Ipsilateral connections of the anterior cingulate cortex with the frontal and medial temporal cortices in the macaque monkey.

The present study was attempted to study ipsilateral corticocortical connections of the anterior part (area 24) of the cingulate cortex of the macaque monkey by means of wheat germ agglutinin-conjugated peroxidase (WGA-HRP) method. In 2 out of 4 Japanese monkeys (Macaca fuscata) that were injected with WGA-HRP into the anterior part of the cingulate cortex, the sites of injection were successfully localized within the cortical regions corresponding to areas 24a and 24b. The results obtained from these monkeys indicate that areas 24a and 24b in the anterior part of the cingulate cortex are reciprocally connected with the prefrontal, premotor, and motor cortical regions, and also with the medial temporal cortical regions. Areas 24a and 24b were strongly connected with the lateral and medial prefrontal cortices and area 6a beta of the premotor cortex, moderately with the remaining premotor cortex, and weakly with the motor cortex. In the medial temporal cortex, areas 24a and 24b were strongly connected with the prosubiculum, entorhinal cortex (area 28), and perirhinal cortex (areas 35 and 36), and weakly with areas TF and TH of the parahippocampal gyrus, throughout their rostrocaudal extent. In addition, areas 24a and 24b projected to the molecular layer of the CAI subfield of Ammon's horn and the external pyramidal layer of the presubiculum. Our findings suggest that areas 24a and 24b of the anterior cingulate cortex may constitute relays in the reciprocal pathways between the prefrontal cortex and the hippocampal, entorhinal and/or perirhinal cortical regions.