Trade-offs in stimulus control in a temporal discrimination task

To study how multiple stimuli may control discriminative behavior, we exposed fifteen pigeons to a symbolic matching-to-sample task with three samples that differed only in duration (2, 6, and 18s) and two keylight colors as comparisons. The pigeons learned to choose one comparison after the shortest sample, and the other comparison after the intermediate and longest samples. A 30-s intertrial interval (ITI), illuminated with the houselight, separated the trials. Previous data has suggested that, in this arrangement, both sample keylight and the ITI houselight influence choice. To assess this joint stimulus control, we introduced two tests. In the no-sample test, the keylight was not illuminated and the comparisons followed the ITI immediately; in the dark-ITI test, the houselight was not illuminated. Results confirmed that both stimuli influenced choice, with an apparent trade-off between them: The more a pigeon relied on one stimulus, the less it seemed to rely on the other. We discuss potential models of joint stimulus in temporal discrimination tasks.