A description of the amygdalo-hippocampal interconnections in the macaque monkey.

The interconnections between the amygdala and the hippocampal formation were investigated in the macaque monkey using anterograde tracers. The hippocampal inputs to the amygdala arose from the subicular and entorhinal cortices and passed through the angular bundle to terminate principally in the medial basal and lateral basal nuclei, with lighter termination in the lateral nucleus, the periamygdaloid cortex, and the cortical-transition area. The majority of these amygdaloid inputs arose from the rostral hippocampal formation although there was equivocal evidence of an additional projection from the caudal hippocampus to the central nucleus. Projections arose from many of the amygdaloid nuclei to terminate in the molecular layer of the amygdalo-hippocampal area and the adjacent stratum moleculare of the uncal portion of the hippocampus. The accessory basal, lateral basal, and medial basal nuclei also projected to the most rostral portions of the stratum moleculare of fields CA1-3, the heaviest termination occurring in field CA3. Additional projections from the basal nuclei terminated in the prosubiculum, presubiculum, and parasubiculum. The heaviest entorhinal inputs arose from the accessory basal and lateral nuclei and terminated in layers I, II, and III of areas 28b, 28i, and the prorhinal cortex. The major amygdaloid input to the perirhinal cortex arose from the lateral basal nucleus.