The role of entorhinal cortex in processing environmental features

• Main Author: Kuruvilla, Maneesh
• Other Authors: Ainge, Jamie ; O'Connor, Akira
• Published: University of St Andrews 2018
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.750127

The medial and lateral entorhinal cortices (MEC; LEC) are widely considered to provide spatial and non-spatial information respectively in the medial temporal lobe, particularly to the hippocampus. However, there is growing evidence to suggest that the functional differences between these two regions are not as clearly delineated as previously thought. This thesis aimed to assess if the two entorhinal sub-regions demonstrated a subtlety in encoding environmental features - landmarks and geometry – that are non-spatial in nature and yet provide an organism with relevant spatial information. In the first experiment, it was hypothesised that varying the scale of landmarks would result in the MEC and LEC encoding global and local spatial frameworks respectively. LEC lesioned animals showed deficits specifically in using a local spatial framework on a spatial memory task. This deficit was not related to processing individual local landmarks but rather the arrangement of cues as a whole. However, MEC lesions did not impair animals on successfully using either spatial framework. Experiment 2 attempted to glean an understanding of the MEC's role in processing environmental features, by investigating a condition where trapezoid geometry has been shown to play a role in distorting spatially modulated MEC grid cells. Rats trained on a distance estimation task demonstrated an overestimation and underestimation of distance when moving towards the converging and diverging ends of a trapezoid respectively, in line with predictions of grid cell function as a distance metric. Experiment 3 replicated the overestimation of distance with humans in a real-world trapezoid arena. Through lesion and behavioural results, this thesis finds evidence for the processing of local environmental features within entorhinal cortex. The LEC appears to use local landmarks to generate spatial frameworks while the MEC, via grid cells, encodes geometric information with implications for how organisms estimate distance in their environments.