Further observations on the reinforcing value of sucrose solutions: Interaction between quantity and concentration

According to the Multiplicative Hyperbolic Model of reinforcer value (MHM), the value of a reinforcer is an increasing hyperbolic function of its size. In addition it is generally accepted that in the case of soluble reinforcers such as sucrose, value is an increasing function of molar concentration. The present experiment examined the interaction between size and concentration on the effectiveness of sucrose reinforcers in an operant choice paradigm. Rats were trained under an adjusting-magnitude schedule in which a response on lever B delivered a fixed volume of a sucrose solution, while a response on lever A delivered a 0.8-M sucrose solution, 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. The concentration and volume of B (c_B, q_B) were varied across six phases of the experiment and the corresponding indifference magnitudes of A (q_A(50)) were measured. Log q_A(50) increased and q_B/q_A(50) declined as approximately linear functions of log c_B, consistent with the supposition that reinforcer value was determined by multiplicative combination of hyperbolic expressions representing concentration and volume.