A, Recording paradigm; data from using unrestrained rats (26), reanalyzed here for speed- and theta-related investigations. B, Example SuM unit (recorded by tetrodes) that positively correlates to speed (i.e. a speed cell). Inset represents these data as a scatter plot. C, Distribution of speed vs. firing rate Pearson r values. Pie chart shows percentage of units for positive (r=0.36±0.023, mean±sem), negative (r=−0.24±0.024), and non-significant cells (r=0.018±0.012). D, Distribution of temporal offsets for positive speed cells. sem=0.19. E, Two example SuM unit spiking activities. Orange cell shows phase-locked firing with respect to hippocampal theta whereas the grey cell does not. F, Quantification of theta-related firing for two example units from E. The top panel shows spike-field coherence and the bottom shows theta-rhythmic spiking. G, Units were clustered into theta cells based on quantification from F. H, Distribution of speed scores among clustered “theta cells” from G.