COGSCI 111 Calvin - Columns and Modules

COGSCI 111 Calvin - Columns and Modules - Author: William...

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Author: William H. Calvin Title: Columns and Modules Each cerebral hemisphere has about 52 "areas" on the basis of differences between the thickness of their layers; on average, a human cortical area is about half the size of a business card. Though Area 17 seems to be a consistent functional unit, other areas prove to contain a half-dozen distinct physiological subdivisions ("maps") on the centimeter scale. Columns are usually subdivisions at the sub-millimeter scale, and modules are thought to occupy the intermediate millimeter scale, between maps and columns. CEREBRAL CORTEX sits atop the white matter, its 2 mm thickness subdivided into about six layers. NEURONs with similar interests tend to cluster. Empirically, a column is simply a sub-mm region where many (but not all) neurons seem to have functional properties in common. They come in two sizes, with separate organizational principles. Minicolumns are about 23-65 µm across, and there are hundreds of them inside any given 0.4 - 1.0 mm macrocolumn. Both may be regarded as the outcomes of a self-organizing tendency during development, patterns that emerge as surely as the hexagons of a honeycomb arise from the pounding of so many hemispherical bee's heads on the soft wax of tunnel walls. The hexagonal shape is an emergent property of such competition for territory; similar competition in cortex may continue throughout adult life, maintaining a shifting mosaic of cortical columns. A column functionally ties together all six layers. Layers II and III can usually be lumped together, as when one talks of the superficial pyramidal neurons. But layer IV has had to be repeatedly subdivided in the visual cortex ( IVa, IVb, IVc , IVc ). Layer IV neurons send most of their outputs up to II and III. Some superficial neurons send messages down to V and VI, though their most prominent connections (either laterally in the layers or via U-fibers in white matter) are within their own layers. Layer
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COGSCI 111 Calvin - Columns and Modules - Author: William...

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