a Mean skin conductance response (SCR) for CS+ and CS− in Control and PTSD groups averaged across all trials. b Compared with the trauma-exposed control group, the PTSD group demonstrated delayed contingency learning, as measured by shock expectancy ratings. On average, subjects’ differential shock expectancy ratings began to diverge from the 2nd trial of each category exemplar in the control group but only from the 7th trial onward in the PTSD group. Linear trend analysis confirmed a strong linear pattern of threat learning (CS+) only in the PTSD group, whereas both groups showed strong linear patterns for safety learning (CS−). c All four runs confirm that learning was achieved in the first run and was maintained for the duration of the experiment. Dark green represents PTSD CS− and that light-red indicates PTSD CS+. Error bars represent mean ± SEM. *P < 0.05, two-tailed t tests. CONT = control.