Effects of Delay and Signals on Choice between Delayed Food Alone and Immediate Food with Delayed Shock

Abstract

Much of the research in the area of self-control has examined choice between small immediate reinforcers and large delayed reinforcers, but many problems result from situations in which a single choice produces consequences of conflicting valence: Those in which the immediate outcome is reinforcing and the delayed outcome is aversive. Recent research has evaluated how preference for a large reinforcer which is followed by a delayed shock changes as a function of the delay to shock and how the intensity and duration of delayed shock affects the value of a large reinforcer. The present set of experiments investigated how the value of a food reinforcer followed by delayed shock changes as a function of the delay to shock. Instead of arranging choice between small and large reinforcers, the present experiments arranged choice between two reinforcers of equal magnitude – one of which was delivered after a delay, and the other was delivered immediately and followed by delayed shock. Rats chose between the consequences by pressing one of two levers. Using an adjusting-delay procedure, adjustments were made in the delay to food based on the rats’ choices. Exclusive choice of delayed food raised the delay to food in subsequent trials; exclusive choice of immediate food with delayed shock reduced the delay to food in subsequent trials. Adjustments continued until the both consequences were chosen equally often and the delay to food stabilized. The mean delay over this stable period was taken as an estimate of the indifference point – the delay at which the delayed food alone was equal in value to the immediate food followed by delayed shock. In Experiment 1, indifference points were identified across conditions with different delays to shock. The shock devalued the immediate food to the greatest extent when the delay to shock was short, and the effects of shock weakened as the delay was raised. In Experiment 2, indifference points were identified across conditions in which either the delay to shock or the presence of a signaling procedure was manipulated. As in Experiment 1, effects of shock were greatest when the delay was short and weakened as the delay was raised. Signaling the delayed shock did not influence effects of the shock systematically. Additional analyses of the adjusting delay, latency to press each lever, and the fit of hyperbolas based on Mazur’s (1987) hyperbolic discounting equation to indifference points were conducted for both experiments. The

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