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Anatomy and Physiology
9
The cortex can be separated into two hemispheres [Teo, et al., 1997 #8]. Area V1 in the left
hemisphere only receives information from the right visual field and vice versa. As explained
before, the cortex in general is a layered structure. For example, area V1 can be subdivided
into 6 layers, it is about 3-4 mm thick (Brian A. Wandell, personal communication, June
2001), and the area is about 4 cm by 8 cm large [Wandell, 1999 #16]. Different layers have
different relative densities of neurons, axons, and synapses. They also interconnect to different
parts of the brain. The underlying white matter consists of axons that carry the signals from
and to the different cortex regions.
Area V1 can be anatomically identified because of a dense collection of myelinated ax-
ons, which causes a striation. Beyond V1, the visual cortex can be divided into many more
different visual areas [Schmidt, 1999 #17]. Different visual areas are specialized to respond to
different pattern of activity, e.g., shape, or moving pattern [Müschen, 2000 #18]. The visual
areas are an important part of cortex organization. Researches try to find out what purposes
they have. Figure 2 (bottom) shows the visual areas V1, V2, V3, V3a, and V4.
The components of the visual pathway are retinotopically arranged, i.e., “they preserve
the map of the visual field that falls on the retina” [Khan, 1999 #19]. For example, the topol-
ogy of the receptive fields in the LGN is similar to the organization in the retina. The same is
true for area V1. Area V1 is retinotopically organized. The full visual field can be mapped
retinotopically in V1. Adjacent points in the visual field map onto adjacent points in the reti-
notopically organized visual cortex [Hurdal and McElwain, 1995 #20]. Not only the primary
visual cortex (V1) shows this retinotopic organization. Nearby areas, such as V2 and V3, are
also retinotopically organized.
Area V1 of each hemisphere represents an entire hemifield (half of the visual field). The
same is true for V2 and V3, but it can be seen that the areas V2 and V3 are each split into two
parts: one on the dorsal1 side (V2d, V3d), and the other one on the ventral1 (V2v, V3v) side.
V2d and V3d each represent the lower part of the visual field; V2v and V3v each represent the
upper part of the visual field.
1
Please refer to the glossary.
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