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: patient cannot perform spontaneous intentional actions

Visual awareness in normal observers

Theories of consciousness

Summary

• Summarize major points