Extrastraite Visual Cortex Flashcards
What is the ventral optic pathway?
A pathway that goes from v1->v2->v4->Infereotemporal cortex
What things are
Where do the dorsal and ventral pathways originate?
The retina
Describe the dorsal optic pathway:
Retina->Superior colliculus-> Pulvinar-> Superior parietal cortex
Where things are
Whats the principle of location in the extrastraite cortex and function?
Regions in the extra striate cortex represent successively more complex stages of visual processing.
Whats the idea of increasing specialisation in the extra striate cortex?
Regions become more specialised (modularity) We see modularity of:
- Colour processing
- Motion processing
- Faces
- Body movement
Adjacent extra striate regions receptive fields are?
They become increasingly larger- integrating features over space.
What does modularity of extra striate regions imply?
That a module can become damaged i.e colour processing region and the person will not see colours despite healthy eyes.
In short what are the key features of the extra striate cortex?
- Increasing complexity of processing
- Modularity
- Integration
Describe the changing properties of the ventral pathway in terms of receptive field and selectivity;
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.
Following the ventral pathway what happens to the receptive field?
It grows and integrates information to sample for more complex information.
Why does information integration need to occur and cells in v1 can’t just sample for faces?
- 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
What does v1 deliver to v2?
An explicit representation of local orientation/motion/contrast
therefore signalling information about contours.
What are the use of contours?
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.
Apart from v1 what else focusses on contours?
The LGN too!
What are contours?
Points in the images where there a sudden change from light to dark or vice versa.
i.e edges, transition in luminance
Where does contours representation occur?
v2
What is the primary input of v2?
Parvocellular pathway (concerned with form)
What are the properties of v2?
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
What is illusory contour?
(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”
What is figure ground?
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.
What is the term used to describe v2 responded to the contour properties?
Border ownership - it is what v2 really cares about (have increased depolarisation rate)
What is border ownership?
- 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.
v2 border ownership extends beyond if light or dark surrounds the contour how?
- This extends to borders arising from transparency.
- ON average border ownership in v2 agrees with the perception.
Whats the function of v3?
Function is not understood, but receives v2 and contributes to the DORSAL stream.
