Effects of immediate extinction training. (A) Mice received extinction training 10 minutes (immediate) or 1 day (delayed) after fear conditioning. All mice then received equivalent extinction training and retrieval testing. (B) In S1 mice, freezing was significantly lower in the immediate than the delayed group during the first extinction trial-block, and then increased across trial-blocks in the immediate but not delayed group. Freezing was similar between groups during extinction retrieval and neither group showed a decrease in freezing on retrieval as compared to the first extinction trial-block (n = 13-19). (C) In B6 mice, freezing significantly decreased across extinction trial-blocks in the delayed but only showed a non-significant trend for a decrease in the immediate group. Freezing was significantly higher in the immediate than delayed group during extinction retrieval, although both groups froze less on retrieval than the first trial-block of extinction training (n = 10). Data are means ± SEM. #P < .05 last (Lst) vs. first (Fst) conditioning trial, †P < .05 Lst extinction trial-block or retrieval vs. Fst extinction trial-block, *P < .05 vs. immediate group at same point in testing.