Primary Visual Cortex Lecture 10 Flashcards
Where does the primary visual pathway originate?
In the retina
Describe the primary visual pathway:
Retina Optic nerve Nasal fibres cross at optic chasm Primary relay station: LGN in thalamus. Optic radiations spread into the primary visual cortices
Where are the left and right sides of the retinas processed?
Left sides of retinas are processed by the the left LGN and Left primary visual cortex. Opposite for right.
What visual fields are processed in what part of the brain?
left retina receives right visual hemisphere input, so the left side of the brain deals with info from the right visual field. and vice versa
What do retinal ganglion cells have in terms of receptive field?
A classic centre-surround receptive field.
Horizontal cell inhibitor of centre bipolar cell, if surround has light on it.
Are the centre and surround receptive fields distinct?
No there is plenty of overlap and it is more of a blurry outline i.e not distinct.
How do retinal cells acquire a receptive field?
Due to the different types of ganglion cells and their SELECTIVE interconnectivity BETWEEN CELLS within the retina, developing the receptive field i.e horizontal cell and amacrine cell input.
What are the types of retinal ganglion cells?
Midget
Parasol
Bistratified
Describe the receptive field of the midget cell?
The midget cells have a very small receptive field and therefore do not receive much input.
Describe the receptive field for the parasol cells.
Moderately sized receptive field with moderate amounts of input
Describe the receptive field of the bistratified cells
large receptive field with extensive interconnections, receiving a lot of input.
What connects each ganglion with photoreceptors?
Horiztonal and amacrine cells create the connectivity and therefore receptive field.
What are the relative amounts of ganglions cells?
80% Midget cells
10% Parasol
10% Bistratified
What is the function of retinal ganglion cells?
They respond to edges of images, and emphasise this to the brain.
Where do majority of ganglion cells project to?
The LGN
What sort of structure is the LGN?
Laminar, therefore is divided into six layers.
What is the input of the LGN layers?
Each layer only receives input from one eye.
Describe the LGN layers input:
1,4,6 receive input from the contralateral eye.
2,3,5 receive input from the ipsilateral eye
What is the function of the LGN?
It is a relay station
How else is the LGN layers specific in input?
Eat LGN layer is specific as to which eye it receives input from and what type of ganglion cell inputs there.
Which layers of the LGN do midget ganglion cells send axons to?
Midget Ganglion Cells send axons to LGN layers 3-6. These are known as parvocellular layers. (p cells)
Miget cells have small cell bodies and this is observed in these layers.
Which layers of the LGN do parasol ganglion cells send axons to?
Parasol Ganglion Cells send axons to LGN layers 1 and 2. This is known as the magnocellular layers (m cells)
Big cell bodies
Which layers of the LGN do bistratified ganglion cells send their axons to?
Bistratified Ganglion cells send their axons to small sublayers between all six LGN layers. Known as the koniocellular layers. K cells.
Very small cell bodies
Draw the LGN showing input from which eye and what type of cell, referring to layers to.
Do it now to show understanding.
This will be examined!!!!!!!!!
The distinct inputs of the LGN i.e types of ganglions, layers, eyes is preserved in the visual pathway, whats the implication of this?
Each cell type has its own unique function to add to the perception of the visual field and this is maintained in the visual pathway structures
What is the function of the Midget ganglion cells+p cells (parvoceullar pathway)?
Colour (differences-few cone input) Fine detail (small receptive field) Poor temporal sensitivity (poor for motion)(take long time to switch on and off)
What is the function of the Parasol ganglion cells + M cells (magnocellular pathway)?
High temporal sensitivity (good for motion) (respond rapidly) Achromatic (don't colour) (too much cone input) Course detail (large receptive fields)
Whats the function of the bistratified ganglion cells + K cells (koniocellular pathway)?
Short wavelengths
Large receptive fields
Not understood well
What happens to the magnocellular, parvocellular and koniocellular pathways in the visual pathway?
They are preserved and run PARALLEL to the primary visual cortex.
What is another name for the primary visual cortex and why?
Striate cortex because it appears striated.
or V1
What can the layers of the straite cortex be grouped into?
Three groups:
- Output to higher cortical areas 1-4b
- Primary input from LGN 4C
- Output to thalamic areas 5,6 (back to LGN)
What inputs into 4C?
Primary input of LGN
So mainly:
- Magnocellular layer (4C alpha) and Parvocellular layer (4C beta)
What v1 areas does the koniocellular pathway project to?
1,2,3
What v1 areas does the magnocellular pathway project to?
4 A, 4 B, 4C alpha and 6
What v1 areas does the parvocellular pathway project to?
4c beta and 6
What sort of mapping is in v1?
Retinotopic Mapping
What is retinotopic mapping?
The right visual field end ups on left retina and in the left visual field.
The v1 is divided by the calcarine sulcus and the mapping is such that the lower visual field, maps to the upper side of the calcarine sulcus and conversely is true.
Also as you move from the back of v1 i.e post to ant, you move from mapping of the fovea to the peripheral vision mapping.
What does retinotopic mapping allow?
To make predictions of visual field loss based on region of damage.
What is selective loss of vision called?
Scartoma
What is it called if you lose half of v1 in one hemisphere?
Quadrantopia
What is it called if you lose all of v1 in one hemisphere?
Hemianopia
Where in the visual pathway are there cells with binocularity?
Not until the primary visual cortex, most cells have monocular dominance / preference
In v1 where are strictly monocular cells found?
IN layer 4c
Outside of layer 4c do cells have a monocular preference?
Yes they have a monocular preference but can receive input from either eye.
Describe ocular dominance in layer 4c and its projections:
Cells in layer 4c form ocular dominance columns that alternate between ipsilateral and contralateral eyes.
The outputs from these columns are combined in layers 2 and 3.
However OD columns appear to extend through the layers.
Describe the density of the retinotopic map for the visual field in v1:
The retinotopic map becomes sparser as you move towards the peripheral visual field. It is more densely packed in the fovea.
What is a log polar retintopic map?
The mathematical representation of the retinotopic map dedicated to parts of the visual field. i.e fovea has the most mapping and is also the most detailed part of your vision.
How are receptive fields in the LGN mapped?
Very similar to receptive fields in the retina. i.e prefer dots of light
What happens to receptive fields in v1?
Cells in v1 now also prefer orientation of light as well as light or dark (centre vs surround)
What is the striate cortex, simple cell selectivity?
This property describes how cells in the v1 prefer light to be of a certain orientation (light bars) and as the bars of light rotate they start to overlap inhibitory regions (centre surround concept except parallel bars) an the overall response of the cell decreases. (as opposed to light bar, correct orientation and dark surround/parrallel bars = max response)
The change in orientation = change in response = tuning response (assuming initially light is only on light cell bar (i.e centre)
Describe orientation tuning in v1:
Orientation tuning progresses in an orderly manner throughout v1
Common orientation preference is columnar
Electrode tack perpendicular to surface= little change in orientation
electrode tack oblique to surface = gradual change in orientation preference.
What does optical imaging of orientation preference in the cortex reveal?
Smooth progression in orientation across the cortex.
Except point called pinwheel fractures where lots of orientations are represented closely to one another.
What do the pinwheels fall close to?
The ocular dominance columns
orientation contours tend to cross OD columns at right angles
What do orientation contours show us?
Each orientation contours servers both eyes for that specific region of visual space.
What are CO blobs?
Cytochrome oxidase stains cells in the cortex with a high metabolic rate.
What cortical layers in v1 have CO blobs?
2,3,5,6
What are the arrangement of CO blobs in v1?
0.5mm apart uniform staining
Neurones that have blobs in v1 receive input from?
- Get LGN input from K cells (kinocellular cells, bistratified ganglions)
- sensitive to colour
- NO orientation tuning
- Have centre surround receive fields
What are hyper columns?
Made up of;
- Columns that share a common 180 degrees orientation preference
- OD column for each eye
- CO blobs across the cortex
What is the function of a hyper column?
Theory is that a hyper column processes all possible combinations of information i.e orientation OD etc for a specific region of visual space
