High-Order Conditioning in Dogs

In a study on classical conditioning, a researcher introduces a new sound (a metronome) to a group of dogs that have previously been conditioned to salivate at the sound of a bell (the conditioned stimulus) paired with food (the unconditioned stimulus). After multiple pairings of the metronome with the presentation of food, the dogs begin to salivate at the metronome alone. The researcher then removes the bell from the experiment and only uses the metronome. Which of the following concepts best explains why the dogs continue to salivate to the metronome even in the absence of the bell?