Delay-related decision-making is measured by various delay discounting procedures (Table 1), each of which generally involving choices between an immediately-delivered small reward and a delayed larger reward; either the magnitude or delay can be varied orthogonally to yield equivalent frequencies of selection of the two options (i.e., the indifference point).175 Choice behavior can be plotted against delays, yielding a hyperbolic curve that estimates the devaluation of reward value by virtue of the delay to its receipt.176 In human studies, these can include hypothetical or actual monetary rewards, as well as hypothetical negative health outcomes and drug rewards.175, 177 Studies in non-human subjects often employ procedures in which operant responses are used to obtain a reinforcer (e.g. food, water, or drug), ranging from maze-based tests to variations of choice procedures involving delay to reinforcement.30, 178, 179 In both contexts, despite differences in the type and magnitude of reinforcement and/or delay, the discount functions obtained across species are similar, i.e. hyperbolic in shape; however the discounting rate, or the steepness of the curve, varies.180
