CS-US temporal relations in blocking.

In four trace-conditioning experiments with rats, the influence on the blocking of differences between the blocking cue-unconditioned stimulus (US) and the blocked cue-US trace intervals was explored. Experiment 1 demonstrated blocking despite the blocked cue's having a shorter trace interval than the blocking cue in both elemental (Phase 1) and compound (Phase 2) training. In Experiment 2, blocking was attenuated when the blocked cue had a longer trace interval than did the blocking cue in both elemental and compound training. In Experiments 3 and 4, the trace intervals of the two cues during compound training were matched (i.e., unlike in Experiments 1 and 2, neither had temporal priority). Blocking was attenuated when the blocking cue trace interval in the elemental phase was shorter (Experiment 3) or longer (Experiment 4) than the compound cue trace during compound training. The findings indicate that subjects encode interstimulus intervals, and they further suggest that cue competition is greatest when the competing cues have the same temporal information as the US.