The basic functional units of the neocortex are vertical nerve cell columns that reach through all six layers of the cerebral cortex and have a diameter between 200 and 300 µm (micrometer). Electrophysiological studies have shown that, in the cortical projection areas, each cell column is connected to a defined peripheral group of sensory cells. Stimulation 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 ipsilateral hemisphere (association fibers, see p. 260) or via the corpus callosum to mostly symmetrically localized columns of the contralateral hemisphere (commissural fibers, see p. 260). Branches of individual fibers terminate in different columns (D21). It is estimated that the neocortex is made up of 4 million columns.