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Visual Awareness
9.012
Bryan C. Russell
What is it?
• Hard to define (it feels like…)
• Francis Crick:
– “There are two rather surprising aspects of
our present knowledge of the visual system.
The first is how much we already know—by
any standards the amount is enormous… The
other surprising thing is that, in spite of all this
work, we really have no clear idea how we
see anything.”
Prior lectures
• Up to now, mostly focused on unconscious
processes
• Vision is deceptively hard
Do we have visual awareness at
the photoreceptor level?
Filling in of scotomas
Questions
• What determines which processes are
conscious ones?
• At what level do these visual experiences
arise?
• How do they arise?
• What is the relation between
consciousness and which neurons are
firing in the brain?
???
• Do answers lie in algorithms or
implementation?
Explanatory gap
• Gap between visual experiences and how
they arise from physical events in the brain
• No serious contender to fill in the gap
Philosophical foundations
Dualism (Descartes)
• Mind and body are separate entities
Mind-body problem
• What is the relation between mental and
physical events?
The problem of other minds
• How can I tell if someone or something is
conscious?
Criteria for consciousness
• Behavioral similarity
– Does the object act like I do?
– Turing test
Criteria for consciousness
• Behavioral similarity
– Does the object act like I do?
– Turing test
What makes you think you would like to kill everyone?
There's a little picture of ED MCMAHON doing BAD THINGS to JOAN
RIVERS in a $200,000 MALIBU BEACH HOUSE!!
Is it because of some problems at home that you are going through all this?
WHOA!! I'm having a RELIGIOUS EXPERIENCE right NOW!!
Why do you say you are having a religious experience right now?
Criteria for consciousness
• Physical similarity
– Does the object look like I do? Does it have
the same physiological structures?
Criteria for consciousness
• Physical similarity
– Does the object look like I do? Does it have
the same physiological structures?
Physical similarity
• Correlational theory
– What physiological structures are responsible
for consciousness?
• Causal theory
– Rigorous scientific explanation for how
physiological mechanisms cause
consciousness
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.’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 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
Marcel’s experiments (1983)
• Used yes/no detection performance as
measure of conscious experience
YELLOW
Marcel’s experiments (1983)
• Used yes/no detection performance as
measure of conscious experience
Pattern
mask
YELLOW
Marcel’s experiments (1983)
• Used yes/no detection performance as measure
of conscious experience
Pattern
mask
YELLOW
• Adjusted word duration to get 60% detection rate
(between 30-80 ms)
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
RED YELLOW GREEN BLUE
• Adjust duration threshold until subject
performs at chance (25%)
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
Theoretical dichotomy
• Functional approach to consciousness:
– Consciousness lies in the algorithms
• Physiological approach to consciousness:
– Consciousness lies in the implementation
• Perhaps answer lies somewhere in
between
Crick/Koch conjectures (1990)
• Neural correlate of consciousness (NCC)
– The lower layers conjectures
– The 40 Hertz conjecture
– The frontal lobes conjecture
The lower layers conjectures
• Need short-term memory of visual neural
activity
• Layers 5 and 6 has reverberatory circuit: a
closed-loop neural circuit
The 40 Hertz conjecture
• The binding problem: how do different
features of different objects get stitched
together?
– Synchronize the firing of all neurons
responding to the same object
– Evidence that this is taking place in the visual
cortex of cats (Gray and Singer, 1989)
The frontal lobes conjecture
• Need to make best interpretation of visual
scene available to motor output planning
• Frontal cortex
– Input: visual areas in the brain
– Output: motor areas of the cortex
Leopold and Logothetis (1996)
Summary
• Visual awareness still a largely open
problem
• Some progress has been made to
separate unconscious visual processing
• Correlational theories seem to be
promising direction towards solution