form of lateral inhibition

46. 20% of signal

47. 10 units 10 10 10 10
Notice that the stronger the response, the bigger the edge enhancement will be

48. Response strength
Cell location

49. Mach bands

50. Physiologically-based illusions
Lateral inhibition

51. Also: two types of ganglions identified: Magno and Parvo cells
Parvo Magno
Small, numerous large, fewer
Small receptive field large receptive field
Slow (20m/s) Fast (40m/s)
Sustained response Transient response
Colour sensitive not colour sensitive
Low contrast sensitivity high contrast sensitivity
Processes form/colour
Processes location, movement

52. From the retina, projections go to two different places:
Superior Colliculus (SC) and Lateral Geniculate Nucleus (LGN)
Two visual pathways: tectopulvinar, and geniculostriate

53. Tectopulvinar pathway
LGN
Pulvinar nucleus
eye
S.C.
Occipital cortex
temporal cortex

54. Geniculostriate pathway

55. Lateral Geniculate Nucleus (LGN)

58. See p.85 and 86 for more diagrams