Richards, J. B., Mitchell, S. H., de Wit, H., & Seiden, L. S. (1997).
Determination of discount functions in rats with an adjusting-amount procedure.
Journal of the Experimental Analysis of Behavior,
67, 353-366.
An adjusting-amount procedure was used to measure discounting of
reinforcer value by delay. Eight rats chose between a varying
amount of immediate water and a fixed amount of water given after
a delay. The amount of immediate water was systematically
adjusted as a function of the rats' previous choices. This
procedure was used to determine the indifference point at which
each rat chose the immediate amount and the delayed amount with
equal frequency. The amount of immediate water at this
indifference point was used to estimate the value of the delayed
amount of water. In Experiment 1, the effects of daily changes in
the delay to the fixed reinforcer (100 µl of water delivered
after 0, 2, 4, 8, or 16 s) were tested. Under these conditions,
the rats reached indifference points within the first 30 trials
of each 60-trial session. In Experiment 2, the effects of water
deprivation level on discounting of value by delay were assessed.
Altering water deprivation level affected the speed of responding
but did not affect delay discounting. In Experiment 3, the
effects of varying the magnitude of the delayed water (100, 150,
and 200 µl) were tested. There was some tendency for the
discounting function to be steeper for larger than for smaller
reinforcers, although this difference did not reach statistical
significance. In all three experiments, the obtained discount
functions were well described by a hyperbolic function. These
experiments demonstrate that the adjusting-amount procedure
provides a useful tool for measuring the discounting of
reinforcer value by delay.
Key words: choice, self-control, impulsivity, delayed
reinforcement, reinforcer magnitude, nose poke, rat