Theory and method in the quantitative analysis of "impulsive choice" behaviour: implications for psychopharmacology

Impulsive choice refers to the selection of small immediate gains in prefer ence to larger delayed gains, or the selection of large delayed penalties i n preference to smaller immediate penalties. Current theoretical interpreta tions of impulsive choice are reviewed, and a synthesis of these ideas, the "multiplicative hyperbolic model of choice", is presented. The model assum es that the value of a positive reinforcer increases as a hyperbolic functi on of its size, and decreases as a hyperbolic function of its delay and the odds against its occurrence. Each hyperbolic function contains a single di scounting parameter which quantifies the organism's sensitivity to the vari able in question. The hyperbolic discounting functions combine multiplicati vely to determine the overall value of the reinforcer. Equivalent functions are postulated to govern the (negative) value of aversive events, the net value of an outcome reflecting the algebraic sum of the positive and negati ve values. The model gives rise to a quantitative methodology for studying impulsive choice, based on a family of linear indifference (null) equations , which describe performance under conditions of indifference, when the val ues of the reinforcers are assumed to be equal. This methodology may be use d to identify individual differences in sensitivity to the magnitude, delay and probability of reinforcement. The methodology is also suitable for the quantitative evaluation of the effects of some pharmacological interventio ns on discounting parameters. Recent psychopharmacological studies of impul sive choice are reviewed, and the utility of indifference equations for ext ending this work, and developing a quantitative psychopharmacology of impul sive choice is discussed.