Neural Population-Level Memory Traces in the Mouse Hippocampus
(A) The top panel shows the spectrogram of local field potentials during a 60-sec epoch of trace recall in mouse #1. The blue bar on the top indicates the non-freezing state, whereas the red bar indicates the freezing state of the animal. The time scale is matched with the spike raster shown below. Colored triangles at the bottom indicate the various moments at which encoding patterns were retrieved. To initiate the trace memory recall, a 2-sec tone was played. (B) Global MDA analysis shows that the data from the recall chamber and the conditioning chamber formed a single tone-pattern ellipsoid. The conditioned tone patterns (see the rectangle in the left panel) were recomputed by a second-step MDA to achieve finer separation for the difference of contextual environments. The purple trajectory is the recall-triggered CS pattern at the time when the tone was played, indicated by the purple triangle with a starred B in (A). (C) Type-I trajectory (purple) was retrieved during the first 1-min trace recall trial. This trajectory occurred at 53.5 seconds, indicated by the purple triangle with a starred C in (A). (D) Type-II trajectory (pink) was retrieved during trace recall. This trajectory occurred at 28.2 seconds, indicated by the pink triangle with a starred D in (A). (E) Additional types of ensemble trajectories were also detected during the trace recall trials. In particular, the dark green trajectory seemed to visit both the tone-at-recall cluster and the tone-at-training cluster. Whereas the light green trajectory visited the subspace between the tone-at-training cluster and the US cluster. (F) The averaged percentages of different types of ensemble patterns retrieved during the trace recall sessions from six recorded mice.