Effects of spaced-CS extinction training. (A) One day after conditioning, mice received 10-trial spaced (20 minute interval), 10-trial massed (5 second interval) or 50-trial massed extinction training. (B) In S1 mice, freezing significantly increased across extinction trial-blocks in the spaced group but did not change in the massed groups, such that freezing was significantly higher in the spaced group than the massed groups by the last trial-block. During extinction retrieval, freezing was significantly higher in the spaced group than the massed groups, but was not different from freezing in non-extinguished retention controls. The spaced group showed a significant increase in freezing on extinction retrieval relative to the first-trial block of extinction training (n = 8-26). (C) In B6 mice, freezing significantly decreased over extinction trial-blocks in the massed groups, but significantly increased in the spaced group, with higher freezing in the spaced group than in the massed groups by the final trial-block. On extinction retrieval, freezing was higher in the spaced group than the massed groups but no different from retention controls, and the massed groups had lower freezing than retention controls. The massed, but not spaced, groups showed significantly less freezing on retrieval than the first extinction training trial-block (n = 8-26). #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. 10-trial massed group at same point in testing, ‡P < .05 vs. retention controls.