[Neural mechanism of hippocampal memory organized by theta rhythm]

The firing of hippocampal principal cells in freely running rats exhibits a progressive phase retardation as the animal passes through a cell's "place" field. It is called "phase precession" and is of interest as a possible neural mechanism of the hippocampus dependent memory. We analyze the complexity of phase-position distributions of spikes by estimating the probability density functions. Although these distributions vary among different cells and regions, this complexity is well described by a superposition of two normal distribution functions, suggesting that the firing behavior consists of two components. In one, the firing phase shifts over a range of about 180 degrees. The functional relevance is further elucidated by analyzing the experience-dependent change of spike distribution. These results suggest that the temporal relation among place cells endows the selective LTP for the memory of temporal sequence. It provides a fundamental neural mechanism of episodic memory.