Lateral geniculate nucleus

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.

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Structure

The left and the right 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".

M, P, K cells

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

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.^[1]

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 as follows:^[2]

• 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 "contrary" to your knowledge of math.

This description applies to the LGN of many primates, but not all. The sequence of layers receiving information from the ipsilateral and contralateral (opposite side of the head) eyes is different in the tarsier ^[3]. Some neuroscientists suggested that "this apparent difference distinguishes tarsiers from all other primates, reinforcing the view that they arose in an early, independent line of primate evolution" ^[4].

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 inputs

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)^[5].

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 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.

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 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 movements can modulate activity in the LGN.

Additional images

References

1. ^ Goodale & Milner, 1993, 1995.
2. ^ Nicholls J., et. al. From Neuron to Brain: Fourth Edition. Sinauer Associates, Inc. 2001.
3. ^ Rosa MG, Pettigrew JD, Cooper HM (1996) Unusual pattern of retinogeniculate projections in the controversial primate Tarsius. Brain Behav Evol 48(3):121-129.
4. ^ Collins CE, Hendrickson A, Kaas JH (2005) Overview of the visual system of Tarsius. Anat Rec A Discov Mol Cell Evol Biol 287(1):1013-1025.
5. ^ 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.