Increasing CS:US trials during FC does not result in PREE. (A) Experimental procedures were carried out as in Figure 1A except for the number of CS:US trials per group. The group exposed to consistent reinforcement (100%) received 6CS:6US pairs, while the partial reinforcement group (50%) received 12CS:6US pairs. (B1) The effect of partial reinforcement on within-session FC acquisition. Partial reinforcement (50%) resulted in higher levels of freezing than consistent reinforcement (100%) during CS:US pairings 2 (P = 0.044), 3 (P < 0.001), and 6 (P = 0.026) on FC1. There was no effect of contingency on FC2 (P > 0.05). Both contingency groups increased freezing relative to baseline (P < 0.001) on both days. (B2) The effect of partial reinforcement on between-session FC acquisition. Conditioned freezing was higher overall in the 50% contingency group (P = 0.048), and both groups exhibited greater freezing on FC2 than FC1 (P < 0.001). (C) Comparison of contingency and CS epoch on FC retrieval. There was a significant effect of CS epoch (P < 0.05), yet no effect of contingency on fear retrieval. (D) No effect of partial reinforcement on FE acquisition. There were no differences in conditioned freezing in any CS-bin between contingencies (P > 0.05). Each number (1–4) along the x-axis represents a bin of three CS; “P” denotes pretone and posttone periods, respectively. (D1) No effect of partial reinforcement on within-session FE acquisition. All groups reduced conditioned freezing within each session (P < 0.001, each). There was no effect of contingency on freezing at any CS bin (P > 0.05). (D2) No effect of partial reinforcement on between-session FE. Freezing decreased across FE days (P < 0.001) without any effect of reinforcement contingency (P > 0.05). (E) The effect of partial reinforcement on FE recall. Consistently reinforced mice displayed greater levels of freezing than those with partial reinforcement (P = 0.038), while freezing increased between 2 and 30 days in both conditions (P < 0.001). Error bars represent ±SEM. Red shaded regions represent CS–US pairings.
