Choice between different concentrations of sucrose in an adjusting-magnitude schedule: Evidence for...

According to the Multiplicative Hyperbolic Model (MHM), an extension of Mazur's (1987) model of delay discounting, the value of a reinforcer is an increasing hyperbolic function of its size (q). This experiment tested a prediction based on a revised version of MHM (MHM-R), according to which the maximum value may differ between different reinforcers, a possibility not envisaged in MHM. Rats were trained under a discrete-trials adjusting-magnitude schedule in which a response on lever B delivered a dilute sucrose reinforcer (0.2 M) of fixed volume, q_B, while a response on lever A delivered a more concentrated sucrose solution (0.4 M) the volume of which, q_A, was adjusted according to the rats' choices. When B was preferred in a given block of trials, q_A was increased in the following block; when A was preferred, q_A was reduced in the following block. q_B was varied across four phases of the experiment and the corresponding indifference magnitudes of A (q_A(50)) were measured. In keeping with expectations based on both MHM and MHM-R, q_A(50) increased approximately linearly with q_B, and 1/q_A(50) with 1/q_B. The ratio q_B/q_A(50) increased linearly with q_B with a positive intercept, a finding that is consistent with MHM-R but not with MHM.