No effect of partial reinforcement on fear extinction acquisition or consolidation. (A) Mice were fear conditioned (FC) in context A for 2 days. One group of mice received consistent reinforcement (100%: 3CS:3US), while another received partial reinforcement (50%: 6CS:3US). Mice were then trained on FE in context B, where they experienced 12 unreinforced CS presentations, once per day, for 3 days. Tone tests (TT) consisting of four unreinforced CS presentations were conducted in context B at 2 and 30 days following the last extinction training day (day 5). (B1) The effect of partial reinforcement on within-session FC acquisition. Comparisons of average percent time freezing for CS–US pairs (pink shaded regions) revealed no effect of contingency (P > 0.05). Freezing increased as a function of CS presentations on both FC1 (P < 0.001) and FC2 (P = 0.002), individual differences between CS shown in the graph. (B2) The effect of partial reinforcement on between-session FC acquisition. Average freezing across all CS was higher for FC2 than FC1 (P < 0.001), with no effect of contingency (P > 0.05). A 25 sec CS elicited greater freezing than 10 sec CS on FC2 (P = 0.013). (C) No effect of partial reinforcement on FC recall. Despite no effect of CS contingency (P > 0.05), all animals showed a significantly higher percent time freezing during the first four CS presentations on FE1 than during FC2 (P < 0.001). (D) The effect of partial reinforcement on FE acquisition. Each number (1–4) along the x-axis represents a bin of three CS (for a total of 12 on each day of extinction); “P” denotes pretone and posttone periods, respectively. Significant differences (P < 0.05) between CS-bins are denoted by an asterisk. (D1) Effects of partial reinforcement on within-session FE acquisition. Percent time freezing decreased across CS bin on each FE day (P < 0.001, each). There were no differences between CS contingencies or CS duration at any CS bin (P > 0.05). (D2) The effect of partial reinforcement on between-session FE acquisition. Average freezing across all CS per day showed reduced levels of freezing as a function of FE training day (P < 0.001), yet there was no effect of contingency. (E) All groups exhibited spontaneous recovery of fear. All groups demonstrated greater levels of freezing at 30 days as compared to 2 days postextinction, regardless of contingency or CS duration, indicating spontaneous recovery (P < 0.05). Four animals were removed from FE recall analysis due to experimenter error. Error bars represent ±standard error of mean (SEM).
