Response factors in delay discounting: evidence for Pavlovian influences on delay discounting in pigeons

Suboptimal choice by pigeons is eliminated when key-pecking behavior is replaced by treadle-pressing

In the study of suboptimal choice, a reliable result is that pigeons strongly prefer an alternative that signals whether a reinforcer will be delivered or not over another alternative without that information even if the first provides a lower probability of reinforcement. In the aforementioned research, key pecking has been the operant response and illuminated keys the discriminative stimuli. In the present study we modified both of these aspects of the procedure in order to analyze the generality of suboptimal preferences of pigeons and to investigate the effect of changes in the incentive salience of the discriminative stimuli. To accomplish this, we presented pigeons a choice situation with the same parameters of reinforcement than previous research, but with treadle pressing as the choice response and ambient lights as discriminative stimuli. Under these conditions, most of the pigeons showed optimal behavior and a high degree of discrimination of the stimuli associated with the discriminative alternative. A control condition with key pecking as choice response and keylights as discriminative stimuli showed that the same pigeons turned to be suboptimal, a result that discards the possibility that the optimality found in the main condition was a consequence of a particular characteristic of our sample of subjects or of our procedure. We discuss the influence that the attribution of incentive salience to the discriminative stimuli has on suboptimal choice in both pigeons and rats.

Towards validation of delay discounting in the pigeon

A series of procedures were conducted in an attempt to assess various forms of validity related to the use of pigeons in research on delay discounting. In separate experimental arrangements, pigeons pressed a treadle, pecked a lit key, pecked a darkened key, or pecked a lit key with a hold as the required response. First, the obtained results were consistent with what would be necessary if the construct of delay discounting were being measured, which provides evidence of face validity. Second, criterion validity was assessed by comparing individual differences in rates of discounting across procedures. Third, to assess internal validity, each pigeon repeated the Treadle, Key Peck, and Dark Key procedures. Again, at both the aggregate and individual levels, the obtained indifference points did not differ systematically between replications. Finally, to assess external validity, discounting was observed regardless of the procedure, where the patterns of data at the aggregate level, and generally at the individual level, were orderly and well described by a hyperbolic function. In addition, rates of discounting were similar when pigeons pecked a lit key, a dark key, or a key with a hold; and each of those rates of discounting tended to be steeper than when treadle pressing. Generally speaking, pigeons that discounted relatively steeply on one procedure also tended to discount relatively steeply on the other procedures. The procedures evidence some of the necessary elements involved with the use of pigeons in research on delay discounting.

Rats behave optimally in a sunk cost task

The sunk cost effect has been defined as the tendency to persist in an alternative once an investment of effort, time or money has been made, even if better options are available. The goal of this study was to investigate in rats the relationship between sunk cost and the information about when it is optimal to leave the situation, which was studied by Navarro and Fantino (2005) with pigeons. They developed a procedure in which different fixed-ratio schedules were randomly presented, with the richest one being more likely; subjects could persist in the trial until they obtained the reinforcer, or start a new trial in which the most favorable option would be available with a high probability. The information about the expected number of responses needed to obtain the reinforcer was manipulated through the presence or absence of discriminative stimuli; also, they used different combinations of schedule values and their probabilities of presentation to generate escape-optimal and persistence- optimal conditions. They found optimal behavior in the conditions with presence of discriminative stimuli, but non-optimal behavior when they were absent. Unlike their results, we found optimal behavior in both conditions regardless of the absence of discriminative stimuli; rats seemed to use the number of responses already emitted in the trial as a criterion to escape. In contrast to pigeons, rats behaved optimally and the sunk cost effect was not observed.

Delay discounting: Pigeon, rat, human--does it matter?

Delay discounting refers to the decrease in subjective value of an outcome as the time to its receipt increases. Across species and situations, animals discount delayed rewards, and their discounting is well-described by a hyperboloid function. The current review begins with a comparison of discounting models and the procedures used to assess delay discounting in nonhuman animals. We next discuss the generality of discounting, reviewing the effects of different variables on the degree of discounting delayed reinforcers by nonhuman animals. Despite the many similarities in discounting observed between human and nonhuman animals, several differences have been proposed (e.g., the magnitude effect; nonhuman animals discount over a matter of seconds whereas humans report willing to wait months, if not years before receiving a reward), raising the possibility of fundamental species differences in intertemporal choice. After evaluating these differences, we discuss delay discounting from an adaptationist perspective. The pervasiveness of discounting across species and situations suggests it is a fundamental process underlying decision making.