Modular architectonic parcellation of the insula in the macaque monkey

We examined the anatomical organization of the insular cortex of the cynomolgus macaque monkey in serial coronal sections stained alternately with Nissl and Gallyas (myelin) techniques, supplemented in some cases with immunohistochemical staining techniques. We observed the same 23 distinct cytoarchitectonic areas in 12 brains. Within the granular, dysgranular, and agranular regions described in prior studies, we identified 4, 11, and 8 distinct areas, respectively. Across brains, these areas have consistent architectonic characteristics, and in flat map reconstructions they display a consistent topological or neighborhood arrangement, despite variations in the size of individual areas between cases. The borders between areas are generally sharply defined, yet particular borders can have a transitional appearance (in part because they are obliquely oriented in coronal sections), which helps explain the more vague conclusions made in earlier studies. The presence of a distinct granular area that straddles the fundus of the superior limiting sulcus over its entire posterior-to-anterior extent is consistent with the available evidence on interoceptive thalamo-cortical projections, and also with the tensile anchor theory of species-specific cortical gyrification. These observations are consonant with the homeostatic afferent model of processing in the primate insula, and they suggest that discrete modules within insular cortex provide the basis for its polymodal integration of all salient activity relevant for ongoing emotional behavior