Delayed stimulus control: recall for single and relational stimuli.

In a discrete-trial symbolic matching-to-sample procedure, pigeons' left-key responses were reinforced following presentation of one center-key sample, and right-key responses were reinforced following presentation of another. Recallability was measured by the difference between log ratios of left to right responses following each sample. In Experiment 1, samples were successively presented same or different wavelengths in the relational discrimination, or individual wavelengths in the single discrimination. The rate at which recallability decreased with increasing delay since sample presentation was the same for single and relational discriminations, but the initial level of performance differed, indicating that the relational discrimination was more difficult. In Experiment 2, recall functions for easy and difficult discriminations between individual wavelengths also differed in levels of initial performance but not in rate of decrement of recallability over time. Recall for stimuli differing in complexity may therefore reflect differences in discrimination difficulty.