Neural Population-Level Memory Traces in the Mouse Hippocampus
(A) The top panel shows the spectrogram of local field potentials during the 60-sec epoch of the contextual recall test 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 color scale represents spectral power. The time scale is matched with the spike raster shown below. The colored triangles at the bottom indicate the moments at which encoding patterns re-appeared. (B) The blue trajectory shows that a representative CS trace (Type I) was retrieved, indicated by the star marked with B at the bottom of the raster in (A). This transient dynamic lasted about one second. (C) The pink trajectory shows a representative US trace that lasts about 0.7 seconds (marked by the pink triangle with C in (A), Type II). (D) The red trajectory is a representative US-CS association trace (the red triangle with D in (A), Type III). This trajectory occurred for about 0.8 seconds. (E) The green trace shows a type of different traces often seen during recall test. It visited the space between the US and CS clusters and lasted about one second. (F) The colored trajectory shows a Type III trace that was retrieved at the time indicated by the red triangle with F in (A). This trajectory occurred in the non-freezing state, and interestingly, had a reversed directionality, namely, moving from the CS ellipsoid to the US ellipsoid. It lasted about three seconds. (G) The percentages of different types of ensemble patterns during contextual recall in six mice. (H) Freezing responses and the total numbers of pattern retrievals were calculated and compared during the entire 5-min contextual recall test (mouse #1). The blue circles are the freezing responses counted in every 30 seconds; the red circles show the averaged numbers of three major types of ensemble traces counted in the same 30 seconds. Cross-correlation analysis shows that the correlation between freezing responses and occurrences of pattern retrievals was significant (r = 0.8840, p<0.001). (I) Averaged freezing responses in six mice are also tightly correlated with their averaged numbers of total pattern retrievals during the contextual recall tests (r = 0.8956, p<0.05). Each circle represents the data from a single mouse. This between-animals plot indicates that the numbers of pattern retrieved is almost in linear proportion to behavioral performances as measured by the amounts of contextual freezing. The animal which had a fewer number of pattern retrieved also showed lower fear memory freezing, whereas the mice which had high numbers of pattern recalled exhibited highest amount of freezing.