Blindsight

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Blindsight

• Patients with scotomas could move eyes to the
  location of a light flash (Poppel et al., 1973).
• Case D.B. (Larry Weizkrantz) hemianopic with
  ability to point to location, detect movement,
  discriminate orientations, discriminate Xs and
  Os.
• Location and motion usually detectable. Other
  things variable including color, flicker
  depending on the case.

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Implicit effects

• Respond more quickly to two than one even if one
  is in the blind field (even for saccades).
• Words in the blind field influence interpretation
  of other words.
• Monkeys could reach for stimuli in blind field
  but didnt report them in a number or
  presence/absence task.

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Temporal distractors have a greater effect in the
blind field. This is consistent with mediation by
the superior colliculus.
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Awareness varies

• Some feel onset of stimuli, jagged or smooth
  feeling (not visual experiences) but intense
  stimuli may say dark shadow.

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Explanations of blindsight

• Vision mediated by remaining striate cortex, e.g.
  scattered light, but how does this explain seeing
  black things (which dont scatter light) on
  white? Or hemidecorticates? (no V1)
• Imaging shows no activation of V1 when saccades
  suppressed by stimuli in blind field. Suppresson
  is seen for temporal hemifield but not nasal
  (temporal dominates for superior colliculus).
  Does this mean subcortical areas are unconscious
  and cortex is conscious?

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Extrastriate mechanism

• Some LGN fibers go to extrastriate areas from
  pulvinar, not just through V1, so can reach MT
  or V4. This suggests vision without awareness is
  related to the quality of representation. Need
  more complete and fully processed representation.
• Supported by data that hemidecorticates cant
  make motion and color judgements. Need the
  associative visual areas for these.
• Variability, but two main mechanisms can
  contribute to blindsight performance subcortical
  and direct LGN-extrastriate projections.

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Apperceptive agnosia

• No awareness (explicit knowledge) of shapes and
  objects, but other visual perception good.
• Good implicit responses to objects. Proper grip
  size, proper slot orientation (not for T objects
  so still shape limited).
• Loss of ventral stream dorsal (where) is ok and
  can mediate action programming. This is a
  privileged role account, since both ventral and
  dorsal stream code for size and orientation, but
  only ventral supports awareness.

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Associative visual agnosia

• Explicit object recognition is impaired despite
  elementary perception abilities. Copy is ok.
• Implicit recognition is shown by turning pictures
  to correct orientation (doesnt know what they
  are).
• Can match related pictures in multiple choice but
  no knowledge of picture. Despite no awareness
  could judge accuracy!
• Multiple choice data support quality of
  representation account. Degraded representations
  support performance when choices are limited, but
  dont support open-ended tasks.

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Unilateral neglect and extinction
Indirect tests show awareness
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• First was a same/different task but it could be
  solved by simpler comparisons as in the degraded
  stimulus experiment with normals also when
  naming choice limited to two were they are ok.
• But another subset of cases cant do two choice
  name but are successful on same/different. Longer
  to respond different if some similarity in the
  two so may be semantic encoding (still without
  awareness).

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More implicit perception in neglect

• Semantic priming responses on right primed by
  left side words, even though no explicit
  perception.
• Flaming house versus normal house preferred
  normal, but didnt know the what difference was.
  But not replicated.

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Neglect and extinction explanations

• Breakdown in flow between conscious and
  non-conscious systems. Privileged property
  explanation (language system is privileged
  conscious system).
• Alternatively, consciousness is a state of
  integration among systems. Damage weakens or
  degrades representation so it doesnt have
  sufficient influence to create a new global state
  in which the representation is integrated.
  Similar to quality of representation explanation.
• Farah favors degraded representation or
  integration account. Not clear that perception is
  normal (needed for privileged property account).
  Degraded stimuli can activate semantic
  representations.

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Transcranial magnetic stimulationand awareness
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Phosphene experiment implicates feedback to V1 in
awareness