Structure of human auditory cortex. III. Statistical analysis of dendritic trees.

3-Dimensional coordinates of Golgi-impregnated neurons in left and right human areas TA, TB and TC were stored using computer microscopy. The tangential extent of neurons was taken to be an estimate of their effective radius, which is a compromise between their maximum radius and the average tangential projection of their dendrites. Analysis of variance and t-test comparisons were made among the areas and between the hemispheres on a total data base of 622 neurons. It was assumed that the neurons are organized in functional columns. The tangential extent of left-hemisphere columns is absolutely larger than on the right, but is smaller relative to the column-column interval. Neuropil on the left is packed more densely with dendrites belonging to the nearest column. It seems that during the course of evolution interconnected units (as in the right hemisphere) have become more disentangled (left hemisphere), yielding perhaps a greater capacity for differentiated responses in the latter case. A more detailed analysis revealed that much of the difference between areas and/or hemispheres was due to the pyramidal neurons. The structure of non-pyramidal cells is relatively consistent within each hemisphere and shows non-systematic differences between the hemispheres.