Choice under concurrent VI schedules: comparison of behavior maintained bycocaine or food

Previous research has shown that concurrent schedule responding maintained by cocaine under short variable-interval (VI) schedules is well described b y the generalized matching law. That is, drug-maintained behavior was appor tioned in accordance with relative frequency of reinforcement. The purpose of the present experiment was to examine the ability of the generalized mat ching law to account for choice under longer VI schedules of cocaine availa bility, and to compare cocaine-maintained to food-maintained behavior in th is regard. One group of rhesus monkeys (n=4) was prepared with indwelling I V catheters and allowed to respond under concurrent VI (conc VI) schedules of cocaine delivery (0.025, 0.05 or 0.1 mg/kg per injection) with an averag e inter-reinforcer interval (IRI) of 10 or 30 min. In a second group of mon keys (n=4), a comparable experiment was conducted but with responding maint ained by different amounts of food (one, two, or four 1-g banana-flavored p ellets). For both groups, the same reinforcer followed responding on either lever, the only difference between the options being the VI schedule, i.e. , frequency of reinforcement. The behavior of the cocaine-maintained group was well predicted by the generalized matching law. While both groups evide nced undermatching of both response and time allocation, lever pressing of monkeys whose behavior was maintained by food showed more undermatching tha n that of the cocaine-maintained group. In addition, a consistent and unexp lained bias in responding toward the right lever developed in the food-main tained, but not the cocaine-maintained monkeys. Considering the present res ults with those of previous experiments, it appears that food-, but not coc aine-maintained behavior, deviates increasingly from strict matching as the IRI is extended. This difference across reinforcers could be due to differ ences between cocaine and food in the mechanisms by which they maintain beh avior, or a direct effect of cocaine on choice performance.