Organization of the reciprocal connections between the subiculum and the entorhinal cortex in the cat: II. An electrophysiological study.

The projection of the entorhinal cortex (EC) to the subiculum (SUB) and the projection in the opposite direction from the SUB to the EC have been studied in the cat by using electrophysiological methods. Field potentials elicited by EC stimulation were recorded in the SUB. Different topographic distributions of the amplitude of the average evoked potentials (AEPs) were obtained depending on the place of stimulation along a lateromedial axis in the EC. Stimulation of the lateral EC elicited the largest AEPs in the septal part of the SUB whereas stimulation of the medial EC evoked maximal responses in the temporal part of the SUB. Field potentials evoked by subicular stimulation were measured in the EC; the topographic distribution of the AEPs was dependent on the place of stimulation along the subicular septotemporal axis. Septal SUB stimulation elicited the largest field potentials in the laterocaudal part of the EC whereas temporal SUB stimulation evoked maximal responses in the mediocaudal part of the EC. These findings indicate that a topographical organization exists in the EC-SUB connections and that these connections are reciprocal. Unit activity both orthodromically and antidromically elicited by SUB stimulation was recorded in the EC. Mean conduction velocities were estimated and found to vary around 1 m/second-1. The physiological evidence is discussed along with the anatomical data reported in the accompanying paper.