Effects of Food Restriction and Pre-Training Length on Delay Discounting in Male Wistar Rats

Abstract

Impulsivity is often measured using delay discounting, a task in which individuals must select between smaller, sooner and larger, later reinforcers. The magnitude effect is the tendency for individuals to discount larger delayed reinforcers less steeply than smaller ones. In terms of food restriction, the magnitude effect predicts that restricted subjects should behave less impulsively, because restriction may increase food reinforcer value and therefore increase subjects’ propensity to select larger, later reinforcers. However, previous research examining the effects of reinforcer restriction has not demonstrated this. In the present research, 12 male Wistar rats were exposed to a delay discounting procedure under both free-feeding and food restriction conditions. Subjects selected more larger, later reinforcers when restricted to ~85% of their free-feeding weights, which is consistent with the magnitude effect. In addition, we systematically examined the effects of shortening the second training phase of a common delay discounting procedure (i.e., Evenden & Ryan, 1996, 1999), during which responses produced the immediate delivery of either one or five reinforcers. The number of sessions in the second training phase did not affect discounting once delays were imposed, indicating that future researchers may shorten this training phase without affecting impulsive responding.