Lateral Geniculate Nucleus (LGN)
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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.
-- 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 V1
LGN
eye
V1
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 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.
- Circular with antagonistic surround ON or OFF center ( 1o
in diameter)
- Each LGN cells receives only a few retinal ganglion cells
(no transformation)
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+ - +
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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
retina
LGN
1 23
V2
V1
From visual field to V1
medial visual field  lateral V1
lower visual field
 anterior V1
upper visual field  posterior V1
V2
Nonuniform representation of the visual field in V1
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.