Ipsilateral and Contralateral Layers
Both the LGN in the right hemisphere and the LGN in the left hemisphere receive input from each eye. However, each LGN only receives information from one half of the visual field. This occurs due to axons of the ganglion cells from the inner halves of the retina (the nasal sides) decussating (crossing to the other side of the brain) through the optic chiasm (khiasma means "cross"). The axons of the ganglion cells from the outer half of the retina (the temporal sides) remain on the same side of the brain. Therefore, the right hemisphere receives visual information from the left visual field, and the left hemisphere receives visual information from the right visual field. Within one LGN, the visual information is divided among the various layers as follows:
- the eye on the same side (the ipsilateral eye) sends information to layers 2, 3 and 5
- the eye on the opposite side (the contralateral eye) sends information to layers 1, 4 and 6.
A simple mnemonic for remembering this is "See I? I see, I see," with "see" representing the C in "contralateral," and "I" representing the I in "ipsilateral."
Another way of remembering this is 2+3=5, which is correct, so ipsilateral side, and 1+4 doesn't equal 6, so contralateral.
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. 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".
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.
Read more about this topic: Lateral Geniculate Nucleus
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