Entorhinal cortex of the monkey: VII. Intrinsic connections

The organization of intrinsic connections within the entorhinal cortex was investigated in Macaca fascicularis monkeys. Anterograde tracers (H-3-amino acids, Phaseolus vulgaris-leucoagglutinin, biotinylated dextran amine, or Fluoro-Ruby) were injected into the deep or superficial layers of the entorhinal cortex in 24 animals. These injections labeled extensive intrinsic projections that terminated throughout all layers of the entorhinal cortex. Labeling was typically continuous i.e., there was no evidence of a patchy or columnar organization. Each injection produced a rostrocaudally oriented band of labeled fibers and terminals that extended for one-third to one-half of the length of the entorhinal cortex. The more extensive distributions of labeled fibers were more typical of caudally placed injection sites. Taken together, the projections identified at least two mediolaterally differentiated bands: a lateral band that encompasses fields Elr, Elc, and the most lateral aspect of fields Ec and Ecl and a wider, medially situated band that encompasses much of fields Er, Ei, Ec, and Ecl. We obtained some evidence that field Eo constitutes a third, very medially placed band. The rostrocaudal organization of labeled fibers and the extent of labeling within the deep and superficial layers were unrelated to the laminar position of the injection. These data suggest that intrinsic associatonal connections in the monkey entorhinal cortex are organized into separate associational networks. Our findings are discussed with reference to the role of interlaminar connections in mediating physiological interactions between the neocortex and the hippocampus.