Ivan Pavlov (1849-1936) was a Russian physiologist who won the Nobel Prize in 1904 for his research on digestion, but whose most enduring legacy is the discovery of classical conditioning. While studying salivary reflexes in dogs, Pavlov noticed that dogs began salivating not only to food but to stimuli that predicted food — the sight of the food dish, the sound of the experimenter's footsteps. His systematic investigation of this "psychic reflex" established the fundamental principles of associative learning.
Classical Conditioning
Pavlov's paradigm involves pairing a neutral stimulus (conditioned stimulus, CS — a bell) with a biologically significant stimulus (unconditioned stimulus, US — food) that naturally elicits a response (unconditioned response, UR — salivation). After repeated pairings, the CS alone elicits a conditioned response (CR — salivation). Pavlov discovered acquisition (learning the association), extinction (CR weakens when CS is presented without US), spontaneous recovery (CR returns after a rest period), stimulus generalization (responding to stimuli similar to the CS), and discrimination (learning to respond to the CS but not to similar stimuli).
Pavlov's discoveries extend far beyond dogs and bells. Classical conditioning underlies drug tolerance (environmental cues associated with drug use trigger compensatory responses), phobias (neutral stimuli paired with frightening events become fear cues), taste aversion learning (a single pairing of a taste with illness produces long-lasting avoidance), and the placebo effect (medical contexts become conditioned stimuli for physiological responses). Modern research has revealed that conditioning involves learning about predictive relationships between events rather than mere temporal pairing, as formalized by the Rescorla-Wagner model.