Fixed-interval schedules of intravenous cocaine presentation in rats.

Scientists gave rats cocaine through a tiny IV line. The drug came only after a fixed time passed. This is called a fixed-interval schedule.

They watched how long the rats waited before pressing the lever again. They also tried different interval lengths and different cocaine doses.

Higher cocaine doses made the rats wait longer. The waiting time grew in a straight line with the dose.

Changing the interval length did almost nothing to the wait. This is odd, because food or shock reinforcers change the pause when you change the interval.

Sievert et al. (1988) later added pentobarbital to the same setup. Cocaine still flattened response rates, while pentobarbital only slowed everything down. The 1973 paper set the baseline for this comparison.

Barber et al. (1977) showed that pentobarbital and d-amphetamine also change fixed-interval performance, but the change depends on how fast the rat was responding before the drug. This extends the 1973 finding to other drugs and shows baseline rate matters.

McKearney (1970) and Morse et al. (1966) proved that fixed-interval scallops happen even when the reinforcer is electric shock. Scull et al. (1973) now shows the same pattern holds with cocaine, but the pause is locked to dose, not interval length.

If you run drug-reduction programs, remember that stimulants like cocaine tighten the pause, and this effect is stubborn across different timing schedules. Use this to spot stimulant use early: watch for unusually long, dose-locked pauses that ignore schedule changes. Pair these signs with rate checks from Barber et al. (1977) to decide if a new medication is helping or hurting your client’s learning.