Trial-to-trial variability of phase precession in the hippocampus

During the crossing of the place field of a pyramidal cell in the rat hippocampus, the firing phase of the cell decreases with respect to the local theta rhythm. This phase precession is usually studied on the basis of data in which many place field traversals are pooled together (Figure 1A). Here, we study the properties of phase precession in single trials (Figure 1B) and compare them to the properties of pooled-trial phase precession. We find that single-trial and pooled-trial phase precession are different with respect to three fundamental properties: phase-position correlation, phase-time correlation, and phase range. While pooled-trial phase precession may span 360°, the most frequent single-trial phase range is only around 180°. Further, an important source of variability of phase precession pooled over trials is the large trial-to-trial variability. Only a part of this trial-to-trial variability may be explained by running speed and firing rate differences across trials, but the larger part of the variability remains to be explained. Finally, comparison with surrogate trials indicates that single trials are not randomly drawn samples from the pooled data and that pooling over trials changes basic measures of phase precession.

Figure 1

(A) Pooled-trial and (B) single-trial phase precession.