Two Visual Systems
Damage to Primary Visual Cortex?
Results in cortical blindness
- Scotoma
- Test by perimetry
Blindsight
– Despite deficit in visual awareness, can
direct actions accurately in space; result of
midbrain
Visual Pathways from eye to brain
Superior
colliculus
Blindsight video
see link in Canvas module
Two Visual Pathways
Dorsal
Frontal
Lobe
Parietal
Lobe
Occipital
Lobe
Temporal Lobe
Ventral
Two Visual Pathways
Dorsal Stream, “where?” or “how?”:
- Ends at parietal lobe
- Where are objects located?
- Guidance of action
Two Visual Pathways
Ventral stream, “what?”:
- ends at temporal lobe
- object identification, recognition
- face recognition
Evidence from monkeys
“What” versus “where”
Double Dissociation
Two different functions involve two different
mechanisms.
- Damage to structure A, impairs function A but
not B
- Damage to structure B, impairs function B but
not A
Double Dissociation
More concretely:
- Damage to temporal lobe impairs “what”
function, but not “where” function
- Damage to parietal lobe impairs “where”
function, but not “what” function
Human Neuropsychology
“What” versus “How”
Visual form agnosia
- Deficit in perceiving simple forms
- Damage to “what” system, temporal lobe
Visual agnosia (more generally)
- Failures in object recognition (cannot be attributed to
deficit in sensory abilities, memory, or speech)
Visual form agnosia
Patient D. F.
-Cannot discriminate objects
-Can pick them up accurately
Another
example:
Task 1: match
the orientation
Task 2: post a
card in an
oriented slot
Results
Optic Ataxia
- Damage to dorsal stream (parietal lobe)
- Can discriminate between objects
- Cannot grasp them accurately
Double Dissociation
SPACE
Parietal
MT
V3
V4
Temporal
FORM
V2
V2
V1
Magno
LGN
M ganglion
cell
V1
Parvo
LGN
P ganglion
cell