In three experiments, two-event sequences were presented as discriminative stimuli for the key pecking of pigeons during a later test period. The purpose of these experiments was to evaluate the stimulus control exerted by saliency, primacy, recency, and, especially, temporal order in discriminations among two-event sequences. In Experiments 1 and 2, reinforcement for responding to the test stimulus depended on the prior occurrence of only one of several sequences of colored lights. In Experiment 3, reinforcement depended both on the sequence of colors and on the particular test stimulus that followed. Two phases in the pigeon's differentiation of two-event sequences were tentatively identified. Control by recency was stronger than by primacy during the first phase. Control by order increased and control by recency decreased during the second phase. The results left little doubt that pigeons can represent and remember the order of a recent series of events.

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Two experiments sought to determine if pigeons could discriminate and remember recent sequences of stimuli and responses. A variant of Konorski's short-term memory procedure involving successive presentation of sample and test stimuli was used. The samples were stimulus-response pairs of the form, (S-R)(1)-(S-R)(2). Differential test responding disclosed memory of the two-item samples, with birds showing earlier and greater control by the second item than the first (Experiment 1). When the retention interval separating the second item of the sample sequence from the test stimulus was lenghtened from .5 to 2.0 or 4.0 sec, a systematic loss of stimulus control resulted; however, when varied over the same temporal range, the interval between the two items of the sample sequence had a much smaller effect, or none at all (Experiment 2). These results support an account of response-sequence differentiation that stresses short-term memory of organized behavior patterns.

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