Amygdalopetal projections in the cat. I. Cortical afferent connections. A study with retrograde and anterograde tracing techniques.

The cortical afferent connections of the amygdaloid complex of the cat have been studied by means of retrograde tracing of horseradish peroxidase and the fluorescent substances bisbenzimid and nuclear yellow. Subsequently, anterograde tracing experiments were carried out in order to define more precisely the termination areas of the corticoamygdaloid fibers. The results of the present study indicate that the main and accessory olfactory bulbs, the anterior olfactory nucleus, the prepiriform cortex and discrete regions of the medial frontal lobe, the insular and temporal cortices, as well as the perirhinal and entorhinal cortices and the ventral subiculum project to the amygdaloid complex. The main termination sites of these projections are the central, basolateral, and lateral amygdaloid nuclei. Neocortical regions project to the lateral nucleus and the lateral division of the lateral central nucleus. The mesocortical regions project predominantly to the basolateral nucleus and a medial division of the lateral central nucleus. In addition, area 35 distributes fibers to the lateral nucleus and the entorhinal cortex projects to the cortical nuclei of the amygdaloid complex. Fibers from the infralimbic area only reach the region of the medial central nucleus. Of the allocortical regions the prepiriform cortex distributes its fibers to the lateral, basolateral, and cortical nuclei, whereas the ventral subiculum projects to the medial division of the lateral central nucleus and the cortical nuclei. In the neocortical and most of the mesocortical regions the cells which project to the lateral and basolateral amygdaloid nuclei lie in layer III, whereas the cells which project to the central nucleus are located in layer V.