Brief History

In 1904, Ivan Pavlov was awarded the Nobel prize in medicine for his pioneering work on the physiology of digestion. Pavlov primarily studied the process of digestion in dogs. This early research set the stage for the discovery of a learning process that would come to be known as classical conditioning. As early as 1880, Pavlov observed that sham feedings, in which food was eaten but failed to reach the stomach (being lost through a surgically implanted esophageal fistula), produced gastric secretions, just like real food did. He immediately understood that this phenomenon must involve the central nervous system and began a program to thoroughly evaluate the parametric features of this learned response.

Pavlov modified his preparation in order to simplify the forthcoming studies. Rather than measure gastric secretions, he began measuring salivation. The first of these studies involved showing the dog a piece of bread as the CS. Pavlov then noticed that in dogs with extensive training, even the act of an experimenter walking into a room could elicit salivation. This finding led to the discovery that a variety of stimuli could induce salivation if paired with meat powder. Accordingly, in a further simplification of his experimental procedures he began using the sound of a bell presented with the meat powder to elicit salivation. It is this preparation that has become synonymous with Pavlov and historical examples of classical conditioning. In fact, Pavlov has become synonymous with classical conditioning because the term Pavlovian conditioning can be used interchangeably with classical conditioning.

Initially, Pavlov referred to the conditioned response as a "psychic secretion'' to distinguish this type of response from the unlearned physiological secretions that would later come to be known as the unconditioned response. In 1903, a student of Pavlov's published a paper that changed the term psychic secretion to conditioned reflex. The terms conditioned reflex and unconditioned reflex were used during the first two decades of the 20th century, during which time this type of learning was often referred to as "reflexology."

Although Pavlov is correctly credited with the discovery of classical conditioning, and with identifying and describing almost all the basic phenomena associated with this form of conditioning, it is worth noting that the phenomenon of classical conditioning was independently discovered by an American graduate student in 1902. Edwin B. Twitmyer made this discovery while finishing his dissertation work on the "knee-jerk" reflex. When the Patellar tendon is lightly tapped with a doctor's hammer, it results in the well-known knee-jerk reflex. Twitmyer's work required many tap-induced reflexes for each subject. Twitmyer, like Pavlov, noticed that eventually the mere sight of the doctor's hammer (the CS) could produce a knee-jerk reflex (the CR). This largely forgotten report was the first example of classical conditioning of a muscle reflex. The potential significance of this finding was not apparent to Twitmyer, and the work was never extended or cast in a theoretical framework as Pavlov had done.

Pavlov's work on classical conditioning was essentially unknown in the United States until 1906 when his lecture "The Scientific Investigation of the Psychical Faculties or Processes in the Higher Animals'' was published in the journal Science. In 1909, Robert Yerkes, who would later become president of the American Psychological Association, and Sergius Morgulis published a thorough review of the methods and results obtained by Pavlov. Although these reports provided a true flavor of the potential value of classical conditioning, the method was not immediately embraced by psychologists. This changed when John B. Watson, who is widely regarded as the founder of a branch of psychology known as behaviorism, championed the use of classical conditioning as a research tool for psychological investigations. Watson's presidential address delivered in 1915 to the American Psychological Association was titled "The Place of the Conditioned Reflex in Psychology.'' Watson was highly influential in the rapid incorporation ofclassical conditioning, as well as other forms of conditioning, into American psychology. In 1920, his work with classical conditioning culminated in the now infamous case of "little Albert.''

Albert B. was an 11-month-old boy who had no natural fear of white rats. Watson and Rosalie Rayner used the white rat as a CS. The US was a loud noise that always upset the child. By pairing the white rat and the loud noise, Albert began to cry and show fear of the white rat—a CR. With successive training sessions over the course of several months, Watson and Rayner were able to demonstrate that this fear of white rats generalized to other furry objects. The plan had been to then systemically remove this fear using methods that Pavlov had shown would eliminate or extinguish the CR—in this case, fear of furry white objects. Unfortunately, little Albert, as he as historically come to be known, was removed from the study by his mother on the day these procedures were to begin. There is no known reliable account of how this experiment on classical conditioning of fear ultimately affected Albert B. Nevertheless, this example of classical conditioning may be the most famous single case in the literature on classical conditioning.

In 1921, a popular textbook on conditioning changed the terms of conditioned and unconditioned reflex to the current terms of conditioned and unconditioned response. This broadened the concept of conditioning to include other behaviors that were not merely automatic reflexes. In 1927, the Anrep (a former student of Pavlov's) translation of Pavlov's Conditioned Reflexes was published, thus making all of his work available in English for the first time. The availability of 25 years worth of Pavlov's research, in vivid detail, led to increased interest in the experimental examination of classical conditioning—an interest that has continued to this day.

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