Ivan Pavlov did not set out to study the mind at all. He was a physiologist, and a formidable one, obsessed with the workings of the digestive system. In his St Petersburg laboratory at the turn of the twentieth century, he was investigating how dogs' bodies prepared to digest food — how the sight and taste of a meal triggered the flow of saliva and gastric juices. To measure it precisely, he performed surgery to divert a dog's salivary duct through a small opening in its cheek, so that every drop of saliva could be collected in a tube and counted. It was for this meticulous work on digestion, not for anything about learning, that Pavlov won the Nobel Prize in Physiology or Medicine in 1904.
The unexpected drool
Then he noticed something that a lesser scientist might have brushed aside as a nuisance. The dogs began to salivate before any food touched their mouths. They drooled at the mere sight of the food bowl, or of the attendant who usually fed them, or even at the sound of that person's footsteps in the corridor. Salivating at food in the mouth was a simple reflex, wired in by nature. But salivating at footsteps was something else — the dog had somehow learned that a particular sound meant dinner was coming. Pavlov called these anticipatory secretions "psychic" salivation, and he grew fascinated enough to abandon much of his digestive work to pursue them.
What he uncovered became the model of a whole kind of learning. Pavlov reasoned that food in the mouth was an unconditioned stimulus — it produced salivation automatically, without any training. If he consistently paired that food with a neutral signal that meant nothing to the dog, the dog would come to associate the two, and eventually the signal alone would set the saliva flowing. The neutral signal, once learned, he called a conditioned stimulus, and the response to it a conditioned reflex. A dog trained this way would salivate at the signal even with no food in sight. He mapped the phenomenon with great rigour: how the response strengthened with repeated pairings, how it faded, or "extinguished," when the signal came repeatedly with no food, and how it could return.
The bell that probably never was
Here the popular story and the historical record part ways. Almost everyone "knows" that Pavlov rang a bell before feeding his dogs until the bell alone made them drool. Bells appear in cartoons, textbooks and everyday speech; to describe a reflex as "Pavlovian" is part of the language. Yet historians who have combed Pavlov's own writings find that the bell has been badly overstated. In his careful experiments Pavlov needed stimuli he could control and time precisely, and he mostly used devices such as a metronome, a buzzer, an electric bell of a controlled kind, a whistle, a ticking sound, or even a flash of light or a touch on the skin. A vague ringing bell was too imprecise for the exact science he was after. The famous bell, in the tidy form the story tells, is largely a simplification introduced by later retellers rather than a fixture of Pavlov's actual bench.
The confusion matters less than the discovery, which was enormous. Pavlov had shown that a form of learning could be studied objectively, measured in drops of saliva and seconds of delay, without any appeal to unobservable thoughts or feelings. That was a revolutionary idea, and it fed directly into the school of psychology called behaviourism, whose champions, led by figures such as John Watson, argued that behaviour could be explained largely through such stimulus-and-response conditioning. Classical conditioning, as Pavlov's process came to be called, remains a cornerstone of psychology, invoked to explain everything from phobias and cravings to why the smell of a hospital can make a former patient feel sick.
Pavlov himself was a complicated figure — a rigorous experimenter who could be tyrannical in the laboratory yet openly critical of the Soviet regime he lived under, protected by his international fame. He kept working into his eighties and died in 1936. His dogs, immobilised in their harnesses with tubes at their cheeks, became some of the most famous animals in the history of science, and gave their name to a phenomenon everyone recognises the moment their own mouth waters at the sound of a kitchen timer.
Quiz nuggets
- Ivan Pavlov was a Russian physiologist who won the 1904 Nobel Prize for his work on digestion, not on conditioning.
- He noticed dogs salivating before food arrived — at sights and sounds they associated with feeding — which he called "psychic" salivation.
- His process is called classical conditioning, using an unconditioned stimulus (food) and a conditioned stimulus (a learned signal).
- The famous "bell" is largely a myth; Pavlov mainly used precise stimuli such as a metronome, buzzer, whistle or light.
- Pavlov's work became a foundation of behaviourism and of the modern understanding of learned responses; he died in 1936.