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The visual system Chapter 10

The visual system Chapter 10. The physical stimulus. Light is a wave… …and a particle. Psychological dimensions of light. Hue Saturation Brightness. The eye. Cornea – the main focusing element Lens – adjustable focusing

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The visual system Chapter 10

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  1. The visual systemChapter 10

  2. The physical stimulus Light is a wave… …and a particle

  3. Psychological dimensions of light Hue Saturation Brightness

  4. The eye • Cornea – the main focusing element • Lens – adjustable focusing • Iris – adjust sensitivity and depth of focus • Retina – photosensitivity and much, much more

  5. Structure of the retina

  6. Visual transduction Photons produce electrical events in photoreceptors (hyperpolarization)

  7. In darkness, there’s a continuous current in the outer segment caused by the circulation of sodium. In light, sodium circulation slows down and receptors hyperpolarize

  8. Disks in outer segments called lamellae contain a photopigment

  9. Rhodopsin -- the magic photopigment

  10. Through the wizardry of biochemistry, sodium channels close

  11. Photoreceptors come in different flavours

  12. Spectral absorption curves

  13. Lateral interactions in the retina help with several problems 1. Contour sharpening 2. Enhancing sensitivity

  14. Mach bands

  15. A slightly misleading illustration • We understand the neural basis of lateral inhibition because of work on the horseshoe crab that is not feasible in mammals

  16. The duplex retina

  17. The cost of the duplex retina

  18. Central visual pathways The lateral geniculate nucleus of the thalamus

  19. Primate lateral geniculate nucleus

  20. Centre-surround antagonism is the mammalian analogue of lateral inhibition.

  21. Hubel and Wiesel’s simple hierarchical model of visual cortical processing Simple cells Complex cells

  22. Columnar organization of VI

  23. Ocular dominance

  24. The hypercolumn

  25. Optical imaging of ocular dominance columns

  26. Optical imaging of orientation tuning

  27. Correlation between optical imaging and electrophysiological results for orientation tuning

  28. Margaret Wong-Riley and • the cytochrome oxidase story • autoradiography and activity • cytochrome oxidase and activity • intrinsic variability in cyo

  29. Cytochrome oxidase in monkey VI and VII -blobs and stripes of every stripe

  30. V2 and cytochrome oxidase stripes

  31. Multiple visual representations in cortex

  32. Visual agnosias • Motion blindness • Prosopagnosia • Cortical colour blindness • Visual object agnosia

  33. Visual processing streams I Schneider’s Experiment Tectal undercut Cortical ablation

  34. Visual processing streams II • Gordon Holmes • single patient studies -- it was obvious that people without conscious vision were not ‘blind’

  35. Visual processing streams III –Weiskrantz and blindsight In a preliminary test, Weiskrantz positioned a stick in D.B.s blind spot, either sideways or straight up and down. He asked D.B. what he saw. The patient said, "I see nothing." Weiskrantz persisted. "Am I holding the stick sideways, or vertically?" D.B.: "I don't know -- I don't see a stick." Weiskrantz: "Guess." D.B.: "Sideways." Weiskrantz: "Now which way am I holding it?" D.B. "I don't see a stick." Weiskrantz: "Guess." This continued for 20 trials in which D.B.'s performance was perfect.

  36. Weiskrantz recounts: "In the interview that followed, and which was recorded, D.B. expressed considerable surprise. 'Did you know how well you had done?', he was asked. 'No,' he replied, 'I didn't -- because I couldn't see anything; I couldn't see a darn thing.' 'Can you say how you guessed -- what it was that allowed you to say whether it was vertical or horizontal?' 'No, I could not because I did not see anything; I just don't know.' (p 24)."

  37. Pohl’s Experiment

  38. Ungerleider and Mishkin’s Two visual cortical streams

  39. Milner and Goodale’s “Posting” experiment

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