Drug discrimination under a concurrent fixed-interval fixed-interval schedule.

The team taught pigeons to tell pentobarbital from saline. Birds pecked on two keys under two time rules. One key paid off every 60 seconds if pentobarbital had been given. The other key paid every 240 seconds if saline had been given.

After training, the birds got test doses of pentobarbital, ethanol, and chlordiazepoxide. The goal was to see if the pigeons would still pick the correct key and how the drugs mixed.

The pigeons almost always chose the drug key after pentobarbital and the saline key after saline. Higher pentobarbital doses made the choice even sharper. Ethanol and chlordiazepoxide also pushed birds toward the drug key, but at different doses.

The results formed clean dose-response curves. The birds showed strong stimulus control, meaning the drug itself acted like a clear cue for which key to hit.

McMillan et al. (1999) swapped the time rules for ratio rules and still got clear drug choices. This conceptual replication shows the signal holds up even when the payoff rule changes.

Bryant et al. (1984) warned that payoff rules alone can nudge birds toward the drug key even with tiny doses. The 1997 study built in checks for this bias, proving the choices were real stimulus control, not just schedule pull.

Lancioni et al. (2009) later added two more choices—dizocilpine and a drug mix—using the same concurrent idea. The four-choice task extends the 1997 method and keeps the clean separation between GABA and NMDA cues.

If you run drug-discrimination assays, this paper gives you a ready template. Concurrent FI schedules keep responding steady and give clear dose curves. You can copy the timing, the key colors, and the test order to screen new compounds or check for abuse potential. The setup also reminds you to watch for schedule bias first; adjust payoff rates before you claim true stimulus control.