The temporal and bilateral structure of hippocampal replay

Abstract

The hippocampus is required for the formation, but not storage, of long-term episodic memories. During memory formation, however, the hippocampus is not a lone actor; rather it works in concert with various structures across the brain. The mechanisms by which diverse populations of cells are coordinated for the formation of a single, coherent memory remain unknown. This thesis is an investigation of the temporal and hemispheric structure of replay events. The timing of replay is investigated at the levels of hippocampal sharp-wave ripples and multi-unit activity. We found that, during sleep, ripples generation is modulated by a 10-15Hz rhythm. We also observed this rhythm in the multi-unit firing rate of hippocampal neurons. Next we investigated and quantified the level of coordination between the hippocampal during replay events. Using bilateral hippocampal recordings from several rats during spatial navigation and subsequent sleep epochs, we directly compared the activity of these two spatially isolated networks at the level of the local field potential and the information encoded by the two neuronal populations. We found that the neural activity of the two hippocampi was highly correlated in some aspects but not others. As previously reported in the mouse, we found that, in the rat, sharp-wave ripples were simultaneously generated spontaneously in both hippocampi and that, although the intrinsic frequencies of ripple oscillations were correlated bilaterally, the phases of the individual ripple wavelets were not. Finally, we found that information encoded by both hippocampal ensembles is highly correlated during replay events.