Back in the early 20th century, Pavlov managed to train man’s best friend to dribble at the sound of a ringing bell. Quite an achievement back in the day.
By continuously pairing the sound of a bell (the conditioned stimulus) with food (the unconditioned stimulus), Pavlov trained his dog to expect the sound of the bell to be followed by food, and therefore salivate each time it heard the bell (the conditioned response). This has since become one of the classic experiments used to train animals to match a conditioned stimulus with an unconditioned stimulus to produce a conditioned response.
These days the same principle is used to train lab rats but the bell has been replaced by a light. Food is still often used and this drops down into a small container. The conditioned response is to go to the container and collect the food. However, researchers have noticed something rather strange.