Pigeons received delayed-matching-to-sample trials in which the samples consisted of a compound of color and location (red on left key, green on right key). The comparisons were either red and green on the right and left keys (location of color counterbalanced over trials) or two yellow keys. On red-green comparison trials, choice of the matching color was reinforced; on yellow comparison trials, choice of the key that matched the sample location was reinforced. Matching accuracy on location trials exceeded that on color trials both during acquisition (Experiment 1) and when the delay interval was varied (Experiment 2). A control experiment (Experiment 3) demonstrated that this superiority was not due to the presence of location competition on color test trials but not color competition on location test trials. These results, in conjunction with other recent reports in the literature, suggest that control by the elements of a compound sample is often asymmetrical.
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