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Lateral Geniculate Nucleus (LGN)

Lateral Geniculate Nucleus (LGN). Overview of central visual pathway Projection from retina to LGN LGN layers: P and M pathways LGN receptive fields Retinotopic Maps. Thalamus -- A large mass of gray matter deeply situated in the forebrain. There is one on either side of the midline.

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Lateral Geniculate Nucleus (LGN)

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  1. Lateral Geniculate Nucleus (LGN) • Overview of central visual pathway • Projection from retina to LGN • LGN layers: P and M pathways • LGN receptive fields • Retinotopic Maps

  2. 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

  3. 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 -

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

  5. LGN V1 eye Visual pathway from retina to V1

  6. Projection from retina to LGN fixation point • 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 fovea 1-6: lesion that produce distinct visual defects

  7. 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!

  8. LGN layers

  9. Lesion studies

  10. (after selective lesion)

  11. 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.

  12. 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.

  13. Anatomical segregation of M and P pathways

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

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

  16. Retinotopic Maps - Adjacent points in the retina project • to adjecent points in the higer order brain regions. • Mapping of LGN: • 1. Recording parallel to the layer showed that adjacent • cells are excited by adjacent retinal cells of the same retina • Recording perpendicular to the layers showed that • cells in different layers are excited by cells in either right • or left retina but having the same receptive field location. • Cells in different layers are in “topographic register”.

  17. FP visual field 1 2 3 left right retina 3 3 2 2 1 1 3 LGN 2 1 2 3 1 V2 V1 V2 From visual field to V1 medial visual field  lateral V1 lower visual field  anterior V1 upper visual field  posterior V1

  18. Fixation point Visual field left right V1 V1 Nonuniform representation of the visual field in V1 Cortical magnification in the fovea ---- The fovea has a larger cortical representation than the peripheral.

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