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3.1 Magno and Parvo in the LGN |
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| As we navigate through our surroundings, a continuous stream of light images impinges our eyes, in the back of each eye is a light-sensitive tissue: the Retina. It converts patterns of light energy into electrical discharges known as action potentials. These signals are conveyed along the axons of retinal ganglion cells to connect for 80% to the LGN (Lateral Geniculate Nucleus) a relay nucleus in the Thalamus and for 20% to the SC (Superior Colliculus). Most of the output of the LGN is relayed directly to the Primary Visual Cortex (V1), and then to surrounding visual association areas."
The Lateral Geniculate Nucleus (LGN) is a part of the Thalamus (Greek, bedroom), a structure in the middle of the brain, it acts as the primary processor of visual information in the Central Nervous System (CNS). It receives signals from the Retina and sends projections to the Primary Visual Cortex (V1). In addition, it receives many strong feedback connections from V1. | 
|  | | The LGN introduces coding efficiencies by
cancelling out redundant information from the Retina,
but much more is going on. Like other areas of the Thalamus,
in particular other relay nuclei, the LGN likely helps the visual
system
to focus its attention. For example, when you
hear a sound slightly to your left, the auditory system "tells"
the visual system, through the LGN, to direct visual attention to that
area.The LGN is also a station that refines certain receptive
fields. Recent experiments using fMRI have found that both
spatial attention and Saccadic eye movements can modulate activity in
the LGN. | 
| | Ganglion cells of the Retina send axons to the LGN through the Optic
Nerve also known as Cranial Nerve II. Rather than a proper nerve, the Optic Nerve is a
tract of the brain, the fibres from the eyes Retinas terminate at the two LGN bodies, one lies in the left hemisphere, the other
lies in the right. All cells in the LGN have concentric
receptive fields, just like the Retina's Ganglion cells. | 
| | Geniculate
means "knee-shaped" and it is build out of 6 layers. Each LGN gets information from only 1 hemifield,
but from 2 eyes and each layer receives inputs from only 1 eye.
The outer 4
layers
(3-4-5-6) are composed of small cells (Parvo) and receive
inputs from the small Ganglion cells of the Retina (Cones), these layers
are called the Parvocellular layers. The inner 2 layers (1-2) are composed of large cells (Magno) and receive their input
from large Ganglion cells (Rods) these layers
are called the Magnocellular layers. Inbetween these layers are
Koniocellular layers and they receive their input from Bistratified
Cells (Rods & Cones). | 
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The segregation of motion and feature vision is a pervasive attribute
of brain organization at all levels of the neuraxis, from the Retina to the Frontal Lobe. "Motion-Orientation" and "Colour-Form"
discrimination is carried out by the separate Magnocellular and
Parvocellular pathways, this
segregated
information
then is transmitted from the LGN to M- and P-related sublayers and
modules in the "Primary Visual Cortex" (V1). The image (right) shows
where all the sections of the Overlapping Visual Fields are regulated to. | 
| MAGNOCELLULAR
- Inner 2 layers ( 1, 2 )
- Input from Parasol Cells.
- Motion and Orientation.
- Rods
- Peripheral of retina
- Responsive to low luminance contrast and spatial freq.
| KONIOCELLULAR
- Inbetween Layers
- Input from Bi- stratified Cells
- Colour: Blue & Yellow.
- Rods & Cones
- Large receptive fields.
| PARVOCELLULAR
- Outer 4 layers ( 3, 4, 5, 6 )
- Input from Midget Cells
- Colour: Red & Green
- Form and Fine details.
- Cones
- Center of retina: foveal
- Responsive to bright, colour, high contrast and spatial freq.
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| V1 is divided into 6 layers. Layer 4,
which receives most visual input from the LGN, is further divided into
4 layers, labelled 4A, 4B, 4Cα, and 4Cβ. Sublamina 4Cα receives most
Magno-cellular input from the LGN, while layer 4Cβ receives input from
Parvocellular pathways. Magnocellular-layers -> V1:
- Contrast and movement
- The P-layers (1-2) neurons send their axons to neurons in the sub-layer 4Cβ
Parvocellular-layers -> V1: - Colour (Red, Green) and fine detail
- The M-layer neurons send their information to neurons in sub layer 4Cα → 4B → V2 → V5.
- Cells in layer 4B
- selective for the direction of movement
- some of these neurons are binocular and sensitive to retinal disparity
Koniocellular-layers -> V1: - Colour information (Blue)
- LGN inbetween layers then project to V1 Blobs
V1 -> LGN:
|   | | [ Next : 3.2 Dyslexia and LGN ]
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The
purpose of this site is to present questions and new ideas about
the above subjects.
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