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Visual Awareness. 9.012 Bryan C. Russell. OUTLINE: Intro stuff. Relate to prior lectures Give philosophical questions Blind spots, etc. OUTLINE: Philosophical foundations. Mind-body problem The problem of other minds. Neuropsychology of visual awareness. Definition of vision. - PowerPoint PPT Presentation
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Visual Awareness
9.012
Bryan C. Russell
OUTLINE: Intro stuff
• Relate to prior lectures
• Give philosophical questions
• Blind spots, etc.
OUTLINE: Philosophical foundations
• Mind-body problem
• The problem of other minds
Neuropsychology of visual awareness
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?
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?
Corpus callosum
• Gustav Fechner (1860): necessary for the unity of consciousness
Evil thought experiment
• Suppose we could sever the corpus callosum
• Would we get a person with two consciences?
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
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!
Patient N.G.
• Roger Sperry (1961), Michael Gazzaniga (1970)
Right visual field (RVF)
Patient N.G.
• Roger Sperry (1961), Michael Gazzaniga (1970)
Left visual field (LVF)
Explanation of N.G. behavior
• Speech centers are located in the left hemisphere (LH)
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?
Blindsight
• Ability of certain patients to perform above chance on visual tasks but report that they cannot see
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?
D.B.’s vision
• D.B. was blind in the LVF
• Tested via point light source in various regions
Weiskrantz et al. (1974)
D.B.’s vision
LVF RVF
Point light source
Horizontalmidline
• 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
Horizontalmidline
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)
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
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?
Two visual systems hypothesis
• Cortical system responsible for awareness
• Colliculus system performed significant non-conscious functions
Two visual systems hypothesis
• Confirmed in three monkeys (Cowey and Stoerig, 1995)
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.
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)
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
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?)
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
Visual awareness in normal observers
Subliminal perception
• Ability to register and process information presented below the threshold of awareness
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
• Used yes/no detection performance as measure of conscious experience
Marcel’s experiments (1983)
YELLOW
• Used yes/no detection performance as measure of conscious experience
Marcel’s experiments (1983)
YELLOWPatternmask
• Used yes/no detection performance as measure of conscious experience
• Adjusted word duration to get 60% detection rate (between 30-80 ms)
Marcel’s experiments (1983)
YELLOWPatternmask
Stroop color-naming task
• Name colors (not text) as fast as you can
Stroop color-naming task
• Name colors (not text) as fast as you can
Stroop color-naming task
• Name colors (not text) as fast as you can
Stroop experiment
RED
Stroop experiment
RED
Suprathreshold trial
Subthreshold trial
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?
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
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
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?
Discussion
• Recall patient D.B. (blindsight)– Ability to “guess” was not considered
awareness
• Both thresholds provide bounds on consciousness
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
Theories of consciousness
Summary
• Summarize major points