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OV2023 visual perception > primary visual (V1) cortex: functional organization > Flashcards

primary visual (V1) cortex: functional organization Flashcards

1
Q

what is the location of V1 cortex?

A
  • the primary visual cortex is located in the calcarine sulcus which is inside the occipital lobe
  • mainly medial, upper and lower banks of the calcarine sulcus in each hemisphere
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2
Q

what is inside the calcarine sulcus?

A
  • gray matter-
  • the calcarine sulcus is not a slit: more a U-bend , with cortical tissue in its depth or ‘fundus’
  • it is 90 degrees to Pareto-occipital sulcus
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3
Q

what is the precise location of area V1?

A
  • corresponds exactly to brodmann cortical area number 17, which is mainly medial, but extends a bit onto the lateral surface of the occipital lobe, where the fovea is represented
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4
Q

do medial and lateral damage cause different VF losses?

A
  • medial vs lateral damage causes different VF losses
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5
Q

what is the cellular organization of area V1?

A
  • Broadmann area 17, based on ‘cyto-architecture’
  • has 6 separate layers of cells ( with very few in layer 1)
  • small granule (or stellate) cells concentrated in layer 4
  • small granules receive input from the thalamus (LGN) and connect to mainly pyramidal cells in the layers directly above and below to continue processing of the visual image
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6
Q

what are the unique features of area 17?

A
  • layer 4 is very wide and has 3 separate sub-layers (A,B,C)
  • with sub-layers 4A and 4B containing pyramidal , not granule cells
  • Layer 4C, with granule cells having separate upper (alpha) and lower (beta) sub-layers
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7
Q

what are the three names of the primary visual cortex?

A
  • V1 - functional name
  • area 17- anatomical name
  • striate
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8
Q

why is primary visual cortex referred to as striate cortex?

A
  • because layer 4c alpha in middle of grey matter to layer 4b contains a band or stripe of myelinated axons
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9
Q

explain V1 cortex: cellular architecture ?

A
  • Grey matter: has 6 major layers ( 2-6 with cell bodies of cortical neurons; layer 1 dendrites and synapses)
  • grey matter is 2mm thick
- wide layer 4 subdivided:
. 4A
. 4B
. 4C alpha
. 4C beta

. 4C alpha and beta receive input from the LGN and connect to cells in the layers above (2-4B) and below (5-6)

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10
Q

what are the 2 main classes of V1 cortical cells?

A
  • granule or stellate cell in layer 4C

- pyramidal cells in layer 2 and layer 3

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11
Q

why is it named a stellate cell?

A
  • dendrites come out of cell body- looks like a star
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12
Q

what is the function of stellate cell?

A
  • small ‘ local circuit’ neurons, which relay information they receive from the LGN via short axons to pyramidal cells in the layers just above or below them
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13
Q

what is the function of pyramidal cells?

A
  • large projection neurons, which relay information to distant targets via long axons
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14
Q

explain the vertical columnar organization of the V1 cortex?

A
  • granule cells axons run mainly vertically across the layers to make excitatory synapses on the prominent vertically-oriented ( primary ) dendrites and dendritic spines of pyramidal cells, local cortical circuits are organized mainly in vertical columns

. columns are 1mm wide

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15
Q

what type of neurons are granule or stellate cells and pyramidal cells ?

A
  • they are excitatory neurons , they use glutamate as their neurotransmitter
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16
Q

what are the three types of excitatory cortical connections?

A

cells in different layers

  1. layer 4C, granule or stellate cells, receive LGN input- have short axons
  2. pyramidal cells in upper layers 2-4B: connect to other nearby ‘extrastriate’ areas beyond V1 of visual association cortex (e.g. V2)
  3. pyramidal cells below layer 4c: have descending sub-cortical connections (e.g. layer 5 connections to superior colliculus: layer 6 send axons back to the LGN)- layer 6 gets input from layer 4c
    - all these cells use glutamate as their neurotransmitter
17
Q

which cells in v1 do NOT use excitatory neurotransmitter?

A
  • basket cells
  • chandelier cells
    . these cells use GABA as their neurotransmitter which is inhibitory
    . they mediate lateral inhibitory processes that also influence the RF properties of nearby pyramidal cells
18
Q

explain the chemical organization of area V1?

A
  • neurons in all cell layers of v1 have mitochondria
  • containing the metabolic enzyme, CYTOCHROME OXIDASE (co)
  • which can be visualized with special staining techniques
19
Q

explain the uniques pattern of CYTOCHROME OXIDASE activity in V1?

A

. columns of intensely CO
stained cells in layers 2-4B, weaker in layers 5-6
. approximately 0.25 mm wide
. separated by columns of less co-stained cells in these layers of similar / wider width
. view horizontally across the layers, the CO-rich cells form a polka-dot arrangement of densely stained cells
. with polka dot-to-dot centre spacing of 1mm- in mitochondria-

20
Q

what are neurons with a lot of CO called?

A

cytochrome oxidase blobs

21
Q

what are neurons with less CO called?

A

interblob cells which surround the blobs

22
Q

what is unique about CO in V1?

A

the cytochrome oxidase ‘blobs’ =surrounded by CO-poor ‘interBlob’ cells

23
Q

what can we see about CO in a cross section of the visual cortex ?

A
  • all the cells in layer 4c show uniformly strong CO staining - this means highly metabolic neurons
  • strong brown staining- indicates a lot of CO in the mitochondria
  • alternating strong versus weaker CO staining- 1/2mm which separates
  • width of these CO strong cells- 0.25mm separated by each other by cells which contain less CO (interblob zone) - with a dimension of 0.5mm
24
Q

what can we see with the polka-dot pattern of CO running across layer 3?

A
  • low power=dark dots = blobs

- interblobs surround blobs

25
Q

what is the functional significance of CO blob and interblob zones?

A
  • electrophysiological microelectrode recordings from single cells in area V1 show that neurons in the Blobs are colour-selective, whereas neurons in the interblobs are contour-selective responding to different lines or edges
26
Q

how did Hubel and Wiesel study the physiology of V1 cortex?

A
  • anaesthetize and paralyze animal, focus eyes on tangent screen
  • record activity of single cells with microelectrode
  • plot RF position to ‘preferred ‘ light stimulus on screen
27
Q

what surprise did Hubel and Wiesel find?

A
  • the cells in V1 don’t have circular RF(receptive fields) like in ganglion cells and LGN
  • RF shapes are rectangular or square
  • cells in v1 don’t like spots of light being flashed on or off- they like lines or edges
28
Q

how are the ‘inter-blob’ cells in V1 orientation-selective?

A
  • rectangle - region of space that particular cell recorded - their RF
  • black line - orientation os stimulus they placed within RF
  • vertical line - shows action potential that was a response of the cells to the stimulus that was being placed
  • this cell is preferentially activated (D) by a line oriented at 45deg placed in its receptive field, with no response to ‘non-preferred’ vertical or horizontal orientations of the same stimulus
  • NB: some cell prefer 90 deg or 180 - not all cells are same
29
Q

explain V1 orientation columns?

A
  • property of orientation is not randomly distributed through interblob zone
  • organized in V1 orientation columns
  • vertical: the same preferred orientation
  • tangential: different orientation preferences, highly systemic changes with complete set 1mm wide-
30
Q

explain the property of orientation and direction selectivity in cells layer 4B?

A
  • this cells is preferentially activated by a vertical line oriented moved left -to-right through its receptive field with no/little response to the ‘non-preferred’ right-to-left direction of motion of the same stimulus
31
Q

explain how blocking inhibitory processes reduces the orientation selectivity of area V1 cells?

A
  • cells with a near-vertical orientation preference (no responses to near-horizontal)
  • this orientation preference is lost in the presence of a GABA-receptor blocker. i.e. it normally depends on local inhibition
  • recovery after blocker removed
32
Q

which LGN layer sends input to layer 4c beta?

A
  • the parvo cellular layers of the LGN send input layer 4c beta
  • those cells make selective connections within the primary visual cortex
  • some cells in 4c activate cells within the CO blobs , which is for red/green colour processing
  • other cell in layer 4c beta make connections with interblob cells , which are responsible for form contours
33
Q

which LGN layer makes connections with layer 4C alpha ?

A
  • the magno cells make connections with layer 4C alpha - those granule cells make connections with either interblob cells responsible for form contours or neurons in layer 4B which contribute to motion processing
34
Q

what do konio cells in LGN make connections with ?

A
  • konio cells in the LGN make connections with blob cells responsible for blue/yellow colour
35
Q

explain the hypercolumns of V1 cortex?

A
  • the surface area of human V1 cortex is 2500-3500 , V1 is conceived as comprising 2500-3500 adjacent hypercolumns
    . each with 1mm x 1mm base= physiological column
    . and spanning across all the grey matter layers
    . each representing a single point in space via
    . a left and a right eye ocular dominance (OD) column
    . across the entire hemi-field map in area V1
36
Q

what do each left and right eye OD column further contains cells with ?

A
  • a full range of orientation preferences in its interblob zones
  • a full range possible direction-selective preferences in layer 4B
  • plus two CO blobs; 1 for red/green and 1 for blue/yellow processing
37
Q

what is the functional significance of this arrangement of the hypercolumns of V1 cortex ?

A
  • the functional significance of this arrangement of the hypercolumns of V1 cortex
    is every possible visiual stimulus-regardless of its form, motion or colour- is encoded by some cells in the hypercolumn
  • all possible stimulus orientation , direction and colours seen by the either eye are represented for every point in the visual hemifield: nothing invisible to V1

OV2023 visual perception (20 decks)

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