Neural Population-Level Memory Traces in the Mouse Hippocampus
(A) Illustration of a fear conditioning chamber for producing a tone-shock trace fear memory. A neutral tone precedes a mild foot-shock via a metal grid floor. (B) The trace interval between the offset of the tone (2 sec) and the onset of the foot shock (0.3 sec) is 20 seconds. (C) Schematic representation of experimental paradigm for trace fear conditioning. On the training day, the experiment begins as follows: one-hour pre-training sleep/rest in home cage; pre-training exploration in the conditioning chamber; trace conditioning in the conditioning chamber; one-hour post-training rest/sleep in home cage; contextual recall in the conditioning chamber; trace conditioning recall in a novel chamber. (D) Immediate freezing in mouse #1 increased as the CS-US pairing repeated during the training phase. (E) Contextual freezing in the 1-h contextual retention test suggests the contextual fear formation in mouse #1; at the end of the 5-min test, freezing decreased as there was no reinforcement shock. (F) Robust trace freezing upon the recall tone. The tone was presented for seven times (trials) with a 1-min interval. (G) As a group (n = 10), there was a significant contextual freezing as revealed by the 1-h contextual retention test (62.5%±5.5%, p<0.001). (H) The same set of mice showed a significant increased freezing during the 1-h trace retention test (n = 10, 61.5%±5.3%, p<0.001).