Some effects of delta-amphetamine and pentobarbital on performance under a long fixed-interval schedule.

Llewellyn et al. (1976) gave lab animals a very long one-hour fixed-interval schedule. The animals had to wait 60 minutes before each food pellet.

The team then injected two drugs: delta-amphetamine (a stimulant) and pentobarbital (a sedative). They watched how the drugs changed the timing and rate of lever presses across the hour.

Low amphetamine doses made the animals press faster. Higher doses later slowed them down again. The effect flipped with dose size.

Pentobarbital did not change the total number of presses. It only moved the presses to different parts of the hour. The animals pressed earlier in the interval after the drug.

Barber et al. (1977) ran almost the same one-hour FI but forced a short pause before each pellet. They saw the same flip: pentobarbital raised low rates and cut high rates. This backs up the idea that baseline rate controls drug impact.

Bacotti (1979) mixed the same two drugs with a second schedule running at the same time. Drug effects reversed when the second schedule changed. This shows the FI results are not fixed; context can flip the outcome.

Sievert et al. (1988) swapped the FI for a random-interval schedule. Pentobarbital simply slowed every response, matching the target paper. The pattern holds across schedule types.

If you track response rate during long sessions, expect drugs to act like a mirror: they exaggerate or flip the baseline you already have. Before changing medication for a client, record steady baseline data for at least a week. Use the same timing and task each day. A clear baseline lets you spot true drug effects instead of daily noise.