Adulteration of sucrose with citric acid: Effect on reinforcing value, examined using an adjusting-magnitude schedule of reinforcement

According to the Multiplicative Hyperbolic Model (MHM), the value of a reinforcer is an increasing hyperbolic function of its size (q). Recently reported results indicated that dilution of a sucrose solution reduced its reinforcing value by increasing the 'size-sensitivity' parameter of this function and reducing its maximum. The present experiment examined whether adulterating a sucrose solution with citric acid would have a similar effect on the hyperbolic function. Rats were trained under an adjusting-magnitude schedule in which a response on lever B delivered a fixed volume of a sucrose/citric-acid mixture, while a response on lever A delivered a 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. q(B) was varied across four phases of the experiment and the corresponding indifference magnitudes of A were measured. The results failed to support the hypothesis that adulteration with citric acid resulted in devaluation of sucrose. An alternative hypothesis derived from MHM, that the value of the mixture reflected the summation of positive (sucrose) and negative (citric acid) values, is discussed.