Stimulus change as a factor in response maintenance with free food available.

Zentall et al. (1975) asked a simple question. If food is free, will rats still press a bar just to hear a click or see a light?

They gave rats free food in one phase. In the next phase, every bar press also produced a brief tone, a flash, or both. No extra food.

The team tracked how many times the rats pressed under each sound-and-light mix.

The rats pressed more when the bar made noise or lit up. The combo of click plus flash kept the highest rate.

Even with food always available, the added sensory payoff kept the response alive.

Blue et al. (1971) already showed that louder sounds alone boost pressing with no extra food. R et al. move the idea forward: the sound must come right after the rat's own response to work as a reinforcer.

Baron et al. (1966) paired white noise with shock relief and turned the noise into a reinforcer. R et al. show the same magic can happen with pleasant clicks and flashes tied to the bar press itself.

Branch (1977) later found that a tone signalling food omission also keeps pigeons working. Together these studies prove that almost any stimulus regularly paired with a key event—food, relief, or the animal's own response—can take on reinforcing power.

You can keep a client working without edible treats. Try adding a click, brief chime, or small visual animation right after each correct response. Pair the two for several trials, then fade the edible reward. The sound or light alone can keep the skill going, letting you reserve food for bigger achievements.