Behavioral contrast in rats with different reinforcers and different response topographies.

Researchers tested behavioral contrast in rats using two different reinforcers and two different ways to respond. They removed milk for some rats and food pellets for others while the animals pressed either a lever or a nose-key.

The team ran three small lab experiments. Each rat lived in a box with two schedules that alternated. When one schedule stopped paying off, the scientists watched if responding jumped on the other schedule.

Taking away milk produced bigger, longer-lasting contrast than taking away pellets. Switching the response from lever press to nose-key still gave contrast, so the effect is not tied to one movement.

In the third try, the same setup produced no contrast at all. The authors note they could not fully repeat their own earlier rat study.

Pear et al. (1971) saw positive contrast that faded over days. The 1975 paper builds on that work but shows the fade can turn into a full no-show when conditions line up wrong.

Honig et al. (1988) later swapped food for alcohol and still saw contrast on concurrent schedules. Their result widens the 1975 finding: contrast can cross very different reinforcer classes, not just milk versus pellets.

Marcucella (1976) adds that how well the reinforcer is signalled matters. Full signalling gives contrast; partial signalling kills it. Together these papers tell us contrast is fragile—change reinforcer, response, or signal and the effect can vanish.

When you run multiple or mixed schedules in the lab, check both what you pay and how the client earns it. A shift from preferred to less-preferred reward, or from one response form to another, can create brief spikes or drops that look like treatment effects but are just contrast. Probe again the next day—if the jump is gone, you are probably seeing the same fragile pattern these rats showed.