Visual Awareness

1. Visual Awareness 9.012 Bryan C. Russell

2. What is it? • Hard to define (it feels like…) • Francis Crick: • “There are two rather surprising aspects of our present knowledge of the visual system. The first is how much we already know—by any standards the amount is enormous… The other surprising thing is that, in spite of all this work, we really have no clear idea how we see anything.”

3. Prior lectures • Up to now, mostly focused on unconscious processes • Vision is deceptively hard

4. Do we have visual awareness at the photoreceptor level?

5. Filling in of scotomas

6. Questions • What determines which processes are conscious ones? • At what level do these visual experiences arise? • How do they arise? • What is the relation between consciousness and which neurons are firing in the brain?

7. ??? • Do answers lie in algorithms or implementation?

8. Explanatory gap • Gap between visual experiences and how they arise from physical events in the brain • No serious contender to fill in the gap

9. Philosophical foundations

10. Dualism (Descartes) • Mind and body are separate entities

11. Mind-body problem • What is the relation between mental and physical events?

12. The problem of other minds • How can I tell if someone or something is conscious?

13. Criteria for consciousness • Behavioral similarity • Does the object act like I do? • Turing test

14. Criteria for consciousness • Behavioral similarity • Does the object act like I do? • Turing test What makes you think you would like to kill everyone? There's a little picture of ED MCMAHON doing BAD THINGS to JOAN RIVERS in a $200,000 MALIBU BEACH HOUSE!! Is it because of some problems at home that you are going through all this? WHOA!! I'm having a RELIGIOUS EXPERIENCE right NOW!! Why do you say you are having a religious experience right now?

15. Criteria for consciousness • Physical similarity • Does the object look like I do? Does it have the same physiological structures?

16. Criteria for consciousness • Physical similarity • Does the object look like I do? Does it have the same physiological structures?

17. Physical similarity • Correlational theory • What physiological structures are responsible for consciousness? • Causal theory • Rigorous scientific explanation for how physiological mechanisms cause consciousness

18. Neuropsychology of visual awareness

19. Definition of vision • “The process of acquiring knowledge about environmental objects and events by extracting information from the light they emit or reflect” • What about visual awareness?

20. Awareness of vision processes • Often, we are not aware of the many vision processes that occur • Is it possible that a full perceptual analysis can occur without visual awareness?

21. Corpus callosum • Gustav Fechner (1860): necessary for the unity of consciousness

22. Evil thought experiment • Suppose we could sever the corpus callosum • Would we get a person with two consciences?

23. Epileptic seizures • Seizure would begin in one hemisphere and move to the other • (1940’s) First surgeries to sever corpus callosum • Reduced frequency and severity of seizures

24. Effect on consciousness • No immediate noticeable effect on consciousness • Karl Lashley: The function of the corpus callosum was simply to hold the two hemispheres together!

25. Patient N.G. • Roger Sperry (1961), Michael Gazzaniga (1970) Right visual field (RVF)

26. Patient N.G. • Roger Sperry (1961), Michael Gazzaniga (1970) Left visual field (LVF)

27. Explanation of N.G. behavior • Speech centers are located in the left hemisphere (LH)

28. N.G. conclusions • It seems that LH is conscious • Is RH visually aware? • Perhaps both LH and RH are visually aware of the object, but only LH can talk about it • Revisit the problem of other minds: what evidence do we need to believe that something is conscious?

29. Blindsight • Ability of certain patients to perform above chance on visual tasks but report that they cannot see

30. Patient D.B. • Had severe migraines due to enlarged blood vessels in the right visual cortex • The part of the brain containing the blood vessels was removed • Migraines stopped • What was the resulting effect on D.B.’s vision?

31. D.B.’s vision • D.B. was blind in the LVF • Tested via point light source in various regions Weiskrantz et al. (1974)

32. Point light source LVF RVF D.B.’s vision Horizontal midline

33. Point light source D.B.’s vision Horizontal midline LVF RVF

34. Point light source D.B.’s vision • D.B. was asked to point to the light source, even if we could not see it Horizontal midline LVF RVF

35. D.B.’s results • D.B. performed remarkably well, given that we was “guessing” when the light was in the LVF Weiskrantz et al. (1974)

36. Other experiments • D.B. (in his LVF) could discriminate between: • “X” versus “O” • Horizontal versus vertical lines • Diagonal versus vertical lines • Performance was improved for larger and longer duration stimuli

37. Other experimental details • D.B. conscientiously reported when he visually saw something • Otherwise, D.B. simply guessed when prompted • How was D.B.’s performance possible?

38. Two visual systems hypothesis • Cortical system responsible for awareness • Colliculus system performed significant non-conscious functions

39. Two visual systems hypothesis • Confirmed in three monkeys (Cowey and Stoerig, 1995)

40. Methodological challenges • D.B.’s eye movements were not tracked • Did not account for light scatter in the eye • Does not agree with experiences of patient C.L.T.

41. Patient C.L.T. • Suffered stroke in right occipital region • MRI showed extensive damage to visual cortex with islands of intact tissue • Superior colliculus unaffected because it uses a different blood stream Fendrich, Wessinger, and Gazzaniga (1992)

42. C.L.T experiments • Eye movement precisely tracked • Stimuli was presented to precise locations • Residual visual function throughout the retina was tested • Performed at chance for most of LVF except for small localizable areas • C.L.T. reported no visual experience in the small localizable areas

43. C.L.T. conclusions • Results challenge theory that unconscious superior colliculus mediates blindsight • However, does not agree with Cowley and Stoerig (1995) experiments • Perhaps monkey mechanisms different from humans (LGN projects to V4 and MT?)

44. Blindsight summary • Patients can perform better than chance on discrimination tasks by “guessing” • Patients cannot “see” based on bottom-up processing of sensory information • Experimenters must provide top-down hypothesis tests; patients cannot do this

45. Blindsight summary • Patients can perform better than chance on discrimination tasks by “guessing” • Patients cannot “see” based on bottom-up processing of sensory information • Experimenters must provide top-down hypothesis tests; patients cannot do this • Blindsight is not helpful: patients cannot perform spontaneous intentional actions

46. Visual awareness in normal observers

47. Subliminal perception • Ability to register and process information presented below the threshold of awareness

48. Subliminal experimentation scheme • Direct task • Subject performs detection task indicating if they see something • If subject performs at chance, then assume they are not visually aware of the stimulus • Indirect task • Subject asked to perform task that uses information from the stimulus of which the subject is not aware

49. Marcel’s experiments (1983) • Used yes/no detection performance as measure of conscious experience YELLOW

50. Marcel’s experiments (1983) • Used yes/no detection performance as measure of conscious experience YELLOW Pattern mask