All vertebrates are capable of generating dissimilar patterns of neuronal activity from similar sensory-driven input patterns, a phenomenon called pattern separation. It is unclear, however, how these separated patterns are transformed into lasting memories that retain the initial discrimination. Using dual-color in-vivo two-photon Ca 2+ imaging, we show that the dentate gyrus, a region implicated in pattern separation, generates in immobile mice sparse, synchronized activity patterns driven by entorhinal cortex activity. These population events are structured and modified by changes in the environment; they incorporate place-and speed cells and are similar to population patterns evoked during self-motion. Inhibiting only granule cells in immobile mice impairs formation of pattern-separated memories. These patterns, thus, support the creation of precise memories by replaying the population codes of the current environment on a short time scale.
Results
Sparse, structured dentate network events in immobile animalsWe imaged the activity of >300 granule cells (GCs) using a Thy1-GCaMP6s mouse line (GP4.12Dkim/J,15 ). In addition, we monitored the activity of the major input system into the dentate gyrus, the medial perforant path (MPP). To this end, we expressed the red-shifted Ca 2+ indicator jRGECO1a 16 in the medial entorhinal cortex using viral gene transfer (see Methods section, Fig. 1A, B, Supplementary Fig. 1A). To allow efficient excitation of both genetically encoded Ca 2+ indicators, we established excitation with two pulsed laser sources at 940 and 1070 nm (see Supplementary Fig. 1B). The mice were placed under a two photon microscope and ran on a linear track (see Supplementary Fig. 1C, Supplementary Movie 1).As previously described, the firing of GCs was generally sparse 7-9,17 , both when animals were immobile and running on a textured belt without additional cues (mean event frequency 1.38±0.19 events/min and 0.97±0.2 events/min, respectively, n=9 mice, Fig. 1B, Supplementary Fig. 2C-F). Despite the sparse activity of granule cells, we observed synchronized activity patterns. To rigorously define such events, we used an algorithm that detects synchronized network events within a 200 ms time window (see Methods). Such synchronous network events could readily be observed in the dentate gyrus ( Fig. 1C, D, network events depicted in different colors, see corresponding Supplementary Movie 2).Network events were sparse, incorporating only 5.7±0.09 % of the active GC population.Notably, network events occurred mainly during immobility and were much less prevalent during running (Fig. 1D, E). Shuffling analysis (see Methods) confirmed that network events cannot arise by chance (Fig 1E, grey bars correspond to shuffled data, ANOVA F (3,25) =30.12, p=4*10 -14 , Bonferroni post-test resting vs. shuffled p=2.2*10 -10 indicated with asterisk, running vs. shuffled p=1). Simultaneous imaging of MPP and GC activity showed that network events were strongly correlated with MPP activity increases (Fig. 1F). Moreover, ...