Lecture 10 - The Two Visual Streams

Question 1

Roles of the Visual System

Answer 1

Perceive the world

Interact within

Question 2

The 2 cortical visual streams

Answer 2

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

Question 3

Dual-pathway model of vision:

Ungerleider and Mishkin’s What vs. Where model

Answer 3

focused on INPUT

VENTRAL visual pathway for object recognition (‘WHAT’)

DORSAL visual pathway for object localization (‘WHERE’)

Question 4

Support for Ungerleider and Mishkin’ model:

Lesion study by Pohl with rhesus monkeys

Answer 4

a) Landmark task
b) Object discrimination task

Parietal lesion disrupts “where” pathway

Temporal lesion disrupts “what” pathway

Question 5

Goodale and Milner’s Perception vs. Action Model

What vs. How

Answer 5

(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

Question 6

Neuropsychological studies

Strange things that patients do

Answer 6

“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

Question 7

Lesion to one eye…

Answer 7

right/left (either or) eye blindness

Question 8

Lesion at optic chiasm

Answer 8

bitemporal blindness (tunnel vision)

Question 9

Lesion to primary visual cortex

Answer 9

LVF Hemianopia (missing part of visual world)

Question 10

Scotoma

Answer 10

small lesion in primary visual cortex

Question 11

Quadrantanopia

Answer 11

larger lesion in primary visual cortex - 1/4 of visual field

Question 12

Hemianopia

Answer 12

entire 1 hemisphere of visual cortex is lesioned

Question 13

Blindsight

Answer 13

multiple visual pathways.

1. geniculostriate pathway – primary visual pathway (eye –> primary visual cortex –> extrastriate cortex)
2. secondary pathways – pathways for blindsight? (eye –> superior colliculus –> pulvinar –> extrastriate cortex)
   by passing primary visual cortex

Question 14

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

Answer 14

• blindsight patients can get sense of movement, even though technically they are blind & not able to perceive the objects

Question 15

Damage to the Ventral stream: Perceptual impairments

Visual Agnosia

Apperceptive Agnosia

Answer 15

Apperceptive agnosia: impairment in matching and copying

Question 16

Damage to the Ventral stream: Perceptual impairments

Visual Agnosia

Associative Agnosia

Answer 16

Associate agnosia: matching & copying relatively well perserved

can’t identify what it is

Question 17

Prosopagnosia

Answer 17

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.

Question 18

2 types of Prosopagnosi

Answer 18

1. Acquired
   - perfectly fine 1 day & then got hit & when he woke up from being unconscious he realized he had problems recognizing faces
2. Developmental
   - had it all their lives & many don’t realize they have it b/c it wasn’t from an accident

Question 19

Patient DF

Answer 19

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

Question 20

DF can do search task for colour but…

Answer 20

shows deficits for orientation

Question 21

Doing without seeing

Impaired perception but intact visuomotor control

Answer 21

she can reach out & somehow use info on how the pencil is orientated to orient her hand

Question 22

The two visual cortical streams each supports different FUNCTIONS

Answer 22

Ventral stream – visual perception (what)

Dorsal stream – visual control of action (how)

Question 23

Ventral stream damage can result in…

Answer 23

perceptual impairments (Agnosia)

Question 24

Doing without seeing

Answer 24

Impaired visual perception but intact visuomotor control

Question 25

Patient DF

Answer 25

damaged ventral stream – visual form agnosia

intact dorsal stream – intact visuomotor control

Question 26

Optic Ataxia

Answer 26

• 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

Question 27

Object processing for action

Answer 27

Dorsal stream processes information about an object’s size to calibrate grip aperture.

Question 28

Perceptual discrimination

Answer 28

DF 52%

RV 90%

Question 29

Object structure

Answer 29

ventral stream – object recognition

dorsal stream – grasp aperture and stability

Question 30

Spatial location

Answer 30

dorsal stream – reach location

ventral stream – ?

Question 31

DF has a deficit in…

Answer 31

DF has a deficit in perceiving the spatial relations between objects
ventral stream processes information about spatial location

Question 32

Double dissociation supporting Goodale & Milner’s

Perception/Action model of vision

Answer 32

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

Question 33

How do the 2 systems work together in the production of adaptive behaviour?

Answer 33

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

Question 34

How is sensory information transformed into both perceptions of the world and useful motor acts?

Is there one general purpose representation of the world?

Answer 34

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).

Question 35

Take home message:

Answer 35

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