Extended Data Fig. 1. Behavioural experiment controls.

A) Schematic to test the temporal window of prospective memory-linking (top). Mice underwent Aversive encoding and then either 5 h, 1d, or 2d later they underwent Neutral encoding. The following day, mice were tested in the previously experienced Neutral context. Mice froze significantly more in the Neutral context when the Neutral context occurred within 5 h of the Aversive context, compared to when it occurred one day or more after Aversive encoding (bottom). Main effect of timepoint (F_2,24 = 3.689, p = 0.04) (5 h, n = 10 mice; 1d, n = 9 mice; 2d, n = 8 mice). Post-hoc tests revealed a trend for higher freezing in the 5 h timepoint compared to the 1d or 2d timepoints: 1d (t_16.38 = 2.137, p = 0.07), 2d (t_13.45 = 2.38, p = 0.07). B) Schematic to test the temporal window of retrospective memory-linking (top). Mice underwent Neutral encoding, followed by Aversive encoding in a separate context 5 h, 1d, or 2d later. The day following Aversive encoding, they were tested in the previously experienced Neutral context. Mice froze no differently in the Neutral context regardless of how long before Aversive encoding the Neutral context was experienced (bottom). No main effect of timepoint (F_2,27 = 0.73, p = 0.49) (5 h, n = 10 mice; 1d, n = 10 mice; 2d, n = 10 mice). C) Schematic of low- vs high-shock retrospective memory-linking experiment (without calcium imaging as a replication – biological replicate). Mice underwent Neutral encoding followed by a low- or high-shock Aversive encoding two days later. In the subsequent 3 days, mice were tested in the Aversive context, and then Neutral and Novel contexts, counterbalanced. D) Mice froze more in the Aversive context in high-shock vs low-shock mice (t₁₄ = 5.04, p = 0.00018) (low-shock, n = 8 mice; high-shock, n = 8 mice). E) High-shock mice exhibited higher freezing in Neutral vs Novel recall, while low-shock mice did not. A priori post-hoc test: high-shock (t₇ = 2.65, p = 0.033), low-shock (t₇ = 1.21, p = 0.133) (low-shock, n = 8 mice; high-shock, n = 8 mice). F) Schematic of temporal window retrospective memory-linking experiment to test whether memory-linking occurs at longer temporal windows. Mice underwent Neutral encoding followed by high-shock Aversive encoding two days later or seven days later. In the subsequent days, mice were tested in the Aversive context, and then Neutral and Novel contexts, counterbalanced. G) Mice froze no differently in the Aversive context in 2-day vs 7-day mice (t_28.81 = 0.72, p = 0.47) (2-day, n = 16 mice; 7-day, n = 15 mice). H) Mice in both 2-day and 7-day groups showed higher freezing in Neutral vs Novel recall (F_1,29 = 63.06, p = 9e-9). There was no difference in freezing in 2-day vs 7-day mice (F_1,29 = 0.16, p = 0.69) and no interaction (F_1,29 = 0.60, p = 0.45) (2-day, n = 16 mice; 7-day, n = 15 mice). I) Schematic to test whether the order of Aversive Recall affects retrospective memory-linking. Mice underwent Neutral encoding followed by high-shock Aversive encoding two days later. In the subsequent three days, mice were tested either in the Aversive context followed by Neutral and Novel, counterbalanced (Aversive First); or, mice were tested in Neutral and Novel, counterbalanced, followed by the Aversive context (Aversive Last). J) Mice froze no differently in the Aversive context if Aversive Recall came first or last (t₄₆ = 0.72, p = 0.48). K) Mice in both groups (Aversive First and Aversive Last) showed higher freezing in Neutral vs Novel recall (F_1,46 = 38.15, p = 1.6e-7). There was no difference in freezing in Aversive First vs Aversive Last groups (F_1,46 = 0.19, p = 0.66) and no interaction (F_1,46 = 0.14, p = 0.71). L) Representative histological verification of viral expression in dorsal and ventral hippocampus. Blue represents DAPI and green represents AAV5-Syn-PSAM-GFP. M) Schematic of the behavioural experiment disrupting hippocampal activity during the offline period. Mice were injected with AAV5-Syn-PSAM-GFP into dorsal and ventral hippocampus. Mice all had a Neutral experience and two days later a strong Aversive experience. Right after Aversive encoding, mice either had the hippocampus inactivated for 12hrs using the PSAM agonist, PSEM, or were given saline as a control. To do this, mice were injected four times, every three hours, to extend the manipulation across a 12-hour period. Two days later, mice were tested in the Neutral or a Novel context for freezing. N) Control (saline-treated) mice displayed retrospective memory-linking (i.e., higher freezing during Neutral vs Novel recall), while mice that received hippocampal inhibition (PSEM-treated) no longer displayed retrospective memory-linking. Significant interaction between Experimental Group (PSEM vs Sal) and Context (Neutral vs Novel) (F_1,42 = 4.00, p = 0.05) (Saline Neutral, n = 12 mice; Saline Novel, n = 10 mice; PSEM Neutral, n = 12 mice; PSEM Novel, n = 12 mice). Post-hoc tests demonstrate higher freezing in Neutral vs Novel contexts in the Sal group (t_19.84 = 2.57, p = 0.03) and no difference in freezing in Neutral vs Novel contexts in the PSEM group (t₂₂ = 0.31, p = 0.76). O) Schematic of the behavioural experiment as above, but this time to test the effects of hippocampal inactivation on Aversive memory recall. Mice all underwent the Neutral and Aversive experiences as before, as well as PSEM or saline injections following Aversive encoding (as in Extended Data Fig. 1m); however, two days following Aversive encoding, mice were tested in the Aversive context to test for an intact aversive memory. P) Mice froze no differently in the Aversive context whether they had received hippocampal inhibition or not (t_13.9 = 0.32, p = 0.748) (Saline, n = 7 mice; PSEM, n = 9 mice).