Sparing of patterned alternation but not partial reinforcement effect after infant and adult hippocampal lesions in the rat.

The effect of hippocampal lesions was assessed in patterned (single) alternation (PA) and the partial reinforcement extinction effect (PREE), the two reward-schedule effects that appear earliest in ontogeny. Three groups of rat pups, with appropriate controls, were tested for each effect: lesion as infant/test as infant, lesion as infant/test as adult, and lesion as adult/test as adult. Hippocampal lesions had no effect on PA in any of the three conditions except for a suggestion that the effect was mildly attenuated in the animals given lesions as infants and tested as adults: The pups were able to discriminate rewarded from nonrewarded trials and to inhibit responding on nonrewarded trials. On the other hand, the PREE was eliminated under all conditions of testing, in each case because of an increase in persistence following continuously reinforced acquisition. The results are discussed in terms of the functional maturation of the hippocampus and a possible dissociation of mechanisms that mediate response suppression in PA and in the PREE in infant rats.