Neural Population-Level Memory Traces in the Mouse Hippocampus
(A) Spectrogram of local field potentials during the third CS-US pairing training. The blue color bar on the top indicates the non-freezing state, whereas the red bar indicates the freezing state of the animal. The color scale represents spectral power. The time scale is matched with the spike raster shown below. (B) Three post-stimulus immediate reactivations of the ensemble encoding patterns during the third learning trial were detected, marked by small triangles at the bottom of the spike raster. The sixty-sec spike-raster plot from mouse #1 is the same as the one shown in Fig. 2D. (C) An immediate reactivation, shown by the blue trajectory, was detected at the moment marked by the blue triangle at the bottom of (B). Similar to its original encoding pattern (shown in Fig. 2E), this reactivation trajectory transiently visited the CS cluster. This CS Trace (type-I) lasted about 0.8 seconds. (D) A second reactivation, shown by the pink trajectory, occurred at ∼9 sec shown by the pink triangle in (B). This trajectory visited the US cluster. This US Trace (Type-II) lasted about 0.7 seconds. (E) A third reactivation (red trajectory) took place at 43 sec, indicated by the red triangle in (B). This 1-sec association trajectory (Type-III), just like the encoding traces shown in Fig. 2F, first visited the US cluster and then toward the CS cluster before returning to the Rest. The same directionality was maintained. (F) Averaged numbers of reactivations (all three major types of traces) in six recorded mice over seven learning trials. The blue open triangles presents the averaged reactivation numbers within the 20-sec interval of the same CS-US pairing, whereas the red open triangles show the averaged reactivation numbers within the 60-sec after the paired US was delivered. The linear regression lines suggest the accumulative effects of pairings on the ensemble trace formation. (G) Numbers of immediate reactivations and the amounts of immediate freezing over seven trials in mouse #1. Cross-correlation analysis shows the correlation between freezing responses and occurrence numbers of reactivations was highly significant (r = 0.9657, p<0.001). Averaged freezing responses and the total numbers of reactivations were calculated from the period between the tone offset and the 60-sec after US presentation. (H) Averaged immediate freezing responses and the total numbers of reactivations were calculated from six mice. The correlation between freezing and reactivations was again highly significant (r = 0.9551, p<0.001).