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

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: patient cannot perform spontaneous intentional actions

30. Visual awareness in normal observers

31. Theories of consciousness

32. Summary Summarize major points