Delay discounting in the pigeon: In search of a magnitude effect.

The magnitude effect, where larger outcomes are discounted proportionally less than smaller outcomes, is a well-established phenomenon in delay discounting by human participants. To this point in the literature magnitude effects have not been reliably evidenced in nonhuman animals. , however, used a concurrent-chains arrangement with pigeon and found evidence for a magnitude effect. Grace et al. suggested that in many delay discounting experimental arrangements with nonhuman animals (e.g., adjusting amount, adjusting delay) the organism is not given the opportunity to directly compare outcomes of different sizes. They suggest that because of the lack of direct comparison it is difficult for the organism to determine the relative size of each outcome, which in turn mutes the effect of the amount differences between outcomes. As a test of this "comparison hypothesis," the present experiment was conducted to assess whether the magnitude effect would be evidenced in pigeon when using an adjusting amount procedure where outcomes of different amounts were presented proximally. In the present arrangement, pigeons were presented two choice panels in an operant chamber where each panel was associated with an independent adjusting amount delay discounting task, but with differing outcome amounts (i.e., a 32-food pellet panel and an 8-food pellet panel). In this arrangement the choice panels alternated in their availability within a session from trial block to trial block. The present findings indicate no reliable effect of amount, even when the outcomes were proximal and thus readily comparable. This result suggests that the lack of magnitude effect is not driven by the organism's ability to compare the difference in amount between choice alternatives.

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.

The domain effect in delay discounting: The roles of fungibility and perishability.

There is a growing body of literature demonstrating domain effects where the rate of temporal discounting depends, in part, on the commodity being evaluated. The commodity of money, for example, is typically discounted much less steeply than commodities of entertainment or food. There are several plausible explanations for domain effects: differences in conditioned reinforcer status, degree of fungibility, and differences in metabolic function. While money can be thought of as a conditioned reinforcer exchangeable for a number of different outcomes (highly fungible), comparing money to food (non-fungible) does not separate whether the difference in rates of discounting are due to food having metabolic importance, being perishable, being less fungible, or all of the above. We systematically manipulated the degree of fungibility and perishability of various outcomes and found that while food outcomes tend to be discounted most steeply, the rate of discounting for these outcomes can be moderated by reducing perishability and by increasing fungibility. Important here is that we have identified two independent means of moderating the effect of delay on the value of the outcome.

Discounting of food, sex, and money.

Discounting is a useful framework for understanding choice involving a range of delayed and probabilistic outcomes (e.g., money, food, drugs), but relatively few studies have examined how people discount other commodities (e.g., entertainment, sex). Using a novel discounting task, where the length of a line represented the value of an outcome and was adjusted using a staircase procedure, we replicated previous findings showing that individuals discount delayed and probabilistic outcomes in a manner well described by a hyperbola-like function. In addition, we found strong positive correlations between discounting rates of delayed, but not probabilistic, outcomes. This suggests that discounting of delayed outcomes may be relatively predictable across outcome types but that discounting of probabilistic outcomes may depend more on specific contexts. The generality of delay discounting and potential context dependence of probability discounting may provide important information regarding factors contributing to choice behavior.

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

Pigeons completed a delay-discounting task where in different conditions the required response was either key pecking or treadle pressing. Because of stimulus-reinforcer relations that are known to form between localized visual cues and the delivery of food (e.g., autoshaping), we predicted that there would be steeper rates of discounting with key pecking than treadle pressing. To account for possible effort differences between key pecking and treadle pressing, pigeons also completed a discounting task where multiple key pecks were required to gain access to the food. The rates of discounting for the key peck and effort-equivalence discounting procedures were similar, and both were steeper than the rate of discounting for the treadle-pressing procedure. While it is tacitly assumed that behavior in choice situations is largely under the control of operant contingencies, the present results suggest that when developing animal analogs to study discounting in a discrete-trial choice procedure, the stimulus-reinforcer relations (Pavlovian conditioning) may need to be taken into consideration.

Estimating the subjective value of future rewards: comparison of adjusting-amount and adjusting-delay procedures.

The present study examined whether equivalent discounting of delayed rewards is observed with different experimental procedures. If the underlying decision-making process is the same, then similar patterns of results should be observed regardless of procedure, and similar estimates of the subjective value of future rewards (i.e., indifference points) should be obtained. Two experiments compared discounting on three types of procedure: adjusting-delay (AD), adjusting-immediate-amount (AIA), and adjusting-delayed-amount (ADA). For the two procedures for which discounting functions can be established (i.e., AD and AIA), a hyperboloid provided good fits to the data at both the group and individual levels, and individuals' discounting on one procedure tended to be correlated with their discounting on the other. Notably, the AIA procedure produced the more consistent estimates of the degree of discounting, and in particular, discounting on the AIA procedure was unaffected by the order in which choices were presented. Regardless of which of the three procedures was used, however, similar patterns of results were obtained: Participants systematically discounted the value of delayed rewards, and robust magnitude effects were observed. Although each procedure may have its own advantages and disadvantages, use of all three types of procedure in the present study provided converging evidence for common decision-making processes underlying the discounting of delayed rewards.

Preference reversals with losses.

People who prefer larger, later gains over smaller, sooner gains when considering outcomes far in the future often reverse their preference as the alternatives become closer in time. This finding, which is contrary to a normative economic account of intertemporal choice, has been interpreted as support for hyperboloid discounting, but the results can also be explained by steeper discounting of smaller amounts. The present study is the first to demonstrate that analogous preference reversals occur with losses: People who preferred a smaller, sooner loss over a larger, later loss when the outcomes were far in the future reversed their preference when these alternatives were closer in time. Because there was no magnitude effect (i.e., smaller losses were not discounted more steeply than larger losses), the present findings strongly support the proposition that reversals in preference between delayed outcomes occur because of the hyperboloid shape of the discounting function.

Do adjusting-amount and adjusting-delay procedures produce equivalent estimates of subjective value in pigeons?

The current experiment examined whether adjusting-amount and adjusting-delay procedures provide equivalent measures of discounting. Pigeons' discounting on the two procedures was compared using a within-subject yoking technique in which the indifference point (number of pellets or time until reinforcement) obtained with one procedure determined the value of the corresponding variable in the yoked condition with the other procedure. Behavior on each procedure was well described by a hyperbolic discounting function. Results revealed no systematic differences in the degree of discounting as measured by the discounting rate parameter of the hyperbola in Experiment 1, which used 20-mg pellets. These results were replicated in Experiment 2 using smaller, 14-mg pellets, which potentially yield more precise measurement of indifference points on the adjusting-amount procedure. The finding that estimates of the k parameter in the hyperbolic discounting function obtained with one procedure did not differ systematically from estimates obtained from the same subjects with the other procedure represents strong support for the hypothesis that the same process underlies the discounting of delayed rewards on both adjusting-amount and adjusting-delay procedures.

Discounting of monetary and directly consumable rewards.

We compared temporal and probability discounting of a nonconsumable reward (money) and three directly consumable rewards (candy, soda, and beer). When rewards were delayed, monetary rewards were discounted less steeply than directly consumable rewards, all three of which were discounted at equivalent rates. When rewards were probabilistic, however, there was no difference between the discounting of monetary and directly consumable rewards. It has been reported that substance abusers discount delayed drug rewards more steeply than delayed money, but this difference may reflect special characteristics of drugs or drug abusers, or it may reflect a general property of consumable rewards. The present findings suggest that abused substances (like beer) share the properties of other directly consumable rewards, whereas delayed monetary rewards are special because they are fungible, generalized (conditioned) reinforcers.

Inhibitory control in children with autism spectrum disorder.

Impairments in executive abilities such as cognitive flexibility have been identified in individuals with autism spectrum disorder (ASD). It remains unclear, however, whether such individuals also experience impairments in another executive ability: inhibitory control. In the present study, we administered three inhibitory tasks to 18 children with ASD, 23 siblings of children with ASD, and 25 typically developing children. After controlling for individual differences in age, overall IQ, and processing speed, children with ASD demonstrated impaired performance on two of the three inhibitory tasks. Results suggest that children with ASD experience circumscribed deficits in some but not all aspects of inhibitory control. More generally, the findings underscore the importance of using multiple measures to assess a putative single cognitive ability.

Differential effects of amount on temporal and probability discounting of gains and losses.

In four experiments, we compared the effects of delay, probability, and monetary amount on the subjective value of gains and losses. For delayed gains, smaller amounts were discounted more steeply than larger amounts, whereas the opposite pattern was observed with probabilistic gains. For both delayed and probabilistic losses, however, amount had much smaller and less reliable effects on discounting. Taken together, the pattern of differential magnitude effects leads to delayed gains' being discounted significantly more steeply than delayed losses, but only at smaller amounts, whereas probabilistic gains are discounted significantly more steeply than probabilistic losses, but only at larger amounts. Even though the same hyperbola-like function described both individual and group discounting of delayed and probabilistic gains and losses, the present findings suggest that different processes are involved in discounting positive and negative outcomes. Raw data may be downloaded from www.psychonomic.org/archive.

Discounting of delayed food rewards in pigeons and rats: is there a magnitude effect?

Temporal discounting refers to the decrease in the present, subjective value of a reward as the time to its receipt increases. Results from humans have shown that a hyperbola-like function describes the form of the discounting function when choices involve hypothetical monetary rewards. In addition, magnitude effects have been reported in which smaller reward amounts are discounted more steeply than larger amounts. The present research examines the cross-species generality of these findings using real rewards, namely food pellets, with both pigeons and rats. As with humans, an adjusting amount procedure was used to estimate the amount of immediate reward judged equal in value to a delayed reward. Different amounts of delayed food rewards (ranging from 5 to 32 pellets in pigeons and from 5 to 20 pellets in rats) were studied at delays varying from 1 s to 32 s. A simple hyperbola, similar to the hyperbola-like mathematical function that describes the discounting of hypothetical monetary rewards in humans, described the discounting of food rewards in both pigeons and rats. These results extend the generality of the mathematical model of discounting. Rates of discounting delayed food rewards were higher for pigeons than for rats. Unlike humans, however, neither pigeons nor rats showed a reliable magnitude effect: Rate of discounting did not vary systematically as a function of the amount of the delayed reward.

Is discounting impulsive?. Evidence from temporal and probability discounting in gambling and non-gambling college students.

Discounting tasks were used to evaluate whether gambling and non-gambling college students (categorized based on their scores on the South Oaks Gambling Screen) differed in the degree to which they discounted delayed and probabilistic rewards. Hyperbola-like functions provided equally good descriptions of discounting in both groups. Both groups discounted large delayed amounts less steeply than small delayed amounts, whereas both groups discounted large probabilistic amounts more steeply than small probabilistic amounts. Gamblers discounted probabilistic rewards less steeply than non-gamblers, suggesting that gamblers are impulsive in the sense that they are less affected by risk than non-gamblers. However, gamblers did not discount delayed rewards more steeply than non-gamblers. The results argue against the view that impulsivity is a general trait that includes both an inability to delay gratification and a tendency to take risks.

Economic and biological influences on key pecking and treadle pressing in pigeons.

Pigeons were studied on a two-component multiple schedule in which the required operant was, in different conditions, biologically relevant (i.e., key pecking) or nonbiologically relevant (i.e., treadle pressing). Responding was reinforced on a variable-interval (VI) 2-min schedule in both components. In separate phases, additional food was delivered on a variable-time (VT) 15-s schedule (response independent) or a VI 15-s schedule (response dependent) in one of the components. The addition of response-independent food had different effects on responding depending on the operant response and on the frequency with which the components alternated. When components alternated frequently (every 10 s), all pigeons keypecked at a much higher rate during the component with the additional food deliveries, whether response dependent or independent. In comparison, treadle pressing was elevated only when the additional food was response dependent; rate of treadling was lower when the additional food was response independent. When components alternated infrequently (every 20 min), pigeons key pecked at high rates at points of transition into the component with the additional food deliveries. Rate of key pecking decreased with time spent in the 20-min component when the additional food was response independent, whereas rate of pecking remained elevated in that component when the additional food was response dependent. Under otherwise identical test conditions, rate of treadle pressing varied only as a function of its relative rate of response-dependent reinforcement. Delivery of response-independent food thus had different, but predictable, effects on responding depending on which operant was being studied, suggesting that animal-learning procedures can be integrated with biological considerations without the need to propose constraints that limit general laws of learning.