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In 3 autoshaping experiments, pigeons were trained with an A+, AB-, ABC+ task in which Stimulus A signaled food, Stimulus Compound AB signaled no food, and Stimulus Compound ABC signaled food. Contrary to the prediction from elemental theories of conditioning, responding on a Stimulus Compound BC trial was not less than responding on a Stimulus C trial in testing. However, Stimulus B attenuated responding to another excitor (Experiment 2), and a separately trained inhibitor could attenuate responding to Stimulus C (Experiment 3). These results were consistent with a configural theory. Pavlov's (1927) conditioned inhibition procedure is one of the most effective ways to establish conditioned inhibition. In this procedure, an excitor (A) is paired with another stimulus (B) without reinforcement (A+, AB-; a plus sign signifies reinforcement and a minus sign nonreinforcement). According to elemental theories of Pavlovian conditioning such as the Rescorla-Wagner model , after A+, AB-training B acquires negative (i.e., inhibitory) associative strength that offsets the positive (i.e., excitatory) associative strength of A . The inhibition to B is evident by its ability to attenuate conditioned responses (CRs) to another excitor (C) on BC trials (subtractive summation), and by its retardation in subsequently becoming an excitor by itself through B + training (Hearst, 1972;Rescorla, 1969).On the other hand, a conflgural model of Pavlovian conditioning (Pearce, 1987) claims that all events perceived simultaneously by animals work together as a unitary stimulus that gains its own associative strength. Furthermore, the active associative strength of each unitary stimulus is determined by an aggregate of its own associative strength and associative strengths generalized from other unitary stimuli through their similarity.Application of the Pearce model to the A+, ABprocedure gives A positive associative strength. As this positive strength generalizes to the nonreinforced AB, it acquires negative associative strength, which in turn generalizes to A. That results in A acquiring more positive strength

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