In the late 1800s the Russian physiologist Ivan Pavlov used dogs to study the neural control of digestive juice secretion. Of course his dogs salivated when they smelled food, but Pavlov observed that they also salivated whenever the technician who routinely fed the dogs entered the room—even when no food was present. Following up this observation, Pavlov substituted a sound stimulus for the technician; a metronome ticked while the dogs were fed. After several trials, the dogs salivated when they heard the metronome, even if no food was offered.

Salivation in response to the sight, smell, or taste of food is a natural reflex response to a stimulus, but salivation in response to a sound was a learned response. The pairing of a sound with the experience of receiving food conditioned the dogs’ nervous systems to generate a response, which Pavlov dubbed the conditioned reflex (Figure 52.1). The natural stimulus—food in this case—became known as the unconditioned stimulus, and the unnatural stimulus was termed the conditioned stimulus. Pavlov received a Nobel prize in 1904 for his work showing that a simple behavior controlled by the nervous system could be modified through experience. This work stimulated much new research because Pavlov had developed an experimental model of learning.

Going beyond the conditioning of autonomic reflexes, the psychologist B. F. Skinner showed that many behaviors could become a conditioned response to a stimulus if a reward was associated with the action and the stimulus. A rat, for example, could be conditioned to press a lever in response to a stimulus if it got a reward when it pressed the lever. Because the animal was conditioned to perform an operation on its environment, this experimental protocol was called operant conditioning and became another model of learning.

The experimental approaches to behavior initiated by Pavlov and Skinner stimulated a lot of research on animal behavior, but the focus of that research was mostly controlled laboratory experiments on learning and memory in only a few species—mostly the white rat. This field of animal behavior became known as behaviorism.