Red color or yellow background indicates data on the light condition, black and gray color or gray background indicates data on the dark condition. (A) Upper panels show the color coded MLE autocorrelogram models for each of n = 20 theta rhythmic grid cells at different running speed intervals. Immobility was defined as running speed <3 cm/s. Neurons are sorted per condition in order of their theta rhythmic firing frequency. Bottom row shows the average across autocorrelogram models per condition. (B) Number of neurons shown in A with statistically significant spiking rhythmicity at alpha = 0.1 determined by the MLE model for different running speeds in light and dark. (C, D) Longitudinal analysis of frequency (C) and magnitude (D) of theta rhythmic firing of neurons with significant theta rhythmic firing across all running speed bins (light: n = 9 neurons; dark: n = 12 neurons). Data show mean ± s.e.m. As shown for all principal neurons (Figure 2—figure supplement 2 and Tables 1 and 2), the frequency of theta rhythmic firing increased as a function of running speed during the light condition (slope = 0.0928, CI = [0.0461 0.1395], t = 4.0058, df = 43, p=0.0002, linear mixed effects model) but not during the dark condition (slope = 0.0168, CI = [–0.0265 0.0601], t = 0.7764, df = 58, p=0.4407, linear mixed effects model), and the difference between slopes in light and dark conditions was significant (slopeLight–Dark = –0.0760, CI = [–0.1385 –0.0135], t = –2.4125, df = 101, p=0.0176, linear mixed effects model). In contrast, the magnitude of theta rhythmic firing did not change as a function of running speed during the light condition (slope = –0.0017, CI = [–0.0091 0.0057], t = –0.4669, df = 43, p=0.6430, linear mixed effects model). The magnitude of theta rhythmic firing decreased slightly as a function of running speed during darkness (slope = –0.0061, CI = [–0.0120 –2.0430], t = –2.0710, df = 58, p=0.0428, linear mixed effects model) but the slope during darkness was not significantly different from the slope in the light condition (slopeLight–Dark = –0.0044, CI = [–0.0136 0.0049], t = –0.9399, df = 101, p=0.3495, linear mixed effects model).