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

19. D.B.’s vision
D.B. was asked to point to the light source, even if we could not see it
Point light source
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
d. The pathway through the superior colliculus is smaller than the cortical system (and hence has lower spatial resolution than the cortical system) which explains why performance increased when the stimulus was larger e. Cowey and Stoerig (1995) experiments on monkeys (look up this paper and contrast with Weiskrantz' work, see above)
They removed area V1 of one cerebral hemisphere from three monkeys
They verified that they had residual visual abilities in the opposite visual half-field
They then trained the monkeys in their intact visual field to discriminate between real visual events and blanks (no stimulus)
They then tested whether the monkeys would respond to a real stimulus in their impaired hemisphere; they responded as if it was a blank
This implies that monkeys do not have visual experiences in the impaired hemifield

24. Two visual systems hypothesis
Confirmed in three monkeys (Cowey and Stoerig, 1995)
d. The pathway through the superior colliculus is smaller than the cortical system (and hence has lower spatial resolution than the cortical system) which explains why performance increased when the stimulus was larger e. Cowey and Stoerig (1995) experiments on monkeys (look up this paper and contrast with Weiskrantz' work, see above)
They removed area V1 of one cerebral hemisphere from three monkeys
They verified that they had residual visual abilities in the opposite visual half-field
They then trained the monkeys in their intact visual field to discriminate between real visual events and blanks (no stimulus)
They then tested whether the monkeys would respond to a real stimulus in their impaired hemisphere; they responded as if it was a blank
This implies that monkeys do not have visual experiences in the impaired hemifield

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?)
In humans, there is evidence that the collicular pathway is involved in blindsight in terms of eye movements: eye movements of blindsighted patients can be influenced by stimulation in the blind field (Rafal, Smith, Krantz, Cohen, and Brennen, 1990)

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
Can we obtain evidence of dissociation between visual processing and visual awareness?

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 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