|
| |
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 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.
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:
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.
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.
| |
|
