visual cortex 1 cell types and orientation selectivity Flashcards
What is a scotoma and what vision loss does it cause?
localised focal damage to lower bank of calcarine sulcus. Will have a blind spot on in right hand side of upper quadrant of visual field. usually because of one of branches of arteries is blocked.
What is quadrantopia?
blocked artery that supplies lower bank gets blocked causes blindness of upper right hand quadrant.
What is Hemianopia?
Thrombus cuts of whole blood supply visual cortex. Causes blindness in half of visual field right hand side.
How is V1 identified?
prominent layer 4b and stria of gennari.
what is in V5?
Area MT
What is in V4 ?
special colour area.
What are different layers of visual cortex?
- layer 6: densely packed, contains pyramidal cells
- Layer 4b: line of gennari, sparse
-layer 4: known as granular layer, main input layer, most geniculate fibres end up here, some in 6 and some in supragranular layers. - Layer 5, 4B, and 1 : loosely packed
- Layer 4c and 6: densely packed.
- Layer 5 and 6: inferogranular layer
- Layer 1 contains no nerve fibres, more dendries. axons and synapses
What technique shows physiology and anatomy?
HRP staining.
2 types of stains
golgi stains and HRP
what are the two major classes of cortical cells?
Pyramidal cells
- stellate cells
what are pyramidal cells?
Occur everywhere except layer 1. Large dendrites radiate from the base; major axon leaves the cortical area. Only these send axons out of the striate cortex.
Where are stellate cells found?
everywhere
Interneurons:
stellate cells , spiny (glutamatergic) and non-spiny (GABAergic), chandelier, double-bouquet, basket.
Where do M, P AND k cells terminate?
P cells: Project from LGN to layers 4cBeta and 4A with sparse projection to layer 6.
M cells project mainly to layer 4Cα, with a sparse projection to layer 6.
K cells project to layers 1 and 3.
What are characterisitcs of simple cells?
Simple cell Complex Cell
Separate On & Off subregions
Small receptive field Larger receptive field
Linear spatial summation (X
like)
Found in layers 4 and 6 of V1
Diffuse light ineffective as stimulus
Most are orientation selective
Some may be direction selectivity
Many are binocularly driven
What are characterisitcs of complex cells?
Overlapping On & Off
subregions
Non-linear properties (Y like)
Mostly outside layer 4
Diffuse light ineffective as stimulus
Most are orientation selective
Some may be direction selectivity
Many are binocularly driven
diff btw complex and simple cells?
simple cells dont respondn to dark bars whereas complex cells do.
Why do hyper complex cells respond poorly to long bars?
Have inhibitory flanks on receptive fields results in endstopping. End-stopping is either a local phenomenon (initiated by layer 6 pyramids) or a
property carried over from the dLGN.
Hubel and Wiesel: Model for simple cell orientation selectivity?
A numebr of genicualte cells with concentric receptive fields transmit excitiatory input that converges on th esimple cell
Hubel and Wiesel’s model for complex cells?
a number of simple cells with the sane orientation preference converge onto complex cells and stimulate them. Some can be on or off?
Hubel and wiesel’s model for hypercomplex cells?
gets input form excitatory complex cell in centre and also inhibitory complex cells at the endzones when presented with long bar.
Limitations of hubel and wiesel models
1 Interms of orientation selectivity: If we remove inhibition we remove orientation selectivity.
- In terms of hierarchy scheme: complex cells often respond to stimuli simple cells do not respond to.
- Both complex cells and hypercomplex cells can be monosynaptically excited from the LGN.
- simple and complex cells end up showing opposite charcteristics.
Is orientation selectivity only in the level of the straite cortex?
Orientation bias exists as early as the retina as well as geniculate. before cortex. Distinction is not as sharp but may be sufficient.
Competition to falsify the original idea. What other schemes proposed?
- Hubel and wiesel’s excitatory convergence. Many excitatoy LGN cells align anf provide excitatory input into oorientation column
- Intracortical cross-orientation inhibition: gets inhibition from another column tuned for ex. horizontal orienations allowing cells to respond to vertical orientations
- **biased geniculate inputs: **cells gives excitatory input and gets inhibitory inputs as well.
- **spatially offset excitatory inputs: **gets input form on and off cells. Gives better response for directions that goes accross both cells
- spatially offset excitatory and inhibitory inputs: cells that are either excitatory or inhibitory.
