Figure 8. Temporal relationship between layer/region-specific firing patterns and theta current sinks in the hippocampus.

Top: Distribution of preferred theta phase. The height of the histograms reflects the proportion of principal neurons significantly modulated in each firing rate category (as in Figure 5A, reference = EC3 theta). Black dashed line, population mean (as in Figure 3B). Instantaneous rate (color coded) was quantified for each spike in ten increments (1, 2, … or ≥10 spikes per 250 msec [~two theta cycles], corresponding to 4, 8, 12,…, ≥ 40 Hz; as in Figure 5). In each region and layer, most principal neurons are silent or fire at low rates, with only a minority of neurons discharging at high frequency at a given time (Figure S19). Bottom: Current-source density (CSD) theta traces, taken from Montgomery et al. (2009), are superimposed on a histological section in the CA1-dentate gyrus axis, with highlighted pyramidal cell and granule cell. Note phase-reversed sinks in CA1 str. lacunosum-moleculare (lm) and dentate molecular layers (ml), and phase-shifted sink (relative to lm sink) in str. radiatum (rad). pyr, pyramidal layer. Arrows indicate the temporal (phase) offsets between the peak of population firing in upstream layers and the theta sinks in the target layers with the expected delays (based on axonal conduction velocity; 30° or ~ 10 msec). Note that while the population peak of an upstream layer correctly predicts the timing of the dendritic sink in its target layer, spiking activity in the downstream target population is substantially delayed.