As illustrated in Figure 6 and Table S2, taking TPSM-phase into account to discriminate between IN-PF and OUT-PF firing (IN versus OUT EpF in the wheel) still provided significant
increase in spatial information content when bursts (taken as successive spikes separated by less than 10 ms) were omitted from the original spike train, as well as when only spikes emitted at high frequency (ISI < 10 ms) or on the contrary at frequencies lower than 25 Hz (ISI > 40 ms) were taken into account. These results suggest that a direct relationship between firing rate and TPSM-phase is unlikely to account for the observed TPSM phase-related gain of spatial information. http://www.selleckchem.com/products/EX-527.html Another possibility is that of a location-dependent modulation of theta power itself. For example, if theta amplitude was systematically maximal within a given place field area, the spikes discharged within this place field would likely be biased toward the corresponding phase of TPSM (i.e., π, for maximal theta power). We therefore computed signal-phase histograms corresponding to the TPSM phases expressed in each place field (i.e., distribution of LFP TPSM-phase relative to physical space; see Experimental Procedures). Although in the open field a significant signal modulation relative
to TPSM phase was observed in 41% of TPSM phase-locked place fields (18 place fields among 44 whose IN-PF spikes were significantly phase locked to TPSM; Rayleigh test, p < Sorafenib price 0.05; Figure 7A), it was most often (13 among 18 place fields) significantly different (p < 0.05, Kuiper test) from the distribution of IN-PF spikes relative to TPSM phase. This observation suggests that the TPSM phase-locking of spikes inside a place field cannot be explained by the preferred TPSM phase of the signal within this same place field. A different situation prevailed in the maze in which, as expected from classical track running experiments,
running was accompanied by a highly reproducible sequence of place cells firing, along with the animal’s stereotypical spatial progression (Pastalkova et al., 2008). As observed in Figures 7B–7D and S3, TPSM was remarkably conserved almost from one run to the other, as was the motor behavior of the animal. Accordingly, we observed that TPSM was in fact phase locked to the environment (Figures 7B–7E), in accordance with a recent study reporting a strong correlation between theta power and animal’s position in a maze (Montgomery et al., 2009). As a result, the relationship between IN-PF spikes and TPSM phase in the maze appears to be tightly related to the coincidence of place field position and phase locking of TPSM to space (Figures 6C and 7C–7E). To further investigate the potential relative influences of time and space on hippocampal activity, we examined TPSM during wheel running, in which although the animal is running, its spatial location does not change (Czurkó et al., 1999; Pastalkova et al., 2008).