Self-control choice with electrical stimulation of the brain as a reinforcer

In a discrete-trials procedure, rats chose between a small reinforcer (a low frequency of electrical stimulation of the brain) and a larger reinforcer (a higher frequency of stimulation). The small reinforcer was delivered after a delay that was constant within a condition but varied across conditions. The delay for the large reinforcer was increased or decreased many times a session in order to estimate an indifference point--a delay at which the two alternatives were chosen about equally often. When the indifference points from several conditions were plotted as a function of the delay for the small reinforcer, the resultant "indifference curves" had positive y-intercepts and slopes greater than 1.0. These results are similar to those obtained in previous studies with pigeons as subjects and food as the reinforcer, and they suggest that a hyperbolic equation describes the relation between a reinforcer's delay and its value or effectiveness. The fact that a large reinforcer delayed several seconds was chosen over a small reinforcer delivered almost immediately after a response provides further evidence against a simple reciprocal relation between delay and value.