The basic functional units of the neocortex are vertical cell columns that reach through all layers and have a diameter of 200 – 300 µm. Electrophysiological studies have shown that, in the cortical projection areas, each cell column is connected to a defined peripheral group of sensory cells. Stimula-tion of the peripheral field always yields a response from the entire column.
Fiber tracts connect the cortical columns with each other (D): the fibers of a column (D20) run either to columns of the ipsi-lateral hemisphere or via the corpus callosum to mostly symmetrically localized columns of the contralateral hemisphere . Branches of individual fibers terminate in different columns (D21). It is estimated that the neocortex is made up of 4 million columns.