primary visual (V1) cortex: functional organization Flashcards
what is the location of V1 cortex?
- 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
what is inside the calcarine sulcus?
- 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
what is the precise location of area V1?
- 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
do medial and lateral damage cause different VF losses?
- medial vs lateral damage causes different VF losses
what is the cellular organization of area V1?
- 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
what are the unique features of area 17?
- 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
what are the three names of the primary visual cortex?
- V1 - functional name
- area 17- anatomical name
- striate
why is primary visual cortex referred to as striate cortex?
- because layer 4c alpha in middle of grey matter to layer 4b contains a band or stripe of myelinated axons
explain V1 cortex: cellular architecture ?
- 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)
what are the 2 main classes of V1 cortical cells?
- granule or stellate cell in layer 4C
- pyramidal cells in layer 2 and layer 3
why is it named a stellate cell?
- dendrites come out of cell body- looks like a star
what is the function of stellate cell?
- 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
what is the function of pyramidal cells?
- large projection neurons, which relay information to distant targets via long axons
explain the vertical columnar organization of the V1 cortex?
- 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
what type of neurons are granule or stellate cells and pyramidal cells ?
- they are excitatory neurons , they use glutamate as their neurotransmitter
what are the three types of excitatory cortical connections?
cells in different layers
- layer 4C, granule or stellate cells, receive LGN input- have short axons
- pyramidal cells in upper layers 2-4B: connect to other nearby ‘extrastriate’ areas beyond V1 of visual association cortex (e.g. V2)
- 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
which cells in v1 do NOT use excitatory neurotransmitter?
- 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
explain the chemical organization of area V1?
- neurons in all cell layers of v1 have mitochondria
- containing the metabolic enzyme, CYTOCHROME OXIDASE (co)
- which can be visualized with special staining techniques
explain the uniques pattern of CYTOCHROME OXIDASE activity in V1?
. 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-
what are neurons with a lot of CO called?
cytochrome oxidase blobs
what are neurons with less CO called?
interblob cells which surround the blobs
what is unique about CO in V1?
the cytochrome oxidase ‘blobs’ =surrounded by CO-poor ‘interBlob’ cells
what can we see about CO in a cross section of the visual cortex ?
- 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
what can we see with the polka-dot pattern of CO running across layer 3?
- low power=dark dots = blobs
- interblobs surround blobs
what is the functional significance of CO blob and interblob zones?
- 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
how did Hubel and Wiesel study the physiology of V1 cortex?
- 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
what surprise did Hubel and Wiesel find?
- 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
how are the ‘inter-blob’ cells in V1 orientation-selective?
- 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
explain V1 orientation columns?
- 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-
explain the property of orientation and direction selectivity in cells layer 4B?
- 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
explain how blocking inhibitory processes reduces the orientation selectivity of area V1 cells?
- 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
which LGN layer sends input to layer 4c beta?
- 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
which LGN layer makes connections with layer 4C alpha ?
- 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
what do konio cells in LGN make connections with ?
- konio cells in the LGN make connections with blob cells responsible for blue/yellow colour
explain the hypercolumns of V1 cortex?
- 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
what do each left and right eye OD column further contains cells with ?
- 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
what is the functional significance of this arrangement of the hypercolumns of V1 cortex ?
- 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
