Neural Correlates of Visual Awareness

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Neural Correlates of Visual Awareness
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A Hard Problem

• Are all organisms conscious?

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A Hard Problem

• Are all organisms conscious?
• If not, whats the difference between those that
  are and those that are not?
• Complexity?
• Language?
• Some peculiar type of memory?
• All of these?

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A Hard Problem

• Really what were asking is

What is it about our brains that makes us
conscious?
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A Hard Problem

• Neuroscientists have deferred some of the
  difficulties of that problem by focusing on a
  subtly different one
• What neural processes are distinctly associated
  with consciousness?
• That is still a pretty hard problem!

What are the neural correlates of consciousness
(NCC)
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Searching for the NCC

• When a visual stimulus appears
• Visual neurons tuned to aspects of that stimulus
  fire action potentials (single unit recording)
• Ensemble depolarizations of pyramidal cells in
  various parts of visual cortex (and elsewhere)
  (ERP, MEG)
• Increased metabolic demand ensues in various
  parts of the visual cortex (and elsewhere) (fMRI,
  PET)
• A conscious visual even occurs

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Searching for the NCC

• We can measure all sorts of neural correlates of
  these processesso we can see the neural
  correlates of consciousness right?
• So whats the problem?

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Searching for the NCC

• We can measure all sorts of neural correlates of
  these processesso we can see the neural
  correlates of consciousness right?
• So whats the problem?
• Not all of that neural activity causes
  consciousness
• We will explore some situations in which neural
  activity is dissociated from awareness

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Dorsal and Ventral Pathways

• But first some review and further consideration
  of visual pathways

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Dorsal and Ventral Pathways

• Different visual cortex regions contain cells
  with different tuning properties represent
  different features in the visual field
• V5/MT is selectively responsive to motion
• V4 is selectively responsive to color

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Dorsal and Ventral Pathways

• V4 and V5 are doubly-dissociated in lesion
  literature Achromatopsia and Akinetopsia,
  respectively

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Dorsal and Ventral Pathways

• V4 and V5 are key parts of two larger functional
  pathways
• Dorsal or Where pathway
• Ventral or What pathway
• Ungerleider and Mishkin (1982)
• Magno and Parvo dichotomy arose at the retina and
  gives rise to two distinct cortical pathways

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Dorsal and Ventral Pathways

• Pohl (1973) Early dissociations of Temporal and
  Parietal functions
• Landmark task
• Monkeys trained to find reward in well near a
  landmark
• once they get the task the contingency is
  switched monkey must find well opposite to the
  landmark
• errors until relearning indicates ability to use
  the spatial relationship information to perform
  task

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Dorsal and Ventral Pathways

• Pohl (1973) Early dissociations of Temporal and
  Parietal functions
• Landmark task
• Dissociates Parietal and Temporal lobes
• Parietal lesions impair relearning of landmark
  task

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Dorsal and Ventral Pathways

• Pohl (1973) Early dissociations of Temporal and
  Parietal functions
• Object task
• Reward location is indicated by one of two
  objects
• contingency is switched monkey must use other
  object
• errors to relearn indicates ability to use
  object distinction to perform task

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Dorsal and Ventral Pathways

• Pohl (1973) Early dissociations of Temporal and
  Parietal functions
• Object task
• Adding this task doubly dissociates Parietal and
  Temporal lesions
• Temporal lesions impair object task

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Dorsal and Ventral Pathways

• do both of these pathways equally contribute
  their contents to visual awareness?

V5
V4
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Agnosia

• Lesions (especially in the left hemisphere) of
  the inferior temporal cortex lead to disorders of
  memory for people and things
• recognition and identification are impaired
• prosopagnosia is a specific kind of agnosia
  inability to recognize faces
• explicit (conscious) decisions about object
  features are disrupted

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Agnosia

• Goodale and Milner Patient DF
• Patient could not indicate the orientation of a
  slot using her awareness
• Patient could move her hand appropriately to
  interact with the slot
• whether visually guided or guided by an internal
  representation in memory

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Agnosia

• Single dissociation of action from conscious
  perception
• Dorsal pathway remained intact while ventral
  pathway was impaired
• Dorsal Pathway seems to guide motor actions, at
  least for ones that need spatial information
• Activity within the Dorsal Pathway seems not to
  be sufficient for consciousness

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Blindsight
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Lesions of Retinostriate Pathway

• Lesions (usually due to stroke) cause a region of
  blindness called a scotoma
• Identified using perimetry
• note macular sparing

X
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Retinocollicular Pathway independently mediates
orienting

• Rafal et al. (1990)
• subjects move eyes to fixate a peripheral target
  in two different conditions
• target alone

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Retinocollicular Pathway independently mediates
orienting

• Rafal et al. (1990)
• subjects move eyes to fixate a peripheral target
  in two different conditions
• target alone
• accompanied by distractor

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Retinocollicular Pathway independently mediates
orienting

• Rafal et al. (1990) result
• Subjects were slower when presented with a
  distracting stimulus in the scotoma (359 ms vs.
  500 ms)

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Retinocollicular Pathway independently mediates
orienting

• Blindsight patients have since been shown to
  posses a surprising range of residual visual
  abilities
• better than chance at detection and
  discrimination of some visual features such as
  direction of motion
• These go beyond simple orienting - how can this
  be?

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Retinocollicular Pathway independently mediates
orienting

• Recall that the feed-forward sweep in not a
  single wave of information and that it doesnt
  only go through V1

• In particular, MT seems to get very early and
  direct input

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Retinocollicular Pathway independently mediates
orienting

• Recall that the feed-forward sweep in not a
  single wave of information and that it doesnt
  only go through V1

• In particular, MT seems to get very early and
  direct input
• Information represented in dorsal pathway guides
  behaviour but doesnt support awareness

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Searching for the NCC

• What is needed is a situation in which a
  perceivers state can alternate between aware and
  unaware in ways that we can correlate with neural
  events
• One such situation is called Binocular Rivalry

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Rivalrous Images

• A rivalrous image is one that switches between
  two mutually exclusive percepts

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Binocular Rivalry

• What would happen if each eye receives
  incompatible input?

Left Eye
Right Eye
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Binocular Rivalry

• What would happen if each eye receives
  incompatible input?
• The percept is not usually the amalgamation of
  the two images. Instead the images are often
  rivalrous.
• Percept switches between the two possible images

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Binocular Rivalry

• Rivalry does not entail suppression of one eye
  and dominance of another it is based on parts
  of objects

Stimuli
Left Eye
Right Eye
Percept
Or
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Binocular Rivalry

• Percept alternates randomly (not regularly)
  between dominance and suppression - on the order
  of seconds
• What factors affect dominance and suppression?

Time -gt
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Binocular Rivalry

• Percept alternates randomly (not regularly)
  between dominance and suppression - on the order
  of seconds
• What factors affect dominance and suppression?
• Several features tend to increase the time one
  image is dominant (visible)
• Higher contrast
• Brighter
• Motion

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Binocular Rivalry

• Percept alternates randomly (not regularly)
  between dominance and suppression - on the order
  of seconds
• What factors affect dominance and suppression?
• Several features tend to increase the time one
  image is dominant (visible)
• Higher contrast
• Brighter
• Motion
• What are the neural correlates of Rivalry?

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Neural Correlates of Rivalry

• What Brain areas experience rivalry?
• Clever fMRI experiment by Tong et al. (1998)
• Exploit preferential responses by different
  regions
• Present faces and buildings in alternation

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Neural Correlates of Rivalry

• What Brain areas experience rivalry?
• Clever fMRI experiment by Tong et al. (1998)
• Exploit preferential responses by different
  regions
• Present faces to one eye and buildings to the
  other

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Neural Correlates of Rivalry

• What Brain areas experience rivalry?
• Apparently activity in areas in ventral pathway
  correlates with awareness
• But at what stage is rivalry first manifested?
• For the answer we need to look to single-cell
  recording

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Neural Correlates of Rivalry

• Neurophysiology of Rivalry
• Monkey is trained to indicate which of two images
  it is perceiving (by pressing a lever)
• One stimulus contains features to which a given
  recorded neuron is tuned, the other does not
• What happens to neurons when their preferred
  stimulus is present but suppressed?

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Neural Correlates of Rivalry

• The theory is that Neurons in the LGN mediate
  Rivalry

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Neural Correlates of Rivalry

• The theory is that Neurons in the LGN mediate
  Rivalry
• NO cells in LGN respond similarly regardless of
  whether their input is suppressed or dominant

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Neural Correlates of Rivalry

• V1? V4? V5?
• YES cells in primary and early extra-striate
  cortex respond with more action potentials when
  their preferred stimulus is dominant relative to
  when it is suppressed
• However,
• Changes are small
• Cells never stop firing altogether

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Neural Correlates of Rivalry

• Inferior Temporal Cortex (Ventral Pathway)?
• YES cells in IT are strongly correlated with
  percept

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Neural Mechanisms of Consciousness?

• So how far does that get us?
• Not all that far we still dont know what is
  the mechanism that causes consciousness
• But we do know that it is probably distributed
  rather than at one locus
• Thus the question is what is special about the
  activity of networks of neurons that gives rise
  to consciousness?