One cannot overstate the significance of the contributions Ivan Pavlov made to the study of predictive learning. Pavlov introduced a level of rigor and precision of measurement of both the independent and dependent variables in animal learning that did not exist at the time. In 1904, Pavlov, a physiologist, was awarded the Nobel Prize in Medicine for his research investigating the digestive process in dogs. He became fascinated by an observation he and his laboratory assistants made while conducting this research. One of the digestive processes they studied was salivation. Saliva contains enzymes that initiate the process of breaking down what one eats into basic nutrients required to fuel and repair the body. The subjects frequently started salivating before being placed in the experimental apparatus. Pavlov described this salivation as a “psychic secretion” since it was not being directly elicited by food. He considered the phenomenon so important that within a few years he abandoned his research program in digestion and dedicated the rest of his professional career to systematically studying the details of this basic learning process.
Figure \(\PageIndex{1}\): Ivan Pavlov. [“Ivan Pavlov NLM3” by National Library of Medicine/Wikimedia Commons is in the public domain.]
This is a wonderful example of what has been described as serendipity, or accidental discovery in science. Dogs have been domesticated for thousands of years. A countless number of people probably observed dogs appearing to predict (i.e., anticipate or expect) food. Pavlov, however, recognized the significance of the observation as an example of a fundamental learning process. We often think of science as requiring new observations. Pavlov’s “discovery” of the classical conditioning process is an example of how this is not necessarily the case. One of the characteristics of an exceptional scientist is to recognize the significance of commonly occurring observations.
We will now review the apparatus, methods, and terminology Pavlov developed for studying predictive learning. He adapted an experimental apparatus designed for one scientific field of inquiry (the physiology of digestion) to an entirely different field (adaptive learning). Pavlov made a small surgical incision in the dog’s cheek and implanted a tube permitting saliva to be directly collected in a graduated test tube. The amount of saliva could then be accurately measured and graphed as depicted in Figure \(\PageIndex{2}\). Predictive learning was inferred when salivation occurred to a previously neutral stimulus as the result of appropriate experience.
Figure \(\PageIndex{2}\): Pavlov’s experiments with a dog. [“Pavlov experiments with dog Wellcome M0014738” by the Wellcome Trust/Wikimedia Commons is licensed under CC BY 4.0.]
VIDEO CLIP
Watch this video describing Pavlov and classical conditioning.
Animals inherit the tendency to make simple responses (i.e., reflexes) to specific types of stimulation. Pavlov’s salivation research was based on the reflexive eliciting of salivation by food (e.g., meat powder). This research was adapted to the study of predictive learning by including a neutral stimulus. By neutral, we simply mean that this stimulus did not initially elicit any behavior related to food. Pavlov demonstrated that if a neutral stimulus preceded a biologically significant stimulus on several occasions, one would see a new response occurring to the previously neutral stimulus. Figure \(\PageIndex{3}\)uses the most popular translation of Pavlov’s terminology (which he wrote in Russian). The reflexive behavior was referred to as the unconditioned response (UR). The stimulus that reflexively elicited this response was referred to as the unconditioned stimulus (US). A novel stimulus, by virtue of being paired in a predictive relation- ship with the food (US), acquires the capacity to elicit a food-related, conditioned response (CR). Once acquiring this capacity, the novel stimulus is considered a conditioned stimulus (CS).
Figure \(\PageIndex{3}\): Pavlov’s classical conditioning procedures and terminology. [This work, “Pavlov’s Dog (English),” is licensed under CC BY-SA 4.0 by Judy Schmitt. It is a derivative of “Pavlov’s dog” by MagentaGreen/ Wikimedia Commons, which is licensed under CC BY-SA 3.0.]
