Lateral Geniculate Nucleus (LGN)PowerPoint PresentationSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Lateral Geniculate Nucleus (LGN)1. Overview of central visual pathway2. Projection from retina to LGN3. LGN layers: P and M pathways4. LGN receptive fields5. Retinotopic MapsThalamus-- 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 -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 V1eyeLGNV1Projection from retina to LGNfixation pointfovea•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 V11-6: 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 layersLesion 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 pathwaysReceptive Fields of LGN neuronsReceptive 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 cortexSpatiotemporal RF: Receptive field is dynamic, containing both space and time infomationRetinotopic 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 retina2. 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 V1lower visual field  anterior V1upper visual field  posterior V112323123123112 3retinaLGNV1FPvisual fieldFrom visual field to V1left rightV2V2Nonuniform representation of the visual field in V1Cortical magnification in the fovea ----The fovea has a larger cortical representation than the peripheral.Fixation pointVisual fieldleft