In the original 1890s experiment, physiologist Ivan Pavlov trained dogs to associate the sound of a click or a bell with the imminent delivery of food by repeatedly feeding them after the sound. The dogs became so used to this pairing that they would salivate when hearing the sound, even if they were not fed.

In the new study, researchers replicated this in rodents by pairing a flash of light with a reward.

In doing so, they discovered a mechanism in the brain that explains how the light—which previously had no inherent value—gains significance as something positive, and the role this mechanism plays in the learning process.