DETERMINATION OF DISCOUNT FUNCTIONS IN RATS WITH AN ADJUSTING-AMOUNT PROCEDURE

An adjusting-amount procedure was used to measure discounting of reinf orcer Value by delay. Eight rats chose between a varying amount of imm ediate water and a fixed amount of water given after a delay. The amou nt of immediate water was systematically adjusted as a function of the rats' previous choices. This procedure was used to determine the indi fference point at which each rat chose the immediate amount and the de layed amount with equal frequency. The amount of immediate water at th is indifference point was used to estimate the value of the delayed am ount of water. In Experiment 1, the effects of daily changes in the de lay to the fixed reinforcer (100 mu l of water delivered after 0, 2, 4 , 8, or 16 s) were tested. Under these conditions, the rats reached in difference points within the first 30 trials of each 60-trial session. In Experiment 2, the effects of water deprivation level on discountin g of value by delay were assessed. Altering water deprivation level af fected the speed of responding but did not affect delay discounting. I n Experiment 3, the effects of varying the magnitude of the delayed wa ter (100, 150, and 200 mu l) were tested. There was some tendency for the discounting function to be steeper for larger than for smaller rei nforcers, although this difference did not reach statistical significa nce. In all three experiments, the obtained discount functions were we ll described by a hyperbolic function. These experiments demonstrate t hat the adjusting-amount procedure provides a useful tool for measurin g the discounting of reinforcer value by delay.