Lateral geniculate body

The lateral geniculate nucleus (LGN) of the thalamus is a part of the brain, which is the primary processor of visual information, received from the retina, in the central nervous system.

The LGN receives information directly from the retina, and sends projections directly to the primary visual cortex. In addition, it receives many strong feedback connections from the primary visual cortex.

Ganglion cells of the retina send axons to the LGN through the optic nerve. Although it is generally considered to be a cranial nerve, and is always listed as cranial nerve II, in reality the retina and optic nerve arise as an outpocketing of the developing diencephalon. Rather than a proper nerve, then, the optic nerve is really a tract of the brain.

Structure
The LGN is a distinctively layered structure ("geniculate" means "bent like a knee"). In most primates, including humans, it has six layers of cell bodies with layers of neuropil in between, in an arrangement something like a club sandwich or layer cake, with cell bodies of LGN neurons as the "cake" and neuropil as the "icing".

These six layers contain two types of cells. The cells in layers 1 and 2 are large, or magnocellular (M); others in layers 3, 4, 5, and 6 are smaller, or parvocellular (P). (The Latin prefix "parvo-" means "small"; some authors prefer the term parvicellular. If you're searching for more information, try both spellings.)

Between each of the M and P layers lies a zone of very small cells: the interlaminar, or koniocellular (K), layers. K cells are functionally and neurochemically distinct from M and P cells and provide a third channel to the visual cortex.

The magnocellular, parvocellular, and koniocellular layers of the LGN correspond with the similarly-named types of ganglion cells.

M, P, K cells
Magnocellular cells have large cell bodies, use a relatively short time to process information. This system operates quickly but without much detail. They are found in layers 1 and 2 of the LGN, those layers more ventrally located which are next to the incoming optic tract fibers.

M Cells are the retinal ganglion cells that project their axons to the magnocellular layers of the LGN.

Parvocellular cells have small cell bodies, and use a relatively long time to process information. This system operates more slowly and with lots of information about details. For example, these cells carry color information while magnocellular cells do not. Parvocellular cells are found in layers 3, 4, 5 and 6.

P Cells are the retinal ganglion cells that project their axons to the parvocellular layers of the LGN.

Koniocellular cells have very small cell bodies and are located in between the layers. The role of the koniocellular system in visual perception is presently unclear, however, it has been linked with integrating somatosensory-proprioceptive information with visual perception, and may also be involved in color perception. K Cells are the retinal ganglion cells that project their axons to the koniocellular layers of the LGN.

It should be noted that the parvo- and magnocellular fibers were previously thought to dominate the Ungerleider-Mishkin ventral and dorsal streams, respectively. However, new evidence has accumulated showing that the two streams appear to feed on a more even mixture of different types of nerve fibers. For a review, see Goodale & Milner, 1993, 1995.

The other major retino-cortical visual pathway is the retinotectal pathway, routing primarily through the superior colliculus and thalamic pulvinar nucleus onto posterior parietal and medial temporal cortices.

Ipsilateral and contralateral layers
In addition, the layers are divided up so that the eye on the same side (the ipsilateral eye) sends information to layers 2, 3 and 5, while the eye on the opposite side (the contralateral eye) sends information to layers 1, 4 and 6. (A simple mnemonic for this is that 2 + 3 = 5 while 1 + 4 does not equal 6, so it is "contra"ry to your knowledge of math.)

Remember that, in visual perception, the right eye gets information from the right side of the world (the right visual field), as well as the left side of the world (the left visual field). You can confirm this by covering your left eye: the right eye still sees to your left and right, although on the left side your field of view is partially blocked by your nose.

In the LGN, the corresponding information from the right and left eyes is "stacked" so that a toothpick driven through the club sandwich of layers 1 through 6 would hit the same point in visual space six different times.

LGN output
Information leaving the LGN travels out on the optic radiations, which form part of the retrolenticular limb of the internal capsule.

The axons that leave the LGN go to V1 visual cortex. Both the magnocellular layers 1-2 and the parvocellular layers 3-6 send their axons to layer 4 in V1, with layer 4c feeding on parvo- and layer 4b on magnocellular input. However, the koniocellular layers (in between layers 1-6) send their axons to layers 2 and 3 in V1.

Axons from layer 6 of visual cortex send information back to the LGN.

Function in visual perception
The function of the LGN is unknown. It has been shown that the LGN introduces coding efficiencies by cancelling out redundant information from the retina, but there is almost certainly much more going on.

Like other areas of the thalamus, particularly other relay nuclei, the LGN likely helps the visual system focus its attention on the most important information. That is, if you hear a sound slightly to your left, the auditory system likely "tells" the visual system, through the LGN, to direct visual attention to that part of space.

The LGN is also a station that refines certain receptive fields.

Recent experiments using fMRI in humans have found that both spatial attention and saccadic eye movements can modulate activity in the LGN.