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

Abstract

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 (qA1 = 8 μl, qA2 = 64 μl) with equal probability, while a response on B delivered a reinforcer whose size qB was adjusted according to the rats’ choices. When A was preferred in a given block of trials, qB was increased in the following block; when B was preferred, qB was reduced in the following block. The oscillating changes in qB, 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 qB (qB(50)) was inversely related to sucrose concentration; it significantly exceeded the arithmetic mean of qA1 and qA2 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 qB 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.