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. 2009 Oct;82(2):214-8.
doi: 10.1016/j.beproc.2009.06.002. Epub 2009 Jun 18.

Delay discounting of saccharin in rhesus monkeys

Kevin B Freeman  1 , Leonard Green, Joel Myerson, William L Woolverton
Affiliations

Delay discounting of saccharin in rhesus monkeys

Kevin B Freeman et al. Behav Processes. 2009 Oct.

Abstract

The value of a reinforcer decreases as the time until its receipt increases, a phenomenon referred to as delay discounting. Although delay discounting of non-drug reinforcers has been studied extensively in a number of species, our knowledge of discounting in non-human primates is limited. In the present study, rhesus monkeys were allowed to choose in discrete trials between 0.05% saccharin delivered in different amounts and with different delays. Indifference points were calculated and discounting functions were established. Discounting functions for saccharin were well described by a hyperbolic function. Moreover, the discounting rates for saccharin in all six monkeys were comparable to those of other non-human animals responding for non-drug reinforcers. Also consistent with other studies of non-human animals, changing the amount of a saccharin reinforcer available after a 10-s delay did not affect its relative subjective value. Discounting functions for saccharin were steeper than we found in a previous study with cocaine, raising the possibility that drugs such as cocaine may be discounted less steeply than non-drug reinforcers.

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Figures

Figure 1
Figure 1
Percentage choice of an immediate saccharin reinforcer as a function of its amount. The volume of the alternative was always 4.0 ml, delivered after the delay indicated by the symbol (see legend). Note that volume is logarithmically scaled.
Figure 2
Figure 2
Subjective value of a saccharin reinforcer as a function of delay to its delivery. In the six upper panels, the solid curves represent a hyperbolic discounting function (i.e., Equation 1 with A=4.0 and s=1.0) fit to the data from each monkey, and the corresponding estimates of the k parameter and the proportion of variance accounted for (R2) are shown. The bottom panel shows the mean relative subjective values (i.e., subjective value as a proportion of actual amount) for delayed 2.0- and 4.0-ml saccharin reinforcers for all monkeys with data at the indicated points. Vertical lines are s.e.m.
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