Callosal Connections Identify the Borders between Human Visual Areas

Clarke and Mikossy studied the organization of human occipital cortex using cyto- and myeloarchi-tecture and their relationships to callosal connections. In addition to the easily identified cytoarchitectonic border of V1 with V2, this border is demarcated by a dense band of callosal connections that is bordered anteriorly by a large acallosal zone (see Fig. 4). On the basis of studies in macaque monkeys, these acallosal zones are thought to contain the lower and upper field representations of area V2, as well as the lower field

Figure 4 (Top) Flat reconstruction of the medial occipital lobe illustrating the tangential distribution of callosal afferents within the gray matter. The reconstructed region corresponds to the shaded portion of the brain insets and also includes cortex buried within the calcarine and other sulci. Sixteen sections are reconstructed with the callosal afferents projected to a point halfway between the pial surface and white matter. The density of degenerating fibers is represented by the different sizes of filled circles. (Bottom) Stippling shows dense regions of callosal afferents. The large callosal-free region labeled I corresponds to area V1. The 17-18 border (heavy dashed line) is marked to a narrow dense band of callosal afferents (regions 1 and 2). A second callosal-free zone is located anterior to region 2 in ventral cortex (region II) and anterior to region 1 in dorsal cortex (region III). These callosal-free zones is contain representations of the horizontal meridian. Anterior to the callosal-free zones is a second dense band of callosal afferents found in ventral cortex (region 4) and in dorsal cortex (region 3). These anterior bands correspond to a second representation of the vertical meridian. The ventral callosal-free zone contains areas V2 and VP and is located within area 18. Similar arguments can be made for areas V3 and V3A in dorsal extrastriate cortex. From Clarke and Miklossy (1990).

Figure 4 (Top) Flat reconstruction of the medial occipital lobe illustrating the tangential distribution of callosal afferents within the gray matter. The reconstructed region corresponds to the shaded portion of the brain insets and also includes cortex buried within the calcarine and other sulci. Sixteen sections are reconstructed with the callosal afferents projected to a point halfway between the pial surface and white matter. The density of degenerating fibers is represented by the different sizes of filled circles. (Bottom) Stippling shows dense regions of callosal afferents. The large callosal-free region labeled I corresponds to area V1. The 17-18 border (heavy dashed line) is marked to a narrow dense band of callosal afferents (regions 1 and 2). A second callosal-free zone is located anterior to region 2 in ventral cortex (region II) and anterior to region 1 in dorsal cortex (region III). These callosal-free zones is contain representations of the horizontal meridian. Anterior to the callosal-free zones is a second dense band of callosal afferents found in ventral cortex (region 4) and in dorsal cortex (region 3). These anterior bands correspond to a second representation of the vertical meridian. The ventral callosal-free zone contains areas V2 and VP and is located within area 18. Similar arguments can be made for areas V3 and V3A in dorsal extrastriate cortex. From Clarke and Miklossy (1990).

representation in area V3 and the upper field representation in area VP. A second dense callosal band marks the anterior border of this acallosal zone, just as is observed at the anterior borders of areas V3 and VP in macaque monkeys.

The border between V2 and presumably V3 in dorsal occipital cortex was difficult to identify on the basis of cytoarchitecture or myeloarchitecture, although macaque V3 has a distinctive pattern of dense myeloarchitecture. In contrast, the ventral acallosal zone can be subdivided into densely myelinated V2 posteriorly and poorly myelinated VP located anteriorly. This border also corresponds to a decrease in CO activity in VP as compared to area V2. Therefore, area V2 is contained within cytoarchitectonic area OB of von Economo, whereas area VP is contained with cortex of the OA1 subtype of area peristriata. On the basis of the published map of callosal afferents, area V2 ranges from approximately 10 mm to more than 20 mm in width.

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