Insula of the Old World Monkey. I. Architectonics in the Insulo-orbito-temporal Component of the Paralimbic Brain

The insula of the rhesus monkey has a surface area of approximately 160 mm2 and can be divided into three architectonic sectors. The agranular sector is coextensive with prepiriform allocortex and is characterized by three agranular cellular strata, a zonal layer of myelinated fibers, and a high level of intracortical acetylcholinesterase (AChE). The dysgranular sector adjoins the agranular sector and shows first the emergence of a granular L4 and then a gradual differentiation of L2. Cortical myelin is low and mostly within deep layers; the AChE level is less than in the agranular sector. The third and granular sector covers the posterior aspect of the insula and contains granular L4 and L2, incipient sublamination of L3, increased cortical myelin with an emergent outer line of Baillarger, and a very low density of AChE. These observations indicate that AChE histochemistry can be used for the architectonic analysis of cortex. The lateral orbital cortex and the temporal pole can also be subdivided into agranular, dysgranular, and granular regions. In the insula as well as in lateral orbital and temporopolar areas, the agranular sector is directly contiguous with prepiriform cortex. When these three brain regions are considered jointly, they are seen to be organized in the form of increasingly more differentiated agranular, dysgranular, granular, and hypergranular sectors arranged concentrically around prepiriform allocortex. The term paralimbic is suggested as a generic term for all regions where such transitions occur from allocortex to granular isocortex. The insula, lateral orbital surface, and temporal pole are paralimbic areas with an olfactory allocortical focus. The parahippocampal, retrosplenial, cingulate, and subcallosal regions constitute a second group of paralimbic areas with a hippocampal-induseal focus. In the most general sense, the functional specilizations of paralimbic areas are predominantly for behaviors which require an integration between extrapersonal stimuli and the internal milieu. The human insula has a plan of organization virtually identical to that in the rhesus monkey. In the human, the insulo-orbito-temporopolar component of the paralimbic brain may become involved in conditions which range from epilepsy to psychosomatic disease.