Choice Between Food and Cocaine Reinforcers Under Fixed and Variable Schedules in Female and Male Rhesus Monkeys

Illicit drugs like cocaine may be uncertain in terms of the time and effort required to obtain them. Behavior maintained by variable schedules resembles excessive drug-taking compared with fixed schedules. However, no prior research has examined fixed versus variable schedules in drug versus nondrug choice. The present study evaluated cocaine versus food choice under fixed- (FR) and variable-ratio (VR) schedules. The simpler food versus food and cocaine versus cocaine arrangements also were included. Adult female (n = 6) and male (n = 7) rhesus monkeys chose between cocaine (0.01-0.18 mg/kg/injection) and food (4 pellets/delivery), food and food (4 pellets/delivery), or cocaine and cocaine (0.018-0.03 mg/kg/injection) under FR and VR 100 and 200 schedules. In cocaine versus food choice, cocaine's potency to maintain choice was greatest when available under a VR 100 or 200 schedule and food under an FR schedule and was lowest when cocaine was available under an FR 200 schedule and food was available under a VR 200 schedule. In food versus food choice, males chose food associated with a VR schedule more than food associated with an FR schedule. In cocaine versus cocaine choice, females and males chose cocaine associated with a VR schedule more than cocaine associated with an FR schedule, particularly under VR 200. These findings suggest that uncertainty in terms of time and effort required to obtain cocaine, or perhaps the occasional low-cost access that results from VR schedules, results in greater allocation of behavior toward drug reinforcers at the expense of more certain, nondrug alternatives. Public Health Significance Illicit drugs like cocaine may be uncertain in terms of their availability, quality, and time and effort required to obtain them compared to other important drivers of behavior, such as a paycheck. Uncertain response requirements (modeling time and effort) may be an important contributing factor in the excessive allocation of behavior toward drug seeking and drug taking at the expense of engaging in nondrug-related activities.

Automated summary

This study evaluates the choice between cocaine and food under fixed and variable schedules, finding that the uncertainty in obtaining cocaine may lead to a greater preference for drug reinforcers over non-drug alternatives. Male monkeys tend to choose food associated with variable schedules, while both female and male monkeys tend to choose cocaine associated with variable schedules.