Ivan Pavlov is best known for his discovery of classical conditioning, a fundamental learning process in which a neutral stimulus becomes associated with a reflex response after repeated pairing with an unconditioned stimulus. His famous experiments with dogs demonstrated that a bell could become a conditioned stimulus for salivation when paired with food. This discovery laid the groundwork for the study of learning, behaviorism, and experimental psychology.

In his most famous experiment, Pavlov observed that dogs naturally salivate (unconditioned response) when presented with food (unconditioned stimulus). He then paired the presentation of food with a neutral stimulus, such as a bell or a metronome. After several pairings, the dogs began to salivate in response to the bell alone, even when no food was presented. This demonstrated that a conditioned reflex could be established, showing that learning occurs through association.

Classical conditioning is a learning process identified by Pavlov in which a neutral stimulus comes to elicit a response after being paired with a stimulus that naturally produces a response. The process involves four key elements: the unconditioned stimulus (which naturally triggers a response), the unconditioned response (the natural reaction), the conditioned stimulus (the previously neutral stimulus), and the conditioned response (the learned reaction). This theory has been foundational in understanding how emotional and physiological responses can be learned.

Pavlov's classical conditioning involves learning through the association of stimuli, where a neutral stimulus is paired with an unconditioned stimulus to elicit a reflexive response. The learner is passive in this process. Skinner's operant conditioning, in contrast, involves learning through the consequences of voluntary behavior, where actions are strengthened or weakened by reinforcement or punishment. While Pavlov focused on reflexive, involuntary behaviors, Skinner emphasized goal-directed, voluntary actions and their outcomes.

Pavlov's most influential works include his lectures on conditioned reflexes, which were published as 'Conditioned Reflexes: An Investigation of the Physiological Activity of the Cerebral Cortex' (1927). He also wrote extensively on digestive physiology, for which he received the Nobel Prize. His research publications and lectures have been widely translated and have had a lasting impact on psychology, neuroscience, and physiology.

Pavlov's work had a profound influence on the development of behaviorism in the United States. His emphasis on observable, measurable behavior and his experimental methods for studying learning provided a scientific model for psychology. Behaviorists, such as John B. Watson and B.F. Skinner, drew on Pavlov's concepts to develop their own theories of learning and behavior. His work helped shift psychology away from introspection and toward objective, experimental methods.

Extinction is the process by which a conditioned response weakens and eventually disappears when the conditioned stimulus is repeatedly presented without the unconditioned stimulus. In Pavlov's experiments, if the bell was repeatedly presented without food, the dogs eventually stopped salivating in response to the bell. This concept has been important in understanding how learned behaviors can be unlearned and has applications in behavioral therapy and treatment of phobias.

Pavlov believed that conditioned reflexes were the result of physiological processes in the cerebral cortex. He proposed that the formation of conditioned reflexes involved the establishment of temporary neural connections between the brain's centers for the conditioned and unconditioned stimuli. His work laid the foundation for the study of neural mechanisms underlying learning and memory. He was a strict physiologist and viewed conditioned reflexes as objective evidence of brain function, avoiding the subjective introspection of mental psychology.