Conditioned Reflexes: An Investigation of the Physiological Activity of the Cerebral Cortex

Pavlov argues that the activity of the cerebral cortex can be studied as a purely physiological process, without invoking subjective or mentalistic concepts. He rejects the introspective method that had dominated psychology and the speculative neurology that attempted to localize mental faculties in specific brain regions. Through decades of experiments on salivation in dogs, he demonstrates that a neutral stimulus repeatedly paired with an unconditioned stimulus acquires the power to elicit the response—the conditioned reflex—and that this process obeys laws of excitation, inhibition, irradiation, and concentration. The book presents a systematic physiology of higher nervous activity, treating learning, discrimination, and the cortical control of behavior as functions of the hemispheres rather than of a mind.

Conditioned Reflexes was originally published in Russian in 1926 under the title Twenty-Five Years of Objective Study of the Higher Nervous Activity (Behaviour) of Animals. The English translation was made by G. V. Anrep, a Russian physiologist working at University College London, and published by Oxford University Press in 1927. Anrep had worked in Pavlov's laboratory and was well positioned to render the technical material accurately. The translation was widely praised for its clarity and fidelity. This edition became the standard English presentation of Pavlov's systematic work, and it shaped the reception of classical conditioning in the English-speaking world. It was followed in 1928 by Lectures on Conditioned Reflexes, translated by W. Horsley Gantt, which presented Pavlov's lectures and papers in a less systematic but historically richer form.

The core experimental procedure involves a dog with a surgically prepared salivary fistula, placed in a soundproofed chamber to minimize extraneous stimuli. A neutral stimulus, such as a bell or light, is presented shortly before food is delivered. Initially, the bell elicits only an orienting response, and salivation occurs only to food. After repeated pairings, the bell alone elicits salivation—the conditioned reflex. Pavlov then systematically varies the parameters: the interval between conditioned and unconditioned stimuli, the number of trials, the intensity of stimuli, and the conditions of reinforcement. He reports the basic laws of conditioning, the phenomena of extinction, spontaneous recovery, generalization, discrimination, and experimental neurosis. The book is organized as a systematic presentation of these findings rather than a chronological record.

Pavlov was awarded the Nobel Prize in Physiology or Medicine in 1904 for his work on the physiology of digestion, particularly the neural regulation of gastric secretion. It was during these studies that he observed what he called 'psychic secretion'—dogs salivating at the sight of food or the sound of the feeder's footsteps, before any food entered their mouths. Rather than dismissing this as a nuisance or a psychological curiosity, Pavlov recognized it as a phenomenon worthy of physiological study. Conditioned Reflexes is the systematic development of that insight. The book thus represents a continuation of his digestive research by other means: the same experimental rigor and surgical preparation are applied to the higher nervous activity of the cerebral cortex.

Pavlov proposes that the cerebral cortex operates through two fundamental processes: excitation and inhibition. Excitation is the process by which a stimulus acquires the power to elicit a response; inhibition is the process by which a response is suppressed. Both processes are subject to laws of irradiation and concentration: they spread across the cortex from their point of origin, then converge back. Internal inhibition, which underlies extinction and discrimination, is an active process that Pavlov distinguishes from simple fatigue. External inhibition, such as a novel stimulus disrupting a conditioned response, is also analyzed. Pavlov extends this framework to sleep, arguing that sleep is a state of widespread cortical inhibition, and to hypnosis, which he treats as a partial inhibition. These laws constitute a unified physiological account of cortical function.

The book became a foundational text for behaviorism, particularly through the work of John B. Watson, who adopted Pavlov's conditioned reflex as a unit of analysis and made classical conditioning central to his program. Watson wrote that Pavlov's work 'made behaviorism definite and concrete' . However, Pavlov himself resisted being classified as a psychologist, insisting that his work was physiology. He was critical of American behaviorism for what he saw as its neglect of physiological mechanisms and its tendency to theorize beyond the data. The relationship between Pavlovian conditioning and Skinnerian operant conditioning became a central distinction in learning theory: classical conditioning involves responses elicited by stimuli, while operant conditioning involves responses controlled by their consequences. The two paradigms together defined the experimental study of learning for much of the twentieth century.