Position reconstruction from an ensemble of hippocampal place cells: Contribution of theta phase coding

Previous analysis of the firing of individual rat hippocampal place cells h as shown that their firing rate increases when they enter a place field and that their phase of firing relative to the ongoing theta oscillation (7-12 Hz) varies systematically as the rat traverses the place held, a phenomeno n termed the theta phase precession. To study the relative contribution of phased-coded and rate-coded information, we reconstructed the animal's posi tion on a linear track using spikes recorded simultaneously from 38 hippoca mpal neurons. Two previous studies of this kind found no evidence that phas e information substantially improves reconstruction accuracy. We have found that reconstruction is improved provided epochs with large, systematic err ors are first excluded. With this condition, use of both phase and rate inf ormation improves the reconstruction accuracy by >43% as compared with the use of rare information alone. Furthermore, it becomes possible to predict the rat's position on a 204-cm track with very high accuracy (error of <3 c m). The best reconstructions were obtained with more than three phase divis ions per theta cycle. These results strengthen the hypothesis that informat ion in rat hippocampal place cells is encoded by the phase of theta at whic h cells fire.