Convergent Projections from Perirhinal and Postrhinal Cortices Suggest a Multisensory Nature of Lateral, but Not Medial, Entorhinal Cortex

The current model of the organization of the medial temporal lobe (MTL) episodic memory system assumes that two functionally different "where" and "what" pathways enter MTL as parallel parahippocampal cortex (PHC)-medial entorhinal cortex (MEC) and perirhinal cortex (PER)-lateral entorhinal cortex (LEC) streams, respectively. With the use of tract tracing and in vitro electrophysiological recordings, we show that, in the rat LEC, all main principal neuron types in layer II receive convergent inputs from PER and postrhinal cortex (POR), homologous to PHC in primates. Projections to MEC from POR are much less prominent than previously assumed. These findings thus challenge the prevailing concept that LEC and MEC are defined by different inputs from the PER and PHC/POR, respectively. Our findings point to LEC as the main parahippocampal multimodal integrative structure whose unique set of external sensory-derived inputs allows its network to represent a continuously fluctuating extrinsic environment.