Final answer:
Stimulus discrimination is suggested to have taken place in Pavlov's dogs as they only salivated to a middle pitch bell, but not to a high pitch bell.
Step-by-step explanation:
The scenario described suggests that stimulus discrimination has taken place. Stimulus discrimination refers to the ability to differentiate between similar stimuli and respond only to specific ones. In this case, the dogs salivated only to the middle pitch bell and not to the high pitch bell, indicating that they have learned to discriminate between the two stimuli.