The Progression of the Cortical Lesions Mimics the Pattern of Myelination in Reverse Order

The primary fields of the neocortex are the point of departure of neocortical evolution and, as such, they command the most space in the cerebral cortex of lower mammals and early primates. Next come the less expansive and less refined first order sensory association and premotor areas. Least spread out are the most elementarily organized highorder sensory association and prefrontal areas. Further along the primate evolutionary scale, the simply organized high order association areas expand relative to that of the primary fields. With growing distance from the primary fields and increasing proximity to the allocortex, the association areas of the neocortex and anterior mesocortex exhibit greater features of structural immaturity (Braak and Braak 1996).

Myelination represents the final step in brain maturation. Functional maturity of projection neurons usually is achieved only after myelination of the axons is completed. In the human brain, myelination of the neocortex is a late-onset and particularly prolonged process that follows a predetermined sequence (van der Knapp et al. 1991; van der Knaap and Valk 1995; Hasegawa et al. 1992; Nieuwenhuys 1999). It commences in the neocortical primary fields and continues via the first order sensory association areas and premotor fields into high order sensory association areas and prefrontal fields, eventually reaching the mesocortex. Exceptionally dense myelina-tion of the primary fields is the end-result in the human adult and, on the average, myelin density declines with increasing distance from the primary areas. Accordingly, the anteromedial temporal mesocortex is very sparsely myelinated.