Abstract
It is now widely accepted that the entorhinal cortex (EC) plays a pivotal role in the processing of spatial information and episodic memory. The EC is segregated into two sub-regions, the medial EC (MEC) and the lateral EC (LEC) but a comprehensive understanding of their roles across multiple behavioral contexts remains unclear. Considering that it is still useful to investigate the impact of lesions of EC on behavior, we review the contribution of lesion approach to our knowledge of EC functions. We show that the MEC and LEC play different roles in the processing of spatial and non-spatial information. The MEC is necessary to the use of distal but not proximal landmarks during navigation and is crucial for path integration, in particular integration of linear movements. Consistent with predominant hypothesis, the LEC is important for combining the spatial and non-spatial aspects of the environment. However, object exploration studies suggest that the functional segregation between the MEC and the LEC is not as clearly delineated and is dependent on environmental and behavioral factors. Manipulation of environmental complexity and therefore of cognitive demand shows that the MEC and the LEC are not strictly necessary to the processing of spatial and non-spatial information. In addition we suggest that the involvement of these sub-regions can depend on the kind of behavior, i.e., navigation or exploration, exhibited by the animals. Thus, the MEC and the LEC work in a flexible manner to integrate the “what” and “where” information in episodic memory upstream the hippocampus.
Highlights
The segregation of the entorhinal cortex (EC) into two main sub-regions, the medial entorhinal cortex (MEC) and the lateral entorhinal cortex (LEC), is classically established in the rat (Krieg, 1946; Blackstad, 1956; Burwell and Amaral, 1998b; Sewards and Sewards, 2003; Kerr et al, 2007)
We review data on the effects of MEC and LEC lesions in spontaneous object exploration tasks, showing a large overlap between MEC and LEC functions. This led us to the conclusion that the MEC and the LEC have distinct functions but this functional dissociation can be modulated by the behavior and/or cognitive demand. Different behaviors such as goal-directed navigation and exploration may result in different involvement of the MEC and LEC for the processing of distal landmarks and proximal landmarks
Considering the allothetic system, a distinction based only on the MEC and the LEC is not sufficient to account for the behavioral effects observed after lesioning either one or the other structure
Summary
The segregation of the entorhinal cortex (EC) into two main sub-regions, the medial entorhinal cortex (MEC) and the lateral entorhinal cortex (LEC), is classically established in the rat (Krieg, 1946; Blackstad, 1956; Burwell and Amaral, 1998b; Sewards and Sewards, 2003; Kerr et al, 2007). The hypothesis of a functional interaction between the two sub-regions comes from lesion (e.g., Hunsaker et al, 2013; Van Cauter et al, 2013; Wilson et al, 2013; Chao et al, 2016), imaging (Beer et al, 2013), and electrophysiological (Deshmukh and Knierim, 2011; Tsao et al, 2013) studies Another aspect that it is worth mentioning is that both the MEC and the LEC only partly contribute to hippocampal place cells activity. LEC lesions abolish hippocampal rate remapping following changes in the shape or the color of the environment suggesting an interaction between spatial and non-spatial processing (Lu et al, 2013) Together these studies suggest that the hippocampal and the MEC place coding systems are relatively independent, and that both the MEC and the LEC are able to influence the hippocampal place cell system. Rats are consistently impaired in both idiothetic and allothetic navigation following MEC lesions, and consistently non impaired following LEC lesions
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