Pavlov noted that once a dog and learned to respond to a conditioned stimulus in a highly dependable way, the conditioned stimulus could be used to reinforce a conditioned response to a new stimulus. Suppose the animal has learned to salivate to a tone as a conditioned stimulus.
This is a first-order conditioned response. If a flashing light is then presented along with only the tone, the flashing light when presented alone will come to elicit the conditioned response. Pavlov called this process second- order conditioning.
The conditioned stimulus of first-order conditioned has become a secondary reinforce. Although second-order conditioning can be established with classical conditioning, it is more easily demonstrated with operant conditioning.