Some studies have found that extinction leaves response structures unaltered; others have found that response variability is increased. Responding by Long-Evans rats was extinguished after 3 schedules. In one, reinforcement depended on repetitions of a particular response sequence across 3 operanda. In another, sequences were reinforced only if they varied. In the third, reinforcement was yoked: not contingent upon repetitions or variations. In all cases, rare sequences increased during extinctionvariability increased-but the ordering of sequence probabilities was generally unchanged, the most common sequences during reinforcement continuing to be most frequent in extinction. The rats' combination of generally doing what worked before but occasionally doing something very different may maximize the possibility of reinforcement from a previously bountiful source while providing necessary variations for new learning.

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