Visual Awareness

1. Visual Awareness 9.012 Bryan C. Russell

2. OUTLINE: Intro stuff • Relate to prior lectures • Give philosophical questions • Blind spots, etc.

3. OUTLINE: Philosophical foundations • Mind-body problem • The problem of other minds

4. Neuropsychology of visual awareness

5. 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?

6. 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?

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

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

9. 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

10. 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!

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

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

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

14. 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?

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

16. 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?

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

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

19. 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

20. 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)

21. 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

22. 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?

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

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

25. 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.

26. 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)

27. 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

28. 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?)

29. 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

30. Visual awareness in normal observers

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

32. 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

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

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

35. Marcel’s experiments (1983) • Used yes/no detection performance as measure of conscious experience • Adjusted word duration to get 60% detection rate (between 30-80 ms) YELLOW Pattern mask

36. Stroop color-naming task • Name colors (not text) as fast as you can

37. Stroop color-naming task • Name colors (not text) as fast as you can

38. Stroop color-naming task • Name colors (not text) as fast as you can

39. Stroop experiment RED

40. Stroop experiment RED

41. Suprathreshold trial

42. Subthreshold trial

43. Marcel experiment conclusions • For subthreshold trial, the words were registered even though the subjects were not aware of them • Did the subjects actually not experience the words?

44. Cheesman and Merikle (1984) • Subjects were too conservative in reporting that they had not seen the words • Direct task: subjects should perform discrimination across color words only • Adjust duration threshold until subject performs at chance (25%) RED YELLOW GREEN BLUE

45. Cheesman and Merikle (1984) • Performed Marcel’s experiments with new threshold • No Stroop effects were found • Marcel’s threshold (Did you see anything or not?): subjective threshold of awareness • Proposed threshold (Which of the words did you see?): objective threshold of awareness

46. Discussion • Near objective threshold, subjects report that they are randomly guessing • Hence, nonconscious processing is included as awareness • Should nonconscious processing be included as awareness?

47. Discussion • Recall patient D.B. (blindsight) • Ability to “guess” was not considered awareness • Both thresholds provide bounds on consciousness

48. Ideal thresholding • Exhaustiveness: threshold should lie at the point where the contents of consciousness is exhausted • Main criticism against Marcel • Exclusiveness: threshold should lie at the point where only conscious experiences occur • Main criticism against Cheesman and Merikle

49. Theories of consciousness

50. Summary • Summarize major points