Lateral Geniculate Nucleus LGN 1 2 3 4

Lateral Geniculate Nucleus (LGN) 1. 2. 3. 4. 5. Overview of central visual pathway Projection from retina to 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. -- 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 V 1 LGN eye V 1

Projection from retina to LGN fixation point • Nasal RGC: axons crossover, project to contralateral LGN fovea • Temporal RGC: axons stay on the same side (ipsilateral) • Left visual field: right LGN, right V 1 • Right visual field: left LGN, left V 1 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. - Circular with antagonistic surround ON or OFF center ( 1 o 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 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 2. 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”.

FP visual field 1 23 left right 32 32 1 retina 1 3 2 LGN 1 V 2 1 23 V 1 From visual field to V 1 medial visual field lateral V 1 lower visual field anterior V 1 upper visual field posterior V 1 V 2

Nonuniform representation of the visual field in V 1 Fixation point Visual field left V 1 right V 1 Cortical magnification in the fovea ---The fovea has a larger cortical representation than the peripheral.