Drug effects on fixed-interval responding with pause requirements for food presentation.

In pigeons performing under a multiple schedule of food presentation, low key-pecking rates (0.18 to 0.29 responses per second) were maintained during 3-min fixed-interval components by requiring a 4-, 5-, or 6-sec pause preceding the food-delivery response (tandem DRL), while higher rates (0.70 to 1.37 responses per second) were maintained in alternative fixed-interval components by requiring a pause of no more than 40 msec preceding the food-delivery response (tandem DRH). Thus, reinforcement density was equal but overall response rates markedly different in the two schedule components. Pentobarbital (3, 10 mg/kg) had effects on overall rates of responding consistent with a rate-dependency interpretation (low rates were increased while higher rates were decreased), but d-amphetamine (0.03 to 3 mg/kg) either failed to increase low overall rates in the tandem DRL components or increased them only slightly. Effects of both drugs on local responding within the fixed-intervals were always related in an orderly way to control response rate, but the extent of rate increases produced by d-amphetamine was modifed in some birds by pause requirements such that the drug increased comparable rates less when these occurred in the tandem DRL component than when they occurred in the tandem DRH components. Control rate is an important determinant of drug effects, independent DRH components. Control rate is an important determinant of drug effects, independent of reinforcement density maintaining rates, and independent of environmental influences, such as response-spacing requirements for food presentation, that can modify the extent of some drug-produced rate changes.