Figure 1.

Experimental design. A, Four experimental groups were studied. In the conditioned group (COND; n = 14), rats received three tone-shock pairs on day 1 and were left in their home cages on day 2. The extinction group (EXT; n = 15) received three tone-shock pairings on day 1 and 10 tones alone on day 2 in the same chamber. A naive control group (NAIVE; n = 12) received neither conditioning nor extinction. On day 1, the pseudoconditioned group (PSEUDO-COND; n = 6) received three tones and three shocks in an unpaired manner. All groups received two test tones on day 3 and then were killed immediately. B, Mean percentage of freezing for all groups across the behavioral procedure. As expected, rats in the COND group showed higher levels of freezing to the test tones, compared with the NAIVE, PSEUDO-COND, and EXT groups (**p < 0.01).