Influence of amount and delay of reward on choice and response rate: A free-operant, multiple-schedule analogue of a discrete-trial procedure

The current study explored a free-operant analogue of discrete-trial procedures to study the effects of amount and delay of reinforcement on choice and response rate. Rats responded on a multiple variable-interval (VI) 45-s, 45-s schedule, with interspersed choice probe trials. Comparison of relative response rates and percentage of choice revealed some discrepancies between the free-operant analogue and discrete-trial procedures. Amount of reward controlled choice behavior when the ratios of delays were similar. When reward delays were more discrepant, delay length controlled choice behavior. Whereas the percentage of choice was larger for the larger magnitude reward, the relative rate of response for the larger magnitude was less than .50. In contrast, when the percentage of choice generally fell to below 50% (with large amount and large delay differences between alternatives), relative response rate indicated a preference for the larger amount alternative. This study shows the feasibility and utility of a free-operant analogue of discrete-choice studies that could be used to develop an analysis of preference.