The insula of Reil in man and monkey: Architectonics, connectivity, and function

During the fetal stages of primate development, the insula is situated on the surface of the cerebral hemisphere. However, in the more advanced primates, the adjacent neocortical areas develop much more extensively than the insula. This leads to massive frontal, parietal, and temporal opercularization and to the formation of the sylvian fissure within which the insula remains buried from the time of birth onwards. In subprimate mammalian species where the neocortex does not develop as extensively, the homolog of the insula remains exposed on the surface of the brain throughout adult life. Two considerations have guided our analysis of the insula. First, the insula is intimately related to adjacent cortical areas of the orbitofrontal and temporopolar regions. The structural and functional similarities among these three regions are so strong that it becomes highly desirable to consider the insula, not in isolation, but as one component of an insulo-orbito-temporopolar complex. Second, the insula as well as the adjacent orbitofrontal and temporopolar areas are characterized by a remarkable heterogeneity in cortical architecture, connectivity, and physiology. This heterogeneity which is a hallmark of paralimbic (mesocortical) parts of the brain, indicates neither a chaotic organization nor a haphazard distribution of behavioral specialization. Instead, evidence indicates that the insula, like other components of the paralimbic brain, acts as a multipurpose region of cortex where a remarkably wide range of neural processes modulate behaviors which primarily depend on interactions between the extrapersonal world and the internal milieu.