Mapping and Deciphering Neural Codes of NMDA Receptor-Dependent Fear Memory Engrams in the Hippocampus
(A) The intertwined memory trace retrievals (underlined brackets) and alternating retrieval (upper brackets) map in a control mouse over seven tone-traced recall trials. Note the timed retrieval of foot shock memory traces around the 22-second time point after the onset of the conditioned tone. (B) Lack of temporally intertwined and alternating recall structures in memory codes of the knockout CA1 region during the tone-traced recall trials. (C) The intertwined memory retrieval in the control group during tone-traced recall exhibited clear exponential decay distribution, but not in the knockout mice (two-sample Kolmogorov-Smirnov test, p = 0.0024). Time window for 50% temporally associational recall is 4.1 sec for control mice and 7.1 sec for mutant mice. (D) Significant reduction in intertwined memory retrievals during tone-traced recall in knockout mice as compared to the control group (Wilcoxon rank sum test, *p<0.05; error bars represent SD). (E) Average number of temporal structures of memory traces as single traces, doublet, triplet the control and mutant mice during trace recall. (F) The alternating retrieval between simple US trace and simple CS memory trace in the control mice exhibited an exponential decay distribution, suggesting that the retrieval occurred as clusters in time domain. Such temporal association is nearly absent in the mutant mice (two-sample Kolmogorov-Smirnov test, p = 0.022). Trace interval for 50% temporally associational recall in the control is 3.8 sec for control mice and 6.6 sec for mutant mice. (G) Differences in alternating retrieval rates during tone-traced recall between the control and knockout mice (Wilcoxon rank sum test, *p<0.05; error bars represent SD).