Transcript Last Lecture http://www.umich.edu/~psycours/345/

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Test 1: Next Monday
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Discussion sections meet in assigned class
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Last Lecture
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More: Retinotopic Map
Parallel Visual Pathways
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Blindsight
Outline
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Blindsight What it is
Blindsight: Why it happens
Parallel Cortical Pathways
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Mishkin & Ungerleider 1982
What versus where
Double Dissociation
Vision requires Area 17…or maybe not?
LORE of neurology until the early 70's...
LGN
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Reports of residual vision in Animals with striate
lesions (hamsters; monkeys):
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Recovery after experimental field defects (cortical ablations)
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Striate
extra
Striate
spared light/dark discrimination
spared localization abilities
Implication: Other pathways can compensate for
some geniculo-striate function.
Can this also be true in humans??
Parallel Pathways
Do cortically blind humans demonstrate
residual vision?
Poppel, Held & Frost (1973).
Task:

light flashed in hemianopic
field
 Patient told ”Look at flash”
 Patient protests: “How can I
look at something I can't
see?"
 Experimenter: GUESS!
Eye position
Saccade Location
Residual visual function
after brain wounds
involving the central visual
pathways in man. Nature.
40
30
20
10
0
5
10
20
30
Target
Location
Blindsight
Blindsight
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More Confirmatory Results
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Weiskrantz and colleagues (1974): X vs. O
discrimination in blind fields by human observers.
Preserved visual behavior (residual vision) in
the absence of visual experience
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How can this occur?
Blindsight
The non-striate hypothesis…
Retinotectal pathway --> direct route for
saccadic eye movement control.
 Tecto-pulvinar-extrastriate --> indirect
route for manual and verbal responses.

LGN
Retinotectal
pathway
Striate
extra
Striate
Pulvinar
S.C.
Tectopulvinar extrastriate pathway
Blindsight
Cortical (Striate)
Islands Hypothesis
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Standard
perimetry
Fendrich, et al. (1992).
detailed (micro) perimetry
revealed islands of vision in
scotoma*
Micro
perimetry
Implications:
Spared tissue in Area 17
can explain residual
vision.
Blindsight
The Two Blindsight Theories
1) Non Striate Theory -- An alternative visual
pathway (retino-tectal or retino-tectal pulvinar)
is responsible for the spared visual behavior.
2) Cortical Islands Theory -- Spared islands of
visual cortex are responsible for the spared
visual behavior.
How do we determine which is correct?
Blindsight
How do we determine which is correct?
Could we use brain imaging to test
predictions from the theories?
 Structural
 Functional

Blindsight
Why is Blindsight blind?
Why doesn’t “normal” awareness
accompany residual vision?

Non striate theory: SC circuitry is too
primitive to mediate "awareness”
 Cortex
is necessary for consciousness
of percepts
Blindsight
Or
Visual signal is weak, below
threshold for consciousness.

non-striate theory- SC only gets
weak retinal input.

Cortical Islands- tissue fragments
are insufficient for consciousness-NOTE: cortex does not equal
consciousness.
Psychological effect
Why is Blindsight blind?
Threshold for
awareness
Signal strength
Blindsight
Lessons from Blindsight
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Some representations are unconscious.
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Implicit (e.g., localization) vs. explicit (experience)
Different representations for different behaviors
Human residual vision is less than animals
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Evident only with experimental methods.
Nonprimate species depend more on tectum
Encephalization of visual (WHAT & WHERE)
function in primates and humans.
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Primates (Humans) depend more on cortex
Blindsight
Outline
•
•
•
Blindsight What it is
Blindsight: Why it happens
Parallel Cortical Pathways
•
•
•
Mishkin & Ungerleider 1982
What versus where
Double Dissociation
Parallel Processing in Vision

Cortical versus Subcortical
Within the Cortex

What versus where
Parallel Pathways
What and where pathways
Parallel Pathways
Ungerleider & Mishkin (1982)
Task 1: Object discrimination
Or
 study an object
 select the familiar object*
Task 2: Landmark discrimination
• Pick the well closer to the
tower
Monkey group 1:
Bilateral temporal lesions
Monkey group 2:
Bilateral parietal lesions
Ungerleider & Mishkin
Results
What can
we conclude
from this
result?
Landmark
task needs
the parietal
lobes?
SINGLE DISSOCIATION
Landmark
task is
harder than
object task?
Another Dissociation is needed…
To rule out
the
possibility
that the
Landmark
task is
harder than
object task.
SINGLE DISSOCIATION
Ta Daaa….Double Dissociation
Implication:
1.Landmark task
needs the parietal
lobes
2.Object task needs
temporal lobes
Ungerleider & Mishkin (1982)
Results Summary:
1) Parietal lesion --> selective deficit, single
dissociation: “where” impaired, “what” intact
2) Temporal lesion --> selective deficit, single
dissociation: “what” impaired, “where” intact
Temoral
lobe: VENTRAL stream endpoint for identity
Parietal lobe: DORSAL stream endpoint for spatial location
Ungerleider & Mishkin
For review– just in case.
VF Left Eye
VF Right Eye
Right
Nasal
Combined Visual Field
Left
Retinotopic Map