In 2 experiments using milk-suckling from an anesthetized dam as the reinforcer, evidence is presented that the transitional age in rat pups for learning of persistence as a result of appetitive partial reinforcement is between 11 and 12 days of age. In Experiment I, pups 12-13 days of age showed the partial reinforcement extinction effect (PREE) whereas 10-11-day olds did not. In Experiment II, pups trained at 11-12 days and tested at Day 13 did show a PREE at Day 13 but those trained at 10-11 days did not.

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Two experiments were performed to determine whether or not persistence acquired under reinforcement conditions of infancy can be retained and transferred to adult reinforcing conditions. In Experiment I 2 groups of rats 14-15 days of age were trained in a runway under a partial (PRF) or continuous (CRF) schedule of milk-suckling reinforcement. A 3rd group was trained under continuous nonreinforcement (NRF) conditions. The PRF and CRF groups acquired the approach response about equally, and the NRF showed a very small increase in speed. In immediate extinction, the PRF group was more resistant to extinction than its CRS counterpart. For the NRF group "extinction" produced a continuation of the low-level "acquisition" performance. Experiment II showed that persistence acquired at 14-15 days with milk-suckling reinforcement can be sustained through a 2-week retention period including a phase of CRF food-pellet reward.

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Five experiments addressed the relation between habituation to shock and subsequent persistence (resistance to extinction and to punished extinction) in preweanling, juvenile, and adult rats. In Experiment 1, a prolonged shock-habituation treatment given between runway acquisition and simple extinction promoted persistence in both preweanling and juvenile rats. Experiment 2 showed even stronger persistence effects in a punished-extinction test given after shock treatment, the effects being stronger in preweanlings than in juveniles. Experiment 3 demonstrated that shock treatment at infant-to-preweanling age resulted in increased persistence that survived a 42-day retention interval in a nonreward extinction test, but not a punished-extinction test. Experiment 4 demonstrated habituation to shock in preweanlings and juveniles in terms of recovery of suppression of milk intake in the shock situation and, compared with controls, in terms of reduced latency to eat in a punished-reward test at several retention intervals. Experiment 5 tested the relation between strength of habituation and persistence in 70-day-old rats by manipulating shock parameters that are known to affect habituation-number of shocks, intershock interval, and gradually increasing versus constant shock durations. In every case, strength of persistence to approach in a runway reflected the presumed degree of habituation induced by varying these factors.