Addition versus deletion as a signal

Subjects typically show superior discriminative performance when a distinguishing feature appears on reinforced rather than nonreinforced trials. The phenomenon is usually attributed to the relative predictiveness of the reinforcer by different stimulus elements. However, stimulus addition may be more effective than stimulus deletion as a signal. By removing the standard intertriai intervals, we made addition and deletion equally predictive of the reinforcer in four operant experiments involving between- and within-subject comparisons. Pigeons consistently performed better on operant discriminations when the addition rather than deletion of an auditory or visual stimulus served as the cue for food. This general finding persisted despite manipulation of the relative duration and localizability of the signal. Thus mere presence as opposed to absence plays a role in the feature-positive superiority, an outcome that may reflect a fundamental, biologically based difference between addition and deletion as effective signals of reinforcement.