Phencyclidine discrimination in the pigeon using color tracking under second-order schedule.

Researchers taught pigeons to tell the difference between phencyclidine and saline.

They used a second-order schedule. A brief color change flashed after every 10 pecks. Food came only after many color flashes.

The color acted like a tiny promise of food later. This kept the birds pecking fast.

The color-tracking trick worked. Birds pecked thousands of times before each meal.

High response counts made it easy to see tiny dose changes. The method gave clear data for drug-discrimination work.

Fantino (1981) tried the same idea with brief tones. Tones also lifted rates, so the color version was a natural next step.

Bryant et al. (1984) later warned that second-order rules can push birds toward the drug key even at tiny doses. They used the same phencyclidine setup. The two papers look opposite—one cheers high rates, the other warns of bias. The gap is purpose, not error. High rates help data clarity; schedule bias can cloud stimulus control. Check both when you plan.

Lancioni et al. (2009) went further. They gave birds four keys to tell phencyclidine, pentobarbital, a mix, and saline apart. The 1982 color method made that finer work possible.

Second-order brief-stimulus schedules are still the go-to when you need lots of responses from one animal. If you run drug-discrimination or any reinforcement study with long sessions, add a brief color, tone, or click after fixed mini-ratios. Watch for the bias trap Bryant et al. (1984) flagged: run probe tests and vary the schedule to be sure the animal is sensing the drug, not just chasing the schedule.