Abstract

Under a fixed-ratio (FR) schedule, every 10th or every 30th key-press response of squirrel monkeys resulted in either presentation of food or intravenous injection of drug. With optimal doses of cocaine or d-amphetamine and with optimal amounts of food, mean response rate was over one per second. Decreasing the cocaine or d-amphetamine dose resulted in irregular responding at reduced rates. Discontinuing food presentation had the same effect. Increasing the cocaine or d-amphetamine dose resulted in a high response rate at the beginning of each session, followed by decreasing response rates as the session progressed; increasing the amount of food had the same effect. Monkeys were then studied under a second-order fixed-interval schedule of FR components. Each FR component completed during a fixed interval of time (five minutes) produced only a brief light. The first FR component completed after the five-minute interval ended produced a brief light and either cocaine injection or food presentation. Mean response rates of about one per second were maintained consistently as the dose of cocaine injected or the amount of food presented was systematically varied over a wide range. Thus, striking parallels between drug-maintained responding and food-maintained responding occurred over a wide range of parameter values under both FR and second-order schedules.