Within-trial contrast: pigeons prefer conditioned reinforcers that follow a relatively more rather than a less aversive event

Abstract

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.

Fig 1. Design of experiment by Clement, Feltus, Kaiser, & Zentall (2000) in which one pair of discriminative stimuli followed 20 pecks and the other pair of discriminative stimuli followed 1 peck during training (top).

Example of two test trials (bottom) involving choice of the two S+ stimuli (left) and choice of the two S– trials (right). Other trials (not shown) involved no initial response requirement or a 20-peck response requirement.

Fig 2. Results obtained by Clement, Feltus, Kaiser, & Zentall (2000).

Pigeons preferred the S+ and the S- that in training followed 20 pecks over the S+ and S– that followed 1 peck. FR 1, 20, and 0 indicate the response requirement that preceded choice between the S+ stimuli from training or between the S– stimuli from training.

Fig 3. Results obtained by Friedrich and Zentall (2004).

Graph shows the increase in preference for the originally nonpreferred feeder as a function of its association with the high-effort (30 peck) response (filled circles). The preferred feeder was associated with the low-effort (1 peck) response. For the control group (open circles), both feeders were associated equally often with the high-effort and low-effort response. The dashed line indicates the baseline preference for the originally nonpreferred feeder.

Fig 4. A model of change in relative value to account for within-trial contrast effects.

According to the model, key pecking results in a negative change in value, V – ΔV₁ for FR 1 and V – ΔV₂₀ for FR 20, whereas obtaining a reinforcer results in a positive change in value, V + ΔV_Rf. The net change in value within a trial depends on the difference between V + ΔV_Rf and V – ΔV₁ on an FR 1 trial, and between V + ΔV_Rf and V – ΔV₂₀ on an FR 20 trial.

Fig 5. Design of experiments by DiGian, Friedrich, & Zentall (2004) in which one pair of discriminative stimuli followed a delay and the other pair of discriminative stimuli followed the absence of a delay.

Top: The delay and absence of delay were unsignaled. Bottom: The delay and absence of delay were signaled.

Fig 6. Design of experiments by Friedrich, Clement, & Zentall (2005) in which one pair of discriminative stimuli followed reinforcement and the other pair of discriminative stimuli followed the absence of reinforcement.

Top: Reinforcement and absence of reinforcement were unsignaled. Bottom: Reinforcement and absence of reinforcement were signaled.

Fig 7. Design of experiment by Clement and Zentall (2002, Experiment 1) in which the effect of a signal for potential (rather than actual) effort on preference for the discriminative stimuli that followed was studied.

Fig 8. Design of experiment by Clement and Zentall (2002, Experiment 2) in which the effect of a signal for potential (rather than actual) reinforcement on preference for the discriminative stimuli that followed was studied.

Fig 9. Design of experiment by Clement and Zentall (2002, Experiment 3), in which whether the effect of a signal for potential (rather than actual) reinforcement on preference for the discriminative stimuli that followed was the result of positive contrast, negative contrast, or both was studied.

Fig 10. Schematic representations of the relation between the discriminative stimuli and reinforcement as a function of the absolute reduction in delay to reinforcement (top) and the relative reduction in delay to reinforcement (bottom) signaled by the discriminative stimuli.

The time needed to make the required number of initial pecks is indicated by the white bars. The time during which the simultaneous discrimination is presented is indicated by the shaded bars. Notice that the absolute time from the presentation of the simultaneous discrimination to reinforcement is the same for both trial types (top) but the relative time (bottom) is longer on trials when fewer responses are required.