Navigation in the Morris swim task as a baseline for drug discrimination: a demonstration with morphine.

Four rats learned to swim to one corner of a round pool when they felt morphine, and to the opposite corner when they felt plain saline. The drug acted like an internal traffic light telling each rat which way to turn.

Each day the animals got an injection before the trial. They had to find the hidden platform that was always paired with the drug state. Correct escapes earned a dry-off period on a warm towel.

Three rats mastered the task. They swam straight to the morphine side after a morphine shot and to the saline side after a saline shot. Higher morphine doses made the choice even clearer.

The fourth rat never caught on, showing that drug cues, like any stimulus, do not control every learner.

HANSON et al. (1961) first showed that a drug could act like a color or tone by using pipradrol to steer pigeons during generalization tests. David et al. simply moved the idea into a water maze and used morphine instead of pipradrol.

Zeiler (1968) taught rats to hold or release a bar depending on whether a light was on or off. The new study swaps the light for an internal drug feeling and swaps bar presses for swimming directions. Both prove one stimulus can set the occasion for two different actions.

Sanders et al. (1971) found that monkeys learn auditory location in three sessions when reinforcement is adjacent to the sound. David’s team used the same adjacency trick: the platform always sat in the corner that matched the drug state, so spatial control emerged just as fast.

If an internal sensation as subtle as morphine can guide navigation, then interoceptive stimuli—heart rate, hunger, anxiety—can also become discriminative stimuli in clinical work. You can teach a client to notice early drug cravings and swim toward the social-skills corner instead of the relapse corner. Start by pairing a clear internal cue with a simple directional response; reinforce the correct turn every time.