Effects of sucrose concentration on choice in an adjusting-magnitude schedule

Rats were trained under a discrete-trials adjusting-magnitude schedule in which a response on lever A delivered either a larger or a smaller sucrose reinforcer (q_A1 = 8 μl, q_A2 = 64 μl) with equal probability, while a response on B delivered a reinforcer whose size q_B was adjusted according to the rats' choices. When A was preferred in a given block of trials, q_B was increased in the following block; when B was preferred, q_B was reduced in the following block. The oscillating changes in q_B, analysed by the Fourier transform, could be described by a power spectrum whose dominant frequency corresponded to a period of 40-50 trial blocks. The equilibrium value of q_B (q_B(50)) was inversely related to sucrose concentration; it significantly exceeded the arithmetic mean of q_A1 and q_A2 when the concentration was 0.2 or 0.4 M, but not when it was 0.8 or 1.6 M. Analysis by mixed-effects modelling revealed a trend for the power of oscillation of q_B to increase monotonically with sucrose concentration; the period of oscillation was not systematically related to sucrose concentration. These results are consistent with predictions derived from a revised version of the multiplicative hyperbolic model of intertemporal choice.