An object touches an infant's lips and she begins sucking. A child opens the window of a car and the blast of wind causes him to blink. The cutting of onions produces tears in the eyes of a cook. In all of these examples, a stimulus reliably produces a change in behavior. Technically the stimulus is said to elicit the response, and the relation between the stimulus and response is called a reflex. More specifically, it is an unconditioned reflex because it does not depend upon any prior learning. The stimulus is called an unconditioned stimulus (US) and the response is called an unconditioned response (UR). For more examples of unconditioned reflexes, see Table 1. All of these "prewired" stimulus-response connections presumably evolved because of their survival value for the individual.
Pavlovian (or Respondent or Classical) Conditioning builds on the unconditioned reflex. Pavlov (1927/1960) studied unconditioned reflexes with dogs. He found that placing meat powder in a dog's mouth (US) elicits salivation (UR). The conditioning part of his experiment involved pairing various innocuous stimuli with the food, such that they preceded food delivery. The fascinating result was that after a number of these pairings, the mere presentation of the paired stimulus elicited salivation.
For example, the sound of a bell elicits very little to no responding; for this reason it is referred to as neutral stimulus (NS). In Pavlov's experiment, a bell was paired with food presentation. The result was that the bell came to produce a reliable change in behavior, salivation. This new relation is called a conditioned reflex. It is "conditioned" in the sense that it depends on a prior relation between the bell and the food.
A previously neutral stimulus, the bell, became an eliciting stimulus. Technically, at this point it is referred to as a conditioned stimulus (CS) and the salivation resulting from it is said to be the conditioned response(CR). Pavlov's experiment is illustrated in Figure 1.
The bell will remain an eliciting stimulus as long as it continues to be paired with the food. If this pairing stops, the bell will return to its neutral status and no longer reliably produce salivation. This phenomenon is known as respondent extinction.
Pavlovian Conditioning is often involved in emotions. In a frequently cited experiment (Watson & Rayner, 1920), an infant boy named Albert was exposed to the sound of striking a suspended steel bar with a hammer. This sound (US) elicited fear-like responses such as crying (UR). Presentations of a white rat reliably preceded this sound; eventually the white rat itself (CS) was sufficient to elicit crying (CR). Albert was not afraid of white rats prior to the experiment; his fear of white rats, or a white rat phobia, developed during the course of the experiment due Pavlovian Conditioning.