Is choice bias in delayed matching a consequence of inter-trial interval effects?

Despite their intended goal of simply separating trials, inter-trial intervals have been found to affect choice behavior in delayed matching-to-sample procedures, leading to choice biases. In the present study, we assessed the effects of delay on choice without the potential influence of inter-trial intervals. Sixteen pigeons learned a symbolic matching-to-sample task with samples differing in duration (3 s vs. 9 s) and no inter-trial intervals. In testing, a delay lasting 2.5, 5, 10 or 20 s was introduced between sample and comparisons. There were two differing delay conditions: for Group Light Off, the delay was spent in darkness and, for Group Light On, the delay was illuminated by a houselight. For Group Light Off, as the delay increased, matching accuracy following both samples decreased towards indifference. For Group Light On, matching accuracy remained relatively high in short-sample trials but decreased abruptly in long-sample trials (choose-short effect). Hence, we found that a choice bias may occur even without inter-trial intervals.