Entorhinal cortex and cognition

https://doi.org/10.1016/j.pnpbp.2009.03.038Get rights and content

Abstract

Understanding the function of the entorhinal cortex (EC) has been an important subject over the years, not least because of its cortical intermediary to and from the hippocampus proper, and because of electrophysiological advances which have started to reveal the physiology in behaving animals. Clearly, a lot more needs to be done but is clear to date that EC is not merely a throughput station providing all hippocampal subfields with sensory information, but that processing within EC contributes significantly to attention, conditioning, event and spatial cognition possibly by compressing representations that overlap in time. These are transmitted to the hippocampus, where they are differentiated again and returned to EC. Preliminary evidence for such a role, but also their possible pitfalls are summarised.

Introduction

Since the historical description of the case H.M. 50 years ago (Scoville and Milner, 1957), a tremendous effort has been dedicated to the study of the medial temporal lobe and its role in learning and memory, in human as well as in non human primates and rodents. The involvement of this area in episodic memory has been convincingly demonstrated, and the unravelling of the underlying mechanisms is rapidly progressing (e.g. Eichenbaum, 2004). The brain structures composing the medial temporal lobe, that is the hippocampus (Ammon's horn and dentate gyrus), the entorhinal and perirhinal cortices are heavily interconnected and their respective contribution to memory is difficult to disentangle. In particular, the Entorhinal Cortex (EC) has long been viewed as a simple interface between cortical regions and the hippocampus, considering that the alteration of EC or hippocampus is functionally equivalent. As illustrated below, this is indeed often the case, indicating that both structures belong to the same processing chain. However, anatomical data demonstrate that the hippocampus and the EC have distinct, although partially overlapping, domains of connectivity. Damage to these brain areas may therefore induce distinct functional and cognitive brain states. Numerous reviews concerning the role of the hippocampus in cognition have been published (e.g. Atallah et al., 2004, Eichenbaum, 2004, Squire, 1992). The present article, based on rodent studies, aims at illustrating cognitive processes which may depend on the EC and downstream areas other than the hippocampus. Various human brain diseases involve pathologies of the EC and are characterised by both common and distinctive cognitive alterations. Improving our knowledge of the cognitive consequences of EC damage in animals may therefore be useful for the understanding of these diseases.

Section snippets

The EC in diseased human brain

In the human brain, the EC (Brodmann areas 28 and 34) is located within the ventromedial portion of the temporal lobe, caudal to prepiriform and piriform cortices, and rostral to the parasubiculum. The EC reliably appears as a shiny verrucuous surface located below the tentorial notch. At this level, the brain is unprotected by dura from this indentation of the tentorium cerebelli, and it was suggested that the EC may suffer mechanical damage in case of traumas or increased intracranial

Anatomy of the EC in rodents

The rodent entorhinal cortex is located at the caudal end of the temporal lobe. It is usually divided into a medial (MEC) and a lateral (LEC) regions. The entorhinal cortex is intermediate in structure between the six-layered neocortex and the two-layered allocortex as a distinguishing characteristic of the EC is the presence of the lamina dissecans which is characterised by the lack of cell bodies where layer IV should be. For this reason, the EC is sometimes classified as a periallocortex.

The

Cognitive deficits in EC-lesioned animals

A basic description of the procedures used for the assessment of cognitive processes mediated by the hippocampal formation is shown in Table 1.

Acknowledgement

The authors wish to thank Dr. Alain Marchand for his careful reading of the manuscript.

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