Operant discrimination of an interoceptive stimulus in rhesus monkeys.

Five rhesus monkeys learned to press a lever when they felt an internal change. The change came from inflating a small balloon in the stomach. When the balloon was up, pressing paid off. When it was down, pressing did not.

The team then reversed the rule. Now balloon-up meant “do not press.” One monkey mastered the switch, showing the signal truly controlled the behavior.

All five monkeys quickly told the two stomach feelings apart. Their presses almost always matched the current rule. The result shows that a gut cue can work just like a light or a tone.

This was the first clear proof that an inside-body signal can become a discriminative stimulus.

Ziegler et al. (2002) repeated the idea with rats. Instead of a balloon, the cue was a morphine dose. The rats learned to swim to the corner that matched how the drug felt. Together, the two studies show that many internal states can guide behavior.

Rapport et al. (1982) and Adams (1980) pushed the idea further. Their pigeons used their own recent actions—peck counts or pauses—as cues for the next choice. These papers widen the lens: anything the body does, inside or out, can serve as a signal once the learner is taught to notice it.

No contradictions appear. The 1965 paper opened the door; later work walked through it with new species and new baselines.

If monkeys can feel a stomach cue, so can people. You can shape clients to notice heart rate, muscle tension, or stomach tightness and then use that cue to start coping skills. Begin with simple yes/no tasks: “Tell me when your chest feels hot.” Pair the hot report with reinforcement. Later, teach the same cue to trigger deep breathing or a break request. Bio-behavioral work starts here.