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Neural Population-Level Memory Traces in the Mouse Hippocampus

Figure 6

The peaked re-appearance of the US patterns at the traced time interval indicates the memory of time.

(A) Representative ensemble tone traces during trace recall (Left panel). The right panel shows the time distribution of the averaged occurrences of this type of trajectories over seven recall-trials in mouse #1. Time zero indicates the moment when the recall tone was delivered. The numbers of trajectories were pooled every 5 seconds. (B) Representative ensemble shock traces during the traced memory recall session are shown (Left panel). The right panel shows the time distribution of the averaged occurrences of this type of trajectories over seven recall-trials in the same mouse. The same analysis reveals a salient peak in the occurrences of the US trajectories around the traced interval time point. Time zero indicates the onset moment when the recall tone was delivered. The tone lasted 2 seconds, thus the absolute amount of time between the offset of the tone and the retrieval peak was around 20 seconds. (C) The peaked occurrences of the US ensemble trajectories were observed in five out of the six recorded mice. Each colored plot represents a mouse. (D) The color matrix shows the retrievals of the US ensemble traces in six individual mice around the traced interval time (20±2.5 sec after the offset of the recall tone) at each of the seven trials. Yellow squares represent the occurrences of the correct US ensemble patterns, whereas blue squares indicate the absence of US patterns at the time point of this trace interval. (E) The exact time distribution of the 31 correct US pattern retrievals around the traced interval time in all six mice. Please note that the 22-sec time point is from the onset of the recall tone (which lasts 2 seconds). (F) Lack of distinct peak in the traced fear freezing over the 1-min recall trials. Blue triangles show the five-second averaged freezing responses over seven recall-trials (1-min per trial) from six recorded mice. Red stars show the US pattern retrieval distribution. Correlation between the averaged freezing response and occurrences of US traces was still significant (r = 0.5851, p<0.05), despite the absence of a distinct peak in freezing.

Figure 6

doi: https://doi.org/10.1371/journal.pone.0008256.g006