Fibers (axons) projected from neurons of the cerebral cortex include the following: (1) fibers projected from the cerebral cortex to gray matter outside the cerebral cortex, including the basal ganglia, thalamus, brain stem, and spinal cord (projection fibers); (2) fibers projected from the cerebral cortex to within the same side of the cerebral cortex (association fibers); and (3) fibers projected to the contralateral side of the cerebral cortex (commissural fibers). The commissural fibers connect the left and right hemispheres of the neocortex through the corpus callosum. The commissural fibers connecting the two cerebral hemispheres of the paleocortex project to the contralateral side through the anterior commissure or through the fornix commissure for the archicortex (hippocampus). The former two are a bundle of fibers that can be clearly observed on the brain surface, whereas the latter are rather difficult to identify macroscopically.
Various fibers project into the cerebral cortex, including those originating from the cerebral cortex as well as those from different gray matter such as the basal ganglia, thalamus, hypothalamus, and the basal nucleus of Meynert.
As described above, fiber connections that form networks that are broadly divided into the network between the two hemispheres of the cerebral cortex and the network outside the cerebral cortex. However, depending on where the axons are being projected, there are various patterns in which the axons are projected from neurons of one of the six cortical layers, and the neural layer that receives input from afferent fibers also varies.
In addition to the transverse structure of the cerebral cortex, it has been found that longitudinal columnar neural units perform a specific role. In other words, the notion that the columnar organization of several millimeters, including layers 1–6, performs a function has been proven on an animal experiment level. In the 1960s, Sperry conducted an experiment using monkeys and found that when a slit of several millimeters was perpendicularly created in the cerebral cortex, no functional damage incurred. For this discovery, he was subsequently awarded the Nobel Prize. Furthermore, on the grounds of this phenomenon, brain surgery is currently performed to block the seizure wave in intractable epilepsy by creating a longitudinal slit in the cerebral cortex. Using this method, the transverse spread of the epilepsy seizure wave is blocked, while preserving the columnar organization, which enables functional impairment, which would originally occur, to be avoided. This procedure is called multiple subpial transection, and it is a revolutionary technique that employs cerebral dissection and physiological findings.
Unlike the horizontal structure, columnar organization cannot be distinguished microscopically; however, the fact that it functionally exists should be kept in mind when examining specimens.