Cortical projections originating from the cat's insular area and remarks on claustrocortical connections.

The cortical projections originating in the cat's insular cortex and claustrum were investigated with the aid of the horseradish peroxidase retrograde tracing technique. Twenty small injections of horseradish peroxidase were distributed along lateral and medial regions of the hemisphere. Labeling in the insular cortex occurred following all injections except those six situated along the lateral gyrus--that is, within the visual cortex. In the claustrum labeled neurons were found following all injections, except following the injection situated in the posterior temporal area. Claustral labeling was frequently more intense than insular labeling. The injections into the occipital cortex that revealed no insular innervation nevertheless received a considerable number of claustral projections. As the insular cortex itself receives at most a minor projection from the claustrum the differing cortical projection patterns of insula and claustrum have to be considered unrelated. Our findings confirm the view that the claustrum projects to most regions of the cerebral cortex; these projections are at least in part topographically organized. A topographical pattern can also be constructed for the insular cortex, though it is less stringent than for the claustrocortical connections. Both the afferent and efferent connections of the insula show similarities to those of the prefrontal cortex. Nevertheless, the insula differs in that it receives strong input from the sensory associative nuclei of the thalamus. Consequently, and in line with behavioral observations following its ablation, we consider the insula as involved in the temporal structuring of perceived patterns.