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

We present a comprehensive analysis of the cortical connections of the insular and adjacent cortical areas in the domestic cat by using microinjections of wheat-germ agglutinin conjugated to horseradish peroxidase. We examined the identity and extent of the cortical fields connected to each area, the relative anatomical weights of the various connections, their laminar origin, and their paths across the cerebral commissures. Our main finding is that despite their relatively small size and close apposition, the connections of the insular and adjacent areas are far more widespread and more specific to each area than previously realized, suggesting that each area is involved in disparate aspects of cortical integration. The granular insular area is linked to a constellation of somatosensory, motor, premotor and prefrontal districts. The dysgranular insular area is chiefly associated with lateral prefrontal and premotor, lateral somatosensory and perirhinal cortices. The dorsal agranular insular area is connected with limbic neocortical fields, 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 prefrontal cortex, and with perirhinal area 36. The parainsular area is linked to non-tonotopic auditory and ventromedial frontal areas. Trajectories followed by the callosal axons of each of the investigated areas are extremely divergent. As a whole, the picture of the insular region that emerges from this and a parallel study (Clascá et al., J Comp Neurol 384:456-482, 1997) is that of an extreme heterogeneity, both in terms of histological architecture and neural connections. Comparison with earlier published reports on primates suggests that most, but not all, of the areas we investigated in cats may have an direct counterpart within the insula of Old World monkeys.