CHOICE MAINTAINED BY COCAINE OR FOOD IN MONKEYS - EFFECTS OF VARYING PROBABILITY OF REINFORCEMENT

Several theoretical models of choice, including matching, maximizing a nd probability matching, have evolved from the experimental analysis o f behavior. However, these models are based largely on results of stud ies involving non-drug reinforcers. The extent to which the choice to self-administer a drug of abuse conforms to these models is largely un known. The present study was designed to examine the effects of varyin g probability of reinforcement of two options on choice by rhesus monk eys maintained under a discrete-trials choice procedure. One group of monkeys (n=5) was prepared with chronic indwelling venous catheters an d responding was maintained by IV injections of cocaine (0.05 or 0.2 m g/kg per injection). In a second group, behavioral conditions were ide ntical, except that responding was maintained by the delivery of food pellets. Two choice options were available to each group. Each option was maintained by the identical dose of cocaine or amount of food, but with varying probabilities of delivery. The percentage of trials on w hich monkeys chose option 1 increased with its probability of reinforc ement for cocaine (0.05 or 0.2 mg/kg per injection) and this effect di d not change with dose of cocaine. When probabilities of reinforcement for option 1/option 2 were 0.75/0.25, choice was exclusive for the hi gher probability option. Results were identical when behavior was main tained by food. The present experiment supports the conclusion that ex clusive choice of the alternative with the higher probability of reinf orcement can be extended to cocaine as a reinforcer. The present resul ts also extend the known conditions under which drug-maintained respon ding is similar to food-maintained responding to situations involving options with different probabilities of reinforcement, i.e., probabili ty choice. Probability matching was not observed, arguing that this mo del does not predict drug or food choice.