The new finding of Witter group that proved the prevailing parallel model of the organization of Parahippocampal Region (PHR), which consist of the “What” and “Where” pathway, was partially incorrect (Doan, et al., 2019). Instead of having these two parallel streams, Lateral Entorhinal Cortex (LEC) is now considered to be the hub which integrates both Postrhinal Cortex (POR) and Perirhinal Cortex (PER) efferent information (Ranganath and Ritchey, 2012; Doan, et al., 2019; Nilssen, et al., 2019).
The aim of this study was to resolve the ‘topological debate’ concerning the terminal distribution of hippocampal projections originating in PER and POR, by analyzing the existing anterograde (AG) tracing experiments used in previous research (Naber et al., 1997; Koganezawa et al., 2015; Doan, et al., 2019) and some added new retrograde (RG) tracer experiments.
For the POR, it is concluded that the projections to the border area between Cornu Ammonis 1 and subiculum (CA1/Sub) mainly arise from the caudolateral area with a preferred origin in the deeper layers, though superficial layers do seem to contribute. Regarding the POR projections preference along the dorsoventral axis of Hippocampal formation (HF), their projection to Entorhinal Cortex (EC) can act as an indicator. If there is no POR projection preference to both Medial Entorhinal Cortex (MEC) and LEC, then no projection preference along the dorsoventral axis will be spotted. If there is a projection preference to ventromedial MEC, then a projection preference to CA1/ Ventral Subiculum (VS) will be evident. A preferred projection to dorsal lateral LEC will be a sign for a POR projection preference for CA1/Dorsal Subiculum (DS).
For the PER samples, it was confirmed that only Caudal Area 35 (A35) will give rise to a CA1/Sub projection. I proposed that there may be a topographical organization for the projection from the Caudal PER A35 to CA1/Sub where the rostro-caudal axis of origin maps onto a ventral-to-dorsal distribution of the projection. However, my RG results which showed that no PER projection to CA1/DS, disagreed with the proposed idea. Further research with more AG PER samples is needed to clarify this issue.