Prospective and Retrospective Memory Processes in Pigeons′ Performance on a Successive Delayed Matching-to-Sample Task

The coding process used by pigeons on a successive delayed conditional discrimination (DCD) was assessed by manipulating the number of sample stimuli (2 or 4), the number of test stimuli (2 or 4), and the interstimulus interval during acquisition (0 or 0.5 s). The stimuli were vertical versus horizontal lines and circle versus triangle shapes. Upon attainment of criterion performance, birds were transferred to interstimulus delays that were 0, 1, 2, and 4 s longer than the training delay. Although there was no effect of number of sample or test stimuli on rate of acquisition, during delay testing, performance was better for groups that had two test stimuli than for those with four test stimuli, regardless of the number of sample stimuli or the training delay. The delay data suggest that the pigeons prospectively coded the test stimulus to which responding was to be reinforced rather than retrospectively coding the sample stimulus.