Backward blocking in first-order conditioning.

Three lick suppression experiments with rats investigated backward blocking in first-order conditioning. As has been suggested in prior studies, the experiments demonstrated that backward blocking is difficult to obtain in conventional first-order conditioning situations. However, the authors demonstrate here that backward blocking is observed in first-order conditioning if the target cue's behavioral control is weak at the time of elemental training of the blocking cue. The target cue's behavioral control was weakened through forward blocking of the target cue by a third cue (Experiment 1), conducting compound and elemental training with backward temporal relationships to the unconditioned stimulus (Experiment 2), and extinguishing the target cue following compound training (Experiment 3). The results of these experiments suggest that weak control of behavior by the blocked cue at the time of elemental training of the blocking cue is a critical determinant of whether blocking can be observed. Prior failures to detect backward blocking in first-order conditioning are seemingly due to a difficulty in decreasing the response-eliciting potential of a cue by indirect means such as associative inflation of a competing cue.