Visual Field map in V1 Flashcards
(28 cards)
What correlation does V1 have with the visual field?
One-to-one. Pick a spot in V1, corresponds to a spot in VF. Move V1 spot, VF spot changes.
Explain the organization of V1. Where does the key component get input from?
6 layers. 2-6 relevant. 4 is divided into 3 parts A/B/C. 4C divided into alpha and beta. 4 gets input from LGN.
What’s the density of V1 compared to anywhere else?
V1 has 2.5 times the density of neurons per chunk.
Why is 4B different?
It has a lot of axons, less cells.
What is diagnostic of V1 in 4B?
Stria of Gennari. Contains intracortical axons (one piece of cortex to next)
What’s the basic model of visual system organization in terms of V1?
Parvo layers of LGN terminate in 4CB. Magno to 4CA. Konio to blobs. Not a lot of crosstalk except 4C(A/B) talking to blobs. Everything above 4C is konio. These all send to extrastriate.
Explain how input is combined/segregated until and in V1.
Everything up to V1 is completely segregated. M and P don’t converge into neurons. In V1, M and P layers send to 4C, which converges into blobs.
What are the cardinal features of neurons in V1?
Orientation selectivity and Ocular Dominance.
Explain ocular dominance.
Neurons in 4C and blobs are monocular. Because axons of cells in any primary layer in LGN terminate in non-overlapping domains in V1. This transfers up and down in cortex. Up to 2, down to 6. Those will be binocular, but dominated still by the eye that feeds the patch.
Middle of column more strongly monocular. Move toward edges of column, more binocularity.
Explain orientation selectivity.
. A neuron in LGN and RGC will have a center and surround. RF will be circular. What happens in V1? RF becomes elongated. Maximal response not to a spot of light but to an elongated bar (dark or light). The bar has to be a specific orientation for cells to respond.
How can you tell a particular spot in an ocular dominance column is innervated indirectly by the fovea?
The patches look like elongated stripes when viewed from above. When the columns disappear or curve onto each other you know it’s fovea.
How can you tell a particular spot in an ocular dominance column is closer to V2?
They get straighter when you approach V1-V2 border.
How does ocular dominance happen?
Axons of cells in 4C send up. Most send directly up, some across columns. Those are weaker and provide signal from contralateral eyes.
What are the properties of a column?
Cannot be simply a map of sensory surface. Columnar property has to be shared between all neurons from layer2-6. Discrete change of property between columns.
Explain the ice cube model.
Map ocular dominance in one dimension and orientation preference in other. As a result every cell will have a particular preference. At the centers of ocular ddominance columns are blobs. 50% blob rest interblob.
What’s a hypercolumn?
Take full set of orientation columns. A column is 10 degrees. 18 of these. 1mm cube of cortex will have 4 blobs. Machine in V1 necessary to analyze all visible features of a parch of IRL. As you move away from fovea, area represented by this gets larger.
Explain how Hubel observed properties of blobs.
He drove electrode into V1, staying in layers 2-3. Sometimes it hit a blob. Show bars of light. Record RF properties of cells. Ablate unusual cells. Whenever unusual, it’s a blob. Orientation selectivity was poor.
Explain how Leventhal observed properties of blobs.
Electrode thru V1. Sinusoidal gradings at different angles. Equally common blob as interblob observed (wrong irl) and equally common orientation tuning or lack of.
Explain how Tootell observed properties of blobs.
Optical imaging. 2-DG. Blobs responded to lower contrast and lower spatial frequency.
What’s the Harvard hypothesis?
K wasn’t discovered yet. M/P go thru LGN. Very little mixture in V1. Flow to separate domains of V2. Compartmental organization of V1/2. Blobs handle color. Go to color are in V2. Interblobs handle spatial, go to spatial place in V2. 4B handles depth, goes to MT. V1 exists strictly to segregate info (not rly true)
What’s at the V1-V2 border?
Representation of vertical meridian where line splits fovea into nasal and temporal halves.
What happens to blobs at V2 border?
Replaced by thin, thick and pale stripes. Thick pale thin pale pattern.
What are thick&thin stripes are innervated by other than V1?
Inferior pulvinar.
What’s the analogy between V1 and V2?
V2 also has hypercolumns, similar organization. Map of orientation. Stripes aren’t uniform. They have bloblike structures too. Thin stripes: color vision. Thick stripes: depth/disparity. Pale stripes: IDK. 1mm hypercolumn in V1-> >1mm hypercolumn in V2.
