Cortical connections of the insular and adjacent parieto-temporal fields in the cat

We present a comprehensive analysis of the cortical connections of the insu lar and adjacent cortical areas in the domestic cat by using microinjection s of wheat-germ agglutinin conjugated to horseradish peroxidase. We examine d the identity and extent of the cortical fields connected to each area, th e relative anatomical weights of the various connections, their laminar ori gin, and their paths across the cerebral commissures, Our main finding is t hat despite their relatively small size and close apposition, the connectio ns of the insular and adjacent areas are far more widespread and more speci fic to each area than previously realized, suggesting that each area is inv olved in disparate aspects of cortical integration. The granular insular ar ea is linked to a constellation of somatosensory, motor, premotor and prefr ontal districts. The dysgranular insular area is chiefly associated with la teral prefrontal and promotor, lateral somatosensory and perirhinal cortice s. The dorsal agranular insular area is connected with limbic neocortical f ields, while the ventral agranular insular area is associated with an array of olfactory allocortical fields. The anterior sylvian area is associated with visual, auditory and multimodal areas, with the dorsolateral prefronta l cortex, and with perirhinal area 36. The parainsular area is linked to no n-tonotopic auditory and ventromedial frontal areas. Trajectories followed by the callosal axons of each of the investigated areas are extremely diver gent. As a whole, the picture of the insular region that emerges from this and a parallel study (Clasca et al., J Comp Neurol 384:456-482, 1997) is th at of an extreme heterogeneity, both in terms of histological architecture and neural connections. Comparison with earlier published reports on primat es suggests that most, but not all, of the areas we investigated in cats ma y have an direct counterpart within the insula of Old World monkeys.