Extrastraite Visual Cortex

Question 1

What is the ventral optic pathway?

Answer 1

A pathway that goes from v1->v2->v4->Infereotemporal cortex

What things are

Question 2

Where do the dorsal and ventral pathways originate?

Answer 2

The retina

Question 3

Describe the dorsal optic pathway:

Answer 3

Retina->Superior colliculus-> Pulvinar-> Superior parietal cortex

Where things are

Question 4

Whats the principle of location in the extrastraite cortex and function?

Answer 4

Regions in the extra striate cortex represent successively more complex stages of visual processing.

Question 5

Whats the idea of increasing specialisation in the extra striate cortex?

Answer 5

Regions become more specialised (modularity) We see modularity of:

• Colour processing
• Motion processing
• Faces
• Body movement

Question 6

Adjacent extra striate regions receptive fields are?

Answer 6

They become increasingly larger- integrating features over space.

Question 7

What does modularity of extra striate regions imply?

Answer 7

That a module can become damaged i.e colour processing region and the person will not see colours despite healthy eyes.

Question 8

In short what are the key features of the extra striate cortex?

Answer 8

• Increasing complexity of processing
• Modularity
• Integration

Question 9

Describe the changing properties of the ventral pathway in terms of receptive field and selectivity;

Answer 9

LGN - Small receptive field, selective for spatial (size) frequency (centre surround) (size because centre is only so big)

V1 - SF orientation (consecutive centre surround receptive fields build orientation)
V2- Contours (curves)
V4- 2D shape, colour
IT - Largest receptive field, Complex objects

As you can see receptive field increases as does selectivity complexity.

Each area selectively samples the previous area.

Question 10

Following the ventral pathway what happens to the receptive field?

Answer 10

It grows and integrates information to sample for more complex information.

Question 11

Why does information integration need to occur and cells in v1 can’t just sample for faces?

Answer 11

• Local information can be ambiguous i.e a picture couldn’t look 3d if it wasn’t for pooling of global info, local info would make it 2d
• Need to pool local information in global cues

Question 12

What does v1 deliver to v2?

Answer 12

An explicit representation of local orientation/motion/contrast

therefore signalling information about contours.

Question 13

What are the use of contours?

Answer 13

Contours are informative:
- Integrating local information into global contours; is what early stages in the brain focus on.

Contours are very informative for example we can be shown a picture of 1% visual information and still be able to tell who it is.

Question 14

Apart from v1 what else focusses on contours?

Answer 14

The LGN too!

Question 15

What are contours?

Answer 15

Points in the images where there a sudden change from light to dark or vice versa.

i.e edges, transition in luminance

Question 16

Where does contours representation occur?

Answer 16

v2

Question 17

What is the primary input of v2?

Answer 17

Parvocellular pathway (concerned with form)

Question 18

What are the properties of v2?

Answer 18

Similar to that of v1 but codes more complex contours information , angles junctions (25% sharp tuning)

Respond to curves
Respond to illusory contour
Respond to figure-ground

Question 19

What is illusory contour?

Answer 19

(an illusion where objects a positioned to imply shapes i.e four corners implies a square even if lines aren’t connected. Parts of v2 that respond to the orientation of the corner lines will fire and say the absent lines are also there. “filling in for visual information from knowledge of the world”

Question 20

What is figure ground?

Answer 20

If you have a whole lot of fragments of letters, you can’t tell what they are. But if you place condors over them (like an ink spill) you can tell what the fragments form.

Question 21

What is the term used to describe v2 responded to the contour properties?

Answer 21

Border ownership - it is what v2 really cares about (have increased depolarisation rate)

Question 22

What is border ownership?

Answer 22

• v2 cares about the contours that ‘owns’ their border. This is the integration of adjacent receptive fields. i.e they care if the vertical edge of a square has the rest of the square on the left or the right and respond differently.
• v2 >50% nuerons care, only >20% v1.

Question 23

v2 border ownership extends beyond if light or dark surrounds the contour how?

Answer 23

• This extends to borders arising from transparency.

- ON average border ownership in v2 agrees with the perception.

Question 24

Whats the function of v3?

Answer 24

Function is not understood, but receives v2 and contributes to the DORSAL stream.

Question 25

Whats the function of v4?

Answer 25

• Primary input is parvocellular
• Colour sensitive
• like v1, size, shape, orientation

Question 26

What does bilateral image of v4 areas lead to?

Answer 26

Cerebral chromatopsia (loss of colour vision)

Question 27

What else does colour sensitivity of v4 extend to?

Answer 27

Colour constancy:

Prevailing/dominant colours can change appearance of colour components.

Brain normally discounts dominant colour of illumination i.e outside blue light illumination but our brain discounts this so colours aren’t impeded by this.

Question 28

How does v4 code for shape?

Answer 28

• Cells code curvature and angular position (i.e where the tip/point of an object points) of features.
• A population(population coding) of them can capture the outlines of shape (each neuron prefers an angular position (0-360degree) (max firing) and then they all summate info into shape outline)

Question 29

What do inferotemporal cells respond to?

Answer 29

Cells here only respond selectively to quite complex forms such as pictures of faces.

• Lesions to this area impact shapes and facial recognition. (prosopagnosia)

Question 30

What exactly is specialised in extra striate specialisations?

Answer 30

The Neurons are specialised.

Question 31

How are v1,2,3 specialised?

Answer 31

They have roughly equal proportions of disparity (stereo information), colour, direction and orientation selective neurons

Question 32

How is v4 specialised?

Answer 32

More colour less direction

Question 33

How is mt/v5 specialised?

Answer 33

More direction, less colour

Question 34

What is disparity?

Answer 34

The difference in measurements between left and right eyes code for depth

Question 35

Describe the modularity of the dorsal stream:

Answer 35

All about the magnocellular stream. (motion)

LGN mango cells project into v1

v1 projects to MT (sensitive to movement)

• Selective projection to MT, motion sensing
• MT movement over large areas of space
• MT -> MST (even larger areas of space)- concerned with self movement in MST
• MST projects to PP which is concerned with knowing where things are.

Question 36

What is the function of V5/MT?

Answer 36

• Primary input is magnocellular
• Part of the dorsal visual stream
• Motion perception
• Motion Integration
• Lesions lead to akinetopsia (motion blindness)

Question 37

What is the motion after effect?

Answer 37

Adaptation of motion receptors, when image stops playing the motion occurs in the opposite direction.

Question 38

What is the motion adaptation due to?

Answer 38

Motion sensors for opposite directions are compared to give final output (opponency)

Adaptation leads to a decrease of firing in that direction

resulting in illusory movement in the opppsoite direction

Adaptation may be due to fatigue or active calibration of the system

Intraocular transfer effects cortical mechanisms.

Question 39

If two directions of motion are combined what happens?

Answer 39

Plaids:

v1 sees the individual direction
mT Sees the plaid direction (combined new direction of two motions)