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Several types of contrast effects have been identified including incentive contrast, anticipatory contrast, and behavioral contrast. Clement, Feltus, Kaiser, and Zentall (2000) proposed a type of contrast that appears to be different from these others and called it within-trial contrast. In this form of contrast the relative value of a reinforcer depends on the events that occur immediately prior to the reinforcer. Reinforcers that follow relatively aversive events are preferred over those that follow less aversive events. In many cases the delay reduction hypothesis proposed by Fantino (1969) also can account for such effects. The current experiments provide a direct test of the delay reduction and contrast hypotheses by manipulating the schedule of reinforcement while holding trial duration constant. In Experiment 1, preference for fixed-interval (FI) versus differential-reinforcement-of-other-behavior (DRO) schedules of reinforcement was assessed. Some pigeons preferred one schedule over the other while others demonstrated a position (side) preference. Thus, no systematic preference was found. In Experiment 2, a simultaneous color discrimination followed the FI or DRO schedule, and following training, preference was assessed by presenting the two positive stimuli simultaneously. Consistent with the contrast hypothesis, pigeons showed a significant preference for the positive stimulus that in training had followed their less preferred schedule.Key words: within-trial contrast, contrast, stimulus preference, delay reduction, cognitive dissonance, key peck, pigeonsContrast refers to a comparison between two conditions in which the difference between them is amplified by the presence of the other. Three forms of contrast have been identified in the literature: incentive contrast, anticipatory contrast, and behavioral contrast.Incentive contrast occurs when there is a sudden, unexpected increase or decrease in reinforcement resulting in an overreaction to the change relative to a control group that experienced the final magnitude of reinforcement from the start. For example, Crespi (1942) trained rats to run for a large food reinforcer and then shifted them to a smaller reinforcer. Another group of rats was trained to run for a small food reinforcer and was then shifted to a larger amount of food. Running speed was compared to rats that had not experienced a shift in reinforcer magnitude. Rats that were shifted to smaller reinforcers ran slower than rats that had been trained to run for small reinforcers from the start (negative incentive contrast), and rats that were shifted to larger reinforcers ran faster than rats that had been trained with large reinforcers from the start (positive incentive contrast).A second form of contrast, anticipatory contrast, is observed when there are repeated predictable changes in reinforcer magnitude. Anticipatory contrast is assessed in anticipation of the change in reinforcement, and the dependent measure is typically a consummatory response rather than running speed. For exampl...

When behavior suggests that the value of a reinforcer depends inversely on the value of the events that precede or follow it, the behavior has been described as a contrast effect. Three major forms of contrast have been studied: incentive contrast, in which a downward (or upward) shift in the magnitude of reinforcement produces a relatively stronger downward (or upward) shift in the vigor of a response; anticipatory contrast, in which a forthcoming improvement in reinforcement results in a relative reduction in consummatory response; and behavioral contrast, in which a decrease in the probability of reinforcement in one component of a multiple schedule results in an increase in responding in an unchanged component of the schedule. Here we discuss a possible fourth kind of contrast that we call within-trial contrast because within a discrete trial, the relative value of an event has an inverse effect on the relative value of the reinforcer that follows. We show that greater effort, longer delay to reinforcement, or the absence of food all result in an increase in the preference for positive discriminative stimuli that follow (relative to less effort, shorter delay, or the presence of food). We further distinguish this within-trial contrast effect from the effects of delay reduction. A general model of this form of contrast is proposed in which the value of a primary or conditioned reinforcer depends on the change in value from the value of the event that precedes it.

Vasconcelos, Urcuioli, and Lionello-DeNolf (2007) report the results of five experiments that fail to replicate the results of our within-trial contrast study (Clement, Feltus, Kaiser, & Zentall, 2000) and suggest that our results may represent a Type I Error. We believe that this conclusion is not warranted because (a) there is considerable evidence in support of the effect and (b) the amount of training that they gave to their pigeons prior to test may not have been sufficient to observe the effect reliably. We suggest that when sufficient training is provided, reliable contrast can be found.