CS memory after trace conditioning

Two experiments investigated successive discrimination learning in rats. Of central interest was the influence of the interval between the reinforced stimulus (S+) and the reinforcer (shock) on discriminative performance. Experiment 1 demonstrated that when shock immediately followed the S+ the discrimination was formed rapidly—subjects quickly came to show conditioned responding exclusively during the S+. However, when there was a 30-s trace interval between the S+ and shock the discrimination was only poorly formed, and conditioned responding occurred on both S+ and nonreinforced (S−) trials. In Experiment 2 subjects received concurrent training on a visual and an auditory discrimination. For one discrimination the S+ was immediately followed by shock and for the second discrimination there was a trace interval between the reinforced cue and shock. The former discrimination was acquired more readily than the latter. Implications of these results for contemporary theories of discrimination learning are explored and parallels between these results and the phenomena of contrast and of “marking” are examined.