Connections between the insular cortex and the amygdala were investigated with tritiated amino acid autoradiography and horseradish peroxidase histochemistry in the rhesus monkey. Our findings revealed widespread reciprocal connections between the insular cortex and almost all subnuclei of the amygdaloid complex. The posterior insula projects predominantly to the dorsal aspect of the lateral and to the central amygdaloid nuclei. In contrast, the anterior insula projects to the anterior amygdaloid area as well as the medial, the cortical, the accessory basal magnocellular, the medial basal and the lateral amygdaloid nuclei. Approximately 90% of the cells of origin of the insulo-amygdala projections are located in layers 2 and 3 with fewer labeled cells in layer 5. Amygdalo-insular projections arise predominantly from medial and anterior parts of the amygdala and reach mostly layers 2 and 5 of the insula. The results demonstrate that the insula and the amygdala have widespread reciprocal connections, which may explain their functional similarities.
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