Pavlovian excitation, internal inhibition, and their interaction with free operant avoidance as a function of age in rats

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Abstract
The central hypothesis was that, in young as compared with adult rats, Pavlovian excitation is weaker and extinguishes more slowly, whereas internal inhibition is weaker but extinguishes more rapidly. In Experiment I, the suitability of free operant (Sidman) avoidance as a baseline for measuring Pavlovian effects was assessed in 5 age groups between 19 and 98 days of age trained in a shuttle box. Although clear age‐related increases were evident in avoidance capacity, efficient “high criterion” avoiders were found in all age groups, indicating that the response was suitable. In Experiment II, groups of 26‐, 38‐, and 95‐day‐old rats first received Pavlovian aversive excitatory and differential inhibitory conditioning, or the conditional stimuli and shock randomly, or no treatment. Three days later they received free avoidance training. Prior Pavlovian conditioning facilitated subsequent avoidance learning in young subjects, but depressed it in adults. Among “high criterion” avoiders, such effects were absent. Following the avoidance training phase, 3 age groups of matched “high criterion” avoiders received conditioned stimuli superimposed on the baseline (a conditioning test). These results confirmed the central hypothesis.