Central Visual Pathways - Week 5 Flashcards
What layers of the LGN are contralateral?
Layers 1, 4 and 6
What layers of the LGN are ipsilateral?
Layers 2,3 and 5
What type of signals do koniocellular layers carry?
S-cone signals
Are the midget/parvo (parasol/magno) channels segregated at the LGN?
Yes
When are the parasol/magno channels no longer segregated?
They remain segregated until the primary visual cortex
What region of the brain is responsible for visual field processing?
Occipital region
How does lower vs upper visual field processing occur in the brain (in terms of location)?
Processing occurs around the calcarine sulcus
- above the sulcus (upper bank) = lower visual field processing
- below the sulcus (lower bank) = upper visual field processing
What part of visual field processing is the actuual calcarine sulcus (i.e. on the sulcus itsefl) responsible for?
Central vision
What is the calcarine sulcus?
A sulcus located at the caudal/posterior - medial part of the brain
Define ventral bank
lower bank
Where do optic radiations terminate?
In the primary visual cortex in the calcarine sulcus
Describe the pathway of the optic radiations towards the primary visual cortex
- optic radiations exit dorsally/upper-posteriorly from the LGN and then they split into 2 branches called ‘fascicles’
- The temporal and parietal fascicles project to the lower and upper banks of the calcarine sulcus respectively
Describe the type of visual field defect acquired in the following scenarios:
A) small lesion in the ventral bank
B) lesion resulting from occlusion of blood vessel projecting to the lower bank
C) lesion resulting from occlusion of blood vessel supplying both lower and upper bank branches
A) Scotoma in upper quadrant
B) Quadrantanopia (upper right or left quadrant)
C) Hemianopia
What is the LGN?
- a relay centre in the thalamus for the visual pathway
- receives major sensory input from the retina
- is the main central connection for the optic nerve to the occipital lobe
Where do the neurons of the LGN send their axons?
Neurons of the LGN send their axons through the optic radiation: a direct pathway to the primary visual cortex
From where does the LGN receive information?
Directly from the ascending retinal ganglion cells via the optic tract
How many layers are in the primary visual cortex?
6 layers. And these layers can be subdivided further (e.g. into a and b layers like 4a)
Note: These are different to the LGN layers!!!! even if they have the same general number
Describe the size of the neurons in the ventral (layers 1 and 2) vs ventral layers (3-6) of the LGN?
The ventral layers of the LGN contain larger neurons
Out of all the layers in the primary visual cortex, which layer is the largest?
Layer 4. It has many subdivisions too
Name the 4 horizontal sub-layers of layer 4 in the primary visual cortex
- 4A
- 4B
- 4Calpha
- 4Cbeta
Which layer, specifically sub-layers, are the major recipients of LGN innervation?
Layer 4. Sublayers 4Calpha and 4Cbeta
Which layer/sub-layers in the primary visual cortex do the Magnocellular and Parvocellular pathways project to respectively?
Magnocellular: project to 4Calpha (with sparse projection to layer 6)
Parvocellular: project mainly to 4Cbeta and 4A (with sparse projection to layer 6)
What is the golgi method of staining?
A silver staining technique that is used to visualise nervous tissue under light microscopy
Can you use golgi staining to visualise nerves in the visual cortex?
Yes, however only a few cells take up the stain. But these cells are able to be seen in their entirety
Not just the soma, but the axons and dendrites too
What layers in the primary visual cortex do koniocelular cells project to?
Layers 1 and 3 (mostly to 3)
Name the 3 parallel pathways from the retina to primary visual cortex
- Koniocellular pathway
- Parvocellular pathway
- Magnocellular pathway
Do cells from the retina and LGN have on and off regions?
Yes
Do cells from the retina and LGN respond to all orientations or only certain orientations?
All orientations. Circular receptive fields
How well do cells from the retina and LGN respond to diffuse light?
Fairly well
How well do cells from the visual cortex respond to diffuse light?
No response. Because the off-surround is strong
How well do cells from the visual cortex respond to small spots of light?
Not very well. Only get small response
What light could you provide to visual cortex cells to get a big response?
A light shaped as a bar/rectangle that is oriented in the same direction as the elongated on-centre region
How do visual cortex cells respond to an orthogonally oriented bar of light?
No response
How did Hubel and Weisel classify cortical cells? (based on receptive field response to light)
Into 3 types:
1/ Simple cells
2/ Complex cells
3/ Hyper-complex cells
How do the on and off subregions in a complex cell receptive field differ to a simple cell?
In a complex cell, the on and of subregions are overlapping
Which has a larger receptive field, simple or complex cells?
Complex
Describe the spatial summation in simple cells and complex cells
simple: linear summation (X-like)
complex: non-linear (Y-like)
In what layers of the primary visual cortex (V1) are simple cells found?
Found in layers 4 and 6 of V1
In what layers of the primary visual cortex are complex cells found?
Mostly outside layer 4
How good are simple and complex cortical cells at responding to diffuse light
Both are poor. They don’t respond.
Are cortical cells mainly binocularly or monocularly driven?
Mostly Binocularly driven
How does the whole visual cortex architecture respond to light of a certain orientation? Why is this?
The orientation the cells preferred keeps changing as you move across the cortex. (or as you penetrate the cortex more obliquely with an electrode)
The conclusion/reason is that: there must be columns of cells that prefer the same orientation and then other different columns that prefer different orientations
In what way can you penetrate the cortical surface with an electrode such that you have no variation in orientation preference of the cortex?
Penetrate and create a track perpendicular to the cortical surface (this was done in the medial track)
- by doing this, all the cells will prefer the same orientation
How do hyper-complex cells respond for a short vs long bar of light?
short: good response
long: no response
Are hyper-complex cells orientation selective?
yes
Are complex cells orientation selective?
yes
What are binocular cells?
Cells that respond to input from both eyes
How do binocular cells differ in their responsiveness?
- respond to either or both eyes
- respond to one eye more than the other and both eyes
- only respond to both eyes
How is ocular dominance vary in the primary visual cortex?
Just like with orientation selectivity, there are columns of cells attuned to a specific ocular dominance (left or right)
- regions of left and right dominance throughout the cortex can be characterised by 4 wedges from left to right, specific for: Left, Right, Left, Right dominance
Does ocular dominance vary with oblique penetration of an electrode?
Yes.
Does ocular dominance vary when testing with perpendicular penetration of an electrode?
No
What type of cells dominate layer 4 of the visual cortex? Monocular or Binocular?
Monocular
How many different visual areas have been identified
35
When using a camera focused about 800 microns below the surface, what areas represent active regions corresponding to the eye exposed to visual stimuli?
Dark areas
Where do orientation domains seem to converge (shown in psueocolour maps)?
at “pinwheel centres”
What are the 2 major streams coming from the primary visual cortex, and where do they go?
- Dorsal stream - heads to posterior parietal cortex (in parietal lobe)
- Ventral stream - heads to infero-temporal cortex (temporal lobe)
What type of information is given by the dorsal and ventral streams?
Dorsal: Action (Where) e.g. reaching out for a glass of water
Ventral: Perception (What)
What happens to your vision if there is a lesion in the primary visual cortex?
Blind Sight. Brain can see, but person cannot
- primary visual cortex is a pathway ‘bottleneck’ for perception, so person does not perceive
How can blindsight exist?
Because even if signals can’t go through the primary visual cortex, there is an alternate pathway:
Retina - SC - Pulvinar - Posterior parietal cortex
What is another name for blindsight?
Visual Agnosia
Agnoisa meaning cannot “perceive”
How did temporal lobe lesions affect object discrimination? (in monkeys)
made it harder/difficult/impossible
How did parietal lobe lesions affect landmark discrimination? (in monkeys)
made it harder/difficult/impossible
