Appetitive learning and extinction in 11-day-old rat pups: Effects of various reinforcement conditions.

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Abstract
The acquisition and extinction of approach behavior in 11-day-old albino rat pups was studied. The reinforcement in experiments 1 and 2 was 15 s of nonnutritive suckling on an anesthetized lactating female. In experiment 1, detention duration, 0 or 15 s in the goal box on nonreward (N) trials, was studied in 2 groups whose acquisition training consisted of alternating blocks of rewarded and nonrewarded trials with a short (5 s) intertrial interval. Experiment 2 combined an examination of the effect of detention length with whether or not the mother was physically present, though inaccessible, on N trials with partially or continuously rewarded groups. Detaining subjects on N trials slowed the rate of extinction when the mother was present but increased rate of extinction when she was absent. There was no evidence of a partial reinforcement extinction effect in any of the groups. The effectiveness of 6 types of reinforcement on promoting acquisition of approach behavior was assessed in experiment 3. Equally good acquisition, but differences in extinction, was obtained with an adult conspecific as the reinforcer, independent of sex and suckling, but there was no acquisition of approach responding when a sibling or no conspecific was in the goal box. These data extend earlier findings of appetitive learning and extinction in infant rats. They do not support the view that inhibitory mechanisms are absent in the rat until the 3rd or 4th week of life.