The ('''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 Cell s of the retina send
Axon s 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 left and the right LGN is a distinctively layered structure ("geniculate" means "bent like a knee"). In most
Primate s, 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
Neuron s as the "cake" and
Neuropil as the "
Icing ".
The magnocellular, parvocellular, and koniocellular layers of the LGN correspond with the similarly-named types of
Ganglion Cell s.
It should be noted that the parvo- and magnocellular fibers were previously thought to dominate the Ungerleider-Mishkin
Ventral Stream and
Dorsal Stream , 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.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.
In addition, the layers are divided up as follows:Nicholls J., ''et. al''. ''From Neuron to Brain: Fourth Edition''. Sinauer Associates, Inc. 2001.
- the eye on the same side (the ''ipsilateral'' eye w.r.t the left or right LGN) sends information to layers 2, 3 and 5
- the eye on the opposite side (the ''contralateral'' eye w.r.t the left or right LGN) 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.
The LGN receives input from the retina.
At least in some species, the LGN also receives some inputs from the
Optic Tectum (also known as the
Superior Colliculus )In Chapter 7, section "The Parcellation Hypothesis" of "Principals of Brain Evolution", Georg Striedter (Sinauer Associates, Sunderland, MA, USA, 2005) states, "...we now know that the LGN receives at least some inputs from the optic tectum (or superior colliculus) in many amniotes". He cites "Wild, J.M. 1989. Pretectal and tectal projections to the homolog of the dorsal lateral geniculate nucleus in the pigeon - an anterograde and retrograde tracing study with cholera-toxin conjugated to horseradish-peroxidase. Brain Res 489: 130-137" and also "Kaas, J.H., and Huerta, M.F. 1988. The subcortical visual system of primates. In: Steklis H. D., Erwin J., editors. Comparative primate biology, vol 4: neurosciences. New York: Alan Liss, pp. 327-391..
Information leaving the LGN travels out on the
Optic Radiation s, which form part of the retrolenticular limb of the
Internal Capsule .
The
Axon s 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 4cβ feeding on parvo- and layer 4cα on magnocellular input. However, the koniocellular layers (in between layers 1-6) send their axons to layers 2 and 3 in V1.
Axon s from layer 6 of
Visual Cortex send information back to the LGN.
The function of the LGN is unknown. It has been shown that while the retina accomplishes spatial decorrelation through center surround inhibition, the LGN accomplishes temporal decorrelation. This spatial-temporal decorrelation makes for much more efficient coding. However, 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 Movement s can modulate activity in the LGN.
