ACQUISITION OF CONDITIONED RESPONSES
At the start of a conditioning procedure, the conditioned stimulus (CS) does not elicit the conditioned response (CR). In fact, the CS may elicit no reaction at all beyond a gen-eral stop-and-look response that organisms produce whenever a new stimulus appears. But after several pairings with the unconditioned stimulus (US), things change, so that a previously neutral CS (say, the sound of a bell) now elicits a CR (salivation).
Let’s emphasize, however, that learning doesn’t take place all at once. Instead, the learning is gradual, and the strength of the CR slowly grows as the animal experiences more and more pairings of CS and US. This pattern is evident in the data shown in Figure 7.6. Once the CS-US relationship is solidly established, though, the CS can be used in other procedures to establish other conditioned stimuli. As one exam-ple, by using meat powder as the US, we can first condition a dog to salivate whenever it sees a light. Once this is done, we can sound a bell and follow that by the light, with-out ever introducing the food. After enough of these pairings, the bell itself will trigger salivation. In this setting, the bell has become a signal for the light, which we’ve already established as a signal for the appearance of food. This sequence is called second-orderconditioning—a procedure in which a neutral stimulus (here, the bell) is paired withsome already established CS (like the light), as shown in Figure 7.7.
Second-order conditioning considerably extends the power and importance of clas-sical conditioning. For example, the sight of your dentist is often paired with the dis-comfort of feeling her drill; as a result, the sight of the dentist (the CS in this case) might become fearful. But other stimuli are in turn associated with the sight of the dentist—the sight of her office, the sound of her voice, the word dentist, and more. Through second-order conditioning, these stim-uli, too, can become fearful—potentially leading to a fear of all things related to dentistry. In this way, second-order conditioning can produce widespread effects that, as in this example, can some-times lead to the highly disruptive fears we call phobias (e.g., Gewirtz
Davis, 2000). More generally, though, mechanisms like higher-order conditioning allow the learning process to play a substantial role in shaping key aspects of our emotional lives.