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.
Right visual field (RVF)
• Roger Sperry (1961), Michael Gazzaniga (1970)
Patient N.G.
Left visual field (LVF)
• Roger Sperry (1961), Michael Gazzaniga (1970)
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
Point light source
Horizontal
midline
LVF
RVF
D.B.’s vision
Point light source
Horizontal
midline
LVF
RVF
• D.B. was asked to point to the light source, even
if we could not see it
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 nonconscious 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: patient cannot
perform spontaneous intentional actions
Visual awareness in normal
observers
Theories of consciousness
Summary
• Summarize major points