Projections from the amygdala to basoventral and mediodorsal prefrontal regions in the rhesus monkey

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Abstract
The sources of ipsilateral projections from the amygdala to basoventral and mediodorsal prefrontal cortices were studied with retrograde tracers (horseradish peroxidase or fluorescent dyes) in 13 rhesus monkeys. The basoventral regions injected with tracers included the orbital periallocortex and proisocortex, orbital areas 13,11, and 12, lateral area 12, and ventral area 46. The mediodorsal regions included portions of medial areas 25, 32, 14, and dorsal area 8. The above sites represent areas within two architectonic series of cortices referred to as basoventral mediodorsal on the basis of their anatomic location. Each series consists of areas that show a gradual increase in the number of layers and their delineation in a direction from the caudal orbital and medial limbic cortices, which have an incipient laminar organization, towards the eulaminated periarcuate cortices (Barbas and Pandya, J. Comp. Neurol. 286:353-375, '89). Labeled neurons projecting to the prefrontal cortex were found in the basolateral, basomedial (also known as accessory basal), lateral, and ventral cortical nuclei, and in the anterior amygdaloid and amygdalopiriform areas. The distribution of labeled neurons differed both quantitatively and qualitatively depending on whether the injection sites were in basoventral or mediodorsal prefrontal cortices. Cases with caudal orbital injections had the most labeled neurons in the amygdala, followed by cases with injections in cortices situated medioventrally. The latter received a high proportion of their amygdaloid projections from the basomedial nucleus. The lateral amygdaloid nucleus sent a robust projection to the least architectonically differentiated orbital periallocortex, and a weaker projection to the adjoining orbital proisocortical regions, but did not appear to project to either medial proisocortical sites to the more differentiated ventrolateral or dorsolateral prefrontal cortices. In addition, there were topographical differences in the origin of projections from one amygdaloid nucleus directed to various prefrontal cortices. These differences were correlated either with the destination of the axons of afferent amygdaloid neurons to basoventral or to mediodorsal prefrontal cortices and/or with their projection to areas with varying degrees of laminar organization within the basoventral or mediodorsal sector. The clearest topography was observed for projections originating in the basolateral nucleus. The results indicate that the least architectonically differentiated basal sites situated in the caudal orbitofrontal region, followed by the comparable medial areas situated ventrally on the medial surface, received the strongest and most widespread projections from the amygdala. Medial proisocortices situated more dorsally and caudally received only a few projections from the amygdala. In addition, areas with a high degree of laminar organization (caudal areas 46 and 8) within both basoventral and mediodorsal regions received few and topographically restricted amygdaloid projections.