Pigeons were trained to match temporal (2 and 8 sec of key light) and color (red and green) samples to vertical and horizontal comparison stimuli. In Experiment 1, samples that were associated with the same correct comparison stimulus displayed similar retention functions, and there was no significant choose-short effect following temporal samples. This finding was replicated in Phase 1 of Experiment 2 for birds maintained on the many-to-one mapping, and it was also obtained in birds that had been switched to a one-to-onemapping by changing the comparison stimuli following color samples. However, in Phase 2 of Experiment 2, when the one-to-one mapping was produced by changing the comparison stimuli following temporal samples, a significant chooseshort effect was observed. In Experiment 3, intratrial interference tests gave evidence of temporal summation effects when either temporal presamples or color presamples preceded temporal targets. This occurred even though these interference tests followed delay tests that failed to reveal significant choose-short effects. The absence of significant choose-short effects in Experiment 1 and in Phase 1 of Experiment 2 indicates that temporal samples are not retrospectively and analogically coded when temporal and nontemporal samples are mapped onto the same set of comparisons. The interference test results suggest that the temporal summation effect arises from nonmemorial properties of the timing system and is independent ofthe memory code being used.Researchers in the area of animal cognition have developed a variety of research approaches in an attempt to determine whether the memory code for visual samples in a delayed matching-to-sample (DMTS) task is a prospective representation of the correct comparison stimulus to respond to, or a retrospective representation of sample stimulus attributes (Honig & Thompson, 1982). These approaches include (1) comparisons of simple delayed and delayed conditional discriminations (Cohen,

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