Lecture 10 - The Two Visual Streams Flashcards
Roles of the Visual System
Perceive the world
Interact within
The 2 cortical visual streams
Dorsal stream - from primary visual cortex to POSTERIOR PARIETAL LOBE
- goes to dorsal part of brain
Ventral stream - goes from primary visual cortex down to INFERIOR TEMPORAL CORTEX
Dual-pathway model of vision:
Ungerleider and Mishkin’s What vs. Where model
focused on INPUT
VENTRAL visual pathway for object recognition (‘WHAT’)
DORSAL visual pathway for object localization (‘WHERE’)
Support for Ungerleider and Mishkin’ model:
Lesion study by Pohl with rhesus monkeys
a) Landmark task
b) Object discrimination task
Parietal lesion disrupts “where” pathway
Temporal lesion disrupts “what” pathway
Goodale and Milner’s Perception vs. Action Model
What vs. How
(perception vs action model)
diff b/t U & M proposed us these is that U & M focused on the INPUT - suggested that the ventral stream is processing info used for perception & dorsal is processing info used for object localization
G & M focused instead on the OUTPUT - felt both streams were processing both types of info, but have using that info in a diff. way
- argued that the ventral stream was def. processing info needed for object perception, but that the dorsal stream was processing info needed for action (interacting with objects)
Dorsal stream: vision-for-action
Ventral stream: vision-for-perception
Neuropsychological studies
Strange things that patients do
“It is not so much the injury that should capture our attention, but how, through injury or disease, normal function is laid bare.”
Head, 1926
Lesion to one eye…
right/left (either or) eye blindness
Lesion at optic chiasm
bitemporal blindness (tunnel vision)
Lesion to primary visual cortex
LVF Hemianopia (missing part of visual world)
Scotoma
small lesion in primary visual cortex
Quadrantanopia
larger lesion in primary visual cortex - 1/4 of visual field
Hemianopia
entire 1 hemisphere of visual cortex is lesioned
Blindsight
multiple visual pathways.
- geniculostriate pathway – primary visual pathway (eye –> primary visual cortex –> extrastriate cortex)
- secondary pathways – pathways for blindsight? (eye –> superior colliculus –> pulvinar –> extrastriate cortex)
by passing primary visual cortex
If you have damage to primary visual cortex, this is where you perceive objects, so you’re knocking out the perception of objects there but the secondary pathway is still getting info about movement to extrastriate areas
- blindsight patients can get sense of movement, even though technically they are blind & not able to perceive the objects
Damage to the Ventral stream: Perceptual impairments
Visual Agnosia
Apperceptive Agnosia
Apperceptive agnosia: impairment in matching and copying
Damage to the Ventral stream: Perceptual impairments
Visual Agnosia
Associative Agnosia
Associate agnosia: matching & copying relatively well perserved
can’t identify what it is
Prosopagnosia
Face recognition deficit associated with damage to the inferior aspect of the occipitotemporal cortex, in the region of the fusiform gyrus.
difficulty even recognizing the faces of their own family.
2 types of Prosopagnosi
- Acquired
- perfectly fine 1 day & then got hit & when he woke up from being unconscious he realized he had problems recognizing faces - Developmental
- had it all their lives & many don’t realize they have it b/c it wasn’t from an accident
Patient DF
Research by Milner and Goodale with APPERCEPTIVE agnosic patient DF, helped lead to the revision of Ungerleider and Mishkin’s model.
Brain damage from carbon monoxide poisoning
MRI shows severe damage to the VENTRAL stream but the primary visual cortex is relatively intact.
Low-level vision is largely in the normal range
- can see texture and colour
Visual form agnosia
- no longer identify objects on the basis of their shape
DF can do search task for colour but…
shows deficits for orientation
Doing without seeing
Impaired perception but intact visuomotor control
she can reach out & somehow use info on how the pencil is orientated to orient her hand
The two visual cortical streams each supports different FUNCTIONS
Ventral stream – visual perception (what)
Dorsal stream – visual control of action (how)
Ventral stream damage can result in…
perceptual impairments (Agnosia)
Doing without seeing
Impaired visual perception but intact visuomotor control
Patient DF
damaged ventral stream – visual form agnosia
intact dorsal stream – intact visuomotor control
Optic Ataxia
- DORSAL stream damage
- inability to point to or grasp targets under visual guidance
“Thus, when asked to grasp a presented object with his right hand, he would miss it regularly and would find it only when his hand knocked against it.”
“… only movements which required visual control were faulty.”
Rudolf Balint, 1909
Optic ataxic patient RV has difficulty with ACTION version of slot task
Object processing for action
Dorsal stream processes information about an object’s size to calibrate grip aperture.
Perceptual discrimination
DF 52%
RV 90%
Object structure
ventral stream – object recognition
dorsal stream – grasp aperture and stability
Spatial location
dorsal stream – reach location
ventral stream – ?
DF has a deficit in…
DF has a deficit in perceiving the spatial relations between objects
ventral stream processes information about spatial location
Double dissociation supporting Goodale & Milner’s
Perception/Action model of vision
Visual from agnosia:
- no perception
- yes action
site of damage was the ventral stream
Optic ataxia
- yes perception
- no action
- site of damage was the dorsal stream
How do the 2 systems work together in the production of adaptive behaviour?
a useful analogy can be found in robotic engineering
tele-operation - slow
tele-assistance - has semi-autonomous robot that is our conscious, us taking a drink
- gets target & goes there
How is sensory information transformed into both perceptions of the world and useful motor acts?
Is there one general purpose representation of the world?
It is unlikely that one general purpose representation of the world could adequately serve both functions of the visual system.
Instead, it is believed that visual information is transformed into the representations needed for visual perception (by the ventral stream) and the visual control of action (by the dorsal stream).
Take home message:
vision is imp. not only in identifying objects, but also in guiding our interactions toward those objects & with those objects so we can achieve our ultimate goal
