Dual pathways connecting the dorsolateral prefrontal cortex with the hippocampal formation and parahippocampal cortex in the rhesus monkey.

Anterograde and retrograde tracing methods including autoradiography, horseradish peroxidase histochemistry and fluorescent dye transport were used to demonstrate that the dorsolateral prefrontal cortex is connected with the hippocampal formation and associated cortical regions by two distinct pathways. Fibers forming a lateral pathway travel in the fronto-occipital fasciculus and connect the dorsolateral prefrontal cortex with the fundus of the rhinal sulcus, posterior subdivisions of the parahippocampal gyrus, and the presubiculum. A larger medial pathway forms in the cingulum bundle and terminates in the most caudal part of the presubiculum, as well as in adjacent transitional cortices. These cortices form a caudomedial promontory that is located between the posterior cingulate and prestriate areas. In all allo- and mesocortical targets of prefrontal cortex, labeled terminals form banding patterns reminiscent of the columnar organization of afferent fiber columns in neocortex. The same cytoarchitectonic areas that receive prefrontal afferents issue reciprocal projections. The largest source is the caudomedial lobule including its presubicular portion. Neurons in the parahippocampal gyrus and adjacent presubiculum also are retrogradely labeled following implants of horseradish peroxidase or injection of fluorescent dyes into prefrontal cortex. In addition, subicular neurons project to the prefrontal cortex although the subiculum does not appear to receive prefrontal afferent input. These findings emphasize that multiple channels of communication link the dorsolateral prefrontal cortex and the hippocampus via the parahippocampal gyrus, subiculum, presubiculum and adjacent transitional cortices. We speculate that each of these prefrontal projections may carry highly specific information into the hippocampus, whereas the reciprocal projections may allow retrieval by prefrontal cortex of memories stored in the hippocampus.