Molecular analyses of the effects of d-amphetamine on fixed-interval schedule performances of rats.

McAuley et al. (1986) gave rats a fixed-interval schedule. The rat got food every 60 seconds if it pressed the lever at least once after the interval ended.

The team then injected d-amphetamine and watched the same animals. They timed how long the rat waited after food before pressing again. They also counted presses in each 10-second slice of the interval.

The drug cut the normal pause after food. Rats started pressing sooner, so the smooth scallop shape flattened.

Local response rates inside the interval also dropped. Together, the data say the rats lost track of time.

Dunlap et al. (1991) saw the same break-down. Their rats worked on a percentile schedule that punishes too many or too few responses. d-Amphetamine wrecked that control too, showing the effect holds across very different schedules.

Garcia (1974) sounds like it disagrees. Pigeons on a conjunctive FI-FR schedule actually sped up when baseline rates were low. The key difference is baseline: low-rate birds increase, but normal FI pauses decrease. The studies fit once you know rate-dependency rules.

Goldman et al. (1979) add that learning tasks suffer first. Their pigeons made more errors on new sequences before any change in old, well-rehearsed patterns. Timing, like learning, is fragile when stimulants hit.

If you run FI schedules in the lab, expect drugs or medications to flatten the scallop. You will see shorter pauses and lower run rates, so mastery looks weaker even when the learner still knows the rule. Track pause length as a quick, sensitive probe of timing problems in any pharmacology study you design.