Figure 7.

Hippocampal peak phase spikes emitted during cued and noncued trials. A, Spatial phase distributions of spikes of a hippocampal neuron that were fired during several theta cycles. Left, Spikes that are plotted on top of the rat's trajectory (gray) are color-coded for phase (peak of cycle is 0°; time interval: 4 s centered on chamber entry; all chambers are pooled; see Materials and Methods). Right, Spatial distribution of regular (black) and peak phase (green, phase range [−40°, 40°]) spikes. B, Firing phase of regular (black) and peak phase (green) spikes as a function of linearized and normalized position of the example neuron shown in A. Red dashed lines indicate that spikes fired at position 0, which is the center of the firing field, occur on both the descending and the peak phase of the theta cycle. C, Example of a recorded hippocampal neuron, plotted with concomitant LFP trace filtered in the theta range (6–12 Hz). Diamonds below trace represent spikes. Gray oval (bottom) represents place field of the neuron. Opaque dots and arrows in the top row indicate the phase of the first spike of a train. Green dots represent peak phase spikes. These spikes (4 and 6) conform to theta phase precession but are intermitted by spikes not conforming to theta phase precession (spike 5). Notice the large phase jump from spike 3 to spike 4 within the range of 2 theta cycles. D, Spikes 1–6 are collapsed into a single theta cycle, now rendered in the phase domain. E, Mean number of peak phase spikes per trial as fraction of the total spikes emitted during cued versus noncued trials (± SEM; not significant). F, The phase distance between the peak phase spikes and their preceding nearest neighboring spikes (green) is significantly larger than the phase distance of trough phase spikes and their prior nearest neighbors (gray; control), indicating that peak phase spikes and the associated phase jumps are a consistent feature of phase coding. *p = 10⁻¹⁷ (Wilcoxon's rank sum test).