2. Visual Thalamus & Cortex

EXAM 2

Lateral Geniculate Nucleus (LGN)

Acts as a relay station for visual information coming from eyes

• located in thalamus

Three layer-categories:

1. Magnocellular

   1. process motion and depth info

   2. black & white cells

2. Parvocellular

   1. process form and color info

   2. red/green cells

3. Koniocellular

   1. process color info

   2. blue/yellow cells

Pathway from retina to LGN

LGN: has six layers (located in the thalamus)

Note* Individual LGN cells receive input from 1 eye (monocular)

LGN layers alternate input from each of the 2 eyes

• Two Broad Categories:

  • Magnocellular layers (1-2): motion, depth— large GC

    • detects gross features

  • Parvocellular layers (3-6): color, fine detail— small GC

    • detects shapes & patterns

Layers 1,4,6 get contralateral input (opposite eye)

Layers 2,3,5 get ipsilateral input (same side)

Pathway from LGN to V1

aka Striate cortex (layer IVC)

Left V1: represents right half of visual field

Right V1: represents left half of visual field

Pathways:

1. M- projects to sublayer 4Calpha

2. P- projects to sublayer 4CB

3. Koniocellular pathways layers 1-3

soo..

Info from LGN layers are sent to different parts of layer 4C in V1

• Patch of layer is called “Ocular Dominance Column”

  • ODC forms zebra-like structure

Info is first combined in layer III (3) of V1

Pathway from V1 to higher-order visual areas

How the visual field reads in both eyes

Note: Nasal half (inner) and temporal half (outer)

Left eye: Nasal half (left), temporal half (right)

Right eye: Nasal half (right), temporal half (left)

At optic chiasm (where nerves cross):

nasal half of each eye’s visual field crosses over, temporal half stays on the same side

Organization selectivity in V1

• Selectivity of cells stays the same as you move perpendicular to surface

• Selectivity changes as you move laterally

• V1 cells respond best to oriented lines

• Many V1 cells are direction-selective

  • will respond to a line in one direction, but not the other

  • come from M cells in LGN, thought to help with processing of object motion

Two types of cells with orientation selection

1. Simple Cells: distinct ON and OFF

   • multiple LGN cells are through to produce simple cell RF

2. Complex Cells: no distinct ON and OFF

   • simple cells w same orientation are thought to provide input to complex cell

Higher-order visual cortical cells are selective for complex objects or motion direction

Blobs, V1, V2, V4, V5/MT

V2, secondary visual cortex

__ cells respond to angles/component lines that form preferred angles

• as a result of summing two V1 cells

V4, fourth visual area

__ cells respond to colored shapes

• are sensitive to form and color

• as a result of summing two V2 cells

V5/MT , fifth visual area/middle temporal area

__ cells respond to stimuli moving in a particular direction

• have preferred motion direction

• have preferred stimulus velocities

Correlated with perception of motion

Blobs

aka Cytochrome Oxidase, distributed across V1

• In layers 2&3, 5&6

• cells in blobs are color-sensitive

  • Thought to contribute to color processing