The explicitly unpaired procedure yields conditioned inhibition whether the CS and the US alternate singly or randomly

Abstract

These experiments examined whether the single and random alternation variants of the explicitly unpaired procedure yielded CSs with equivalent inhibitory strength and sought evidence on whether animals conditioned with the single alternation variant learn the sequence of singly alternating CS and US trials. The explicitly unpaired procedure proved effective in making the CS a conditioned inhibitor, as assessed by summation and retardation of acquisition tests, whether the CS (60 s white noise) and US (0.5 s, 0.8 mA footshock) alternated singly or alternated randomly, with both moderate (20 sessions in Experiment 1) and extensive (60 sessions in Experiments 2) amounts of conditioning. Although the two variants of the explicitly unpaired procedure yielded CSs with equivalent inhibitory strength, examination of the operant baselines during the conditioning phase (Experiment 2) revealed that there was greater acceleration of lever-pressing during the explicitly unpaired CS that randomly alternated with footshock than during the CS that singly alternated with footshock. However, in both variants there was greater acceleration of lever-pressing during the explicitly unpaired CSs than during the zero-contingency CS. There was no evidence that animals had learned the sequence of singly alternating CS and US trials even when the intertrial interval was substantially shortened (from 420 s in Experiments 1 and 2 to 55 and 25 s in Experiment 3) to facilitate sequence learning. The explicitly unpaired procedure in which CS and US alternated singly yielded an inhibitory CS across all experiments.