Lateral Geniculate Nucleus (LGN)

1. Overview of central visual pathway

2. Projection from retina to LGN

3. LGN layers: P and M pathways

4. LGN receptive fields

5. Retinotopic Maps

Thalamus

-- A large mass of gray matter deeply situated in the forebrain. There is one on either side of the midline.

-- Axons from every sensory system (except olfaction) synapse here as the last relay site before the information reaches the cerebral cortex.

-- Lateral geniculate nucleus (LGN) is responsible for relaying visual information

• Three subcortical areas in the visual pathway: - Pretectal area, superior colliculus, and lateral geniculate nucleus

(LGN)

Superior colliculus controls saccadic eye movements:

Coordinates visual, somatic and auditory information, adjusting movement of the head and eyes towards a stimulus

1. Superior colliculus – brain stem – eye muscles (oculomotor reflex) 2. Superior colliculus – tectospinal and tectopontine tracts – head and

neck muscles

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Pretectal area mediates pupillary light reflex

Retina – pretectal area – Edinger- Westphal nuclei (on both sides) – IIIrd cranial nerve – pupillary constrictor muscles.

Visual pathway from retina to V1

eye

LGN

V1

Projection from retina to LGN

fixation point

fovea

• Nasal RGC: axons crossover, project to contralateral LGN

• Temporal RGC: axons stay on the same side (ipsilateral)

• Left visual field: right LGN, right V1

• Right visual field: left LGN, left V1

1-6: lesion that produce distinct visual defects

• Parvocellular layers: 3-6

(input from P type RGCs)

• Magnocellular layers: 1,2 (input from M type RGCs)

• Contralateral eye: 1,4,6

• Ipsilateral eye: 2,3,5

• But all LGN layers represent contralateral visual field!

LGN layers

Lesion studies

(after selective lesion)

• Parvocellular layers (form and color):

-- small cells, color sensitive, high spatial resolution (small RF), low temporal resolution (does not see fast flickers of light). They receive inputs from P type RGC cells.

• Magnocellular layers (motion)

-- large cells, color blind, low spatial resolution (large RF), high temporal resolution (good for processing motion stimuli). They receive inputs from M type RGC cells.

Interlaminar koniocellular (K) Layers - between each of the M and P layers. K cells are functionally and neurochemically distinct from M and P cells and provide a third channel to the visual cortex.

Function of LGN: Unknown

Possibilities

--gating visual information flow, via different modes (oscillations and bursting/tonic firing)

--feedback regulation of visual information flow; for example, spatial attention and saccadic eye movements can modulate activity in the LGN.

Anatomical segregation of M and P pathways

Receptive Fields of LGN neurons

Receptive field -- Part of the retina (visual field) in which light can evoke response from a cell.

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- Circular with antagonistic surround ON or OFF center ( 1o in diameter)

- Each LGN cells receives only a few retinal ganglion cells (no transformation)

Note: Only 20% of inputs to LGN are from retina, the rest from other areas, e.g. brain stems and cortex.

-M Layers (1 &2) receive feedback inputs from extrastriate cortex

Spatiotemporal RF: Receptive field is dynamic, containing both space and time infomation

Retinotopic Maps - Adjacent points in the retina projectto adjecent points in the higer order brain regions.

Mapping of LGN:

1. Recording parallel to the layer showed that adjacentcells are excited by adjacent retinal cells of the same retina

2. Recording perpendicular to the layers showed thatcells in different layers are excited by cells in either rightor left retina but having the same receptive field location.

Cells in different layers are in “topographic register”.

medial visual field lateral V1

lower visual field anterior V1

upper visual field posterior V1

1 2 3

23

1

23

1

23

1

1 2 3

retina

LGN

V1

FPvisual field

From visual field to V1

left right

V2V2

Nonuniform representation of the visual field in V1

Cortical magnification in the fovea ----

The fovea has a larger cortical representation than the peripheral.

Fixation point

Visual field

left right

V1

V1