## Chapter 18, Table 5

Color coding of the two layers with the largest numbers of neurons per 1 mm2 column for each cytoarchitectonic area at each age-point. The following color key is used: "?" = cells in which the data for all six layers were not available. "Red" = cells in which layers III and VI have the most neurons. "Green" = cells in which layers II and IV have the most neurons. "Violet" = cells in which layers IV and VI have the most neurons. "Magenta" = cells in which layers II and VI have the most neurons. "Light Blue" = cells in which layers III and IV have the most neurons. "Dark Blue" = cells in which layers II and III have the most neurons. "Yellow" = cells in which layers I and II have the most neurons. Of the 15 possible ways of combining any two of the six layers (without regard to order), only 7 ways are used at all, and of these 7, only 4 are used frequently. Layers III and VI (red), layers II and IV (green), and layers III and IV (light blue) are the dominant permutations. The interpretation of these data presume that, for a given cortical area, the number of neurons in a layer is a rough measure of its overall computational power relative to the numbers of neurons in the other layers. | From this perspective, one can see that early on, primary motor cortex emphasizes the processing of long cortico-cortical (layer III), cortico-cortical feedback and cortico-thalamic (layer VI) information. Such integration would be useful in coordinating the survival of essential movements of the newborn child. As another example, one can see that temporal and parietal association cortical areas emphasize the processing of short cortico-cortical (layer II), cortico-cortical feedforward and thalamo-cortical (layer IV) information, but at 15 months or later, shift to emphasizing the processing of long cortico-cortical (layer III), cortico-cortical feedback and cortico-thalamic (layer VI) information (similar to what primary motor and other cortical areas do as development proceeds from birth to 72 months). In general, primary sensory cortices emphasize layers long cortico-cortical (layer III), cortico-cortical feedforward and thalamo-cortical (layer IV) information processing, which is sensible considering their importance in receiving sensory information from the outside world to layer IV. Because each cortical layer has a specific function, these data suggest that the function of each cortical area relates to the relative number of processing units (neurons) it has in each of the cortical layers. Since these relative numbers change in each cortical area during postnatal development, the function of these cortical areas presumably also changes accordingly.

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