DescriptionThe focus of my thesis was the assessment of the presence and dynamics of non-specific, putatively homeostatic, as well as specific memory-related, modifications in the structure of hippocampal firing patterns during sleep. As a step toward understanding sleep function, I developed an appropriate, open-source Matlab-based application for the visualization, annotation and detailed sleep scoring of long-time scale electrophysiological data relevant to brain state dynamics. In my study of putatively homeostatic changes in excitability I found that the overall firing rates of hippocampal CA1 neurons decreased across sleep concurrent with an increased recruitment of neuronal spiking to brief ‘ripple’ episodes, resulting in a net increase in neural synchrony. Unexpectedly, within non-REM episodes overall firing rates were found to gradually increase together with a decrease in recruitment of spiking to ripples. The rate increase within non-REM rate episodes was counteracted by a larger rapid decrease of discharge frequency during the interleaving REM episodes. Both the decreasing firing rates and the increasing synchrony during the course of sleep were correlated with the power of theta activity during REM episodes. These findings suggest a prominent role of REM sleep in sleep-related neuronal plasticity. Lastly, in order to gauge the interaction between non-specific and memory-specific contributions to sleep-related firing dynamics, I recorded hours of sleep in rats before and after exposure to a completely novel maze environment. While both replay and 'pre-play' were observed, both were found to be dominated by non-local (different silicon-probe shank) interactions. However, while replay was observed in both pair-wise and higher-order interactions, pre-play was surprisingly specifically restricted to higher-order sequential interactions. This analysis also included the assessment of several traditional as well as novel methods for measuring replay, leading to significant methodological insights into their sensitivity to the established non-stationary nature of excitability in sleep, and to several suggestions for future work.