Temporal discounting and inter-temporal choice in rhesus monkeys

Jaewon Hwang

doi:10.3389/neuro.08.009.2009

Abstract

Humans and animals are more likely to take an action leading to an immediate reward than actions with delayed rewards of similar magnitudes. Although such devaluation of delayed rewards has been almost universally described by hyperbolic discount functions, the rate of this temporal discounting varies substantially among different animal species. This might be in part due to the differences in how the information about reward is presented to decision makers. In previous animal studies, reward delays or magnitudes were gradually adjusted across trials, so the animals learned the properties of future rewards from the rewards they waited for and consumed previously. In contrast, verbal cues have been used commonly in human studies. In the present study, rhesus monkeys were trained in a novel inter-temporal choice task in which the magnitude and delay of reward were indicated symbolically using visual cues and varied randomly across trials. We found that monkeys could extract the information about reward delays from visual symbols regardless of the number of symbols used to indicate the delay. The rate of temporal discounting observed in the present study was comparable to the previous estimates in other mammals, and the animal's choice behavior was largely consistent with hyperbolic discounting. Our results also suggest that the rate of temporal discounting might be influenced by contextual factors, such as the novelty of the task. The flexibility furnished by this new inter-temporal choice task might be useful for future neurobiological investigations on inter-temporal choice in non-human primates.

Highlights

The rewards that humans and animals seek to obtain are often not delivered immediately after the required actions are completed
We found that monkeys could extract the information about reward delays from visual symbols regardless of the number of symbols used to indicate the delay.The rate of temporal discounting observed in the present study was comparable to the previous estimates in other mammals, and the animal’s choice behavior was largely consistent with hyperbolic discounting
MODELS OF TEMPORAL DISCOUNTING Reward resulting from a particular action is often delayed in real life

Summary

Introduction

The rewards that humans and animals seek to obtain are often not delivered immediately after the required actions are completed In such cases, the subjective desirability or utility of the expected reward decreases with its delay, and this is referred to as temporal discounting. During inter-temporal choice in which the decision makers choose between rewards delivered after unequal delays, they might in some cases prefer a small but immediate reward to a larger but more delayed reward. Such impulsive choices can be often parsimoniously accounted for by a discount function, which is defined as the fraction of the subjective value of a delayed reward relative to that of the same reward delivered immediately. High discount rate underlies a number of psychiatric disorders, including substance abuse and pathological gambling (see Reynolds, 2006)

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Results

Conclusion