[1] Ivan Pavlov conducted multiple experiments investigating digestion in dogs in which neutral, unconditioned, and conditioned stimuli were used.
In these experiments, the neutral stimulus was the sound of a bell ringing.
This sound was presented to the dogs along with food, which acted as an unconditioned stimulus.
The presentation of a neutral stimulus does not result in any particular response, but the presentation of an unconditioned stimulus results in an unconditioned response, which was the dogs salivating in Pavlov's experiments.
[2] Pavlov later used the sound of a metronome as a neutral stimulus in studies on cerebral cortex activity.