Polydrug abuse: Choice between drugs as a function of concurrent nonindependent ratio sizes

Polydrug abuse is the persistent self-administration of more than one reinforcing drug. The present study provided rhesus monkeys concurrent access to two drugs: 8% alcohol and solutions of either cocaine or methadone. The liquids were available under concurrent nonindependent fixed-ratio (FR) schedules across increasing and then decreasing ratio sizes. These schedules generate high rates of changeover responses and yield a dependent variable of responses per delivery that is not rigidly tied to the ratio-schedule value. The programmed schedule size was equal for both liquids and increased in the sequence 8, 16, 32, and so on until responding decreased, whereupon the schedule size was decreased in reversed order to the original steps. Eight percent alcohol was strongly preferred at the nonindependent FR 8 FR 8 baseline. As schedule size increased, intake of the 8% alcohol solution decreased and intake of the alternative liquid increased. Consumption of the alternative liquid generally remained elevated over initial values when schedule size decreased. The data can be analyzed in several ways, including consumption as a function of price (behavioral economics) and log of relative response rates as a function of log of relative deliveries (matching), thereby providing an interface between behavioral economics and matching analyses.