Primary Visual (V1) Cortex - Location & Organisation Flashcards
Where is the primary visual cortex located?
In the Occipital lobes - more specifically the medial, upper and lower banks of the Calcarine Sulcus in each hemisphere
What is the deep bit of a sulcus known as?
Its fundus
What is area V1 also known as?
Area 17 or Striate Cortex
What does damage to V1 cause in terms of vision?
Unilateral damage causes hemi-anopsia
What does V1 on the right side of the brain visualise?
The left side of your field of view
Where is the fovea represented in V1?
Right at the posterior pole - so literally right at the back of your head
Why did brodmann think area 17 was unique?
- Layer 4 is very wide & has 3 separate sub-layers (A, B, C)
- Sub-layers 4A & 4B contain pyramidal, not granule, cells (which is different to other parts of the cortex
-Layer 4C, had granule cells, and a separate upper (alpha) & lower (beta) sub-layer
• Layer 4B contains a band or stripe of MYELINATED AXONS (which gives rise to the name the ‘Striate cortex’)
Why is V1 referred to as the Striate Cortex?
Because in the fourth layer ( specifically 4Cα-4B) there is a band or stripe of myelinated axons which give the appearance of a striated cortex.
How is the primary cortex similar to other cortical areas?
- Has 6 separate layers of cells (with very few cell bodies in layer 1)
- Small granule (or stellate) cells concentrated in layer 4
What is the pial surface?
Pial surface [r] : A surface representing the boundary between grey matter and cerebrospinal fluid, as reconstructed from neuroimaging data; roughly corresponds to the position of the pia mater.
Why is layer 1 of the primary cortex not heavily stained?
The stain picks up cell bodies. In layer one of V1 (and in all regions of the cerebral cortex) there are hardly any cell bodies- there are mainly dendrites and synapses of cortical cells that sit below.
What is the difference between granule and stellate cells?
Nothing it’s two different names for the same cell.
(Sometimes called a stellate cell because the cell bodies look a little like a star).
What are the two main classes of V1 cortical cells and what do they do?
Granule/Stellate cells- these are small ‘local circuit’ neurones, which relay information they recieve from the LGN to pyramidial cells in layers above or below them.
Pyramidial Cells (triangular shape of cell body) - are neurones with large ‘projections’ which relay information to distant targets via long axons.
Why may we say V1 has a vertical columnar organisation?
Because granule cell axons run up and down to layers above and below them and connect onto the long vertical projection of the pyramidal cell via excitatory synapses from the dendritic spines.
Columns tend to be 0.5mm wide.
What are the three types/examples of excitatory cortical connections you would find in V1?
[Side note- cells above 4C make cortical connections , cells below 4C make sub-cortical connections- i.e. to out the cortex).
Are all cortical cells excitatory?
NO - Basket cells and Chandelier cells are inhibitatory
How is the chemical organisation of V1 unique to all other cortical areas?
The pattern of CYTOCHROME OXIDASE activity in V1 is unique - it is non-uniform in layers above and below layer 4C.
There are columns of intensely stained CO- cells in layers 2-4B, and weak columns in layers 5 and 6. These CO- rich cells were called ‘blob’ cells.
These are seperated by less stained CO-cells in columns of a similar width to the heavily stained column. The CO weak cells were called ‘interblob’ cells.
[If you cut across the tissue horizontally you get a regular polka dot formation of blob cells]
Are cells in V1 interested in form processing?
Yes ‘Inter-blob’ Cells are Orientation-Selective. (Essentially you can think of it as certain cells prefer lines at certain angles)
[The picture attached is showing you how stimulated certain cells are getting by certain lines of different orientation].
Were orientation preferences of V1 cells random?
NO - Hubel & Wiesel (from 1938) discovered that all V1 cells vertically ontop of eachother prefered the same orientation and that adjacent (/tangenital- like diagonally) cells prefered different orientations.
[However in layer 4c cells do not have orientation preferences, they have centre - surround receptivity]
What’s special about V1 cells of layer 4B?
Layer 4B has a highly concentrated amount of cells that are not only orientation selective but direction sensitive too!
Summarise parallel imaging pathways to and within area V1 and its different layers.
Summarise the V1 cortex hypercolumn concept
(Don’t need to know the numbers - just the concept).
The surface area of human V1 cortex is 2500-3500 mm2
V1 is conceived as comprising 2500-3500 adjacent hypercolumns
- Each with 1mm x 1mm base & spanning across the grey matter layers
- Each representing a single point in space via
- A left & a right eye ocular dominance (OD) column
- Across the entire hemi-field map in area V1
Each left & 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 & 1 for blue/yellow processing
Functional Significance =
•Every possible visual stimulus - regardless of its form, motion or colour – is encoded by some cells in the hypercolumn
•So nothing is ‘invisible’ to cells in V1!
Describe parallel processing to and in the area V1:
