Blocking the development of stimulus control when stimuli indicate periods of nonreinforcement.

To learn whether prior discrimination training based on one stimulus would block learning about a subsequently added stimulus, rats were first trained to press a bar on a variable-interval schedule of food reinforcement. Occasional stimuli were presented during which no reinforcement was available. Responding became suppressed in the presence of these stimuli. Stimuli could be noise, light, or a compound of noise plus light. A group trained with noise in Phase 1, then trained with the compound in Phase 2, showed less suppression to light in a subsequent test than a group that had the same compound training in Phase 2 but only variable-interval training in Phase 1. This showed that prior training with noise blocked the development of control by light during compound training. Two further groups showed that noise training following compound training did not have the same effect on control by light.