Sources of visual interference in delayed matching-to-sample with pigeons.

In two experiments, independent groups of pigeons were trained on an identity matching task involving line orientations as sample and comparison stimuli. For some birds an overhead houselight was illuminated continuously throughout each training session. For other birds the houselight was never illuminated during training sessions. During subsequent testing, the lighting conditions during the delay were the same as in training on some trials, but on other trials they were opposite those of training during either the entire delay (Experiment 1) or during a portion of the delay (Experiment 2). In birds trained with the houselight off, turning the houselight on during the delay produced a large and enduring disruption in matching accuracy. On the other hand, in birds trained with the houselight on, turning the houselight off during the delay produced only a moderate and temporary disruption in matching accuracy. These findings are inconsistent with the prevailing view that retroactive interference in pigeons is a function of a change in illumination level relative to that which prevailed during training. In pigeons, as in monkeys, sustained retoactive interference effects obtain only when the level of illumination is increase during the delay interval.