The core of Pavlovian conditioning is the pairing (association) of stimuli to elicit responses. Food (meat powder) placed in a dog's mouth naturally produces salivation. Pavlov called the food an unconditioned stimulus (US) and salivation, elicited by the food, the unconditioned response (UR). When a neutral stimulus—for example, a tone that does not naturally elicit salivation—is repeatedly followed by presentation of food, the tone alone eventually evokes salivation. Pavlov labeled the tone a conditioned stimulus (CS) and the response (salivation) elicited by it the conditioned response (CR).

Pavlov's formulation can be summarized as follows:

Before conditioning:

Food (US) elicits Salivation (UR) Conditioning procedure:

Neutral Stimulus (Tone) plus Food (US) elicits Salivation (UR) After conditioning:

Tone (CS) elicits Salivation (CR)

Pavlov believed that conditioned responses were identical to unconditioned responses. That is usually not the case. For example, conditioned responses may be less pronounced (weaker) or a bit more lethargic than unconditioned responses.

Several phenomena turn up in studies of Pavlovian conditioning. Extinction, generalization, and discrimination are among the most important. Extinction refers to the procedure as well the elimination of a CR. If the CS is repeatedly presented without the US, extinction occurs: The dog stops salivating to the tone. During the course of extinction, the CR may return from time to time until it is finally extinguished. Pavlov called the occasional return of the CR "spontaneous recovery."

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Ivan Pavlov.

(The Nobel Foundation)

Stimulus generalization refers to responding not only to a particular CS but also to different but similar stimuli. Further, the magnitude (amount of salivation) of a generalized response tends to decline as stimuli become less and less like the CS. For example, a dog trained to salivate to a 5,000-cycle-per-second (cps) tone is likely to salivate also to 5,300 cps and 4,700 cps tones without specific training to do so (stimulus generalization). Responses tend to weaken in an orderly way as tones become more and more unlike the CS. As the tones move away from the CS in both directions, say, to 4,400 cps from 4,100 cps, and 5,600 cps to 5,900 cps, the flow of salivation becomes less and less.

Stimulus generalization in effect extends the number of stimuli that elicit a conditioned response. Discrimination procedures restrict that number by conditioning a subject not to generalize across stimuli. The procedure involves two processes: acquisition and extinction. The CS is paired repeatedly with the US (acquisition) while the US is withheld as generalized stimuli are presented repeatedly (extinction). If the dog now salivates to the CS and not to the generalized stimuli, the dog has learned to discriminate or to act discriminatingly. Pavlov reported that some dogs displayed a general breakdown in behavior patterns (experimental neurosis) when called upon to make discriminations that were too difficult.

Pavlov's work on what he called the second-signal system implies that conditioning principles are relevant to human as well as to animal learning. Once, say, a tone is established as a CS in first-order conditioning, the tone can be paired with a neutral stimulus to establish a second-order CS. Thus, in the absence of food, a light might precede the tone (CS) several times until the light itself begins to function as a CS. Second-order conditioning appears to follow many of the same rules as first-order conditioning.

Pavlov's work has clearly provided one way to study the learning process in great detail. It has also provided the kind of data and theory that have affected research in other areas of learning, such as instrumental conditioning and, subsequently, cognitive science and neuroscience.

Range of Pavlovian Conditioning

Pavlovian phenomena have been demonstrated with different kinds of organisms and a wide variety of stimuli and responses far beyond those studied by Pavlov. Stimuli that precede such unconditioned stimuli as sudden loud noises (leading to rapid heart rate), a puff of air delivered to the eye (evoking blinking), or a large temperature increase (eliciting sweating) may become conditioned stimuli, capable of eliciting conditioned responses on their own. The idea of second-order (higher-order) conditioning is profoundly important because it suggests how rewards such as money or words of praise are established apart from primary (biologically necessary) rewards, such as food and water. It also may, in part, explain the power of films, plays, novels, and advertisements to evoke strong emotion in the absence of direct experience with primary (unconditioned) stimuli. Studies con cerned with conditioned emotional reactions (CER), especially fear and anxiety in people—a subject much more complex than simple reflexes— have been of special interest to researchers and therapists for many years.

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