Constructing the human cerebral cortex during infancy and childhood: Typesand numbers of cortical columns and numbers of neurons in such columns at different age-points

This study examines JL Conel's data on neuron numbers in 35 human cortical areas for right age paints from 0 (birth) to 72 months, to analyze cortical columns, the presumed functional units of the cortex. For each cortical ar ea at each age point, cortical surface divided by the square root of the ar ea's neuron number gives cross-sectional areas with radii ranging from 180 mu m at birth to 250 mu m at 72 months. For the prefrontal carter at birth and 48 months, these radii are approximately 2.10 and 1.19 times the longes t radial basal dendrites, suggesting similar dimensions between these two m easures of column radius. The logarithm of neuron number per cortical area and age point was examined in relation to the Weber-Fechner law governing t he relationship between stimulus intensity and perception. A mechanism for this law consistent with the cortical model of Douglas et nl. illustrates t he importance of local circuit neurons. The cross-sectional areas of hexago nal columns for prefrontal cortex, using as radius, the longest radial exte nt of layer 5 pyramidal neuron basal dendrites, ranging from 0.013 mm(2) at birth to 0.064 mm(2) at 48 months, suggests that functional cortical colum ns increase cross-sectional area during development. These cross-sectional areas are 55-100-fold larger at birth, and 229-277-fold larger at 48 months , than those computed from somal width in prefrontal, layer 5 pyramidal neu rons. Comparison of radial extent of pyramidal basal dendrites to their som a-to-soma distances shows that layer 3 pyramidal basal dendrites reach 1.5 and 4.0 other pyramidal neurons at 15 and 60 months, respectively, while la yer 5, extra-large pyramidal basal dendrites reach 1.14 and 1.72 other such neurons at birth and 48 months, respectively. Lf such a relationship holds for other cortical areas, then the Conel data can be used to estimate basa l dendrite extent, for which there currently is a paucity of data.