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

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Searching for the NCC

• Weve seen several examples of visual system
  activity in which no awareness ensues, yet
  information is represented and processed
• Blindsight
• Object Substitution Masking
• Neglect

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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?