Figure 1.

Illustration of the experimental paradigm. (A) Subjects received fear conditioning on Day 1, which consisted of three CS-US pairings. On Day 2, subjects received either a retrieval CS or no retrieval CS, followed 1 h later by extinction (19 total CSs) with either fixed ITIs or variable ITIs. Average ITIs were 1 min in Experiment 1 and 2 min in Experiment 2 min. (B) In Experiment 2 min, the ITI variability was divided into three different categories: no variability (fixed), small variability and large variability. On Day 3, subjects received two unpaired USs (reinstatement). On Day 4, subjects received three unpaired CSs as a probe for reinstatement of fear. CS, conditional stimulus; US, unconditional stimulus; ITI, inter trial interval; ret, retrieval; min, minute.