Figure 4

Extinction of a second-order conditioned cue. (A) All mice first received CS1xUS fear conditioning, 1 day prior to exposure to either CS2 paired with CS1 or CS2 exposure alone. All mice then received equivalent extinction training and retrieval testing. (B). S1 mice significantly increased freezing across CS1xUS pairings during conditioning. Freezing significantly increased across CS2 trials when the CS2 was paired with the CS1 and not when presented alone. Freezing to the CS2 did not change across extinction trial-blocks in either group and was significantly higher in the CS2-CS1 paired group than the CS2 only group during extinction retrieval (n = 5-10). (C). B6 mice significantly increased freezing across CS1xUS pairings during conditioning. Freezing significantly increased across CS2 trials when the CS2 was paired with the CS1 and not when presented alone. Freezing to the CS2 significantly decreased across extinction trial-blocks in the CS2-CS1 paired group. Freezing was significantly higher in the CS2-CS1 paired group than the CS2 only group during extinction retrieval, but was lower relative to the first trial-block of extinction training (n = 5-10). #P < .05 last (Lst) vs. first (Fst) conditioning trial, †P < .05 Lst vs. Fst trial-block during 2nd order conditioning or extinction, *P < .05 vs. CS2-only at same point in testing.