Figure 2.

Learned suppression of contextual fear during the CS−. (A) Percent time spent freezing in trials with the CS− in the testing session with the conditioning context (day 4). The freezing level is markedly lower during the CS− than during the PreCS−, indicating inhibition of contextual fear during the CS−. (B) Percentage of time spent freezing in trials with the CS+ in the testing session on day 4. On average, no significant change of the freezing level is detected during the CS+ comparerd to that during the PreCS+. (C) Changes in freezing levels represented as the percent time spent freezing during the CS subtracted by that during the corresponding PreCS of the trial. A negative change (i.e., reduction) of freezing levels for trials with the CS− and a positive change (i.e., enhancement) of freezing levels for trials with the CS+ indicate opposing effects of the CSs on emotional responses. Note that the testing session on day 4 is performed in the conditioning context (context A), whereas the testing session on day 5 is carried out in a novel context (context B). (D) Comparison of freezing levels between the time during the PreCS− and during the CS− across sessions. The freezing level during the CS− becomes significantly lower than that during the PreCS− on day 3 as well as on day 4. Gray dots with lines (A,B) and circles (C) represent the median of data sets from three trials for individual mice (n = 12 mice). All data in bar graphs in A–C and the line graph in D are depicted as mean ± SEM. (*) P < 0.05, (**) P < 0.01, (***) P < 0.001. P values were obtained using a two-tailed paired t-test (A,B,D) and a one-tailed paired t-test (C).