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The effectiveness of a stimulus as a conditioned reinforcer depends on the temporal context of reinforcement, that is, the overall rate of reinforcement in the situation. The dominant view has been that context determines the learned value of a stimulus directly, according to delay-reduction theory. By contrast, the contextual choice model (CCM) maintains that value is independent of context and incorporates the effects of context on choice in the framework of the matching law. The authors report 2 experiments with pigeons as subjects that use transfer tests to assess the value of stimuli in the concurrent-chains procedure. Results strongly support the assumption of CCM that pigeons learn the temporal relations between events independently of context but that context modulates the expression of that learning as choice.

Four experiments using rats as subjects investigated the claim of Williams (1996) that cue competition results from an associative acquisition deficit, rather than a performance deficit. In Experiment 1, extinction of an overshadowing stimulus following overshadowing treatment increased responding to the overshadowed stimulus, thereby replicating prior observations with new parameters. In Experiment 2, an overshadowed stimulus failed to support second-order conditioning unless the overshadowing stimulus received prior extinction treatment. Experiment 3 replicated the recovery from overshadowing effect seen in Experiment 1 using a sensory preconditioning procedure. Most important, in Experiment 4 an overshadowed stimulus failed to block conditioned responding to a novel CS, but blocking by the overshadowed cue was observed following posttraining extinction of the overshadowing stimulus. These results, as well as those of Williams, are discussed in terms of traditional and more recent acquisition-focused models as well as an extension of the comparator hypothesis ( Denniston, Savastano, & Miller, 2001). CUE COMPETITION AS A RETRIEVAL DEFICITStudents of associative learning have long been interested in the conditions under which conditioned stimuli (CSs) trained together will compete with one another for behavioral control. One example of cue competition is the so-called "blocking" effect (Kamin, 1968). In a typical blocking experiment, a CS (A) is paired with an unconditioned stimulus (US) in Phase 1 (i.e., A→US); then in Phase 2, CS A and another stimulus (CS X) are paired with the US (i.e., AX→US). Blocking is evidenced by reduced behavioral control by CS X relative to subjects that received Phase 1 training with an irrelevant stimulus (i.e., B→US). Most contemporary theories of learning explain blocking and other forms of cue competition as a deficit in acquiring an X-US association. For example, Rescorla and Wagner (1972) explained blocking as a result of CS A accruing much of the associative value supportable by the US during Phase 1. This strong A-US association leaves less potential associative strength to the US available for assignment during Phase 2 than if both A and X were novel at the time of Phase 2 training.

In two experiments, pigeons were trained with concurrent‐chains schedules, wherein responding to equal initial links measured preference between variable‐interval terminal links. Absolute terminal‐link duration was varied by keeping constant the difference between the terminal‐link delays and forcing their ratio to change. Delay‐reduction theory scales value relative to a common temporal context and requires that delay differences control choice. Thus, preference should remain invariant. Most competing accounts, including the matching law and a strong form of Weber's law, require that preference vary with the delay ratio. Experiment 1 employed standard concurrent chains, in which terminal‐link position and color were confounded. Although average preference remained constant, individual preferences were highly variable and inconsistent, possibly due to carryover of position biases across conditions. In an attempt to reduce variability, Experiment 2 used a modified concurrent‐chains procedure. Preference at different terminal‐link durations was assessed simultaneously to prevent order effects, and terminal‐link position was alternated randomly across trials to minimize the impact of position biases. In Experiment 2, both individual and mean preferences showed the constant‐difference invariance. Overall, choice was controlled by terminal‐link differences, not ratios.