Contextual fear conditioning was examined using the startle reflex in two g
roups of participants over two sessions separated by 1/2 h. The conditioned
stimulus (CS) was paired (paired group) or not (unpaired group) with an un
pleasant shock during conditioning. The paired group showed conditioning to
the CS that was well retained over the retention interval. Session 1 inter
trial interval startles-a measure of contextual conditioning-were greater i
n the unpaired compared to the paired group. Context conditioning was retai
ned in Session 2 and was present before the shock electrodes were attached.
Self-rating of state anxiety, arousal, and pleasure indicated differential
changes in mood from Session 1 to Session 2 in the two groups, with the un
paired group showing relatively greater negative affects compared to the pa
ired group. These results indicate that unpredictable shocks lead to greate
r context conditioning as measured by startle and self-reports.