Stimuli that signal the absence of reinforcement are paid more attention than are irrelevant stimuli.

According to established theories of attention (e.g., Mackintosh, 1975; Sutherland & Mackintosh, 1971), simple discriminations of the form AX+ BX- result in an increase in attention to stimuli A and B, which are relevant to the outcome that follows them, at the expense of X, which is irrelevant. Experiments that have apparently shown such changes in attention have failed to determine whether attention is enhanced to both A and B, which signal reinforcement and nonreinforcement, respectively, or just to A. In Experiments 1 and 2, pigeons were trained with a number of discriminations of the kind AX+ BX-, before compounds that had been consistently nonreinforced were involved in a subsequent discrimination. Both experiments provided support for theories that propose that more attention is paid to stimuli that consistently signal nonreinforcement than to irrelevant stimuli in simple discriminations.