Network mechanisMS that represent novelty and familiarity in the medial Entorhinal Cortex (mEC)

Abstract

Grid cells in the medial entorhinal cortex (mEC) fire action potentials whenever the animal is positioned at the vertices of a tessellating hexagonal grid. Changes to the animal’s environment can alter grid orientation, break its symmetry along specific axes, and increase or decrease the spatial scale of the grid. In this work, we focus on a trans- formation of the spatial scale of the grid as a function of the novelty of the environment. The grid pattern formed by an individual grid cell shows visible changes in novel envi- ronments - its hexagonal symmetry is distorted, and there is an increase in the spatial scale of the entire grid. As the animal spends more time in its environment the spa- tial scale of the grid pattern contracts, and the ’gridness’, a measure of symmetry, gradually increases. We study this phenomenon using biophysically realistic neuronal networks. The neurons in our network are conductance-based neurons modeled as layer II stellate cells that are coupled via inhibitory interneurons. Previous experiments in brain slices have discovered Spike timing dependent plasticity (STDP) in the In- hibitory synpases of the mEC, which could potentially reshape the topology of grid cell networks present in this region. In our study, we demonstrate that changes in the topol- ogy of the neuronal network, brought about by STDP, in conjuction with the modulation of the theta rhythm from the medial septum can replicate the dynamics observed in the mEC as the animal becomes progressively familiar with its environment.