RTG 2175 Perception in Context and its neural Basis
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Dr. Michaela Poth

Defense in July 2020

PhD Thesis

Spatial Representation in the Mammalian Brain - Field-to-Field Variability of Grid Cells

Cognitive maps are neuronal representations of the world. These are required for animals to efficiently navigate. Grid cells in the medial entorhinal cortex are heavily involved in forming a basis of such cognitive maps. They are active at multiple locations of the environment and these locations form an imaginary hexagonal grid tessellating the explored space.
While firing fields in two-dimensional environments are regu- larly spaced, grid cells seem to respond different for movements along a linear track, a quasi one-dimensional environment. On such a linear track, they show multiple firing fields which are not periodically arranged and whose shape and position change when the running direction is reversed. In both, one- and two- dimensional environments, the firing rates of a grid cell vary widely from field to field.
In this thesis, we investigate possible reasons that lead to the field-to-field variability of grid cell recordings in d and d.
The research is presented in the from of two articles; one accepted paper and one manuscript. Both articles are included as single chapters preceded by a brief summary, each. The following sections give a short overview of the thesis.
The introduction provides a recapitulation of animals’ spatial behavior leading to the assumption that they have a cognitive map. We review a few of the most important observations about spatial representations in the brain. To this end, we describe the anatomical organisation of the hippocampal formation, a brain region containing most of the cells involved in spatial navigation and representation.
In the published paper, the field-to-field variability of the grid- cell activity along a linear track is studied. For each running direction, firing fields turn out to be compatible with a slice through a two-dimensional ( D) hexagonal pattern. We show that a single hexagonal pattern can explain the one-dimensional data if a translational shift is allowed at the movement turning point.
In the manuscript, a possible role of the burst activity for the field-to-field variability in two-dimensional environments is investigated (chapter ). We show that burst activity plays no role for this variability or for rate remapping. Furthermore, we demonstrate that theta-phase coding is preserved but we do not observe differences between the first and second half of the theta cycle.

Publications

Pröll M, Häusler S, Herz AVM. "Grid-cell activity on linear tracks indicates purely translational remapping of D firing patterns at movement turning points." In: Journal of Neuroscience, accepted June 2020.

Poth M and Herz AVM "Burst activity plays no role for the field-to-field variability and rate remapping of grid cells." bioRxiv preprint https://doi.org/10.1101/2020.03.28.013318