Choice between delayed reinforcers in a discrete-trials schedule: the effect of deprivation level.

Choice between two reinforcers differing in magnitude and delay was investigated in rats using a discrete-trials schedule in which the two reinforcers were associated with two levers (A and B); in each session 5 free-choice trials (A and B both available) were interspersed among 44 forced-choice trials (A alone, 22 trials; B alone, 22 trials). In Experiment 1, preference for the more concentrated of two sucrose solutions declined as the delay to that reinforcer was progressively increased. In Experiment 2, progressively increasing the delay to both reinforcers by the same amount resulted in a shift in preference away from the less concentrated solution. In Experiment 3, it was found that the decline in preference for the more concentrated solution as a function of the delay to that reinforcer was steeper when the rats were maintained at 90% than when they were maintained at 80% of their free-feeding body weights. This effect of deprivation level on choice is inconsistent with some current models of "self-control".