14 rats were maintained at 80% of their free-feeding weights and trained to press a bar on a chained variable-interval 1/2 fixed-ratio 20 schedule of reinforcement. In Exp. I a discriminative stimulus paired with the ratio component of the chain was shown to control a high rate of responding. In Exp. II, it was demonstrated that this stimulus, when superimposed on a stimulus which had been associated with punishment and therefore controlled a low rate of bar-pressing, facilitated recovery from the effects of punishment.
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