Dr. frolov classically conditioned a dog to flex his right hind leg at the sound of a bell by pairing the ringing of a bell with a mild tug to that leg. in this example, the ringing bell is the:

Dr. Frolov classically conditioned a dog to flex his right hind leg at the sound of a bell by pairing the ringing of a bell with a mild tug to that leg. The ringing bell is the neutral stimulus. What Dr. Frolov did is a sample of respondent conditioning or a learning procedure that results from pairing a biologically potent stimulus with a previously neutral stimulus.

The neutral stimulus (ringing bell) comes to elicit a response (flexing of right hind) that is usually similar to the one elicited by the potent stimulus.