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Anatomy and Physiology
7
consist of axons that connect the cortical neurons with other regions in the brain. The myelin
sheaths of the axons give them a white look. The term white matter refers therefore to axon
tracts.
The human brain can be subdivided into four lobes: frontal, parietal, temporal, and
occipital. They are shown in Figure 1. Please refer also to the glossary. A finer separation is
possible. The separation can be based on purely anatomical criteria, such as the microanatomy
of cell types and their organization. The lobes can also be subdivided into brain areas of
groups of neurons that function together. For example, the occipital lobe can be subdivided
into several visual areas, such as V1, V2, V3, V3a, etc. Brain areas are connected to certain
other parts of the brain.
2.3 Visual System
The primate visual system processes information in a parallel and hierarchical fashion. Infor-
mation is first processed in the retina and later in the visual cortex where the information is
directed to specialized cortex regions [Watson, 2000 #14].
The visual signal originates at the light receptors of the retina in the eye. It travels along
the optic nerve, which consists of retinal ganglion cells, to the lateral geniculate nucleus
(LGN). The LGN is located in the thalamus. Some fibers end up in the midbrain. Between the
retina and the LGN, about half of the axons change their side at the optic chiasm, so that ax-
ons from corresponding points from the two retinas are close together. The information at the
left (right) LGN comes from both eyes but from only the right (left) visual field. Before the
optic chiasm, the collection of axons is called optic nerve, after the optic chiasm it is called
optic tract. From the LGN, the information is carried to the visual cortex by a new set of neu-
rons via the optic radiation pathway. This pathway is located in the white matter of the cortex.
Most of the axons go to the primary visual cortex, which can be found at the occipital lobe of
the cortex within and around the calcarine sulcus. The primary visual cortex is also called area
V1 [Watson, 2000 #14].
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