Genome-wide association study of delay discounting in Heterogeneous Stock rats.

Delay discounting refers to the behavioral tendency to devalue rewards as a function of their delay in receipt. Heightened delay discounting has been associated with substance use disorders and multiple co-occurring psychopathologies. Human and animal genetic studies have established that delay discounting is heritable, but only a few associated genes have been identified. We aimed to identify novel genetic loci associated with delay discounting through a genome-wide association study (GWAS) using Heterogeneous Stock (HS) rats, a genetically diverse outbred population derived from eight inbred founder strains. We assessed delay discounting in 650 male and female HS rats using an adjusting amount procedure in which rats chose between smaller immediate sucrose rewards or a larger reward at various delays. Preference switch points were calculated and both exponential and hyperbolic functions were fitted to these indifference points. Area under the curve (AUC) and the discounting parameter k of both functions were used as delay discounting measures. GWAS for AUC, exponential k, and one indifference point identified significant loci on chromosomes 20 and 14. The gene Slc35f1, which encodes a member of the solute carrier family, was the sole gene within the chromosome 20 locus. That locus also contained an eQTL for Slc35f1, suggesting that heritable differences in the expression might be responsible for the association with behavior. Adgrl3, which encodes a latrophilin subfamily G-protein coupled receptor, was the sole gene within the chromosome 14 locus. These findings implicate novel genes in delay discounting and highlight the need for further exploration.

Individual differences in degree of discounting: Do different procedures and measures assess the same construct?

The present study compared the two most prominent procedures for measuring delay discounting, the Adjusting-Amount procedure and the Monetary Choice Questionnaire (MCQ). Of interest was whether the two procedures measure the same construct. Results obtained from two online samples recruited using the Prolific (N = 150) and MTurk (N = 243) platforms revealed generally similar results for both procedures. Regardless of the procedure, the sample, the reward amount, and whether the discounting measure used was theoretically based (i.e., log k) or was atheoretical (i.e., area under the curve, proportion of choices of the delayed reward option), the degree of discounting on the Adjusting-Amount procedure was highly correlated with the degree of discounting on the MCQ, consistent with the hypothesis that both procedures assess the same construct.

Genome-wide association study of delay discounting in Heterogenous Stock rats.

Delay discounting refers to the behavioral tendency to devalue rewards as a function of their delay in receipt. Heightened delay discounting has been associated with substance use disorders, as well as multiple co-occurring psychopathologies. Genetic studies in humans and animal models have established that delay discounting is a heritable trait, but only a few specific genes have been associated with delay discounting. Here, we aimed to identify novel genetic loci associated with delay discounting through a genome-wide association study (GWAS) using Heterogenous Stock rats, a genetically diverse outbred population derived from eight inbred founder strains. We assessed delay discounting in 650 male and female rats using an adjusting amount procedure in which rats chose between smaller immediate sucrose rewards or a larger reward at variable delays. Preference switch points were calculated for each rat and both exponential and hyperbolic functions were fitted to these indifference points. Area under the curve (AUC) and the discounting parameter k of both functions were used as delay discounting measures. GWAS for AUC, exponential k, and indifference points for a short delay identified significant loci on chromosomes 20 and 14. The gene Slc35f1, which encodes a member of the solute carrier family of nucleoside sugar transporters, was the only gene within the chromosome 20 locus. That locus also contained an eQTL for Slc35f1, suggesting that heritable differences in the expression of that gene might be responsible for the association with behavior. The gene Adgrl3, which encodes a member of the latrophilin family of G-protein coupled receptors, was the only gene within the chromosome 14 locus. These findings implicate novel genes in delay discounting and highlight the need for further exploration.

Pigeons' delay discounting functions established using a concurrent-chains procedure.

Several studies have examined discounting by pigeons and rats using concurrent-chains procedures, but the results have been inconsistent. None of these studies, however, has established that discounting functions derived from estimates of indifference points can be obtained with a concurrent-chains procedure, so their validity remains in doubt. The present study used a concurrent-chains procedure within sessions combined with an adjusting-amount procedure across sessions to determine the present, subjective values of food reinforcers to be obtained after a delay. Discounting was well described by the hyperbolic discounting function, suggesting that the concurrent-chains procedure and the more typical adjusting-amount procedure are measuring the same process. Consistent with previous studies with rats and pigeons using adjusting-amount procedures, no significant effect of the amount of the delayed reinforcer on the degree of discounting was observed, suggesting that the amount effect may be unique to humans although consistent with the view that animals' choices are controlled by the relative, rather than the absolute, value of reinforcers.

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.

Delay discounting in Lewis and Fischer 344 rats: steady-state and rapid-determination adjusting-amount procedures.

Lewis rats have been shown to make more impulsive choices than Fischer 344 rats in discrete-trial choice procedures that arrange fixed (i.e., nontitrating) reinforcement parameters. However, nontitrating procedures yield only gross estimates of preference, as choice measures in animal subjects are rarely graded at the level of the individual subject. The present study was designed to examine potential strain differences in delay discounting using an adjusting-amount procedure, in which distributed (rather than exclusive) choice is observed due to dynamic titration of reinforcer magnitude across trials. Using a steady-state version of the adjusting-amount procedure in which delay was manipulated between experimental conditions, steeper delay discounting was observed in Lewis rats compared to Fischer 344 rats; further, delay discounting in both strains was well described by the traditional hyperbolic discounting model. However, upon partial completion of the present study, a study published elsewhere (Wilhelm & Mitchell, 2009) demonstrated no difference in delay discounting between these strains with the use of a more rapid version of the adjusting-amount procedure (i.e., in which delay is manipulated daily). Thus, following completion of the steady-state assessment in the present study, all surviving Lewis and Fischer 344 rats completed an approximation of this rapid-determination procedure in which no strain difference in delay discounting was observed.

Risky choice in pigeons: preference for amount variability using a token-reinforcement system.

Pigeons were given repeated choices between variable and fixed numbers of token reinforcers (stimulus lamps arrayed above the response keys), with each earned token exchangeable for food. The number of tokens provided by the fixed-amount option remained constant within blocks of sessions, but varied parametrically across phases, assuming values of 2, 4, 6, or 8 tokens per choice. The number of tokens provided by the variable-amount option varied between 0 and 12 tokens per choice, arranged according to an exponential or rectangular distribution. In general, the pigeons strongly preferred the variable option when the fixed option provided equal or greater numbers of tokens than the variable amount. Preference for the variable amount decreased only when the alternatives provided widely disparate amounts favoring the fixed amount. When tokens were removed from the experimental context, preference for the variable option was reduced or eliminated, suggesting that the token presentation played a key role in maintaining risk-prone choice patterns. Choice latencies varied inversely with preferences, suggesting that local analyses may provide useful ancillary measures of reinforcer value. Overall, the results indicate that systematic risk sensitivity can be attained with respect to reinforcer amount, and that tokens may be critical in the development of such preferences.

¿El método modula la relación entre descuentos temporal, probabilístico y social? / Do methods modulate the relation among delay, probability and social discounting?

Resumen Los descuentos temporal, probabilístico y social se refieren a la disminución del valor de una recompensa en función de su demora de entrega, probabilidad o distancia social, respectivamente. Una pregunta en el área es si los 3 tipos de descuento comparten un mismo proceso. Por otra parte, se ha sugerido que el método empleado modula las tasas de descuento. Por lo tanto, el propósito del presente estudio fue averiguar si el método empleado contribuye a la correlación entre los 3 tipos de descuento. Cuarenta y cuatro participantes respondieron tareas de descuento temporal, probabilístico y social con 2 métodos diferentes, elección binaria y ajuste de la magnitud. Se encontró que, con el método elección binaria, el descuento social correlacionó con los descuentos temporal y probabilístico, pero los últimos 2 no correlacionaron entre sí. Con el método ajuste de la magnitud, solo se encontró una correlación entre los descuentos temporal y probabilístico. Estos resultados sugieren que el método empleado contribuye a la búsqueda de un proceso común entre los 3 tipos de descuento. Incluso, un proceso común de toma decisiones subyacente a los 3 tipos de descuento podría depender del contexto en que se toma la decisión.

Abstract Delay, probability and social discounting refer to the decrease in the subjective value of a reward as a function of its delay of delivery, probability or social distance, respectively. One question in the area is whether the three types of discounting share the same process or whether they are held by different processes. This question has been investigated by looking at the correlations between the three types of discounting. Moreover, it has been suggested that the method used to obtain the indifference points could modulate the discounting rates. However, the search of a common process underlying the three types of discounting should not be influenced by the method used to obtain the indifference points. Therefore, the purpose of this study was to determine whether the method used to obtain the indifference points contributes to the correlation between the three types of discounting. Forty-four college students responded tasks of delay, probability and social discounting —which used a hypothetical monetary reward— with two different methods: Binary choice and adjusting amount. We found significant correlations between binary-choice and adjusting-amount methods for the three types of discounting. Albeit, a significantly higher delay discounting rate was found with the adjusting-amount method rather than with the binary-choice method, but there were no significant differences between the probability and social discounting rates obtained with both methods. In addition to the previous results, with the binary-choice method it was found that social discounting correlated with delay and probability discounting, but the last two did not correlate between them. In contrast, with the adjusting-amount method, it was only found a correlation between delay and probability discounting. The latter result suggests that the method used to obtain the indifference points contributes to the search of a common process between the three types of discounting. Actually a common decision-making process underlying the three types of discounting could depend on the context in which the choice is made.