Separating the effects of response rate and reinforcement frequency in the rate-dependent effects of amphetamine and scopolamine on the schedule-controlled performance of rats and pigeons.
Rats and pigeons responded under three-component multiple schedules of food reinforcement. Each component was associated with a separate stimulus condition and a schedule that arranged food delivery intermittently for appropriately spaced responding. In the experiment with pigeons, three response rates were maintained with equal reinforcement frequency in the three components. With rats, the lowest response rate was associated with a higher reinforcement frequency. The effects of d-amphetamine in rats (0.05-0.6 mg) and pigeons (0.1-3.0 mg) were closely associated with the dose, and with the response rates that occurred under nondrug control conditions. Small doses of d-amphetamine increased low response rates proportionately more than moderate rates; moderate rates were increased proportionately more than were high rates. With larger doses, low rates were decreased proportionately less than were moderate rates, which in turn were decreased proportionately less than were high rates. Similar relations between drug effects and control rates were obtained in rats with scopolamine (0.0125-0.4 mg), with the exception that constant effects appeared at doses of 0.1 mg and greater. The present experiments, through manipulating response rate independently of reinforcement frequency, show that the rate-dependent effects of d-amphetamine and scopolamine are primarily response-rate-dependent drug effects.