Insula of the old world monkey. II: Afferent cortical input and comments on the claustrum

The afferent connections of the insula in the rhesus monkey were studied with axonal transport methods. Injections of horseradish peroxidase (HRP) in the insula revealed labeled neurons in the prefrontal cortex, the lateral orbital region, the frontoparietal operculum, the cingulate gyrus and adjacent medial cortex, the prepiriform olfactory cortex, the temporal pole, the cortex of the superior temporal sulcus, the rhinal cortex, the supratemporal plane, and the posterior parietal lobe. Tritiated amino acid (TAA) injections in some of the cortical regions which contained retrogradely labeled neurons confirmed projections to the insula from prefrontal granular cortex, orbital frontal cortex, prepiriform cortex, temporal pole, rhinal cortex, cingulate gyrus, frontal operculum, and parietal cortex. In these studies, cortical areas that projected to the insula also projected to the claustrum. However, the topographic and quantitative relationships between the projections into the insula and those into the claustrum were inconsistent. Moreover, the claustrum has additional connections which it does not share with the insula. A selected review of the literature suggests that the claustrum and insula differ widely also with respect to ontogenesis and functional specialization.