Extrastriate Cortex

Question 1

What are visual cortical areas identified by?

Answer 1

o Physiology
o Architecture
o Connections
o Topography

Question 2

Describe secondary/extrastriate areas?

Answer 2

• 30 secondary visual areas after V1
• Most lie in occipital lobe
• But some are in parietal (where) and temporal (what) lobes

Question 3

Describe architecture in the extrastriate cortex?

Answer 3

• Cytoarchitecture (cell size, cell density, number of layers, density of axons, etc)
  o Revealed through different stains – can change from on visual area to next – helping to identify boundaries between distinct brain regions
• Cytochrome Oxidase – enzyme used in metabolism
  o Staining gives characteristically different patterns in different visual areas
  o In V1 it shows puffs or blobs
  o In V2 it yields a striped pattern
  o In V3 it yields a dense strip
  o In MT it yields a dense region

Question 4

What are the components of topography?

Answer 4

• Retinotopy: radial component
• Retinotopy: angular component
• Superimpose angular and radial components together

Question 5

What are the parietal and temporal pathways?

Answer 5

• 2 distinct streams (parallel pathways):
  o Going to parietal lobe
  o Going to temporal lobe
• Functions:
  o Parietal lesions lead to deficits in spatial orientation attention (this is the ‘where’ pathway)
   Parietal pxs often unaware of their deficits
  o Temporal lobe lesions lead to deficits in object recognition (this is the ‘what’ pathway)
   Pxs with temporal lobe lesions are aware that there’s a problem and they develop strategies to compensate for it

Question 6

Why is such a large area of the cortex dedicated to vision?

Answer 6

• Compared to other sensory modalities of motor system, much larger cortical area dedicated to vision
• Humans are visual animals and most info about external world is obtained through vision
• Vision important for all human behaviour – interacting with people, walk, drive, working, reading
• Visual system is accurate and reliable in wide variety of operating conditions under different illumination conditions, at different viewing angles and distances
  o Biggest strength of visual system
• Extraneous variables – lighting, pose, viewing distance, clutter – strongly influence retinal image and responses of V1 neurons
• Visual system (specifically extrastriate cortex) has figured out ways to disentangle intrinsic properties of object – shape, colour, motion direction etc – from a dynamic and ever-changing environment so objects are always recognisable
• Extrastriate areas comprise roughly 30-50% of cortex

Question 7

How to study what the brain perceives?

Answer 7

• Lesion experiments – portion of cortex removed and resulting loss in function assessed
  o Tell you what coarse function of area is but not how it’s achieved
• Anatomical tracing studies – connections between different areas are mapped
  o Provides clues about direction of info flow but no direct evidence about function
• Single cell neurophysiology experiments – responses of single neurons in different regions of brain are studied to ascertain role of neurons
  o Provides v goo spatial resolution (activity of single neurons) but results are correlational (not causal like in lesion studies) & and only looking at one neuron at a time rather than network as a whole
• Microstimulation experiments – provide answers about causation
  o Small amount of current injected into localised brain area (causing excitation of those neurons)
  o Results assess how this current injection affects behavioural performance

Question 8

What does V1 analyse and what do we percieve? What are the 4 theories as to why this happens?

Answer 8

V1 analyses line drawing, spatial frequencies etc but we perceive complex 3D image
1. Do they encode features? – was proposed that some cells were specifically for identifying your grandmother
2. By changing either orientation or speed of movement would equally diminish level of activity in cells
3. Population Encoding
4. Parallel Processing

Question 9

+ves & -ves of grandmother cell hypothesis?

Answer 9

• Cells in extrastriate areas do have more specific stimulus requirements
• Cells have been found in temporal cortex which respond to v specific stimuli such as faces, hands etc
• Against: there can’t be a cell for every object that we perceive
  o How would we be able to incorporate perception of ‘new’ objects?
• Most extrastriate areas have reciprocal connections to lower areas -> perceived by lower areas but processed by higher areas
• Output of feature detectors is ambiguous i.e. the same firing rate of a cell can be produced by many different stimuli (thinner vertical bar, slightly tilted bar, edge, curved bar, fuzzy bar, a few spots)

Question 10

Describe population encoding?

Answer 10

• Neurons that are tuned (to particular value, location or property) are also rate coded: amplitude of their response varies as a function of stimulus intensity as well as frequency
• Causes a problem: because tuned neurons also use a rate code, single neurons are inherently ambiguous
• Neurons are noisy
• A population of cells can reduce noise & increase reliability (signal to noise ratio) – signal becomes bigger, noise gets smaller
• Increase coding capacity
• More cells = more dynamic range
• You can represent a wider range of stimuli
• Need balance between synchronous and asynchronous firing

Question 11

Which do V1 neurons repond to faster: Luminance changes or contrast change?

Answer 11

Luminance changes

Question 12

Describe parallel processing?

Answer 12

• V1 “subcontracts” processing to extrastriate areas
• These areas process information relating to form, colour, motion
• Zeki suggested there’s 4 main pathways working in parallel:
  o Colour: parvocellular – V1 (blobs) – V2 – V4
  o Motion: magnocellular – V1 (4B) – V2 – V5
  o Form System 1: linked to P1 and colour (Blobs) – V2 – V4
  o Form System 2: linked to M1 and motion V1 (4b) – V2 – V3
• Parvo = ventral stream
• Magno = dorsal stream