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?

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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.

• 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.’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 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

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