Learning to ignore irrelevant stimuli: Variations within and between displays

Pigeons were exposed to stimuli differing in orientation. Some birds received both horizontal and vertical on every trial; others received two horizontals on some trials and two verticals on other trials. These stimuli were irrelevant, i.e., did not predict reliably the availability of reinforcement. When required to learn a discrimination between horizontal and vertical, the birds that had experienced the stimulus variation within each display learned more slowly. This was true whether the test problem was a simultaneous or a successive discrimination. It is argued that this result implies that animals will actively learn to ignore stimuli that are irrelevant.