Blindsight
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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.
Implicit effects
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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 didn’t report them in a
number or presence/absence task.
Temporal distractors have a greater effect in the blind field.
This is consistent with mediation by the superior colliculus.
Awareness varies
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Some ‘feel’ onset of stimuli, jagged
or smooth feeling (not visual
experiences); but intense stimuli
may say ‘dark shadow’.
Explanations of blindsight
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Vision mediated by remaining striate
cortex, e.g. scattered light, but how does
this explain seeing black things (which
don’t 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’?
Extrastriate mechanism
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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
can’t 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.
Apperceptive agnosia
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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.
Associative visual agnosia
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Explicit object recognition is impaired
despite elementary perception abilities.
Copy is ok.
Implicit recognition is shown by turning
pictures to correct orientation (doesn’t
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 don’t
support open-ended tasks.
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 can’t 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).
More implicit perception in neglect
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Semantic priming: responses on
right primed by left side words, even
though no explicit perception.
Flaming house versus normal house:
preferred normal, but didn’t know
the what difference was. But not
replicated.
Neglect and extinction explanations
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Breakdown in flow between conscious and nonconscious 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 doesn’t 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.
Transcranial magnetic stimulation
and awareness
Phosphene experiment implicates
feedback to V1 in awareness