The periallocortex is formed at transition areas where any of the other two subtypes of allocortex borders with the neocortex (which is also called isocortex).
One subtype is called peripaleocortex, which is formed at borders between paleocortex and neocortex.
Areas considered to belong to peripaleocortex are for example anterior insular cortex.
[3] No one allocortex or even periallocortex area borders, contacts or transitions immediately to the so-called true isocortex.
Thus, at borders between isocortex and allocortex there are two transitional areas, one from allocortex side and histologically more resembling allocortex, so it is called periallocortex, and another from isocortex side, histologically more resembling true isocortex, so it is called proisocortex.