Jeewajee, A.A.; (2008) Computational analyses of the role of hippocampal oscillations in familiar and novel environments. Doctoral thesis , University of London.

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Abstract

The hippocampal formation is known to support several different types of spatial representation, and to play an important role in detecting environmental novelty. A striking aspect of hippocampal physiology is the theta rhythm, recorded in the electroencephalogram (EEG). This thesis investigates the role of the theta rhythm as an organising principle of the hippocampal formation, and in particular its role in the detection of novelty. In the first experimental chapter, the behavioural correlates of principal cells across several regions of the hippocampal formation - head direction and conjunctive place by direction cells in the presubiculum, place cells in CA1 and grid cells in the MEC - are studied, with an emphasis on the contribution of running speed. Subpopulations of cells, both positively and a minority which are negatively modulated by speed are found in each region. The second experimental chapter investigates the predictions of a recent model of entorhinal cortical grid cells. The model, based on the interference between somatic and dendritic oscillators, predicts that intrinsic firing frequency should exceed EEG theta frequency by a greater amount for small grids than for large grids and by a greater amount during fast running compared to slow. These relationships are confirmed in electrophysiological recordings in freely-moving rats. The third experimental chapter reports the results of a detailed examination of the rodent EEG under conditions of novelty and familiarity. The oscillatory interference model predicts a reduction in the theta frequency in a novel environmental context. This is substantiated by the data and reveals a new mechanism for signalling novelty in the brain. The fourth experimental chapter probes the specific roles played by the subiculum and CA1. There is debate as to whether the subiculum is an input as well as, traditionally considered, an output of the HF. I propose that subiculum is capable of informing CA1 and data is presented to show that subicular firing occurs earlier in the theta cycle and anticipates position further ahead than firing in CA1 does, and that this difference is modulated by familiarity with the environment.

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