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 CNS.
Schematic diagram of the primate lateral geniculate nucleus.
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.
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 ; others in layers 3, 4, 5, and 6 are smaller, or parvocellular. (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.
M and P Cells
Magnocellular cells (commonly called M cells) have large cell bodies, use a relatively short time to process information, and are part of a visual processing system that tells the brain where something is. 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.
Parvocellular cells (commonly called P cells) have small cell bodies, use a relatively long time to process information, and are part of a visual processing system that tells the brain what something is. 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.
Ipsilateral and Contralateral Layers
Additionally, 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, but on the left side, your vision 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.
Information leaving the LGN travels out on the optic radiations, which form part of the retrolenticular limb of the internal capsule.
The axons which leave the LGN go to V1 visual cortex and generally end in layer IV.
Axons from layer VI 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.
Recent experiments using fMRI in humans have found that both spatial attention and saccadic eye movements can modulate activity in the LGN.
Blohm G and Schreiber C. LGN in the visual pathway (http://www.auto.ucl.ac.be/EYELAB/neurophysio/light_perception/High_order_visual_proc.html). Retrieved September 1, 2004.
Harvey R. Really cool movie of a 3D reconstruction of monkey LGN (http://www.utoronto.ca/cat/services/services_other/vizProjects/LGN.html). Retrieved September 1, 2004.
Malpeli J. Malpeli Lab Home Page (http://soma.npa.uiuc.edu/labs/malpeli/home.html). Retrieved September 1, 2004.