Reduced blocking as a result of increasing the number of blocking cues.

Weak behavioral control (blocking) occurs when a target stimulus (X) is paired with an outcome in the presence of a well-established signal for the outcome (i.e., a blocking stimulus). Conventional Pavlovian conditioning theories explain this effect by asserting that a discrepancy between expected and experienced outcomes is necessary for learning about X and that no such discrepancy exists in blocking situations. These theories anticipate that the effect of additional well-established signals for the unconditioned stimulus (US) should be additive. In two conditioned barpress suppression experiments using rats as subjects, the opposite result was observed. Experiment 1 provided evidence that blocking was reduced when two blocking stimuli were present during X-US pairings relative to when one blocking stimulus was present. Experiment 2 elaborated on the mechanisms underlying the observations in Experiment 1, while explaining the discrepancy between the results of Experiment 1 and prior reports of the additivity of blocking stimuli.