Neural Population-Level Memory Traces in the Mouse Hippocampus
(A) Sixty-sec spike rasters of a selective set of CA1 units during the first CS-US pairing (105 units shown out of 208 simultaneously recorded units from mouse #1 for demonstration). The different colors represent several groups of cells with different response properties. The units listed at the bottom were selected from the non-responsive group (see details in SF 4). (B) MDA analysis shows ensemble patterns and dynamic trajectories in response to the first CS before the US arrived. The grey, blue and red ellipsoids represent the resting state, tone, and foot-shock clusters, respectively. The grey trajectory shows a transient (∼1-sec) ensemble dynamics at the moment marked by the arrow B near the zero time point in (A) in response to the first tone before the paired US arrived. This trajectory remained largely within the rest cluster, indicating that CA1 did not respond to the neutral tone initially. (C) A robust dynamic trajectory moving from the Rest cluster to Shock cluster was detected when the first paired US was delivered at the time point marked by the arrow C in (A). This transient trace lasted about 1.5 seconds. (D) The sixty-sec spike rasters demonstrate neural activity of the same set of cells shown in (A) during the third CS/US pairing. (E) The CS presentation in this third trial elicited a robust pattern (blue trajectory). The duration corresponds to the blue bar length in (D) marked by the arrow E. (F) The third paired US triggered an associative trajectory that transversed the shock cluster and the conditioned tone cluster. The color-coded trajectory, spanning for about 3 seconds, corresponds to the moment indicated by the colored bar in (D). (G) Two more examples of the unique US-CS associative trajectories elicited by foot shock at the fourth and sixth trials in the same animal (mouse #1) are shown here. The arrows indicate the moving directions of the ensemble trajectories. (H) The six-by-seven color matrix shows the occurrences of US-CS association patterns over seven trials from the six recorded mice. The blue squares indicate that foot shock (US) only triggered US ensemble trajectories early on, whereas the yellow squares indicate the US-CS association trajectory was elicited by foot shock, which became more prevalent after pairings.