The CEREBRAL CORTEX sits atop the white matter of the brain, its 2 mm thickness subdivided into about six layers. Neurons with similar interests tend to cluster. Columns are usually subdivisions at the submillimeter scale, and modules are thought to occupy the intermediate millimeter scale, between maps and columns. Empirically, a column is simply a submillimeter 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. Each cerebral hemisphere has about 52 "areas" distinguished 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.
Both columns and modules 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 the 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 VI sends messages
back down to the THALAMUS via the white matter, while V sends
signals to other deep and distant neural structures, sometimes even
the spinal cord.
For any column of cortex, the bottom layers are like a subcortical outgoing mailbox, the middle layer like an inbox, and the superficial layers somewhat like an interoffice mailbox spanning the columns and reaching out to other cortical areas (Calvin and Ojemann 1994). Indeed, Diamond (1979) argues that the "motor cortex" isn"t restricted to the motor strip but is the fifth layer of the entire cerebral cortex. That"s because V, whatever the area, contains neurons that at some stage of development send their outputs down to the spinal cord, with copies to the brain stem, BASAL GANGLIA, and hypothalamus. Likewise the fourth layer everywhere is the "sensory cortex," and the second and third layers everywhere are the true "association cortex" in this view.
Minicolumns appear to be formed from dendritic bundles. Ramón y CAJAL saw connect-the-dots clusters of cell bodies, running from white matter to the cortical surface; these hair-thin columns are about 30 m apart in human cortex. It now appears that a column is like a stalk of celery, a vertical bundle containing axons and apical dendrites from about 100 neurons (Peters and Yilmaz 1993) and their internal microcircuitry.
Macrocolumns may, in contrast, reflect an organization of the input wiring, for example, corticocortical terminations from different areas often terminate in interdigitating zones about the width of a thin pencil lead. In 1957, Mountcastle and his coworkers discovered a tendency for somatosensory strip neurons responsive to skin stimulation (hair, light touch) to alternate with those specializing in joint and muscle receptors about every 0.5 mm. It now appears that there is a mosaic organization of similar dimensions, the neurons within each macrocolumn (or "segregate") having a receptive field optimized for the same patch of body surface (Favorov and Kelly 1994).
Hubel and Wiesel, recording in monkey visual cortex, saw curtainlike clusters ("ocular dominance columns") that specialized in the left eye, with an adjacent cluster about 0.4 mm away specializing in the right eye. As seems appropriate for an outcome of self-organization, average size varies among individuals over 0.4-0.7 mm; those with smaller ocular dominance columns have more of them (Horton and Hocking 1996).
Orientation columns are of minicolumn dimensions, within which the neurons prefer lines and edges that are tilted about the same angle from the vertical; there are many such minicolumns specializing in various angles within an ocular dominance macrocolumn (Hubel and Wiesel 1977). The relationships between minicolumns and macrocolumns are best seen in VISUAL CORTEX, though it may be hazardous to generalize from this because ocular dominance columns themselves are less than universal; for instance, they are not a typical feature of New World monkeys.
Color blobs are clusters of COLOR-sensitive neurons in the cortex at macrocolumnar spacing but involving only neurons of the superficial layers and not extending throughout all cortical layers, as in a proper column. Recently, recurrent excitation in the superficial layers has been identified as a coordinating (and perhaps self-organizing) principle among distant minicolumns (Calvin 1995). The superficial pyramids send myelinated axons out of the cortical layers into the white matter; their eventual targets are typically the superficial layers of other cortical areas when of the "feedback" type; when "feedforward" they terminate in IV and deep III.
But superficial pyramidal neurons also send unmyelinated collaterals sideways, with an unusual patterning that suggests a columnar organizing principle. Like an express train that skips intermediate stops, the collateral axon travels a characteristic lateral distance without giving off any terminal branches; then it produces a tight terminal cluster (see fig. 5 in Gilbert 1993). The axon may continue for many millimeters, repeating such clusters about every 0.43 mm in primary visual cortex, 0.65 mm in the secondary visual areas, 0.73 mm in sensory strip, and 0.85 mm in motor cortex of monkeys (Lund, Yoshioka, and Levitt 1993). Because of this local standard for axon length, mutual re-excitation becomes probable among some cell pairs. Macrocolumns of similar emphasis are seen to be connected by such synchronizing excitation. Calvin (1996) argues that these express connections could implement a Darwinian copying competition among Hebbian cell-assemblies on the time scale of thought and action, providing one aspect of CONSCIOUSNESS.
Though COMPUTATIONAL NEUROANATOMY has proved more complex, it has been widely expected that cerebral cortex will turn out to have circuits which, in different cortical patches, are merely repeats of a standard "modular" pattern, something like modular kitchen cabinets. Columns, barrels, blobs, and stripes have all been called modules, and the term is loosely applied to any segmentation or repeated patchiness (Purves, Riddle, and LaMantia 1992) and to a wide range of functional or anatomical collectives. The best candidate for a true module was the "hypercolumn" (Hubel and Wiesel 1977): two adjacent ocular dominance columns, each containing a full set of orientation columns, suggested similar internal wiring, whatever the patch of visual field being represented. However, newer mapping techniques have shown that ocular dominance repeats are somewhat independent of orientation column repeats (Blasdel 1992), making adjacent hypercolumns internally nonidentical, that is, not iterated circuitry. Module remains a fuzzy term for anything larger than a macrocolumn but smaller than a map -- though one increasingly sees it used as a trendy word denoting any cortical specialization, for example, modules as the foundation for "multiple intelligences."
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