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The Speed Dependence of Hippocampal Neural Oscillations


Hippocampus plays important roles in episodic memory formation, learning and spatial navigation. In rodents, hippocampal theta and gamma rhythms become prominent during locomotion and disappear during immobility. Many studies have been done to understand their functional roles. However, how these rhythms transit between mobile and immobile states is largely unknown. The most intuitive way to address this problem is to look at their dynamics as a function of the running speed of the animal. This thesis is focused on the speed dependence of theta and gamma rhythm as well as their coupling. In chapter 2, I showed that hippocampal gamma rhythm can be further divided into two subbands whose amplitudes are differentially correlated with the running speed. This speed dependence of gamma amplitude was restricted to a narrow range of theta phases. The preferred theta phase for slow gamma showed a precession with speed while the preferred theta phase for fast gamma remains unchanged. These results demonstrate a novel influence of speed on the amplitude and timing of the hippocampal gamma rhythm which could contribute to learning of temporal sequences and navigation. In chapter 3, I found that LFP-theta modulation of spikes (TMoS) is the largest during immobility when theta is weak and irregular. This can be explained by the fact that the hippocampal gamma oscillations become stronger with running speed and take over the control of spike-timing while reducing theta modulation through transient phase-phase coupling. This transition leads to improved precision of spike timing within theta cycle at high speeds which could facilitate precise spike-timing based neural computations and learning. Finally, in chapter 4, I am trying to explain these novel speed dependence of theta and gamma as well as theta-gamma coupling. The simulation results suggest that there could be just one parameter that speed modulates in order to generate all these results. This hypothesis, although needs to be confirmed by the experiments, provides a really profound possibility on how the running speed of the animal may influence the neural oscillations and thus control the neural activities.

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