Discriminative and reinforcing properties of two types of food pellets.

The team asked a simple question. Can the food pellet itself tell the bird when to peck?

They set up a multiple schedule. In one part, only red pellets dropped after pecks. In the other part, only white pellets dropped. The birds never saw lights or colors on the key. The pellets were the only cue.

Later they flipped the rules so the birds had to learn a reversal. They also tested if the birds could tell left-from-right based only on pellet type.

Every bird learned the game. Peck rates rose and fell with the pellet type that was due. When the rules flipped, the birds flipped too.

The pellets had become full discriminative stimuli. The food was not just a reward; it was also the signal.

Stretch et al. (1966) did the same trick with live rats. One rat in the chamber meant food, another rat meant no food. Both studies show that almost anything can gain stimulus control if it predicts reinforcement.

Tracey et al. (1974) went further. They used pictures made of many parts. Birds pecked the piece that was different, not the part that stayed the same. Together these papers say: pick the cue that stands out, whether it is a pellet, a rat, or a tiny color spot.

Okouchi (1999) replaced pellets with words. Instructions like "go fast" or "go slow" also worked as cues. The idea keeps spreading: if it predicts payoff, it can become an SD.

You may be teaching with edible rewards right now. This study warns that the snack itself can accidentally steer the learner. If you switch from cheddar cubes to raisins, the child might think the task changed. Keep pellet, cracker, or candy type the same while you want steady performance. When you do need a new cue, change the food on purpose and re-train. Treat the reinforcer as part of the stimulus set, not background noise.