INSULA OF THE OLD-WORLD MONKEY .1. ARCHITECTONICS IN THE INSULO-ORBITO-TEMPORAL COMPONENT OF THE PARALIMBIC BRAIN

The insula of the rhesus monkey has a surface area of .apprx. 160 mm2 and can be divided into 3 architectonic sectors. The agranular sector is coextensive with prepiriform allocortex and is characterized by 3 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 L[lumbar]4 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 3rd 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 3 brain regions are considered jointly, they are 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 2nd group of paralimbic areas with a hippocampal-induseal focus. In the most general sense, the functional specializations 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.