Delay discounting of qualitatively different reinforcers in rats

Abstract

Humans discount larger delayed rewards less steeply than smaller rewards, whereas no such magnitude effect has been observed in rats (and pigeons). It remains possible that rats' discounting is sensitive to differences in the quality of the delayed reinforcer even though it is not sensitive to amount. To evaluate this possibility, Experiment 1 examined discounting of qualitatively different food reinforcers: highly preferred versus nonpreferred food pellets. Similarly, Experiment 2 examined discounting of highly preferred versus nonpreferred liquid reinforcers. In both experiments, an adjusting-amount procedure was used to determine the amount of immediate reinforcer that was judged to be of equal subjective value to the delayed reinforcer. The amount and quality of the delayed reinforcer were varied across conditions. Discounting was well described by a hyperbolic function, but no systematic effects of the quantity or the quality of the delayed reinforcer were observed.

Keywords: behavioral economics; choice; delay discounting; lever press, rat; magnitude effect; reinforcer amount; reinforcer quality.

Fig 1

Results of the preference tests for each rat in Experiment 1. Bars show the percentage choice (upper panels) and percentage consumed (lower panels) of the alternative that appears first in the pairs noted along the x-axis. The upper panels show results from the operant-choice preference tests; the lower panels show results from the home-cage preference tests.

Fig 2

Delay discounting functions for the 10-pellet (left panels) and 30-pellet (right panels) conditions for each rat in Experiment 1. Symbols show subjective values of the three types of food reinforcers expressed as a proportion of the delayed amount. Curves represent the best-fitting discounting functions (Equation 1).

Fig 3

Area under the discounting curve for each type of food reinforcer for each rat in Experiment 1. Steeper discounting is indicated by lower values.

Fig 4

Results of the preference tests for each rat in Experiment 2. Bars show the percentage consumed of the alternative that appears first in the pairs noted along the x-axis.

Fig 5

Delay discounting functions for the 100-µL (left panels) and 500-µL (right panels) amount conditions for each rat in Experiment 2. Symbols show subjective values of the saccharin and quinine liquid reinforcers expressed as a proportion of the delayed amount. Curves represent the best-fitting discounting functions (Equation 1).

Fig 6

Area under the discounting curve for the saccharin and for the quinine liquid reinforcers for each rat in Experiment 2. Steeper discounting is indicated by lower values.