Overshadowing of situational cues with variable but not fixed intertrial intervals

In two experiments, we examined the conditions under which signaling an unc onditioned stimulus (US) with a nominal conditioned stimulus (CS) interfere s with the conditioning of situational cues in defensive freezing in the ra t. Subjects received footshock USs that were (1) either signaled or unsigna led and (2) either varied or fixed in their temporal location within the co nditioning session. Experiment 1, with only one trial per session, yielded no evidence that signaling affected pretrial freezing using either a fixed or variable interval between placement in the context and shock onset. In a test in which no CSs or footshocks were presented, groups that previously had received footshock at a fixed temporal location showed greatest freezin g at around that same time. For groups that had received footshocks at vari ous times, freezing declined across the test session. Experiment 2 showed o vershadowing of pretrial freezing after more extensive conditioning with ma ny trials per session, but only if the intershock intervals were variable r ather than fixed.