Mapping and Deciphering Neural Codes of NMDA Receptor-Dependent Fear Memory Engrams in the Hippocampus
(A) MDA projection of CA1 ensemble firing patterns in two representative control mice (left and right plots, respectively). Ensemble activity patterns form distinct states for the rest period prior to CS (dots, grey ellipsoid), conditioned tone state (CS, square, blue ellipsoid) and CS-paired foot-shock state (US, circle, red ellipsoid). The top three most discriminant subspaces were plotted to show the encoding patterns revealed by MDA-based dimensionality-reduction method. (B) MDA projection of CA1 ensemble firing patterns from two representative knockout mice (left and right plots, respectively). (C) Discriminant distances between points within the Rest cluster, separation distances between the Rest and CS or US clusters, or between the CS and US clusters. Distances were calculated by averaging the mean distances between point to point belonging to different clusters or the same cluster (Rest) over animals. (Student t-test, **p<0.01). The knockout mice can still form CS and US ensemble representation, but exhibited lower pattern separation at the CA1 level.