visual cortex 1 cell types and orientation selectivity

Question 1

What is a scotoma and what vision loss does it cause?

Answer 1

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.

Question 2

What is quadrantopia?

Answer 2

blocked artery that supplies lower bank gets blocked causes blindness of upper right hand quadrant.

Question 3

What is Hemianopia?

Answer 3

Thrombus cuts of whole blood supply visual cortex. Causes blindness in half of visual field right hand side.

Question 4

How is V1 identified?

Answer 4

prominent layer 4b and stria of gennari.

Question 5

what is in V5?

Answer 5

Area MT

Question 6

What is in V4 ?

Answer 6

special colour area.

Question 7

What are different layers of visual cortex?

Answer 7

• 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

Question 8

What technique shows physiology and anatomy?

Answer 8

HRP staining.

Question 9

2 types of stains

Answer 9

golgi stains and HRP

Question 10

what are the two major classes of cortical cells?

Answer 10

Pyramidal cells
- stellate cells

Question 11

what are pyramidal cells?

Answer 11

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.

Question 12

Where are stellate cells found?

Answer 12

everywhere

Question 13

Interneurons:

Answer 13

stellate cells , spiny (glutamatergic) and non-spiny (GABAergic), chandelier, double-bouquet, basket.

Question 14

Where do M, P AND k cells terminate?

Answer 14

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.

Question 15

What are characterisitcs of simple cells?

Answer 15

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

Question 16

What are characterisitcs of complex cells?

Answer 16

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

Question 17

diff btw complex and simple cells?

Answer 17

simple cells dont respondn to dark bars whereas complex cells do.

Question 18

Why do hyper complex cells respond poorly to long bars?

Answer 18

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.

Question 19

Hubel and Wiesel: Model for simple cell orientation selectivity?

Answer 19

A numebr of genicualte cells with concentric receptive fields transmit excitiatory input that converges on th esimple cell

Question 20

Hubel and Wiesel’s model for complex cells?

Answer 20

a number of simple cells with the sane orientation preference converge onto complex cells and stimulate them. Some can be on or off?

Question 21

Hubel and wiesel’s model for hypercomplex cells?

Answer 21

gets input form excitatory complex cell in centre and also inhibitory complex cells at the endzones when presented with long bar.

Question 22

Limitations of hubel and wiesel models

Answer 22

1 Interms of orientation selectivity: If we remove inhibition we remove orientation selectivity.

2. 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.

Question 23

Is orientation selectivity only in the level of the straite cortex?

Answer 23

Orientation bias exists as early as the retina as well as geniculate. before cortex. Distinction is not as sharp but may be sufficient.

Question 24

Competition to falsify the original idea. What other schemes proposed?

Answer 24

• 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.