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 Horizontal midline

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

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 d. The pathway through the superior colliculus is smaller than the cortical system (and hence has lower spatial resolution than the cortical system) which explains why performance increased when the stimulus was larger e. Cowey and Stoerig (1995) experiments on monkeys (look up this paper and contrast with Weiskrantz' work, see above) They removed area V1 of one cerebral hemisphere from three monkeys They verified that they had residual visual abilities in the opposite visual half-field They then trained the monkeys in their intact visual field to discriminate between real visual events and blanks (no stimulus) They then tested whether the monkeys would respond to a real stimulus in their impaired hemisphere; they responded as if it was a blank This implies that monkeys do not have visual experiences in the impaired hemifield

Two visual systems hypothesis Confirmed in three monkeys (Cowey and Stoerig, 1995) d. The pathway through the superior colliculus is smaller than the cortical system (and hence has lower spatial resolution than the cortical system) which explains why performance increased when the stimulus was larger e. Cowey and Stoerig (1995) experiments on monkeys (look up this paper and contrast with Weiskrantz' work, see above) They removed area V1 of one cerebral hemisphere from three monkeys They verified that they had residual visual abilities in the opposite visual half-field They then trained the monkeys in their intact visual field to discriminate between real visual events and blanks (no stimulus) They then tested whether the monkeys would respond to a real stimulus in their impaired hemisphere; they responded as if it was a blank This implies that monkeys do not have visual experiences in the impaired hemifield

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?) In humans, there is evidence that the collicular pathway is involved in blindsight in terms of eye movements: eye movements of blindsighted patients can be influenced by stimulation in the blind field (Rafal, Smith, Krantz, Cohen, and Brennen, 1990)

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 Can we obtain evidence of dissociation between visual processing and visual awareness?

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 YELLOW Pattern mask

Marcel’s experiments (1983) Used yes/no detection performance as measure of conscious experience Adjusted word duration to get 60% detection rate (between 30-80 ms) YELLOW Pattern mask

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